3.8.2 Chillers

Chiller Name
Applicability All chillers
Definition A unique descriptor for each chiller
Units Text, unique
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans.
Baseline Rules Chillers are only designated when the baseline system is of type 7 (VAV with reheat) or 8 (VAV with PFP Boxes) (see Table G3.1.1B).
Chiller Type
Applicability All chillers
Definition The type of chiller, either a vapor-compression chiller or an absorption chiller.
 
Vapor compression chillers operate on the reverse-Rankine cycle, using mechanical energy to compress the refrigerant, and include:
  • Reciprocating – uses pistons for compression
  • Screw – uses two counter rotating screws for compression
  • Scroll – uses two interlocking spirals or scrolls to perform the compression
  • Centrifugal – uses rotating impeller blades to compress the air
  • Absorption chillers – use heat to vaporize a working fluid (usually either ammonia or lithium bromide)
  • Single Effect Absorption – use a single generator & condenser
  • Double Effect Absorption – use two generators/concentrators and condensers, one at a lower temperature and the other at a higher temperature. It is more efficient than the single effect, but it must use a higher temperature heat source.
Units List (see above)
Input Restrictions As designed
Baseline Rules

The baseline building chiller is based on the design capacity of the proposed design as follows from ASHRAE 90.1 Appendix G:
[table title="Type and Number of Chillers" id="type-and-number-of-chillers"]

Building Peak
Cooling Load
Number and Type of Chillers (s)
$\leq$300 tons 1 water-cooled screw chiller
>300 tons, <600 tons 2 water-cooled screw chillers sized equally
$\geq$600 tons 2 water-cooled centrigugal chillers minimum with chillers added so that no chiller is larger than 800 tons, all sized equally
Number of Identical Chiller Units
Applicability All chillers
Definition The number of identical units for staging.
Units None
Input Restrictions As designed. Default is 1.
Baseline Rules From [bookref id="type-and-number-of-chillers"] above, there is one chiller if the cooling load is 300 tons or less and two equally sized chillers for loads between 300 and 600 tons. For loads above 600 tons, two or more chillers of equal size are used, with no chiller larger than 800 tons.
Chiller Fuel
Applicability All chillers
Definition The fuel source for the chiller. The choices are:
  • Electricity (for all vapor-compression chillers)
  • Gas (Absorption units only, designated as direct-fired units)
  • Oil (Absorption units only, designated as direct-fired units)
  • Hot Water (Absorption units only, designated as indirect-fired units)
  • Steam (Absorption units only, designated as indirect-fired units)
Units List (see above)
Input Restrictions As designed.
Baseline Rules Electricity
Chiller Rated Capacity
Applicability All chillers
Definition The cooling capacity of a piece of heating equipment at rated conditions.
Units Btu/h or tons
Input Restrictions As designed. If unmet load hours are greater than 300, the chiller may have to be made larger.
Baseline Rules Determine loads for baseline building and oversize by 15%.
Chiller Rated Efficiency
Applicability All chillers
Definition The Coefficient of Performance (COP) at ARI rated conditions.
Units Ratio
Input Restrictions As designed
Baseline Rules With the ASHRAE Standard 90.1-2007 baseline, use the minimum values of efficiency from either Table 6.8.1C for various types of chillers or the values from Tables 6.8.1H, 6.8.1I or 6.8.1J for centrifugal chillers. With the ASHRAE Standard 90.1-2001 baseline, use the minimum values of efficiency from either Table 6.2.1C for various types of chillers, or the values from Tables 6.2.1H, 6. 2.1I or 6. 2.1J for centrifugal chillers.
Chiller Minimum Unloading Ratio
Applicability All chillers
Definition

The minimum unloading capacity of a chiller expressed as a fraction of the rated capacity. Below this level the chiller must cycle to meet the load.


[table title="Default Minimum Unloading Ratios" id="default-minimum-unloading-ratios"]

Chiller Type Default Unloading Ratio
Reciprocating 25%
Screw 15%
Centrifugal 10%
Scroll 25%
Single Effect Absorption 10%
Double Effect Absorption 10%
Units Percent (%)
Input Restrictions As designed. If the user does not employ the default values, supporting documentation is required.
Baseline Rules Use defaults listed above.
Chiller Cooling Capacity Adjustment Curve
Applicability All chillers
Definition

A curve or group of curves or other functions that represent the available total cooling capacity as a function of evaporator and condenser conditions and perhaps other operating conditions. The default curves are given as follows:

(6.8.2-1)

$$ Q_{available} = CAP \_ FT \times Q_{rated}$$

For air-cooled chillers:

(6.8.2-2)

$$ CAP \_FT = a + b \times t_{chws} + c \times \left. t_{chws}\right. ^2 + d \times t_{odb} + e \times \left. t_{odb}\right. ^2 + f \times t_{chws} \times t_{odb}$$

For water-cooled chillers:

(6.8.2-3)

$$ CAP \_FT = a + b \times t_{chws} + c \times \left. t_{chws} \right. ^2 + d \times t_{cws} + e \times \left. t_{cws} \right. ^2 + f \times t_{chws} \times t_{cws}$$

where

Qavailable Available cooling capacity at present evaporator and condenser conditions (MBH)
tchws The chilled water supply temperature (°F)
tcws The condenser water supply temperature (°F)
todb The outside air dry-bulb temperature (°F)
Qrated Rated capacity at ARI conditions (MBH)

Note: If an air-cooled unit employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

[table title="Default Capacity Coefficients – Electric Air-Cooled Chillers" id="default-capacity-coefficients-–-electric-air---cooled-chillers"]

Coefficient Scroll Recip Screw Centrifugal
a 0.40070684 0.57617295 -0.09464899 N/A
b 0.01861548 0.02063133 0.03834070 N/A
c 0.00007199 0.00007769 -0.00009205 N/A
d 0.00177296 -0.00351183 0.00378007 N/A
e -0.00002014 0.00000312 -0.00001375 N/A
f -0.00008273 -0.00007865 -0.00015464 N/A

[table title="Default Capacity Coefficients – Electric Water-Cooled Chillers" id="default-capacity-coefficients-–-electric-water---cooled-chillers"]

Coefficient Scroll Recip Screw Centrifugal
a 0.36131454 0.58531422 0.33269598 -0.29861976
b 0.01855477 0.01539593 0.00729116 0.02996076
c 0.00003011 0.00007296 -0.00049938 -0.00080125
d 0.00093592 -0.00212462 0.01598983 0.01736268
e -0.00001518 -0.00000715 -0.00028254 -0.00032606
f -0.00005481 -0.00004597 0.00052346 0.00063139

[table title="Default Capacity Coefficients – Fuel- & Steam-Source Water-Cooled Chillers" id="default-capacity-coefficients-–-fuel--&-steam---source-water---cooled-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption Engine Driven Chiller
a 0.723412 -0.816039 1.000000 0.573597
b 0.079006 -0.038707 0.000000 0.0186802
c -0.000897 0.000450 0.000000 0.000000
d -0.025285 0.071491 0.000000 -0.00465325
e -0.000048 -0.000636 0.000000 0.000000
f 0.000276 0.000312 0.000000 0.000000
Units Data structure
Input Restrictions User may input curves, other appropriate functions, or use default curves. If the default curves are overridden, supporting documentation is required.
Baseline Rules Use default curves.
Electric Chiller Cooling Efficiency Adjustment Curves
Applicability All chillers
Definition

A curve or group of curves that varies the cooling efficiency of an electric chiller as a function of evaporator conditions, condenser conditions and part-load ratio. The default curves are given as follows:

(6.8.2-4)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ EIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ air-cooled \enspace EIR\_FT = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 + \ d \times t_{odb} + \ e \times \left. t_{odb}\right. ^2+ \ f \times t_{chws} \times t_{odb}$$ $$ water-cooled \enspace EIR\_FT = a + b \times t_{chws}+ c \times \left. t_{chws}\right. ^2 + d \times t_{cws} + e \times \left. t_{cws}\right. ^2+ f \times t_{chws} \times t_{cws}$$ $$P_{operating} = P_{rated} \times EIR\_FPLR \times EIR\_FT \times CAP\_FT $$

where

PLR Part load ratio based on available capacity (not rated capacity)
Qoperating Present load on chiller (Btu/h)
Qavailable Chiller available capacity at present evaporator and condenser conditions (Btu/h)
tchws The chilled water supply temperature (°F)
tcws The condenser water supply temperature (°F)
todb The outside air dry-bulb temperature (°F)
Prated Rated power draw at ARI conditions (kW)
Poperating Power draw at specified operating conditions (kW)

Note: If an air-cooled chiller employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

[table title="Default Efficiency EIR-FT Coefficients – Air-Cooled Chillers" id="default-efficiency-eir---ft-coefficients-–-air---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.99006553 0.66534403 0.13545636 N/A
b -0.00584144 -0.01383821 0.02292946 N/A
c 0.00016454 0.00014736 -0.00016107 N/A
d -0.00661136 0.00712808 -0.00235396 N/A
e 0.00016808 0.00004571 0.00012991 N/A
f -0.00022501 -0.00010326 -0.00018685 N/A

[table title="Default Efficiency EIR-FT Coefficients – Water-Cooled Chiller" id="default-efficiency-eir--ft-coefficients-–-water---cooled-chiller"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 1.00121431 0.46140041 0.66625403 0.51777196
b -0.01026981 -0.00882156 0.00068584 -0.00400363
c 0.00016703 0.00008223 0.00028498 0.00002028
d -0.00128136 0.00926607 -0.00341677 0.00698793
e 0.00014613 0.00005722 0.00025484 0.00008290
f -0.00021959 -0.00011594 -0.00048195 -0.00015467

[table title="Default Efficiency EIR-FPLR Coefficients – Air-Cooled Chillers" id="default-efficiency-eir---fplr-coefficients-–-air---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.06369119 0.11443742 0.03648722 N/A
b 0.58488832 0.54593340 0.73474298 N/A
c 0.35280274 0.34229861 0.21994748 N/A

[table title="Default Efficiency EIR-FPLR Coefficients – Water-Cooled Chillers" id="default-efficiency-eir---fplr-coefficients-–-water---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.04411957 0.08144133 0.33018833 0.17149273
b 0.64036703 0.41927141 0.23554291 0.58820208
c 0.31955532 0.49939604 0.46070828 0.23737257
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, supporting documentation is required.
Baseline Rules Use default curves.
Fuel and Steam Chiller Cooling Efficiency Adjustment Curves
Applicability All chillers
Definition

A curve or group of curves that varies the cooling efficiency of a fuel-fired or steam chiller as a function of evaporator conditions, condenser conditions, and part-load ratio. The default curves are given as follows:
Default Curves for Steam-Driven Single and Double Effect Absorption Chillers

(6.8.2-5)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ FIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ FIR\_FT = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 + \ d \times t_{cws} + \ e \times \left. t_{cws}\right. ^2+ \ f \times t_{chws} \times t_{cws}$$ $$Fuel_{partload} = Fuel_{rated} \times FIR\_FPLR \times FIR\_FT \times CAP\_FT $$

Default Curves for Direct-Fired Double Effect Absorption Chillers

(6.8.2-6)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ FIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ FIR\_FT1 = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 $$ $$ FIR\_FT2 = d + e \times t_{cws} + \ f \times \left. t_{cws}\right. ^2 $$ $$Fuel_{partload} = Fuel_{rated} \times FIR\_FPLR \times FIR\_FT1 \times FIR\_FT2 \times CAP\_FT $$

The default curves for engine driven chillers are the same format as those for the Steam-Driven Single and Double Effect Absorption Chillers but there are three sets of curves for different ranges of operation based on the engine speed.

