6.7.5 Cooling Systems

General

This group of building descriptors applies to all cooling systems.

Cooling Source
Applicability All systems
Definition The source of cooling for the system. The choices are:
  • Chilled water
  • Direct expansion (DX)
  • Other
Units List (see above)
Input Restrictions As designed
Baseline Rules The baseline building cooling source is shown in Table 6.7.5-1. See Figure 6.1.2-1 for HVAC system mapping.

Table 6.7.5-1: "Cooling Source for Baseline Building System"

Baseline building System Cooling Source
System 1 – PTAC Direct expansion (DX)
System 2 – PTHP Direct expansion (DX)
System 3 – PSZ-AC Direct expansion (DX)
System 4 – PSZ-HP Direct expansion (DX)
System 5 – Packaged VAV with Reheat Direct expansion (DX)
System 6 – Packaged VAV with PFP boxes Direct expansion (DX)
System 7 – VAV with Reheat Chilled water
System 8 – VAV with PFP boxes Chilled water
Total Cooling Capacity
Applicability All cooling systems
Definition The total cooling capacity (both sensible and latent) of a cooling coil or packaged DX system at ARI conditions. The building descriptors defined in this chapter assume that the fan is modeled separately, including any heat it adds to the air stream. The cooling capacity specified by this building descriptor should not consider the heat of the fan.
Units kBtu/h
Input Restrictions

As designed. For packaged equipment that has the fan motor in the air stream such that it adds heat to the cooled air, the software shall adjust the total cooling capacity as follows:

(6.7.5-1)

TeX Embedding failed!

where

Qt,adj The adjusted total cooling capacity of a packaged unit (kBtu/h)
Qt,rated The ARI rated total cooling capacity of a packaged unit (kBtu/h) from manufactures' literature
BHPsupply The supply fan brake horsepower (bhp).

If the number of unmet load hours in the proposed design exceeds 300, the software shall warn the user to resize the equipment.

Baseline Rules The total cooling capacity of the baseline building is oversized by 15%. However, the cooling equipment may need to be subsequently downsized such that the difference in unmet load hours between the proposed design and the baseline building is less than 50 (see Chapter 2). Sizing calculations shall be based on 1% dry-bulb and 1% wet-bulb design conditions.
Sensible Cooling Capacity
Applicability All cooling systems
Definition The sensible heat cooling capacity of the coil or packaged equipment at ARI conditions. The building descriptors defined in this chapter assume that the fan is modeled separately, including any heat it adds to the air stream. The cooling capacity specified by this building descriptor should not consider the heat of the fan.
Units kBtu/h
Input Restrictions

As designed. For packaged equipment that has the fan motor located in the air stream such that it adds heat to the cooled air, the software shall adjust the sensible cooling capacity as follows:

(6.7.5-2)

TeX Embedding failed!

where

Qs,adj The adjusted sensible cooling capacity of a packaged unit (kBtu/h)
Qs,rated The ARI rated sensible cooling capacity of a packaged unit (kBtu/h)
BHPsupply The supply fan brake horsepower (bhp).

If the number of unmet load hours in the proposed design exceeds 300, the software shall warn the user to resize the equipment.

Baseline Rules The sensible cooling capacity of the baseline building is oversized by 15%. However, the cooling equipment may need to be subsequently downsized such that the difference in unmet load hours between the proposed design and the baseline building is less than 50 (see Chapter 2). Sizing calculations shall be based on 1% dry-bulb and 1% wet-bulb design conditions.
Cooling Capacity Adjustment Curves
Applicability All cooling systems
Definition

A curve that represents the available total cooling capacity as a function of cooling coil and/or condenser conditions. The common form of these curves is given as follows:

(6.7.5-3)

TeX Embedding failed!

For air cooled direct expansion

(6.7.5-4)

TeX Embedding failed!

For water cooled direct expansion

(6.7.5-5)

TeX Embedding failed!

