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6.8.3 Cooling Towers

Baseline Building Summary. Baseline building systems 7 and 8 have one or more cooling towers. One tower is assumed to be matched to each baseline building chiller. The number of baseline building chillers is determined in 6.8.2. Each baseline building chiller has its own condenser water pump that operates when the chiller is brought into service. The range between the condenser water return (CWR) and condenser water supply (CWS) is 10 F so the condenser water flow is a constant 2.5 gpm per cooling ton1 when the chiller is in service. The baseline building pumping energy is assumed to be 19 W/gpm. The baseline building cooling tower is assumed to have a two-speed fan that is controlled to provide a CWS of 70 F when weather permits. The tower fan cycles to one-speed or off to maintain a CWS of 70 F at low wetbulb conditions. Under cooling conditions closer to design conditions, the CWS floats up to a maximum of 85 F (the design condition).

Cooling Tower Name
Applicability All cooling towers
Definition A unique descriptor for each cooling tower
Units Text, unique
Input Restrictions User entry. Where applicable, this should match the tags that are used on the plans.
Baseline Rules Descriptive name that keys the baseline building plant
Cooling Tower Type
Applicability All cooling towers
Definition The type of cooling tower employed. The choices are:
  • Open tower, centrifugal fan
  • Open tower, axial fan
  • Closed tower, centrifugal fan
  • Closed tower, axial fan

Open cooling towers collect the cooled water from the tower and pump it directly back to the cooling system. Closed towers circulate the evaporated water over a heat exchanger to indirectly cool the system fluid.

Units List (see above)
Input Restrictions As designed
Baseline Rules The baseline cooling tower is an open tower axial fan device with a two-speed fan (See PRM G3.1.3.11)
Cooling Tower Capacity
Applicability All cooling towers
Definition The tower thermal capacity per cell adjusted to CTI (Cooling Technology Institute) rated conditions of 95 F condenser water return, 85 F condenser water supply, and 78 F wetbulb with a 3 gpm/nominal ton  water flow. The default cooling tower curves below are at unity at these conditions.
Units Btu/h
Input Restrictions As designed
Baseline Rules The baseline building chiller is autosized and increased by 15%. The tower is sized to deliver 85 F condenser water supply at design conditions for the oversized chiller.
Cooling Tower Number of Cells
Applicability All cooling towers
Definition The number of cells in the cooling tower. Each cell will be modeled as equal size. Cells are subdivisions in cooling towers into individual cells, each with their own fan and water flow, and allow the cooling system to respond more efficiently to lower load conditions.
Units Numeric: integer
Input Restrictions As designed
Baseline Rules One cell per tower and one tower per chiller.
Cooling Tower Total Fan Horse Power
Applicability All cooling towers
Definition The sum of the nameplate rated horsepower (hp) of all fan motors on the cooling tower. Pony motors should not be included.
Units Horsepower (hp)
Input Restrictions As designed. For minimum compliance with ASHRAE Standard 90.1-2007, must be at least 38.2 gpm/hp for an axial fan cooling tower and at least 20.0 gpm/hp for a centrifugal fan cooling tower. (Table 6.8.1G)
Baseline Rules Not applicable since pump power is specified as 19 watts/gpm.
Cooling Tower Design Wet-Bulb
Applicability All cooling towers
Definition The design wet-bulb temperature that was used for selection and sizing of the cooling tower.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed.
Baseline Rules Same as proposed design
Cooling Tower Design Entering Water Temperature
Applicability All cooling towers
Definition The design condenser water supply temperature (leaving tower) that was used for selection and sizing of the cooling tower.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Default to 85°F.
Baseline Rules 85°F or 10°F above the design wet-bulb temperature, whichever is lower (Table 6.8.1G)
Cooling Tower Design Return Water Temperature
Applicability All cooling towers
Definition The design condenser water return temperature (entering tower) that was used for selection and sizing of the cooling tower.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Default to 95°F.
Baseline Rules Set to 95°F for a range of 10 F. (Table 6.8.1G)
Cooling Tower Capacity Adjustment Curve(s)
Applicability All cooling towers
Definition

A curve or group of curves that represent the available total cooling capacity as a function of outdoor air wet-bulb, condenser water supply and condenser water return temperatures. The default curves are given as follows:

(6.8.3-1)

$$ t_R = t_{cwr} - t_{cws}$$
$$ t_A = t_{cws} - t_{owb}$$
$$ t_A = a + b \times t_R + c \times \left. t_R\right. ^2 + d \times FRA + e \times FRA^2 + f \times t_R \times FRA$$
$$ FRA = \frac{- d - f \times t_R + \sqrt{\left ( d+ f \times t_R \right )^2 - 4 \times e \times \left ( a + b \times t_R + c \times \left. t_R \right. ^2 - t_A\right )}}{2 \times e} $$
$$FW\!B = a + b \times FRA + c \times FRA^2 + d \times t_{owb} + e \times \left. t_{owb} \right. ^2 + f \times FRA \times t_{owb}$$
$$ Q_{available} = Q_{rated} \times FW\!B \times \left ( \frac{t_R}{10} \right )$$

where

Qavailable Available cooling capacity at present outside air and condenser water conditions (MBH)
Qrated Rated cooling capacity at CTI test conditions (MBH)
tcws The condenser water supply temperature (in °F)
tcwr
The condenser water return temperature (in °F)
towb The outside air wet-bulb temperature (°F)
tR The tower range (in °F)
tA The tower approach (in °F)
FRA An intermediate capacity curve based on range and approach
FWB The ratio of available capacity to rated capacity (gpm/gpm).