where

PLR Part load ratio based on available capacity (not rated capacity)
FIR-FPLR A multiplier on the fuel input ratio (FIR) to account for part load conditions
FIR-FT A multiplier on the fuel input ratio (FIR) to account for the chiller water supply temperature and the condenser water temperature
FIR-FT1 A multiplier on the fuel input ratio (FIR) to account for chilled water supply temperature
FIR-FT2 A multiplier on the fuel input ratio (FIR) to account for condenser water supply temperature
CAP-FT A multiplier on the capacity of the chiller (see Equation (6.8.2-2))
Qoperating Present load on chiller (in Btu/h)
Qavailable Chiller available capacity at present evaporator and condenser conditions (in Btu/h)
tchws The chilled water supply temperature (in °F)
tcws The condenser water supply temperature (in °F)
todb The outside air dry-bulb temperature (°F)
Fuelrated Rated fuel consumption at ARI conditions (in Btu/h)
Fuelpartload Fuel consumption at specified operating conditions (in Btu/h)

[table title="Default FIR-FPLR coefficients – Fuel- & Steam-Source Water-Cooled Absorption Chillers" id="default-fir---fplr-coefficients-–-fuel---&-steam---source-water---cooled-absorption-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption
a 0.098585 0.013994 0.13551150
b 0.583850 1.240449 0.61798084
c 0.560658 -0.914883 0.24651277
d -0.243093 0.660441 0.00000000

[table title="Default FIR-FPLR coefficients – Engine Driven Chillers" id="default-fir---fplr-coefficients-–-engine-driven-chillers"]

Coefficient %Speed$\leq$Min. %Speed>Min.
%Speed<60%
%Speed>60%
a 0.3802 1.14336 1.38861
b 2.3609 0.022889 -0.388614
c 0.0000 0.0000 0.0000
d 0.0000 0.0000 0.000

[table title="Default FIR-FT coefficients – Fuel- & Steam-Source Water-Cooled Absorption Chillers" id="default-fir---ft-coefficients-–-fuel---&-steam---source-water---cooled-absorption-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption
a 0.652273 1.658750 4.42871284
b 0.000000 0.000000 -0.13298607
c 0.000000 0.000000 0.00125331
d -0.000545 -0.290000 0.86173749
e 0.000055 0.000250 -0.00708917
f 0.000000 0.000000 0.0010251

[table title="Default FIR-FT coefficients – Engine Driven Chillers" id="default-fir---ft-coefficients-–-engine-driven-chillers"]

Coefficient %Speed$\leq$Min. %Speed>Min.
%Speed<60%
%Speed>60%
a 1.0881500 1.2362400 1.2362400
b 0.0141064 0.0168923 0.0168923
c 0.0000000 0.0000000 0.0000000
d -0.00833912 -0.0115235 -0.0115235
e 0.0000000 0.0000000 0.0000000
f 0.0000000 0.0000000 0.0000000
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, supporting documentation is required.
Baseline Rules Use default curves.
Chilled Water Supply Temperature
Applicability All chillers
Definition The chilled water supply temperature of the chiller at design conditions
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules The baseline chilled water supply temperature is set to 44°F.
Chilled Water Return Temperature
Applicability All chillers
Definition The chilled water return temperature setpoint
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules The baseline chilled water return temperature is set to 56°F.
Chilled Water Supply Temperature Reset
Applicability All chillers
Definition The reset schedule for the chilled water supply temperature. The chilled water setpoint may be reset based on demand or outdoor air temperature.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. The default is [bookref id="chilled-water-supply-temperature-reset-schedule"].

Baseline Rules

The baseline chilled water supply temperature is reset from 44°F to 54°F based on outdoor air temperature as shown in the figure below.

[figure title="Chilled Water Supply Temperature Reset Schedule" id="chilled-water-supply-temperature-reset-schedule"]Chilled Water Supply Temperature Reset Schedule[/figure]

Condenser Type
Applicability All chillers
Definition The type of condenser for a chiller. The choices are:
  • Air-Cooled
  • Water-Cooled
  • Evaporatively-Cooled
Air-cooled chillers use air to cool the condenser coils. Water-cooled chillers use cold water to cool the condenser and additionally need either a cooling tower or a local source of cold water. Evaporatively-cooled chillers are similar to air-cooled chillers, except they use a water mist to cool the condenser coil which makes them more efficient.
Units List (see above)
Input Restrictions As designed
Baseline Rules The baseline chiller is always assumed to have a water-cooled condenser, although the chiller type will change depending on the design capacity. If the chiller size is less than 600 tons, the baseline chiller is a water-cooled screw; if the capacity is greater than or equal to 600 tons, the baseline chiller is a water-cooled centrifugal chiller.
Air-Cooled Condenser Power
Applicability All chillers
Definition The energy usage of the condenser fan(s) at design conditions on an air-cooled chiller. This unit should only be used for chillers composed of separate evaporator and condenser sections where the fan energy is not part of the chiller COP.
Units Kilowatts (kW)
Input Restrictions As designed. The user must enter data for remote air-cooled condensing units.
Baseline Rules Not applicable, since all baseline chillers have water-cooled condensers.
90.1-2007
Chiller Name
Applicability All chillers
Definition A unique descriptor for each chiller
Units Text, unique
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans.
Baseline Rules Chillers are only designated when the baseline system is of type 7 (VAV with reheat) or 8 (VAV with PFP Boxes) (see Table G3.1.1B).
Chiller Type
Applicability All chillers
Definition The type of chiller, either a vapor-compression chiller or an absorption chiller.
 
Vapor compression chillers operate on the reverse-Rankine cycle, using mechanical energy to compress the refrigerant, and include:
  • Reciprocating – uses pistons for compression
  • Screw – uses two counter rotating screws for compression
  • Scroll – uses two interlocking spirals or scrolls to perform the compression
  • Centrifugal – uses rotating impeller blades to compress the air
  • Absorption chillers – use heat to vaporize a working fluid (usually either ammonia or lithium bromide)
  • Single Effect Absorption – use a single generator & condenser
  • Double Effect Absorption – use two generators/concentrators and condensers, one at a lower temperature and the other at a higher temperature. It is more efficient than the single effect, but it must use a higher temperature heat source.
Units List (see above)
Input Restrictions As designed
Baseline Rules

The baseline building chiller is based on the design capacity of the proposed design as follows from ASHRAE 90.1 Appendix G:
[table title="Type and Number of Chillers" id="type-and-number-of-chillers"]

Building Peak
Cooling Load
Number and Type of Chillers (s)
$\leq$300 tons 1 water-cooled screw chiller
>300 tons, <600 tons 2 water-cooled screw chillers sized equally
$\geq$600 tons 2 water-cooled centrigugal chillers minimum with chillers added so that no chiller is larger than 800 tons, all sized equally
Number of Identical Chiller Units
Applicability All chillers
Definition The number of identical units for staging.
Units None
Input Restrictions As designed. Default is 1.
Baseline Rules From [bookref id="type-and-number-of-chillers"] above, there is one chiller if the cooling load is 300 tons or less and two equally sized chillers for loads between 300 and 600 tons. For loads above 600 tons, two or more chillers of equal size are used, with no chiller larger than 800 tons.
Chiller Fuel
Applicability All chillers
Definition The fuel source for the chiller. The choices are:
  • Electricity (for all vapor-compression chillers)
  • Gas (Absorption units only, designated as direct-fired units)
  • Oil (Absorption units only, designated as direct-fired units)
  • Hot Water (Absorption units only, designated as indirect-fired units)
  • Steam (Absorption units only, designated as indirect-fired units)
Units List (see above)
Input Restrictions As designed.
Baseline Rules Electricity
Chiller Rated Capacity
Applicability All chillers
Definition The cooling capacity of a piece of heating equipment at rated conditions.
Units Btu/h or tons
Input Restrictions As designed. If unmet load hours are greater than 300, the chiller may have to be made larger.
Baseline Rules Determine loads for baseline building and oversize by 15%.
Chiller Rated Efficiency
Applicability All chillers
Definition The Coefficient of Performance (COP) at ARI rated conditions.
Units Ratio
Input Restrictions As designed
Baseline Rules With the ASHRAE Standard 90.1-2007 baseline, use the minimum values of efficiency from either Table 6.8.1C for various types of chillers or the values from Tables 6.8.1H, 6.8.1I or 6.8.1J for centrifugal chillers.
Chiller Minimum Unloading Ratio
Applicability All chillers
Definition

The minimum unloading capacity of a chiller expressed as a fraction of the rated capacity. Below this level the chiller must cycle to meet the load.


[table title="Default Minimum Unloading Ratios" id="default-minimum-unloading-ratios"]

Chiller Type Default Unloading Ratio
Reciprocating 25%
Screw 15%
Centrifugal 10%
Scroll 25%
Single Effect Absorption 10%
Double Effect Absorption 10%
Units Percent (%)
Input Restrictions As designed. If the user does not employ the default values, supporting documentation is required.
Baseline Rules Use defaults listed above.
Chiller Cooling Capacity Adjustment Curve
Applicability All chillers
Definition

A curve or group of curves or other functions that represent the available total cooling capacity as a function of evaporator and condenser conditions and perhaps other operating conditions. The default curves are given as follows:

(6.8.2-1)

$$ Q_{available} = CAP \_ FT \times Q_{rated}$$

For air-cooled chillers:

(6.8.2-2)

$$ CAP \_FT = a + b \times t_{chws} + c \times \left. t_{chws}\right. ^2 + d \times t_{odb} + e \times \left. t_{odb}\right. ^2 + f \times t_{chws} \times t_{odb}$$

For water-cooled chillers:

(6.8.2-3)

$$ CAP \_FT = a + b \times t_{chws} + c \times \left. t_{chws} \right. ^2 + d \times t_{cws} + e \times \left. t_{cws} \right. ^2 + f \times t_{chws} \times t_{cws}$$

where

Qavailable Available cooling capacity at present evaporator and condenser conditions (MBH)
tchws The chilled water supply temperature (°F)
tcws The condenser water supply temperature (°F)
todb The outside air dry-bulb temperature (°F)
Qrated Rated capacity at ARI conditions (MBH)

Note: If an air-cooled unit employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

[table title="Default Capacity Coefficients – Electric Air-Cooled Chillers" id="default-capacity-coefficients-–-electric-air---cooled-chillers"]

Coefficient Scroll Recip Screw Centrifugal
a 0.40070684 0.57617295 -0.09464899 N/A
b 0.01861548 0.02063133 0.03834070 N/A
c 0.00007199 0.00007769 -0.00009205 N/A
d 0.00177296 -0.00351183 0.00378007 N/A
e -0.00002014 0.00000312 -0.00001375 N/A
f -0.00008273 -0.00007865 -0.00015464 N/A

[table title="Default Capacity Coefficients – Electric Water-Cooled Chillers" id="default-capacity-coefficients-–-electric-water---cooled-chillers"]

Coefficient Scroll Recip Screw Centrifugal
a 0.36131454 0.58531422 0.33269598 -0.29861976
b 0.01855477 0.01539593 0.00729116 0.02996076
c 0.00003011 0.00007296 -0.00049938 -0.00080125
d 0.00093592 -0.00212462 0.01598983 0.01736268
e -0.00001518 -0.00000715 -0.00028254 -0.00032606
f -0.00005481 -0.00004597 0.00052346 0.00063139

[table title="Default Capacity Coefficients – Fuel- & Steam-Source Water-Cooled Chillers" id="default-capacity-coefficients-–-fuel--&-steam---source-water---cooled-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption Engine Driven Chiller
a 0.723412 -0.816039 1.000000 0.573597
b 0.079006 -0.038707 0.000000 0.0186802
c -0.000897 0.000450 0.000000 0.000000
d -0.025285 0.071491 0.000000 -0.00465325
e -0.000048 -0.000636 0.000000 0.000000
f 0.000276 0.000312 0.000000 0.000000
Units Data structure
Input Restrictions User may input curves, other appropriate functions, or use default curves. If the default curves are overridden, supporting documentation is required.
Baseline Rules Use default curves.
Electric Chiller Cooling Efficiency Adjustment Curves
Applicability All chillers
Definition