For chilled water coils

(6.7.5-6)

TeX Embedding failed!

where

Qt,available Available cooling capacity at specified evaporator and/or condenser conditions (MBH)
Qt,adj Adjusted capacity at ARI conditions (Btu/h) (see Equation(6.7.5-1)
CAP_FT A multiplier to adjust Qt,adj
twb The entering coil wet-bulb temperature (°F)
tdb The entering coil dry-bulb temperature (°F)
twt The water supply temperature (°F)
todb The outside-air dry-bulb temperature (°F)

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.

Software may represent the relationship between cooling capacity and temperature in ways other than the equations given above.

Table 6.7.5-2: "Cooling Capacity Curve Coefficients"

 

Co-
efficient
Air Cooled Direct Expansion Water Cooled Direct Expansion Chilled Water Coils
Air-Source
(PTAC)
Air-Source
(Other DX)
Water-Source
(Heat Pump)
Water-Source
(Other DX)
Fan-Coil Other Chilled Water
a 1.1839345 0.8740302 -0.2780377 0.9452633 0.5038866 2.5882585
b -0.0081087 -0.0011416 0.0248307 -0.0094199 -0.0869176 -0.2305879
c 0.0002110 0.0001711 -0.0000095 0.0002270 0.0016847 0.0038359
d -0.0061435 -0.0029570 -0.0032731 0.0004805 0.0336304 0.1025812
e 0.0000016 0.0000102 0.0000070 -0.0000045 0.0002478 0.0005984
f -0.0000030 -0.0000592 -0.0000272 -0.0000599 -0.0010297 -0.0028721
Note: These curves are the DOE-2.1E defaults, except for Water-Source (Other DX), which is taken from the “ECB Compliance Supplement, public review draft prepared by the SSPC 90.1 ECB Panel, Version 1.2, March 1996.
Units Data structure
Input Restrictions As designed. The equations and coefficients given above are the default.
Baseline Rules Use the default curves or equivalent data for other models.
Coil Bypass Factor
Applicability All cooling systems
Definition The ratio of air that bypasses the cooling coil at design conditions to the total system airflow.
Units Ratio
Input Restrictions

As designed. Default values are given in Table 6.7.5-2.

Table 6.7.5-3: "Default Coil Bypass Factors"

 

System Type Default Bypass Factor
Packaged Terminal Air-conditioners and Heat Pumps 0.241
Other Packaged Equipment 0.190
Multi-Zone Systems 0.078
All Other 0.037
Baseline Rules Defaults
Coil Bypass Factor Adjustment Curve
Applicability All cooling systems
Definition Adjustments for the amount of coil bypass due to the following factors:
  • Coil airflow rate as a percentage of rated system airflow
  • Entering air wet-bulb temperature
  • Entering air dry-bulb temperature
  • Part load ratio
Units Data structure
Input Restrictions

Default to the simulation engine defaults based on HVAC system type. The following default values shall be used for the adjustment curves:

(6.7.5-7)

TeX Embedding failed!

(6.7.5-8)

TeX Embedding failed!

(6.7.5-9)

TeX Embedding failed!

(6.7.5-10)

TeX Embedding failed!

where

CBFrated The coil bypass factor at ARI rating conditions
CBFadj The coil bypass factor adjusted for airflow and coil conditions
CFMR The ratio of airflow to design airflow
COIL-BF-FFLOW A multiplier on the rated coil bypass factor to account for variation in air flow across the coil (take coefficients from Table 6.7.5-3
COIL-BF-FT A multiplier on the rated coil bypass factor to account for a variation in coil entering conditions (take coefficients from Table 6.7.5-5)
COIL-BF-FPLR A multiplier on the rated coil bypass factor to account for the part load ratio (take coefficients from Table 6.7.5-6)
Twb The entering coil wet-bulb temperature (°F)
Tdb The entering coil dry-bulb temperature (°F)
PLR Part load ratio

And the coefficients are listed in the tables below.