Table 6.8.3-1: "Default Capacity Coefficients – Cooling Towers"

Coefficient FRA FWB
a -2.22888899 0.60531402
b 0.16679543 -0.03554536
c -0.01410247 0.00804083
d 0.03222333 -0.02860259
e 0.18560214 0.00024972
f 0.24251871 0.00490857
Units Data structure
Input Restrictions User may input curves or use default curves. If defaults are overridden, the rating software must indicate that supporting documentation is required on the output forms.
Baseline Rules Use default curves.
Cooling Tower Set Point Control
Applicability All cooling towers
Definition The type of control for the condenser water supply. The choices are:
  • Fixed
  • Wet-bulb reset

A fixed control will modulate the tower to provide the design supply water temperature at all times. A wet-bulb reset control will reset according to the following control scheme:

(6.8.3-2)

$$ t_{cws} = t_{owb} + t_A + RR \times \left ( t_{dwb} - t_{owb}\right )$$

where

tcws The condenser water supply setpoint (in °F)
towb The outside air wet-bulb temperature (°F)
tdwb The design outside air wet-bulb temperature (°F).
tA The tower design approach (in °F).
RR The reset ratio (default is 0.29)

A reset ratio (RR) of 0 will force the tower to always attempt a fixed approach to the outdoor wet-bulb temperature. A reset ratio (RR) of 1 will cause the system to perform as if it had fixed condenser water controls.
Units List (see above)
Input Restrictions As designed. If the user does not use the default curve, supporting documentation is required.
Baseline Rules Fixed at 70°F
Cooling Tower Capacity Control
Applicability All cooling towers
Definition Describes the modulation control employed in the cooling tower. Choices include:
  • Fluid Bypass provides a parallel path to divert some of the condenser water around the cooling tower at part-load conditions
  • Fan Cycling is a simple method of capacity control where the tower fan is cycled on and off. This is and is often used on multiple-cell installations.
  • Two-Speed Fan/Pony Motor. From an energy perspective, these are the same. A lower horsepower pony motor is an alternative to a two-speed motor; the pony motor runs at part-load conditions (instead of the full sized motor) and saves fan energy when the tower load is reduced. Additional buildling descriptors are triggered when this method of capacity control is selected.
  • Variable Speed Fan. A variable frequency drive is installed for the tower fan so that the speed can be modulated.
Units List (see above)
Input Restrictions As designed.
Baseline Rules Two-speed fan
Cooling Tower Low-Speed Airflow Ratio
Applicability All cooling towers with two-speed or pony motors
Definition The percentage full load airflow that the tower has at low speed or with the pony motor operating. This is equivalent to the percentage full load capacity when operating at low speed.
Units Ratio
Input Restrictions As designed.
Baseline Rules 0.50
Cooling Tower Low-Speed kW Ratio
Applicability All cooling towers
Definition The percentage full load power that the tower fans draw at low speed or with the pony motor operating
Units Ratio
Input Restrictions As designed.
Baseline Rules 0.30
Cooling Tower Power Adjustment Curve
Applicability All cooling towers with VSD control
Definition

A curve that varies the cooling tower fan energy usage as a function of part-load ratio for cooling towers with variable speed fan control. The default curve is given as follows:

(6.8.3-3)

$$ PLR = \frac{Q_{operating}}{Q_{available}\left ( t_R, t_A, t_{OW\!B}\right )}$$
$$ TW\!R\!\_F\!AN\!\_F\!P\!L\!R = a + b \times PLR + c \times PLR^2$$
$$ P_{operating} = P_{rated} \times TW\!R\!\_F\!AN\!\_F\!P\!L\!R $$

where

PLR Part load ratio based on available capacity (not rated capacity)
Qoperating Present load on tower (in Btu/h)
Qavailable Tower available capacity at present range, approach, and outside wet-bulb conditions (in Btu/h).
towb The outside air wet-bulb temperature (°F)
tR The tower range (°F)
tA The tower approach (°F)
Prated Rated power draw at CTI conditions (kW)
Poperating Power draw at specified operating conditions (kW)

Table 6.8.3-2: "Default Efficiency TWR-FAN-FPLR Coefficients – VSD on Cooling Tower Fan"

Coefficient TWR-FAN-FPLR
a 0.33162901
b -0.88567609
c 0.60556507
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 from DOE 2, given above.
Cooling Tower Minimum Speed
Applicability All cooling towers with a VSD control
Definition The minimum fan speed setting of a VSD controlling a cooling tower fan expressed as a ratio of full load speed.
Units Ratio
Input Restrictions As designed. The default is 0.40.
Baseline Rules Not applicable
  • 1. Cooling capacity is related to flow and delta-T through the equation Q = 500 * GPM * Delta-T. When Q is one ton (12,000 Btu/h), GPM = 24 / Delta-T and Delta-T = 24 / GPM