A curve or group of curves that varies the cooling efficiency of an electric chiller as a function of evaporator conditions, condenser conditions and part-load ratio. The default curves are given as follows:

(6.8.2-4)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ EIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ air-cooled \enspace EIR\_FT = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 + \ d \times t_{odb} + \ e \times \left. t_{odb}\right. ^2+ \ f \times t_{chws} \times t_{odb}$$ $$ water-cooled \enspace EIR\_FT = a + b \times t_{chws}+ c \times \left. t_{chws}\right. ^2 + d \times t_{cws} + e \times \left. t_{cws}\right. ^2+ f \times t_{chws} \times t_{cws}$$ $$P_{operating} = P_{rated} \times EIR\_FPLR \times EIR\_FT \times CAP\_FT $$

where

PLR Part load ratio based on available capacity (not rated capacity)
Qoperating Present load on chiller (Btu/h)
Qavailable Chiller available capacity at present evaporator and condenser conditions (Btu/h)
tchws The chilled water supply temperature (°F)
tcws The condenser water supply temperature (°F)
todb The outside air dry-bulb temperature (°F)
Prated Rated power draw at ARI conditions (kW)
Poperating Power draw at specified operating conditions (kW)

Note: If an air-cooled chiller employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

[table title="Default Efficiency EIR-FT Coefficients – Air-Cooled Chillers" id="default-efficiency-eir---ft-coefficients-–-air---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.99006553 0.66534403 0.13545636 N/A
b -0.00584144 -0.01383821 0.02292946 N/A
c 0.00016454 0.00014736 -0.00016107 N/A
d -0.00661136 0.00712808 -0.00235396 N/A
e 0.00016808 0.00004571 0.00012991 N/A
f -0.00022501 -0.00010326 -0.00018685 N/A

[table title="Default Efficiency EIR-FT Coefficients – Water-Cooled Chiller" id="default-efficiency-eir--ft-coefficients-–-water---cooled-chiller"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 1.00121431 0.46140041 0.66625403 0.51777196
b -0.01026981 -0.00882156 0.00068584 -0.00400363
c 0.00016703 0.00008223 0.00028498 0.00002028
d -0.00128136 0.00926607 -0.00341677 0.00698793
e 0.00014613 0.00005722 0.00025484 0.00008290
f -0.00021959 -0.00011594 -0.00048195 -0.00015467

[table title="Default Efficiency EIR-FPLR Coefficients – Air-Cooled Chillers" id="default-efficiency-eir---fplr-coefficients-–-air---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.06369119 0.11443742 0.03648722 N/A
b 0.58488832 0.54593340 0.73474298 N/A
c 0.35280274 0.34229861 0.21994748 N/A

[table title="Default Efficiency EIR-FPLR Coefficients – Water-Cooled Chillers" id="default-efficiency-eir---fplr-coefficients-–-water---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.04411957 0.08144133 0.33018833 0.17149273
b 0.64036703 0.41927141 0.23554291 0.58820208
c 0.31955532 0.49939604 0.46070828 0.23737257
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, supporting documentation is required.
Baseline Rules Use default curves.
Fuel and Steam Chiller Cooling Efficiency Adjustment Curves
Applicability All chillers
Definition

A curve or group of curves that varies the cooling efficiency of a fuel-fired or steam chiller as a function of evaporator conditions, condenser conditions, and part-load ratio. The default curves are given as follows:
Default Curves for Steam-Driven Single and Double Effect Absorption Chillers

(6.8.2-5)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ FIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ FIR\_FT = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 + \ d \times t_{cws} + \ e \times \left. t_{cws}\right. ^2+ \ f \times t_{chws} \times t_{cws}$$ $$Fuel_{partload} = Fuel_{rated} \times FIR\_FPLR \times FIR\_FT \times CAP\_FT $$

Default Curves for Direct-Fired Double Effect Absorption Chillers

(6.8.2-6)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ FIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ FIR\_FT1 = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 $$ $$ FIR\_FT2 = d + e \times t_{cws} + \ f \times \left. t_{cws}\right. ^2 $$ $$Fuel_{partload} = Fuel_{rated} \times FIR\_FPLR \times FIR\_FT1 \times FIR\_FT2 \times CAP\_FT $$

The default curves for engine driven chillers are the same format as those for the Steam-Driven Single and Double Effect Absorption Chillers but there are three sets of curves for different ranges of operation based on the engine speed.

where

PLR Part load ratio based on available capacity (not rated capacity)
FIR-FPLR A multiplier on the fuel input ratio (FIR) to account for part load conditions
FIR-FT A multiplier on the fuel input ratio (FIR) to account for the chiller water supply temperature and the condenser water temperature
FIR-FT1 A multiplier on the fuel input ratio (FIR) to account for chilled water supply temperature
FIR-FT2 A multiplier on the fuel input ratio (FIR) to account for condenser water supply temperature
CAP-FT A multiplier on the capacity of the chiller (see Equation (6.8.2-2))
Qoperating Present load on chiller (in Btu/h)
Qavailable Chiller available capacity at present evaporator and condenser conditions (in Btu/h)
tchws The chilled water supply temperature (in °F)
tcws The condenser water supply temperature (in °F)
todb The outside air dry-bulb temperature (°F)
Fuelrated Rated fuel consumption at ARI conditions (in Btu/h)
Fuelpartload Fuel consumption at specified operating conditions (in Btu/h)

[table title="Default FIR-FPLR coefficients – Fuel- & Steam-Source Water-Cooled Absorption Chillers" id="default-fir---fplr-coefficients-–-fuel---&-steam---source-water---cooled-absorption-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption
a 0.098585 0.013994 0.13551150
b 0.583850 1.240449 0.61798084
c 0.560658 -0.914883 0.24651277
d -0.243093 0.660441 0.00000000

[table title="Default FIR-FPLR coefficients – Engine Driven Chillers" id="default-fir---fplr-coefficients-–-engine-driven-chillers"]

Coefficient %Speed$\leq$Min. %Speed>Min.
%Speed<60%
%Speed>60%
a 0.3802 1.14336 1.38861
b 2.3609 0.022889 -0.388614
c 0.0000 0.0000 0.0000
d 0.0000 0.0000 0.000

[table title="Default FIR-FT coefficients – Fuel- & Steam-Source Water-Cooled Absorption Chillers" id="default-fir---ft-coefficients-–-fuel---&-steam---source-water---cooled-absorption-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption
a 0.652273 1.658750 4.42871284
b 0.000000 0.000000 -0.13298607
c 0.000000 0.000000 0.00125331
d -0.000545 -0.290000 0.86173749
e 0.000055 0.000250 -0.00708917
f 0.000000 0.000000 0.0010251

[table title="Default FIR-FT coefficients – Engine Driven Chillers" id="default-fir---ft-coefficients-–-engine-driven-chillers"]

Coefficient %Speed$\leq$Min. %Speed>Min.
%Speed<60%
%Speed>60%
a 1.0881500 1.2362400 1.2362400
b 0.0141064 0.0168923 0.0168923
c 0.0000000 0.0000000 0.0000000
d -0.00833912 -0.0115235 -0.0115235
e 0.0000000 0.0000000 0.0000000
f 0.0000000 0.0000000 0.0000000
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, supporting documentation is required.
Baseline Rules Use default curves.
Chilled Water Supply Temperature
Applicability All chillers
Definition The chilled water supply temperature of the chiller at design conditions
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules The baseline chilled water supply temperature is set to 44°F.
Chilled Water Return Temperature
Applicability All chillers
Definition The chilled water return temperature setpoint
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules The baseline chilled water return temperature is set to 56°F.
Chilled Water Supply Temperature Reset
Applicability All chillers
Definition The reset schedule for the chilled water supply temperature. The chilled water setpoint may be reset based on demand or outdoor air temperature.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. The default is [bookref id="chilled-water-supply-temperature-reset-schedule"].

Baseline Rules

The baseline chilled water supply temperature is reset from 44°F to 54°F based on outdoor air temperature as shown in the figure below.

[figure title="Chilled Water Supply Temperature Reset Schedule" id="chilled-water-supply-temperature-reset-schedule"]Chilled Water Supply Temperature Reset Schedule[/figure]

Condenser Type
Applicability All chillers
Definition The type of condenser for a chiller. The choices are:
  • Air-Cooled
  • Water-Cooled
  • Evaporatively-Cooled
Air-cooled chillers use air to cool the condenser coils. Water-cooled chillers use cold water to cool the condenser and additionally need either a cooling tower or a local source of cold water. Evaporatively-cooled chillers are similar to air-cooled chillers, except they use a water mist to cool the condenser coil which makes them more efficient.
Units List (see above)
Input Restrictions As designed
Baseline Rules The baseline chiller is always assumed to have a water-cooled condenser, although the chiller type will change depending on the design capacity. If the chiller size is less than 600 tons, the baseline chiller is a water-cooled screw; if the capacity is greater than or equal to 600 tons, the baseline chiller is a water-cooled centrifugal chiller.
Air-Cooled Condenser Power
Applicability All chillers
Definition The energy usage of the condenser fan(s) at design conditions on an air-cooled chiller. This unit should only be used for chillers composed of separate evaporator and condenser sections where the fan energy is not part of the chiller COP.
Units Kilowatts (kW)
Input Restrictions As designed. The user must enter data for remote air-cooled condensing units.
Baseline Rules Not applicable, since all baseline chillers have water-cooled condensers.
90.1-2010
Chiller Name
Applicability All chillers
Definition A unique descriptor for each chiller
Units Text, unique
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans.
Baseline Rules Chillers are only designated when the baseline system is of type 7 (VAV with reheat) or 8 (VAV with PFP Boxes) (see Table G3.1.1B).
Chiller Type
Applicability All chillers
Definition The type of chiller, either a vapor-compression chiller or an absorption chiller.
 
Vapor compression chillers operate on the reverse-Rankine cycle, using mechanical energy to compress the refrigerant, and include:
  • Reciprocating – uses pistons for compression
  • Screw – uses two counter rotating screws for compression
  • Scroll – uses two interlocking spirals or scrolls to perform the compression
  • Centrifugal – uses rotating impeller blades to compress the air
  • Absorption chillers – use heat to vaporize a working fluid (usually either ammonia or lithium bromide)
  • Single Effect Absorption – use a single generator & condenser
  • Double Effect Absorption – use two generators/concentrators and condensers, one at a lower temperature and the other at a higher temperature. It is more efficient than the single effect, but it must use a higher temperature heat source.
Units List (see above)
Input Restrictions As designed
Baseline Rules

The baseline building chiller is based on the design capacity of the proposed design as follows from ASHRAE 90.1 Appendix G:

 

Table 6.8.2-1: Type and Number of Chillers

Building Peak
Cooling Load
Number and Type of Chillers (s)
$\leq$300 tons 1 water-cooled screw chiller
>300 tons, <600 tons 2 water-cooled screw chillers sized equally
$\geq$600 tons 2 water-cooled centrigugal chillers minimum with chillers added so that no chiller is larger than 800 tons, all sized equally
Number of Identical Chiller Units
Applicability All chillers
Definition The number of identical units for staging.
Units None
Input Restrictions As designed. Default is 1.
Baseline Rules From Table 7.8.2-1 above, there is one chiller if the cooling load is 300 tons or less and two equally sized chillers for loads between 300 and 600 tons. For loads above 600 tons, two or more chillers of equal size are used, with no chiller larger than 800 tons.
Chiller Fuel
Applicability All chillers
Definition The fuel source for the chiller. The choices are:
  • Electricity (for all vapor-compression chillers)
  • Gas (Absorption units only, designated as direct-fired units)
  • Oil (Absorption units only, designated as direct-fired units)
  • Hot Water (Absorption units only, designated as indirect-fired units)
  • Steam (Absorption units only, designated as indirect-fired units)
Units List (see above)
Input Restrictions As designed.
Baseline Rules Electricity
Chiller Rated Capacity
Applicability All chillers
Definition The cooling capacity of a piece of heating equipment at rated conditions.
Units Btu/h or tons
Input Restrictions As designed. If unmet load hours are greater than 300, the chiller may have to be made larger.
Baseline Rules Determine loads for baseline building and oversize by 15%.
Chiller Rated Efficiency
Applicability All chillers
Definition The Coefficient of Performance (COP) at ARI rated conditions.
Units Ratio
Input Restrictions As designed
Baseline Rules With the ASHRAE Standard 90.1-2007 baseline, use the minimum values of efficiency from either Table 6.8.1C for various types of chillers or the values from Tables 6.8.1H, 6.8.1I or 6.8.1J for centrifugal chillers.
Chiller Minimum Unloading Ratio
Applicability All chillers
Definition

The minimum unloading capacity of a chiller expressed as a fraction of the rated capacity. Below this level the chiller must cycle to meet the load.