Table 6.7.5-4: "Coil Bypass Factor Airflow Adjustment Factor"

Co-
efficient
COIL-BF-FFLOW (PTAC) COIL-BF-FFLOW (HP) COIL-BF-FFLOW (PSZ/other)
a -2.277 -0.8281602 -0.2542341
b 5.21140 14.3179150 1.2182558
c -1.93440 -21.8894405 0.0359784
d   9.3996897  

Table 6.7.5-5: "Coil Bypass Factor Temperature Adjustment Factor"

Co-
efficient
COIL-BF-FT (PTAC) COIL-BF-FT (HP) COIL-BF-FT (PSZ, other)
a -1.5713691 -29.9391098 1.0660053
b 0.0469633 0.8753455 -0.0005170
c 0.0003125 -0.0057055 0.0000567
d -0.0065347 0.1614450 -0.0129181
e 0.0001105 0.0002907 -0.0000017
f -0.0003719 -0.0031523 0.0001503

Table 6.7.5-6: "Coil Bypass Factor Part Load Adjustment Factor"

Co-
efficient
COIL-BF-FPLR (All Systems)
a 0.00
b 1.00
Baseline Rules Use defaults as described above.

Direct Expansion

Direct Expansion Cooling Efficiency
Applicability Packaged equipment
Definition

The cooling efficiency of a direct expansion (DX) cooling system at ARI rated conditions as a ratio of output over input in Btu/h per W, excluding fan energy. The software must accommodate user input in terms of either the Energy Efficiency Ratio (EER) or the Seasonal Energy Efficiency Ratio (SEER). For equipment with SEER ratings, EER shall be taken from manufacturers’ data when it is available. When it is not available it shall be calculated as follows:

(6.7.5-11)

TeX Embedding failed!

For all unitary and applied equipment where the fan energy is part of the equipment efficiency rating, the EER shall be adjusted as follows:

(6.7.5-12)

TeX Embedding failed!

where

EERadj The adjusted Energy Efficiency Ratio for simulation purposes
EER The rated Energy Efficiency Ratio
Qt,rated The ARI rated total cooling capacity of a packaged unit (kBtu/h)
BHPsupply The supply fan brake horsepower (bhp) shall be taken from manufacturers’ literature when available, otherwise use Equation(6.7.3-2).
Units Btu/h-W
Input Restrictions As designed. When possible, specify the SEER and EER for packaged equipment with cooling capacity less than 65,000 Btu/h. For equipment with capacity above 65,000 Btu/h, specify EER.
Baseline Rules For the purpose of green building ratings, look up the requirement from Table 6.8.1A and Table 6.8.1B in ASHRAE Standard 90.1-2007. For the purpose of tax deduction calculations, look up the requirement from Table 6.2.1A and 6.2.1B in ASHRAE Standard 90.1-2001. Use the total cooling capacity of the proposed design to determine the size category.
Direct Expansion Cooling Efficiency Adjustment Curve
Applicability Packaged DX equipment
Definition

A curve or group of curves that varies the cooling efficiency of a direct expansion (DX) coil as a function of evaporator conditions, condenser conditions and part-load ratio. The default curves are given as follows as adjustments to the energy input ratio (EIR)1:

(6.7.5-13)

TeX Embedding failed!

(6.7.5-14)

TeX Embedding failed!

(6.7.5-15)

For air-cooled DX systems:

TeX Embedding failed!

(6.7.5-16)

For water-cooled DX systems:

TeX Embedding failed!

(6.7.5-17)

TeX Embedding failed!

where

PLR Part load ratio based on available capacity (not rated capacity)
EIR-FPLR A multiplier on the EIR to account for the part load ratio
EIR-FT A multiplier on the EIR to account for the wet-bulb temperature entering the coil and the outdoor dry-bulb temperature
Qoperating Present load on heat pump (Btu/h)
Qavailable Heat pump available capacity at present evaporator and condenser conditions (in Btu/h).
twb The entering coil wet-bulb temperature (°F)
twt The 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 unit employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.