Table 6.8.2-2: Default Minimum Unloading Ratios

Chiller Type Default Unloading Ratio
Reciprocating 25%
Screw 15%
Centrifugal 10%
Scroll 25%
Single Effect Absorption 10%
Double Effect Absorption 10%
Units Percent (%)
Input Restrictions As designed. If the user does not employ the default values, supporting documentation is required.
Baseline Rules Use defaults listed above.
Chiller Cooling Capacity Adjustment Curve
Applicability All chillers
Definition

A curve or group of curves or other functions that represent the available total cooling capacity as a function of evaporator and condenser conditions and perhaps other operating conditions. The default curves are given as follows:

(6.8.2-1)

$$ Q_{available} = CAP \_ FT \times Q_{rated}$$

For air-cooled chillers:

(6.8.2-2)

$$ CAP \_FT = a + b \times t_{chws} + c \times \left. t_{chws}\right. ^2 + d \times t_{odb} + e \times \left. t_{odb}\right. ^2 + f \times t_{chws} \times t_{odb}$$

For water-cooled chillers:

(6.8.2-3)

$$ CAP \_FT = a + b \times t_{chws} + c \times \left. t_{chws} \right. ^2 + d \times t_{cws} + e \times \left. t_{cws} \right. ^2 + f \times t_{chws} \times t_{cws}$$

where

Qavailable Available cooling capacity at present evaporator and condenser conditions (MBH)
tchws The chilled water supply temperature (°F)
tcws The condenser water supply temperature (°F)
todb The outside air dry-bulb temperature (°F)
Qrated Rated capacity at ARI conditions (MBH)

Note: If an air-cooled unit employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

 

Table 6.8.2-3: Default Capacity Coefficients – Electric Air-Cooled Chillers

Coefficient Scroll Recip Screw Centrifugal
a 0.40070684 0.57617295 -0.09464899 N/A
b 0.01861548 0.02063133 0.03834070 N/A
c 0.00007199 0.00007769 -0.00009205 N/A
d 0.00177296 -0.00351183 0.00378007 N/A
e -0.00002014 0.00000312 -0.00001375 N/A
f -0.00008273 -0.00007865 -0.00015464 N/A

Table 6.8.2-4: Default Capacity Coefficients – Electric Water-Cooled Chillers

Coefficient Scroll Recip Screw Centrifugal
a 0.36131454 0.58531422 0.33269598 -0.29861976
b 0.01855477 0.01539593 0.00729116 0.02996076
c 0.00003011 0.00007296 -0.00049938 -0.00080125
d 0.00093592 -0.00212462 0.01598983 0.01736268
e -0.00001518 -0.00000715 -0.00028254 -0.00032606
f -0.00005481 -0.00004597 0.00052346 0.00063139

Table 6.8.2-5: Default Capacity Coefficients – Fuel- & Steam-Source Water-Cooled Chillers

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption Engine Driven Chiller
a 0.723412 -0.816039 1.000000 0.573597
b 0.079006 -0.038707 0.000000 0.0186802
c -0.000897 0.000450 0.000000 0.000000
d -0.025285 0.071491 0.000000 -0.00465325
e -0.000048 -0.000636 0.000000 0.000000
f 0.000276 0.000312 0.000000 0.000000
Units Data structure
Input Restrictions User may input curves, other appropriate functions, or use default curves. If the default curves are overridden, supporting documentation is required.
Baseline Rules Use default curves.
Electric Chiller Cooling Efficiency Adjustment Curves
Applicability All chillers
Definition

A curve or group of curves that varies the cooling efficiency of an electric chiller as a function of evaporator conditions, condenser conditions and part-load ratio. The default curves are given as follows:

(6.8.2-4)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ EIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ air-cooled \enspace EIR\_FT = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 + \ d \times t_{odb} + \ e \times \left. t_{odb}\right. ^2+ \ f \times t_{chws} \times t_{odb}$$ $$ water-cooled \enspace EIR\_FT = a + b \times t_{chws}+ c \times \left. t_{chws}\right. ^2 + d \times t_{cws} + e \times \left. t_{cws}\right. ^2+ f \times t_{chws} \times t_{cws}$$ $$P_{operating} = P_{rated} \times EIR\_FPLR \times EIR\_FT \times CAP\_FT $$

where

PLR Part load ratio based on available capacity (not rated capacity)
Qoperating Present load on chiller (Btu/h)
Qavailable Chiller available capacity at present evaporator and condenser conditions (Btu/h)
tchws The chilled water supply temperature (°F)
tcws The condenser water supply temperature (°F)
todb The outside air dry-bulb temperature (°F)
Prated Rated power draw at ARI conditions (kW)
Poperating Power draw at specified operating conditions (kW)

Note: If an air-cooled chiller employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

 

Table 6.8.2-6: Default Efficiency EIR-FT Coefficients – Air-Cooled Chillers

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.99006553 0.66534403 0.13545636 N/A
b -0.00584144 -0.01383821 0.02292946 N/A
c 0.00016454 0.00014736 -0.00016107 N/A
d -0.00661136 0.00712808 -0.00235396 N/A
e 0.00016808 0.00004571 0.00012991 N/A
f -0.00022501 -0.00010326 -0.00018685 N/A

Table 6.8.2-7: Default Efficiency EIR-FT Coefficients – Water-Cooled Chiller

Coefficient Scroll Reciprocating Screw Centrifugal
a 1.00121431 0.46140041 0.66625403 0.51777196
b -0.01026981 -0.00882156 0.00068584 -0.00400363
c 0.00016703 0.00008223 0.00028498 0.00002028
d -0.00128136 0.00926607 -0.00341677 0.00698793
e 0.00014613 0.00005722 0.00025484 0.00008290
f -0.00021959 -0.00011594 -0.00048195 -0.00015467

Table 6.8.2-8: Default Efficiency EIR-FPLR Coefficients – Air-Cooled Chillers

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.06369119 0.11443742 0.03648722 N/A
b 0.58488832 0.54593340 0.73474298 N/A
c 0.35280274 0.34229861 0.21994748 N/A

Table 6.8.2-9: Default Efficiency EIR-FPLR Coefficients – Water-Cooled Chillers

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.04411957 0.08144133 0.33018833 0.17149273
b 0.64036703 0.41927141 0.23554291 0.58820208
c 0.31955532 0.49939604 0.46070828 0.23737257
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, supporting documentation is required.
Baseline Rules Use default curves.
Fuel and Steam Chiller Cooling Efficiency Adjustment Curves
Applicability All chillers
Definition

A curve or group of curves that varies the cooling efficiency of a fuel-fired or steam chiller as a function of evaporator conditions, condenser conditions, and part-load ratio. The default curves are given as follows:
Default Curves for Steam-Driven Single and Double Effect Absorption Chillers

(6.8.2-5)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ FIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ FIR\_FT = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 + \ d \times t_{cws} + \ e \times \left. t_{cws}\right. ^2+ \ f \times t_{chws} \times t_{cws}$$ $$Fuel_{partload} = Fuel_{rated} \times FIR\_FPLR \times FIR\_FT \times CAP\_FT $$

Default Curves for Direct-Fired Double Effect Absorption Chillers

(6.8.2-6)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ FIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ FIR\_FT1 = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 $$ $$ FIR\_FT2 = d + e \times t_{cws} + \ f \times \left. t_{cws}\right. ^2 $$ $$Fuel_{partload} = Fuel_{rated} \times FIR\_FPLR \times FIR\_FT1 \times FIR\_FT2 \times CAP\_FT $$

The default curves for engine driven chillers are the same format as those for the Steam-Driven Single and Double Effect Absorption Chillers but there are three sets of curves for different ranges of operation based on the engine speed.

where

PLR Part load ratio based on available capacity (not rated capacity)
FIR-FPLR A multiplier on the fuel input ratio (FIR) to account for part load conditions
FIR-FT A multiplier on the fuel input ratio (FIR) to account for the chiller water supply temperature and the condenser water temperature
FIR-FT1 A multiplier on the fuel input ratio (FIR) to account for chilled water supply temperature
FIR-FT2 A multiplier on the fuel input ratio (FIR) to account for condenser water supply temperature
CAP-FT A multiplier on the capacity of the chiller (see Equation (6.8.2-2))
Qoperating Present load on chiller (in Btu/h)
Qavailable Chiller available capacity at present evaporator and condenser conditions (in Btu/h)
tchws The chilled water supply temperature (in °F)
tcws The condenser water supply temperature (in °F)
todb The outside air dry-bulb temperature (°F)
Fuelrated Rated fuel consumption at ARI conditions (in Btu/h)
Fuelpartload Fuel consumption at specified operating conditions (in Btu/h)

Table 6.8.2-10: Default FIR-FPLR coefficients – Fuel- & Steam-Source Water-Cooled Absorption Chillers

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption
a 0.098585 0.013994 0.13551150
b 0.583850 1.240449 0.61798084
c 0.560658 -0.914883 0.24651277
d -0.243093 0.660441 0.00000000

Table 6.8.2-11: Default FIR-FPLR coefficients – Engine Driven Chillers

Coefficient %Speed$\leq$Min. %Speed>Min.
%Speed<60%
%Speed>60%
a 0.3802 1.14336 1.38861
b 2.3609 0.022889 -0.388614
c 0.0000 0.0000 0.0000
d 0.0000 0.0000 0.000

Table 6.8.2-12: Default FIR-FT coefficients – Fuel- & Steam-Source Water-Cooled Absorption Chillers

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption
a 0.652273 1.658750 4.42871284
b 0.000000 0.000000 -0.13298607
c 0.000000 0.000000 0.00125331
d -0.000545 -0.290000 0.86173749
e 0.000055 0.000250 -0.00708917
f 0.000000 0.000000 0.0010251

Table 6.8.2-13: Default FIR-FT coefficients – Engine Driven Chillers

Coefficient %Speed$\leq$Min. %Speed>Min.
%Speed<60%
%Speed>60%
a 1.0881500 1.2362400 1.2362400
b 0.0141064 0.0168923 0.0168923
c 0.0000000 0.0000000 0.0000000
d -0.00833912 -0.0115235 -0.0115235
e 0.0000000 0.0000000 0.0000000
f 0.0000000 0.0000000 0.0000000
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, supporting documentation is required.
Baseline Rules Use default curves.
Chilled Water Supply Temperature
Applicability All chillers
Definition The chilled water supply temperature of the chiller at design conditions
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules The baseline chilled water supply temperature is set to 44°F.
Chilled Water Return Temperature
Applicability All chillers
Definition The chilled water return temperature setpoint
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules The baseline chilled water return temperature is set to 56°F.
Chilled Water Supply Temperature Reset
Applicability All chillers
Definition The reset schedule for the chilled water supply temperature. The chilled water setpoint may be reset based on demand or outdoor air temperature.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. The default is Figure 6.8.2-1.

Baseline Rules

The baseline chilled water supply temperature is reset from 44°F to 54°F based on outdoor air temperature as shown in the figure below.

 

Chilled Water Supply Temperature Reset Schedule

 

Figure 6.8.2-1: Chilled Water Supply Temperature Reset Schedule

Condenser Type
Applicability All chillers
Definition The type of condenser for a chiller. The choices are:
  • Air-Cooled
  • Water-Cooled
  • Evaporatively-Cooled
Air-cooled chillers use air to cool the condenser coils. Water-cooled chillers use cold water to cool the condenser and additionally need either a cooling tower or a local source of cold water. Evaporatively-cooled chillers are similar to air-cooled chillers, except they use a water mist to cool the condenser coil which makes them more efficient.
Units List (see above)
Input Restrictions As designed
Baseline Rules The baseline chiller is always assumed to have a water-cooled condenser, although the chiller type will change depending on the design capacity. If the chiller size is less than 600 tons, the baseline chiller is a water-cooled screw; if the capacity is greater than or equal to 600 tons, the baseline chiller is a water-cooled centrifugal chiller.
Air-Cooled Condenser Power
Applicability All chillers
Definition The energy usage of the condenser fan(s) at design conditions on an air-cooled chiller. This unit should only be used for chillers composed of separate evaporator and condenser sections where the fan energy is not part of the chiller COP.
Units Kilowatts (kW)
Input Restrictions As designed. The user must enter data for remote air-cooled condensing units.
Baseline Rules Not applicable, since all baseline chillers have water-cooled condensers.
90.1-2016 BM

Chiller Name

Applicability

All chillers

Definition

A unique descriptor for each chiller

Units

Text, unique

Input Restrictions

User entry. Where applicable, this should match the tags that are used on the plans.

Baseline Rules

Baseline HVAC  systems  7, 8, 11, 12, and 13 have chillers.

 

Chiller Type

Applicability

All chillers

Definition

The type of chiller, either a vapor-compression chiller or an absorption chiller.
 
Vapor compression chillers operate on the reverse-Rankine cycle, using mechanical energy to compress the refrigerant, and include:

  • Positive displacement – includes reciprocating (piston-style), scroll and screw compressors
  • Centrifugal – uses rotating impeller blades to compress the refrigerant and impart velocity
  • Single Effect Absorption – use a single generator & condenser
  • Double Effect Absorption – use two generators/concentrators and condensers, one at a lower temperature and the other at a higher temperature. It is more efficient than the single effect, but it must use a higher temperature heat source.
  • Double Effect Absorption, Indirect-Fired
  • Gas Engine Driven Chiller

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

The baseline building chiller is based on the baseline building peak cooling load, as follows:

Table 3.8.2-1: Chiller Type

 Building Peak Cooling Load

Number and Type of Chillers (s)

≤300 tons

One water-cooled screw chiller

>300 tons, <600 tons

Two water-cooled screw chillers sized equally

≥600 tons

A minimum of two (2) water-cooled centrifugal chillers minimum with chillers added so that no chiller is larger than 800 tons, all sized equally

 

Number of Identical Chiller Units

Applicability

All chillers

Definition

The number of identical units for staging.

Units

Unitless integer

Input Restrictions

As designed. Default is 1.

Baseline Rules

There is one chiller if the cooling load is 300 tons or less and two equally sized chillers for loads between 300 and 600 tons. For loads above 600 tons, two or more chillers of equal size are used, with no chiller larger than 800 tons. See Table 3.8.2-1.

 

Chiller Fuel

Applicability

All chillers

Definition

The fuel source for the chiller. The choices are:

  • Electricity (for all vapor-compression chillers)
  • Gas (absorption units only, designated as direct-fired units)
  • Oil (absorption units only, designated as direct-fired units)
  • Hot Water (absorption units only, designated as indirect-fired units)
  • Steam (Absorption units only, designated as indirect-fired units)

Units

List (see above)

This input is restricted, based on the choice of Chiller Type, according to the following rules:

Table 3.8.2-2: Chiller Fuel

 

Electricity [SG1] 

Gas/Oil

Hot Water

Steam

Reciprocating

Allowed

 

 

 

Scroll

Allowed

 

 

 

Screw

Allowed

 

 

 

Centrifugal

Allowed

 

 

 

Single Effect Absorption

 

Allowed

Allowed

Allowed

Direct Fired Double Effect Absorption

 

Allowed

Allowed

Allowed

Indirect Fired Absorption

 

Allowed

Allowed

Allowed

Input Restrictions

As designed.

Baseline Rules

Electricity

 

Chiller Rated Capacity

Applicability

All chillers

Definition

The cooling capacity of a piece of heating equipment at rated conditions.

Units

Btu/h or tons

Input Restrictions

As designed. If unmet load hours are greater than 300, the chiller may have to be made larger.

Baseline Rules

Determine loads for baseline building and oversize by 15%.

 

Chiller Rated Efficiency

Applicability

All chillers

Definition

The efficiency of the chiller expressed as EER for air cooled chillers, kW/ton for water cooled, positive displacement chillers, and COP for fuel-fired and heat driven chillers. COP is also sometimes used for electric chillers. The applicable test procedure is AHRI Standard 550/590. The test conditions for the full load (FL) rating are summarized below:

  • 44°F leaving chilled-fluid temperature
  • 2.4 gpm/ton evaporator fluid flow
  • 85°F entering condenser-fluid temperature
  • 3.0 gpm/ton condenser-fluid flow

Chillers are also rated at part load conditions known as IPLV (integrated part load value). The IPLV is the weighted average of the chiller efficiency at 100%, 75%, 50% and 25%. The corresponding weights for these part-load conditions are are 17%, 39%, 33%, and 11%.

Units

kW/ton, EER or COP, depending on chiller type

Input Restrictions

As designed, but chillers must meet minimum efficiency standards

Baseline Rules

The baseline chiller efficiencies are as follows:

Table 3.8.2-3: Baseline Chiller Rated Efficiency

Type

Size

FL

IPLV

Test Procedure

Screw

< 75 tons

≤ 0.75

≤ 0.60

ARI 550/590

≥ 75 tons and < 150 tons

≤ 0.72

≤ 0.56

≥ 150 tons and < 300 tons

≤ 0.66

≤ 0.54

≥ 300 tons and < 600 tons

≤ 0.61

≤ 0.52

≥ 600 tons

≤ 0.56

≤ 0.50

Centrifugal

< 150 tons

≤ 0.61

≤ 0.55

≥ 150 tons and < 300 tons

≤ 0.61

≤ 0.55

≥ 300 tons and < 400 tons

≤ 0.56

≤ 0.52

≥ 400 tons and < 600 tons

≤ 0.56

≤ 0.50

≥ 600 tons

≤ 0.56

≤ 0.50

FL = Full Load

IPLV = Integrated Part Load Value

                                   

Chiller Minimum Unloading Ratio

Applicability

All chillers

Definition

The minimum unloading capacity of a chiller expressed as a fraction of the rated capacity. Below this level the chiller must cycle to meet the load. The following are default values for various types of chillers

Table 3.8.2-4: Baseline Chiller Minimum Unloading Ratio

Chiller Type

Default Unloading Ratio

Reciprocating

25%

Screw

15%

Centrifugal

10%

Scroll

25%

Single Effect Absorption

10%

Double Effect Absorption

10%

Units

Percent (%)

Input Restrictions

As designed. If the user does not employ the default values, supporting documentation is required.

Baseline Rules

The baseline HVAC chiller will be either a screw or centrifugal. Use the defaults for these chiller types from the above table: 15% for screw and 10% for centrifugal.

 

Chiller Cooling Capacity Adjustment Curve

Applicability

All chillers

Definition

A curve or group of curves or other functions that represent the available total cooling capacity as a function of evaporator and condenser conditions and perhaps other operating conditions. The default curves are given as follows:

(Equation 3.8.2-1)

$$Q_{available} = CAP\_FT \times Q_{rated}$$

For air-cooled chillers:

(Equation 3.8.2-2)

$$CAP\_FT =  a + b \cdot t_{chws} + c \cdot t_{chws}^{2} + d \cdot t_{odb} + e \cdot t_{odb}^{2} +f \cdot t_{chws} \cdot t_{odb}$$

For water-cooled chillers:

(Equation 3.8.2-3)

$$CAP\_FT =  a + b \cdot t_{chws} + c \cdot t_{chws}^{2} + d \cdot t_{cws} + e \cdot t_{cws}^{2} +f \cdot t_{chws} \cdot t_{cws}$$

where

Qavailable

Available cooling capacity at present evaporator and condenser conditions (MBH)

tchws

The chilled water supply temperature (°F)

tcws

The condenser water supply temperature (°F)

todb

The outside air dry-bulb temperature (°F)

Qrated

Rated capacity at ARI conditions (MBH)

a, b, c, d, e, f

Default coefficients are selected from Appendix H

Note: If an air-cooled unit employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit. 

Units

Data structure

Input Restrictions

User may input curves, other appropriate functions, or use default curves. If the default curves are overridden, supporting documentation is required.

Baseline Rules

Use default equations and coefficients for the baseline chillers..

 

Electric Chiller Cooling Efficiency Adjustment Curves

Applicability

All chillers

Definition

A curve or group of curves that varies the cooling efficiency of an electric chiller as a function of evaporator conditions, condenser conditions and part-load ratio. Note that for variable-speed chillers, the part-load cooling efficiency curve is a function of both part-load ratio and leaving condenser water temperature. The default curves are given as follows:

(Equation 3.8.2-4)

$$PLR = \frac{Q_{operating}}{Q_{available}(t_{chws},t_{cws/odb})}$$

$$EIR\_FPLR = a + b \cdot PLR + c \cdot PLR^{2}$$

Variable speed

$$EIR\_FPLR = a + b \cdot PLR + c \cdot PLR^{2} + d \cdot t_{cws} + e \cdot t_{cws}^{2} + f \cdot PLR \cdot t_{cws} + g \cdot PLR^{3} + h \cdot t_{cws}^{3} + i \cdot PLR^{2} \cdot t_{cws} + j \cdot PLR \cdot t_{cws}^{2}$$

Air cooled

$$EIR\_FT = a + b \cdot t_{chws} + c \cdot t_{chws}^{2} + d \cdot t_{odb} + e \cdot t_{odb}^{2} + f \cdot t_{chws}\cdot t_{odb} $$

Water cooled

$$EIR\_FT = a + b \cdot t_{chws} + c \cdot t_{chws}^{2} + d \cdot t_{cws} + e \cdot t_{cws}^{2} + f \cdot t_{chws}\cdot t_{cws} $$

Where

PLR

Part load ratio based on available capacity (not rated capacity)

Qoperating

Present load on chiller (Btu/h)

Qavailable

Chiller available capacity at present evaporator and condenser conditions (Btu/h)

tchws

The chilled water supply temperature (°F)

tcws

The condenser water supply temperature (°F)

todb

The outside air dry-bulb temperature (°F)

Prated

Rated power draw at ARI conditions (kW)

Poperating

Power draw at specified operating conditions (kW)

a, b, c, d, e, f

Coefficients selected from Appendix H

Note: If an air-cooled chiller employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

Units

Data structure

Input Restrictions

User may input curves or use default curves. If defaults are overridden, supporting documentation is required.

Baseline Rules

Use default equations and coefficients for the baseline HVAC chillers.