Table 6.7.5-7: "Cooling System Coefficients for EIR-FPLR"

Co-
efficient
Water-Source
(Heat Pump)
Water-Source
(Other)
Air-Source (PTAC) Air-Source (Other)
a 0.1250000 0.2012301 0.1250000 0.2012301
b 0.8750000 -0.0312175 0.8750000 -0.0312175
c 0.0000000 1.9504979 0.0000000 1.9504979
d 0.0000000 -1.1205105 0.0000000 -1.1205105

Table 6.7.5-8: "Cooling System Coefficients for EIR-FT"

Co-
efficient
Water-Source
(Heat Pump)
Water-Source
(Other)
Air-Source (PTAC) Air-Source (Other)
a 2.0280385 -1.8394760 -0.6550461 -1.0639310
b -0.0423091 0.0751363 0.0388910 0.0306584
c 0.0003054 -0.0005686 -0.0001925 -0.0001269
d 0.0149672 0.0047090 0.0013046 0.0154213
e 0.0000244 0.0000901 0.0001352 0.0000497
f -0.0001640 -0.0001218 -0.0002247 -0.0002096
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, the software must indicate that supporting documentation is required on the output forms.
Baseline Rules Use default curves.
Minimum Unloading Ratio
Applicability Packaged systems which use hot-gas bypass during low load conditions
Definition The upper end of the hot-gas bypass operating range. This is the percentage of peak cooling capacity below which hot-gas bypass will operate.
Units Ratio
Input Restrictions As designed. The user must enter this descriptor for each DX cooling system. If hot-gas bypass is not employed, a value of 0 may be entered. A maximum of 0.5 is allowed for units with a peak cooling capacity of 240 kBtu/h (20 tons) or less, and a maximum value of 0.25 is allowed for units with a peak cooling capacity greater than 240 kBtu/h.
Baseline Rules Not applicable
Minimum HGB Ratio
Applicability Packaged systems which use hot-gas bypass during low load conditions
Definition The lower end of the hot-gas bypass operating range. The percentage of peak cooling capacity below which hot-gas bypass will no longer operate (i.e. the compressor will cycle).
Units Ratio
Input Restrictions As designed. The user must enter this descriptor for each DX cooling system. If hot-gas bypass is not employed, a value of 0 may be entered.
Baseline Rules Not applicable
Condenser Type
Applicability All direct expansion systems including heat pumps
Definition The type of condenser for a direct expansion (DX) cooling system. The choices are:
  • Air-Cooled
  • Water-Cooled
  • Air-Cooled with Evaporative Pre-cooler
Units List (see above)
Input Restrictions As designed
Baseline Rules Based on the prescribed system type. Refer to the HVAC System Map in Figure 6.1.2-1.

Table 6.7.5-9: "Baseline Building Condenser Type"

Baseline building System Condenser Type
System 1 – PTAC Air-cooled
System 2 – PTHP Air-cooled
System 3 – PSZ-AC Air-cooled
System 4 – PSZ-HP Air-Cooled
System 5 – Packaged VAV with Reheat Air-cooled
System 6 – Packaged VAV with PFP boxes Air-cooled
System 7 – VAV with Reheat N/A
System 8 – VAV with PFP boxes N/A
Condenser Flow Type
Applicability All direct expansion systems including heat pumps
Definition Describes water flow control for a water-cooled condenser. The choices are:
  • Fixed Flow
  • Two-position
  • Variable Flow
Units List (see above)
Input Restrictions Default to fixed flow. If the variable-flow is selected, the software must indicate that supporting documentation is required on the output forms.
Baseline Rules Always fixed flow

Evaporative Cooler

This is equipment that pre-cools the outside air that is brought into the building. It may be used with any type of cooling system that brings in outside air. This equipment is not applicable for the baseline building.

Evaporative Cooling Type
Applicability Systems with evaporative pre-cooling
Definition

The type of evaporative pre-cooler, including:

  • None
  • Non-Integrated Indirect
  • Non-Integrated Direct/Indirect
  • Integrated Indirect
  • Integrated Direct/Indirect

An integrated pre-cooler can operate together with the compression or CHW cooling. A non-integrated pre-cooler will shut down the evaporative cooling whenever it is unable to provide 100% of the cooling required.

 

In all cases, the evaporative pre-cooler must be modeled with 100% of the outside air routed through the pre-cooler.