 

Fuel and Steam Chiller Cooling Efficiency Adjustment Curves

Applicability

All chillers

Definition

A curve or group of curves that varies the cooling efficiency of a fuel-fired or steam chiller as a function of evaporator conditions, condenser conditions, and part-load ratio. The default curves are given as follows:

Default Curves for Steam-Driven Single and Double Effect Absorption Chillers

(Equation 3.8.2-5)

$$PLR = \frac{Q_{operating}}{Q_{available}(t_{chws},t_{cws/odb})}$$

$$FIR\_FPLR = a + b \cdot PLR + c \cdot PLR^{2}$$

$$FIR\_FT = a + b \cdot t_{chws} + c \cdot t_{chws}^{2} + d \cdot t_{cws} + e \cdot t_{cws}^{2} + f \cdot t_{chws} \cdot t_{cws}$$

Default Curves for Direct-Fired Double Effect Absorption Chillers

(Equation 3.8.2-6)

$$PLR = \frac{Q_{operating}}{Q_{available}(t_{chws},t_{cws/odb})}$$

$$FIR\_FPLR = a + b \cdot PLR + c \cdot PLR^{2}$$

$$FIR\_FT1 = a + b \cdot t_{chws} + c \cdot t_{chws}^{2}$$ 

$$FIR\_FT2 = d + e \cdot t_{cws} + f \cdot t_{cws}^{2}$$

$$Fuel_{partload} =  Fuel_{rated} \cdot FIR\_FPLR \cdot FIR1\_FT1  \cdot FIR1\_FT2 \cdot CAP\_FT$$

The default curves for engine driven chillers are the same format as those for the Steam-Driven Single and Double Effect Absorption Chillers but there are three sets of curves for different ranges of operation based on the engine speed.

where

PLR

Part load ratio based on available capacity (not rated capacity)

FIR-FPLR

A multiplier on the fuel input ratio (FIR) to account for part load conditions

FIR-FT

A multiplier on the fuel input ratio (FIR) to account for the chiller water supply temperature and the condenser water temperature

FIR-FT1

A multiplier on the fuel input ratio (FIR) to account for chilled water supply temperature

FIR-FT2

A multiplier on the fuel input ratio (FIR) to account for condenser water supply temperature

CAP-FT

A multiplier on the capacity of the chiller (see Equation (Equation 3.8.2-2))

Qoperating

Present load on chiller (in Btu/h)

Qavailable

Chiller available capacity at present evaporator and condenser conditions (in Btu/h)

tchws

The chilled water supply temperature (in °F) (For air cooled chillers, the permitted range is 40 °F to 54 °F.)

tcws

The condenser water supply temperature (in °F)

todb

The outside air dry-bulb temperature (°F) (For air cooled chillers, the permitted range is 40 °F to 115 °F.)

Fuelrated

Rated fuel consumption at ARI conditions (in Btu/h)

Fuelpartload

Fuel consumption at specified operating conditions (in Btu/h)

a, b, c, d, e, f

Default coefficients are selected from Appendix H

 

Units

Data structure

Input Restrictions

User may input curves or use default curves. If defaults are overridden, supporting documentation is required.

Baseline Rules

Not applicable. The baseline HVAC system has electric chillers.

 

Chilled Water Supply Temperature

Applicability

All chillers

Definition

The chilled water supply temperature of the chiller at design conditions

Units

Degrees Fahrenheit (°F)

Input Restrictions

As designed

Baseline Rules

The baseline chilled water supply temperature is set to 44°F.

 

Chilled Water Return Temperature

Applicability

All chillers

Definition

The chilled water return temperature setpoint

Units

Degrees Fahrenheit (°F)

Input Restrictions

As designed

Baseline Rules

The baseline chilled water return temperature is set to 56°F.

 

Chilled Supply Temperature Control Type

Applicability

All chillers

Definition

The method by which the chilled water setpoint temperature is reset. The chilled water setpoint may be reset based on demand or outdoor air temperature.

Units

List: demand or outdoor air temperature

Input Restrictions

As designed

Baseline Rules

Outdoor air temperature

 

Chilled Water Supply Temperature Reset

Applicability

All chillers

Definition

The reset schedule for the chilled water supply temperature. The chilled water setpoint may be reset based on demand or outdoor air temperature.

Units

Data structure

Input Restrictions

As designed. The default to reset based on outdoor air temperature as follows:

  • 44°F at OAT 80°F and above
  • 54°F at OAT 60°F and below
  • Ramped linearly between 44°F and 54°F at temperatures between 80°F and 60°F

chiller_supply_temp_reset.jpg

Baseline Rules

The baseline chilled water supply temperature reset uses the defaults.

However, if the baseline chilled-water system serves a computer room HVAC system, the supply chilled-water temperature shall be reset higher based on the HVAC system requiring the most cooling; i.e., the chilled-water setpoint is reset higher until one cooling coil valve is nearly wide open. The maximum reset chilled-water supply temperature shall be 54°F.

Temperature reset is not required for systems served by purchased chilled water.

 

Condenser Type

Applicability

All chillers

Definition

The type of condenser for a chiller. The choices are:

  • Air-Cooled
  • Water-Cooled
  • Evaporatively-Cooled

Air-cooled chillers use air to cool the condenser coils. Water-cooled chillers use cold water to cool the condenser and additionally need either a cooling tower or a local source of cold water. Evaporatively-cooled chillers are similar to air-cooled chillers, except they use a water mist to cool the condenser coil which makes them more efficient.

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

The baseline chiller is always assumed to have a water-cooled condenser, although the chiller type will change depending on the design capacity.

 

Air-Cooled Condenser Power

Applicability

All chillers with air-cooled condensers where fan energy is not part of the COP.

Definition

The energy usage of the condenser fan(s) at design conditions on an air-cooled chiller. This unit should only be used for chillers composed of separate evaporator and condenser sections where the fan energy is not part of the chiller COP.

Units

Kilowatts (kW)

Input Restrictions

As designed. The user must enter data for remote air-cooled condensing units.

Baseline Rules

Not applicable, since all baseline chillers have water-cooled condensers.

 

Building EQ
Chiller Name
Applicability All chillers
Definition A unique descriptor for each chiller
Units Text, unique
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans.
Chiller Type
Applicability All chillers
Definition The type of chiller, either a vapor-compression chiller or an absorption chiller.
 
Vapor compression chillers operate on the reverse-Rankine cycle, using mechanical energy to compress the refrigerant, and include:
  • Reciprocating – uses pistons for compression
  • Screw – uses two counter rotating screws for compression
  • Scroll – uses two interlocking spirals or scrolls to perform the compression
  • Centrifugal – uses rotating impeller blades to compress the air
  • Absorption chillers – use heat to vaporize a working fluid (usually either ammonia or lithium bromide)
  • Single Effect Absorption – use a single generator & condenser
  • Double Effect Absorption – use two generators/concentrators and condensers, one at a lower temperature and the other at a higher temperature. It is more efficient than the single effect, but it must use a higher temperature heat source.
Units List (see above)
Input Restrictions As designed
Baseline Rules

The baseline building chiller is based on the design capacity of the proposed design as follows from ASHRAE 90.1 Appendix G:
[table title="Type and Number of Chillers" id="type-and-number-of-chillers"]

Building Peak
Cooling Load
Number and Type of Chillers (s)
$\leq$300 tons 1 water-cooled screw chiller
>300 tons, <600 tons 2 water-cooled screw chillers sized equally
$\geq$600 tons 2 water-cooled centrigugal chillers minimum with chillers added so that no chiller is larger than 800 tons, all sized equally
Number of Identical Chiller Units
Applicability All chillers
Definition The number of identical units for staging.
Units None
Input Restrictions As designed. Default is 1.
Chiller Fuel
Applicability All chillers
Definition The fuel source for the chiller. The choices are:
  • Electricity (for all vapor-compression chillers)
  • Gas (Absorption units only, designated as direct-fired units)
  • Oil (Absorption units only, designated as direct-fired units)
  • Hot Water (Absorption units only, designated as indirect-fired units)
  • Steam (Absorption units only, designated as indirect-fired units)
Units List (see above)
Input Restrictions As designed.
Chiller Rated Capacity
Applicability All chillers
Definition The cooling capacity of a piece of heating equipment at rated conditions.
Units Btu/h or tons
Input Restrictions As designed. If unmet load hours are greater than 300, the chiller may have to be made larger.
Chiller Rated Efficiency
Applicability All chillers
Definition The Coefficient of Performance (COP) at ARI rated conditions.
Units Ratio
Input Restrictions As designed
Chiller Minimum Unloading Ratio
Applicability All chillers
Definition

The minimum unloading capacity of a chiller expressed as a fraction of the rated capacity. Below this level the chiller must cycle to meet the load.


[table title="Default Minimum Unloading Ratios" id="default-minimum-unloading-ratios"]

Chiller Type Default Unloading Ratio
Reciprocating 25%
Screw 15%
Centrifugal 10%
Scroll 25%
Single Effect Absorption 10%
Double Effect Absorption 10%
Units Percent (%)
Input Restrictions As designed. If the user does not employ the default values, supporting documentation is required.
Chiller Cooling Capacity Adjustment Curve
Applicability All chillers
Definition

A curve or group of curves or other functions that represent the available total cooling capacity as a function of evaporator and condenser conditions and perhaps other operating conditions. The default curves are given as follows:

(6.8.2-1)

$$ Q_{available} = CAP \_ FT \times Q_{rated}$$

For air-cooled chillers:

(6.8.2-2)

$$ CAP \_FT = a + b \times t_{chws} + c \times \left. t_{chws}\right. ^2 + d \times t_{odb} + e \times \left. t_{odb}\right. ^2 + f \times t_{chws} \times t_{odb}$$

For water-cooled chillers:

(6.8.2-3)

$$ CAP \_FT = a + b \times t_{chws} + c \times \left. t_{chws} \right. ^2 + d \times t_{cws} + e \times \left. t_{cws} \right. ^2 + f \times t_{chws} \times t_{cws}$$

where

Qavailable Available cooling capacity at present evaporator and condenser conditions (MBH)
tchws The chilled water supply temperature (°F)
tcws The condenser water supply temperature (°F)
todb The outside air dry-bulb temperature (°F)
Qrated Rated capacity at ARI conditions (MBH)

Note: If an air-cooled unit employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

[table title="Default Capacity Coefficients – Electric Air-Cooled Chillers" id="default-capacity-coefficients-–-electric-air---cooled-chillers"]

Coefficient Scroll Recip Screw Centrifugal
a 0.40070684 0.57617295 -0.09464899 N/A
b 0.01861548 0.02063133 0.03834070 N/A
c 0.00007199 0.00007769 -0.00009205 N/A
d 0.00177296 -0.00351183 0.00378007 N/A
e -0.00002014 0.00000312 -0.00001375 N/A
f -0.00008273 -0.00007865 -0.00015464 N/A

[table title="Default Capacity Coefficients – Electric Water-Cooled Chillers" id="default-capacity-coefficients-–-electric-water---cooled-chillers"]

Coefficient Scroll Recip Screw Centrifugal
a 0.36131454 0.58531422 0.33269598 -0.29861976
b 0.01855477 0.01539593 0.00729116 0.02996076
c 0.00003011 0.00007296 -0.00049938 -0.00080125
d 0.00093592 -0.00212462 0.01598983 0.01736268
e -0.00001518 -0.00000715 -0.00028254 -0.00032606
f -0.00005481 -0.00004597 0.00052346 0.00063139

[table title="Default Capacity Coefficients – Fuel- & Steam-Source Water-Cooled Chillers" id="default-capacity-coefficients-–-fuel--&-steam---source-water---cooled-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption Engine Driven Chiller
a 0.723412 -0.816039 1.000000 0.573597
b 0.079006 -0.038707 0.000000 0.0186802
c -0.000897 0.000450 0.000000 0.000000
d -0.025285 0.071491 0.000000 -0.00465325
e -0.000048 -0.000636 0.000000 0.000000
f 0.000276 0.000312 0.000000 0.000000
Units Data structure
Input Restrictions User may input curves, other appropriate functions, or use default curves. If the default curves are overridden, supporting documentation is required.
Electric Chiller Cooling Efficiency Adjustment Curves
Applicability All chillers
Definition

A curve or group of curves that varies the cooling efficiency of an electric chiller as a function of evaporator conditions, condenser conditions and part-load ratio. The default curves are given as follows:

(6.8.2-4)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ EIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ air-cooled \enspace EIR\_FT = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 + \ d \times t_{odb} + \ e \times \left. t_{odb}\right. ^2+ \ f \times t_{chws} \times t_{odb}$$ $$ water-cooled \enspace EIR\_FT = a + b \times t_{chws}+ c \times \left. t_{chws}\right. ^2 + d \times t_{cws} + e \times \left. t_{cws}\right. ^2+ f \times t_{chws} \times t_{cws}$$ $$P_{operating} = P_{rated} \times EIR\_FPLR \times EIR\_FT \times CAP\_FT $$

where

PLR Part load ratio based on available capacity (not rated capacity)
Qoperating Present load on chiller (Btu/h)
Qavailable Chiller available capacity at present evaporator and condenser conditions (Btu/h)
tchws The chilled water supply temperature (°F)
tcws The condenser water supply temperature (°F)
todb The outside air dry-bulb temperature (°F)
Prated Rated power draw at ARI conditions (kW)
Poperating Power draw at specified operating conditions (kW)

Note: If an air-cooled chiller employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

[table title="Default Efficiency EIR-FT Coefficients – Air-Cooled Chillers" id="default-efficiency-eir---ft-coefficients-–-air---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.99006553 0.66534403 0.13545636 N/A
b -0.00584144 -0.01383821 0.02292946 N/A
c 0.00016454 0.00014736 -0.00016107 N/A
d -0.00661136 0.00712808 -0.00235396 N/A
e 0.00016808 0.00004571 0.00012991 N/A
f -0.00022501 -0.00010326 -0.00018685 N/A

[table title="Default Efficiency EIR-FT Coefficients – Water-Cooled Chiller" id="default-efficiency-eir--ft-coefficients-–-water---cooled-chiller"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 1.00121431 0.46140041 0.66625403 0.51777196
b -0.01026981 -0.00882156 0.00068584 -0.00400363
c 0.00016703 0.00008223 0.00028498 0.00002028
d -0.00128136 0.00926607 -0.00341677 0.00698793
e 0.00014613 0.00005722 0.00025484 0.00008290
f -0.00021959 -0.00011594 -0.00048195 -0.00015467

[table title="Default Efficiency EIR-FPLR Coefficients – Air-Cooled Chillers" id="default-efficiency-eir---fplr-coefficients-–-air---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.06369119 0.11443742 0.03648722 N/A
b 0.58488832 0.54593340 0.73474298 N/A
c 0.35280274 0.34229861 0.21994748 N/A

[table title="Default Efficiency EIR-FPLR Coefficients – Water-Cooled Chillers" id="default-efficiency-eir---fplr-coefficients-–-water---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.04411957 0.08144133 0.33018833 0.17149273
b 0.64036703 0.41927141 0.23554291 0.58820208
c 0.31955532 0.49939604 0.46070828 0.23737257
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, supporting documentation is required.
Fuel and Steam Chiller Cooling Efficiency Adjustment Curves
Applicability All chillers
Definition

A curve or group of curves that varies the cooling efficiency of a fuel-fired or steam chiller as a function of evaporator conditions, condenser conditions, and part-load ratio. The default curves are given as follows:
Default Curves for Steam-Driven Single and Double Effect Absorption Chillers

(6.8.2-5)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ FIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ FIR\_FT = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 + \ d \times t_{cws} + \ e \times \left. t_{cws}\right. ^2+ \ f \times t_{chws} \times t_{cws}$$ $$Fuel_{partload} = Fuel_{rated} \times FIR\_FPLR \times FIR\_FT \times CAP\_FT $$

Default Curves for Direct-Fired Double Effect Absorption Chillers

(6.8.2-6)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ FIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ FIR\_FT1 = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 $$ $$ FIR\_FT2 = d + e \times t_{cws} + \ f \times \left. t_{cws}\right. ^2 $$ $$Fuel_{partload} = Fuel_{rated} \times FIR\_FPLR \times FIR\_FT1 \times FIR\_FT2 \times CAP\_FT $$

The default curves for engine driven chillers are the same format as those for the Steam-Driven Single and Double Effect Absorption Chillers but there are three sets of curves for different ranges of operation based on the engine speed.

where

PLR Part load ratio based on available capacity (not rated capacity)
FIR-FPLR A multiplier on the fuel input ratio (FIR) to account for part load conditions
FIR-FT A multiplier on the fuel input ratio (FIR) to account for the chiller water supply temperature and the condenser water temperature
FIR-FT1 A multiplier on the fuel input ratio (FIR) to account for chilled water supply temperature
FIR-FT2 A multiplier on the fuel input ratio (FIR) to account for condenser water supply temperature
CAP-FT A multiplier on the capacity of the chiller (see Equation (6.8.2-2))
Qoperating Present load on chiller (in Btu/h)
Qavailable Chiller available capacity at present evaporator and condenser conditions (in Btu/h)
tchws The chilled water supply temperature (in °F)
tcws The condenser water supply temperature (in °F)
todb The outside air dry-bulb temperature (°F)
Fuelrated Rated fuel consumption at ARI conditions (in Btu/h)
Fuelpartload Fuel consumption at specified operating conditions (in Btu/h)

[table title="Default FIR-FPLR coefficients – Fuel- & Steam-Source Water-Cooled Absorption Chillers" id="default-fir---fplr-coefficients-–-fuel---&-steam---source-water---cooled-absorption-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption
a 0.098585 0.013994 0.13551150
b 0.583850 1.240449 0.61798084
c 0.560658 -0.914883 0.24651277
d -0.243093 0.660441 0.00000000

[table title="Default FIR-FPLR coefficients – Engine Driven Chillers" id="default-fir---fplr-coefficients-–-engine-driven-chillers"]

Coefficient %Speed$\leq$Min. %Speed>Min.
%Speed<60%
%Speed>60%
a 0.3802 1.14336 1.38861
b 2.3609 0.022889 -0.388614
c 0.0000 0.0000 0.0000
d 0.0000 0.0000 0.000

[table title="Default FIR-FT coefficients – Fuel- & Steam-Source Water-Cooled Absorption Chillers" id="default-fir---ft-coefficients-–-fuel---&-steam---source-water---cooled-absorption-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption
a 0.652273 1.658750 4.42871284
b 0.000000 0.000000 -0.13298607
c 0.000000 0.000000 0.00125331
d -0.000545 -0.290000 0.86173749
e 0.000055 0.000250 -0.00708917
f 0.000000 0.000000 0.0010251

[table title="Default FIR-FT coefficients – Engine Driven Chillers" id="default-fir---ft-coefficients-–-engine-driven-chillers"]

Coefficient %Speed$\leq$Min. %Speed>Min.
%Speed<60%
%Speed>60%
a 1.0881500 1.2362400 1.2362400
b 0.0141064 0.0168923 0.0168923
c 0.0000000 0.0000000 0.0000000
d -0.00833912 -0.0115235 -0.0115235
e 0.0000000 0.0000000 0.0000000
f 0.0000000 0.0000000 0.0000000
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, supporting documentation is required.
Chilled Water Supply Temperature
Applicability All chillers
Definition The chilled water supply temperature of the chiller at design conditions
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Chilled Water Return Temperature
Applicability All chillers
Definition The chilled water return temperature setpoint
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Chilled Water Supply Temperature Reset
Applicability All chillers
Definition The reset schedule for the chilled water supply temperature. The chilled water setpoint may be reset based on demand or outdoor air temperature.
Units Degrees Fahrenheit (°F)
Input Restrictions

As designed. The default is [bookref id="chilled-water-supply-temperature-reset-schedule"].

[figure title="Chilled Water Supply Temperature Reset Schedule" id="chilled-water-supply-temperature-reset-schedule"]Chilled Water Supply Temperature Reset Schedule[/figure]

Condenser Type
Applicability All chillers
Definition The type of condenser for a chiller. The choices are:
  • Air-Cooled
  • Water-Cooled
  • Evaporatively-Cooled
Air-cooled chillers use air to cool the condenser coils. Water-cooled chillers use cold water to cool the condenser and additionally need either a cooling tower or a local source of cold water. Evaporatively-cooled chillers are similar to air-cooled chillers, except they use a water mist to cool the condenser coil which makes them more efficient.
Units List (see above)
Input Restrictions As designed
Air-Cooled Condenser Power
Applicability All chillers
Definition The energy usage of the condenser fan(s) at design conditions on an air-cooled chiller. This unit should only be used for chillers composed of separate evaporator and condenser sections where the fan energy is not part of the chiller COP.
Units Kilowatts (kW)
Input Restrictions As designed. The user must enter data for remote air-cooled condensing units.
Energy Star
Chiller Name
Applicability All chillers
Definition A unique descriptor for each chiller
Units Text, unique
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans.
Chiller Type
Applicability All chillers
Definition The type of chiller, either a vapor-compression chiller or an absorption chiller.
 
Vapor compression chillers operate on the reverse-Rankine cycle, using mechanical energy to compress the refrigerant, and include:
  • Reciprocating – uses pistons for compression
  • Screw – uses two counter rotating screws for compression
  • Scroll – uses two interlocking spirals or scrolls to perform the compression
  • Centrifugal – uses rotating impeller blades to compress the air
  • Absorption chillers – use heat to vaporize a working fluid (usually either ammonia or lithium bromide)
  • Single Effect Absorption – use a single generator & condenser
  • Double Effect Absorption – use two generators/concentrators and condensers, one at a lower temperature and the other at a higher temperature. It is more efficient than the single effect, but it must use a higher temperature heat source.
Units List (see above)
Input Restrictions As designed
Baseline Rules

The baseline building chiller is based on the design capacity of the proposed design as follows from ASHRAE 90.1 Appendix G:
[table title="Type and Number of Chillers" id="type-and-number-of-chillers"]

Building Peak
Cooling Load
Number and Type of Chillers (s)
$\leq$300 tons 1 water-cooled screw chiller
>300 tons, <600 tons 2 water-cooled screw chillers sized equally
$\geq$600 tons 2 water-cooled centrigugal chillers minimum with chillers added so that no chiller is larger than 800 tons, all sized equally
Number of Identical Chiller Units
Applicability All chillers
Definition The number of identical units for staging.
Units None
Input Restrictions As designed. Default is 1.
Chiller Fuel
Applicability All chillers
Definition The fuel source for the chiller. The choices are:
  • Electricity (for all vapor-compression chillers)
  • Gas (Absorption units only, designated as direct-fired units)
  • Oil (Absorption units only, designated as direct-fired units)
  • Hot Water (Absorption units only, designated as indirect-fired units)
  • Steam (Absorption units only, designated as indirect-fired units)
Units List (see above)
Input Restrictions As designed.
Chiller Rated Capacity
Applicability All chillers
Definition The cooling capacity of a piece of heating equipment at rated conditions.
Units Btu/h or tons
Input Restrictions As designed. If unmet load hours are greater than 300, the chiller may have to be made larger.
Chiller Rated Efficiency
Applicability All chillers
Definition The Coefficient of Performance (COP) at ARI rated conditions.
Units Ratio
Input Restrictions As designed
Chiller Minimum Unloading Ratio
Applicability All chillers
Definition

The minimum unloading capacity of a chiller expressed as a fraction of the rated capacity. Below this level the chiller must cycle to meet the load.