Units None
Input Restrictions As designed
Baseline Rules Not applicable
Direct Stage Effectiveness
Applicability Systems with evaporative pre-cooling
Definition

The effectiveness of the direct stage of an evaporative cooling system. Effectiveness is defined as follows:

(6.7.5-18)

TeX Embedding failed!

where

DirectEFF The direct stage effectiveness
Tdb The entering air dry-bulb temperature
Twb The entering air wet-bulb temperature
Tdirect The direct stage leaving dry-bulb temperature
Units Numeric
Input Restrictions As designed
Baseline Rules Not applicable
Indirect Stage Effectiveness
Applicability Systems with evaporative pre-cooling
Definition

The effectiveness of the indirect stage of an evaporative cooling system. Effectiveness is defined as follows:

(6.7.5-19)

TeX Embedding failed!

where

IndEFF The indirect stage effectiveness
Tdb The entering air dry-bulb temperature of the supply air
Twb The entering air wet-bulb temperature of the “scavenger air”
Tind The supply air leaving dry-bulb temperature
Units Numeric
Input Restrictions As designed
Baseline Rules Not applicable
Evaporative Cooling Performance Curves
Applicability Systems with evaporative cooling
Definition

A curve that varies the evaporative cooling effectiveness as a function of primary air stream airflow. The default curves are given as follows:

(6.7.5-20)

TeX Embedding failed!
TeX Embedding failed!

where

PLR Part load ratio of airflow based on design airflow
EFF-FFLOW A multiplier on the evaporative cooler effectiveness to account for variations in part load
CFMoperating Operating primary air stream airflow (cfm)
CFMdesign Design primary air stream airflow (cfm)

Table 6.7.5-10: "Part Load Curve Coefficients – Evaporative Cooler Effectiveness"

Co-
efficient
Direct Indirect
a 1.1833000 1.0970000
b -0.2575300 -0.1650600
c 0.0742450 0.0680690
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, the software must indicate that supporting documentation is required on the output forms.
Baseline Rules Not used.
Auxiliary Evaporative Cooling Power
Applicability Systems with evaporative cooling
Definition The auxiliary energy of the indirect evaporative cooler fan, and the pumps for both direct and indirect stages
Units kW/cfm
Input Restrictions As designed
Baseline Rules Not applicable
Evaporative Cooling Scavenger Air Source
Applicability Systems with evaporative cooling
Definition The source of scavenger air for an indirect section of an evaporative cooler. Options include:
  • Return Air
  • Outside Air
Units List (see above)
Input Restrictions As designed
Baseline Rules Not applicable

Evaporative Condenser

Evaporative Condenser Power
Applicability Direct expansion systems with an evaporatively cooled condenser
Definition The power of the evaporative precooling unit. This includes any pump(s) and/or fans that are part of the precooling unit.
Units Kilowatts (kW)
Input Restrictions As designed
Baseline Rules Not applicable
Evaporative Condenser Effectiveness
Applicability Direct expansion systems with an evaporatively cooled condenser
Definition The effectiveness of the evaporative precooling unit for a condenser. Effectiveness is defined as follows:

(6.7.5-21)

TeX Embedding failed!

where

DirectEFF The direct stage effectiveness
Tdb The entering air dry-bulb temperature
Twb The entering air wet-bulb temperature
Tdirect The direct stage leaving dry-bulb temperature
Units Ratio
Input Restrictions As designed
Baseline Rules Not applicable
Evaporative Condenser Operation Range
Applicability Direct expansion systems with an evaporatively cooled condenser.
Definition

The temperature range within which the evaporative condenser operates. Two values are provided:

Tmaximum    The threshold outside air dry-bulb temperature below which evaporative condenser operates.

Tminimum    The threshold outside air dry-bulb temperature above which evaporative condenser operates.

Units Degrees Fahrenheit (°F)
Input Restrictions As designed
Baseline Rules Not applicable
  • 1. The EIR is the ratio of energy used by the system to cooling capacity in the same units. It is the reciprocal of the coefficient of performance (COP).