[table title="Default Minimum Unloading Ratios" id="default-minimum-unloading-ratios"]

Chiller Type Default Unloading Ratio
Reciprocating 25%
Screw 15%
Centrifugal 10%
Scroll 25%
Single Effect Absorption 10%
Double Effect Absorption 10%
Units Percent (%)
Input Restrictions As designed. If the user does not employ the default values, supporting documentation is required.
Chiller Cooling Capacity Adjustment Curve
Applicability All chillers
Definition

A curve or group of curves or other functions that represent the available total cooling capacity as a function of evaporator and condenser conditions and perhaps other operating conditions. The default curves are given as follows:

(6.8.2-1)

$$ Q_{available} = CAP \_ FT \times Q_{rated}$$

For air-cooled chillers:

(6.8.2-2)

$$ CAP \_FT = a + b \times t_{chws} + c \times \left. t_{chws}\right. ^2 + d \times t_{odb} + e \times \left. t_{odb}\right. ^2 + f \times t_{chws} \times t_{odb}$$

For water-cooled chillers:

(6.8.2-3)

$$ CAP \_FT = a + b \times t_{chws} + c \times \left. t_{chws} \right. ^2 + d \times t_{cws} + e \times \left. t_{cws} \right. ^2 + f \times t_{chws} \times t_{cws}$$

where

Qavailable Available cooling capacity at present evaporator and condenser conditions (MBH)
tchws The chilled water supply temperature (°F)
tcws The condenser water supply temperature (°F)
todb The outside air dry-bulb temperature (°F)
Qrated Rated capacity at ARI conditions (MBH)

Note: If an air-cooled unit employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

[table title="Default Capacity Coefficients – Electric Air-Cooled Chillers" id="default-capacity-coefficients-–-electric-air---cooled-chillers"]

Coefficient Scroll Recip Screw Centrifugal
a 0.40070684 0.57617295 -0.09464899 N/A
b 0.01861548 0.02063133 0.03834070 N/A
c 0.00007199 0.00007769 -0.00009205 N/A
d 0.00177296 -0.00351183 0.00378007 N/A
e -0.00002014 0.00000312 -0.00001375 N/A
f -0.00008273 -0.00007865 -0.00015464 N/A

[table title="Default Capacity Coefficients – Electric Water-Cooled Chillers" id="default-capacity-coefficients-–-electric-water---cooled-chillers"]

Coefficient Scroll Recip Screw Centrifugal
a 0.36131454 0.58531422 0.33269598 -0.29861976
b 0.01855477 0.01539593 0.00729116 0.02996076
c 0.00003011 0.00007296 -0.00049938 -0.00080125
d 0.00093592 -0.00212462 0.01598983 0.01736268
e -0.00001518 -0.00000715 -0.00028254 -0.00032606
f -0.00005481 -0.00004597 0.00052346 0.00063139

[table title="Default Capacity Coefficients – Fuel- & Steam-Source Water-Cooled Chillers" id="default-capacity-coefficients-–-fuel--&-steam---source-water---cooled-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption Engine Driven Chiller
a 0.723412 -0.816039 1.000000 0.573597
b 0.079006 -0.038707 0.000000 0.0186802
c -0.000897 0.000450 0.000000 0.000000
d -0.025285 0.071491 0.000000 -0.00465325
e -0.000048 -0.000636 0.000000 0.000000
f 0.000276 0.000312 0.000000 0.000000
Units Data structure
Input Restrictions User may input curves, other appropriate functions, or use default curves. If the default curves are overridden, supporting documentation is required.
Electric Chiller Cooling Efficiency Adjustment Curves
Applicability All chillers
Definition

A curve or group of curves that varies the cooling efficiency of an electric chiller as a function of evaporator conditions, condenser conditions and part-load ratio. The default curves are given as follows:

(6.8.2-4)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ EIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ air-cooled \enspace EIR\_FT = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 + \ d \times t_{odb} + \ e \times \left. t_{odb}\right. ^2+ \ f \times t_{chws} \times t_{odb}$$ $$ water-cooled \enspace EIR\_FT = a + b \times t_{chws}+ c \times \left. t_{chws}\right. ^2 + d \times t_{cws} + e \times \left. t_{cws}\right. ^2+ f \times t_{chws} \times t_{cws}$$ $$P_{operating} = P_{rated} \times EIR\_FPLR \times EIR\_FT \times CAP\_FT $$

where

PLR Part load ratio based on available capacity (not rated capacity)
Qoperating Present load on chiller (Btu/h)
Qavailable Chiller available capacity at present evaporator and condenser conditions (Btu/h)
tchws The chilled water supply temperature (°F)
tcws The condenser water supply temperature (°F)
todb The outside air dry-bulb temperature (°F)
Prated Rated power draw at ARI conditions (kW)
Poperating Power draw at specified operating conditions (kW)

Note: If an air-cooled chiller employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

[table title="Default Efficiency EIR-FT Coefficients – Air-Cooled Chillers" id="default-efficiency-eir---ft-coefficients-–-air---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.99006553 0.66534403 0.13545636 N/A
b -0.00584144 -0.01383821 0.02292946 N/A
c 0.00016454 0.00014736 -0.00016107 N/A
d -0.00661136 0.00712808 -0.00235396 N/A
e 0.00016808 0.00004571 0.00012991 N/A
f -0.00022501 -0.00010326 -0.00018685 N/A

[table title="Default Efficiency EIR-FT Coefficients – Water-Cooled Chiller" id="default-efficiency-eir--ft-coefficients-–-water---cooled-chiller"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 1.00121431 0.46140041 0.66625403 0.51777196
b -0.01026981 -0.00882156 0.00068584 -0.00400363
c 0.00016703 0.00008223 0.00028498 0.00002028
d -0.00128136 0.00926607 -0.00341677 0.00698793
e 0.00014613 0.00005722 0.00025484 0.00008290
f -0.00021959 -0.00011594 -0.00048195 -0.00015467

[table title="Default Efficiency EIR-FPLR Coefficients – Air-Cooled Chillers" id="default-efficiency-eir---fplr-coefficients-–-air---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.06369119 0.11443742 0.03648722 N/A
b 0.58488832 0.54593340 0.73474298 N/A
c 0.35280274 0.34229861 0.21994748 N/A

[table title="Default Efficiency EIR-FPLR Coefficients – Water-Cooled Chillers" id="default-efficiency-eir---fplr-coefficients-–-water---cooled-chillers"]

Coefficient Scroll Reciprocating Screw Centrifugal
a 0.04411957 0.08144133 0.33018833 0.17149273
b 0.64036703 0.41927141 0.23554291 0.58820208
c 0.31955532 0.49939604 0.46070828 0.23737257
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, supporting documentation is required.
Fuel and Steam Chiller Cooling Efficiency Adjustment Curves
Applicability All chillers
Definition

A curve or group of curves that varies the cooling efficiency of a fuel-fired or steam chiller as a function of evaporator conditions, condenser conditions, and part-load ratio. The default curves are given as follows:
Default Curves for Steam-Driven Single and Double Effect Absorption Chillers

(6.8.2-5)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ FIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ FIR\_FT = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 + \ d \times t_{cws} + \ e \times \left. t_{cws}\right. ^2+ \ f \times t_{chws} \times t_{cws}$$ $$Fuel_{partload} = Fuel_{rated} \times FIR\_FPLR \times FIR\_FT \times CAP\_FT $$

Default Curves for Direct-Fired Double Effect Absorption Chillers

(6.8.2-6)

$$ PLR = \frac {Q_{operating}}{Q_{available}\left ( t_{chws} , t_{cws/odb}\right )}$$ $$ FIR\_FPLR = a + b \times PLR + c \times PLR^2$$ $$ FIR\_FT1 = a + \ b \times t_{chws}+ \ c \times \left. t_{chws}\right. ^2 $$ $$ FIR\_FT2 = d + e \times t_{cws} + \ f \times \left. t_{cws}\right. ^2 $$ $$Fuel_{partload} = Fuel_{rated} \times FIR\_FPLR \times FIR\_FT1 \times FIR\_FT2 \times CAP\_FT $$

The default curves for engine driven chillers are the same format as those for the Steam-Driven Single and Double Effect Absorption Chillers but there are three sets of curves for different ranges of operation based on the engine speed.

where

PLR Part load ratio based on available capacity (not rated capacity)
FIR-FPLR A multiplier on the fuel input ratio (FIR) to account for part load conditions
FIR-FT A multiplier on the fuel input ratio (FIR) to account for the chiller water supply temperature and the condenser water temperature
FIR-FT1 A multiplier on the fuel input ratio (FIR) to account for chilled water supply temperature
FIR-FT2 A multiplier on the fuel input ratio (FIR) to account for condenser water supply temperature
CAP-FT A multiplier on the capacity of the chiller (see Equation (6.8.2-2))
Qoperating Present load on chiller (in Btu/h)
Qavailable Chiller available capacity at present evaporator and condenser conditions (in Btu/h)
tchws The chilled water supply temperature (in °F)
tcws The condenser water supply temperature (in °F)
todb The outside air dry-bulb temperature (°F)
Fuelrated Rated fuel consumption at ARI conditions (in Btu/h)
Fuelpartload Fuel consumption at specified operating conditions (in Btu/h)

[table title="Default FIR-FPLR coefficients – Fuel- & Steam-Source Water-Cooled Absorption Chillers" id="default-fir---fplr-coefficients-–-fuel---&-steam---source-water---cooled-absorption-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption
a 0.098585 0.013994 0.13551150
b 0.583850 1.240449 0.61798084
c 0.560658 -0.914883 0.24651277
d -0.243093 0.660441 0.00000000

[table title="Default FIR-FPLR coefficients – Engine Driven Chillers" id="default-fir---fplr-coefficients-–-engine-driven-chillers"]

Coefficient %Speed$\leq$Min. %Speed>Min.
%Speed<60%
%Speed>60%
a 0.3802 1.14336 1.38861
b 2.3609 0.022889 -0.388614
c 0.0000 0.0000 0.0000
d 0.0000 0.0000 0.000

[table title="Default FIR-FT coefficients – Fuel- & Steam-Source Water-Cooled Absorption Chillers" id="default-fir---ft-coefficients-–-fuel---&-steam---source-water---cooled-absorption-chillers"]

Coefficient Single Stage Absorption Double Stage Absorption Direct-Fired Absorption
a 0.652273 1.658750 4.42871284
b 0.000000 0.000000 -0.13298607
c 0.000000 0.000000 0.00125331
d -0.000545 -0.290000 0.86173749
e 0.000055 0.000250 -0.00708917
f 0.000000 0.000000 0.0010251

[table title="Default FIR-FT coefficients – Engine Driven Chillers" id="default-fir---ft-coefficients-–-engine-driven-chillers"]

Coefficient %Speed$\leq$Min. %Speed>Min.
%Speed<60%
%Speed>60%
a 1.0881500 1.2362400 1.2362400
b 0.0141064 0.0168923 0.0168923
c 0.0000000 0.0000000 0.0000000
d -0.00833912 -0.0115235 -0.0115235
e 0.0000000 0.0000000 0.0000000
f 0.0000000 0.0000000 0.0000000
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, supporting documentation is required.
Chilled Water Supply Temperature
Applicability All chillers
Definition The chilled water supply temperature of the chiller at design conditions
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Chilled Water Return Temperature
Applicability All chillers
Definition The chilled water return temperature setpoint
Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Chilled Water Supply Temperature Reset
Applicability All chillers
Definition The reset schedule for the chilled water supply temperature. The chilled water setpoint may be reset based on demand or outdoor air temperature.
Units Degrees Fahrenheit (°F)
Input Restrictions

As designed. The default is [bookref id="chilled-water-supply-temperature-reset-schedule"].

[figure title="Chilled Water Supply Temperature Reset Schedule" id="chilled-water-supply-temperature-reset-schedule"]Chilled Water Supply Temperature Reset Schedule[/figure]

Condenser Type
Applicability All chillers
Definition The type of condenser for a chiller. The choices are:
  • Air-Cooled
  • Water-Cooled
  • Evaporatively-Cooled
Air-cooled chillers use air to cool the condenser coils. Water-cooled chillers use cold water to cool the condenser and additionally need either a cooling tower or a local source of cold water. Evaporatively-cooled chillers are similar to air-cooled chillers, except they use a water mist to cool the condenser coil which makes them more efficient.
Units List (see above)
Input Restrictions As designed
Air-Cooled Condenser Power
Applicability All chillers
Definition The energy usage of the condenser fan(s) at design conditions on an air-cooled chiller. This unit should only be used for chillers composed of separate evaporator and condenser sections where the fan energy is not part of the chiller COP.
Units Kilowatts (kW)
Input Restrictions As designed. The user must enter data for remote air-cooled condensing units.