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:
|
| 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
[table title="Default Capacity Coefficients – Cooling Towers" id="default-capacity-coefficients-–-cooling-towers"]
|
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| 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:
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
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:
|
| 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
[table title="Default Efficiency TWR-FAN-FPLR Coefficients – VSD on Cooling Tower Fan" id="default-efficiency-TWR-FAN-FPLR-coefficients-–-VSD-on-cooling-tower-fan"]
|
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| 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 |
- 1Cooling 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
Baseline HVAC Tower Characteristics
Baseline building systems 7, 8, 11, 12, and 13 have one or more cooling towers.
The baseline heat rejection device shall be an axial-fan open-circuit cooling tower with variable-speed fan control and shall have an efficiency of 38.2 gpm/hp at the conditions specified in Table 6.8.1-7 of Standard 90.1-21016.
Condenser water design supply temperature shall be calculated using the cooling tower approach to the 0.4% evaporation design wet-bulb temperature as generated by the formula below, with a design temperature rise of 10°F.
$$Approach\ 10°F\ Range = 25.72 - (0.24 \times WB)$$
where
WB is the 0.4% evaporation design wet-bulb temperature in °F; valid for wet bulbs from 55°F to 90°F.
The tower shall be controlled to maintain a leaving water temperature based on climate zone (see below), floating up to the design leaving water temperature for the cooling tower.
|
Climate Zone |
Leaving Water Temperature |
|
5B, 5C, 6B, 8 |
65°F |
|
0B, 1B, 2B, 3B, 3C, 4B, 4C, 5A, 6A, 7 |
70°F |
|
3A,4A |
75°F |
|
0A, 1A, 2A |
80°F |
The baseline building design condenser-water pump power shall be 19 W/gpm and modeled as constant volume. For computer room systems using System 11 with an integrated water-side economizer, the baseline building design condenser water-pump power shall be increased by 3 W/gpm for flow associated with the water-side economizer. Each chiller shall be modeled with separate condenser water and chilled-water pumps interlocked to operate with the associated chiller.
|
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 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 variable speed drive. |
|
Cooling Tower Capacity1 |
|
|---|---|
|
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 auto-sized and increased by 15%. The tower is sized to supply 85 °F condenser water or 10 °F approach to wet bulb, whichever is lower, 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 |
Gpm/hp or unitless if energy input ratio (EIR) is specified (If the nominal tons but not the condenser water flow is specified, the condenser design water flow shall be 2.4 gpm per nominal cooling ton.) |
|
Input Restrictions |
As designed, but the cooling towers shall meet minimum performance requirements in Table 6.8.1G of Standard 90.1-2016 and must be at least 38.2 gpm/hp for an axial fan cooling tower and at least 20 gpm/hp for a centrifugal fan cooling tower. |
|
Baseline Rules |
38.2 gpm/hp |
|
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 |
1% wet-bulb design conditions if the baseline systemhas a cooling tower |
|
Cooling Tower Design Leaving 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 |
|
Cooling Tower Design Entering 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 10 °F above the cooling tower design leaving water temperature. |
|
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: (Equation 3.8.3-1) $$t_{R} = t_{cwr} - t_{cws}$$ $$t_{A} = t_{cws} - t_{owb}$$ $$t_{A} = a + b \cdot t_{R} + c \cdot t_{R}^{2} + d \cdot FRA + e \cdot FRA^{2} + f \cdot t_{R} \cdot FRA$$ $$FRA = \frac{-d -f \cdot t_{R} + \sqrt{(d + f +t_{R})^{2} - 4 \cdot e \cdot \left ( a+b \cdot t_{R} + c \cdot t_{R}^{2} - t_{A} \right )}}{2 \times e}$$ $$FWB = a + b \cdot FRA + c \cdot FRA^{2} + d \cdot t_{OWB} + e \cdot t_{OWB}^{2} + f \cdot t_{OWB} \cdot FRA$$ $$Q_{available} = Q_{rated} \cdot FWB \cdot \left ( \frac{t_{R}}{10} \right )$$ Where
|
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|
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:
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: (Equation 3.8.3-2) $$t_{cws} = t_{owb} + t_{A} + RR \cdot \left ( t_{dwb} + t_{owb} \right )$$ where
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 |
Controlled to 70°F when weather permits, floating up to leaving water temperature at design conditions |
||||||||||
|
Cooling Tower Capacity Control |
|
|---|---|
|
Applicability |
All cooling towers |
|
Definition |
Describes the modulation control employed in the cooling tower. Choices include:
|
|
Units |
List (see above) |
|
Input Restrictions |
As designed. |
|
Baseline Rules |
Variable 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 between 0 and 1.0 |
|
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 between 0 and 1.0 |
|
Input Restrictions |
As designed. |
|
Baseline Rules |
0.30 |
|
Cooling Tower Fan 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: (Equation 3.8.3-3) $$PLR = \frac{Q_{operating}}{Q_{available}(t_{R},t_{A}, t_{owb})}$$ $$TWR\_FAN\_FPLR = a + b \cdot PLR + c \cdot PLR^{2} + d \cdot PLR^{3}$$ $$P_{operating} = P_{rated} \cdot TWR\_FAN\_FPLR$$
where
|
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|
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 and coefficients, 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 between 0 and 1.0 |
|
Input Restrictions |
As designed. The default is 0.40. |
|
Baseline Rules |
Not applicable |
- 1Cooling 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
Baseline building systems 7, 8, and 11 through 13, have one or more cooling towers. One tower is assumed to be matched to each baseline building chiller. 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 that condenser water flow is a constant 2.5 gpm per cooling ton.5 The baseline building pumping energy is assumed to be 19 W/gpm. The baseline building cooling tower is assumed to have a variable speed fan that is controlled to provide a CWS equal to 70°F when weather permits. The tower fan would operate to maintain a CWS of 70 °F at low wet-bulb conditions. Under cooling conditions closer to design conditions, the CWS floats up to a maximum of 85 °F (the design condition).
The baseline building condenser water design supply temperature shall be calculated using the cooling tower approach to the 0.4% evaporation design wet-bulb temperature as generated by the formula below, with a design temperature rise of 10°F.
Approach 10°F Range = 25.72 – (0.24 × WB)
where WB is the 0.4% evaporation design wet-bulb temperature in °F; valid for wet bulbs from 55°F to 90°F.
The tower shall be controlled to maintain a leaving water temperature based on climate zone (see below), floating up to the design leaving water temperature for the cooling tower.
| Climate Zone | Leaving Water Temperature |
|---|---|
| 5B, 5C, 6B, 8 | 65°F |
| 0B, 1B, 2B, 3B, 3C, 4B, 4C, 5A, 6A, 7 | 70°F |
| 3A,4A | 75°F |
| 0A, 1A, 2A | 80°F |
| 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 Building | 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 Building | The baseline cooling tower is an open tower axial fan device with a two speed fan |
| 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 wet-bulb with a 3 gpm/nominal ton water flow. The default cooling tower curves below are at unity at these conditions. |
| Units | But/h |
| Input Restrictions | As designed |
| Baseline Building | The baseline building chiller is autosized and increased by 15%. The tower is sized to supply 85°F condenser water or 10°F approach to wet bulb, whichever is lower, 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, each with its 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 Building | The baseline building shall be modeled with one cooling tower with one cell. |
| 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 | gpm/hp or unitless if EIR is specified. (If the nominal tons but not the condenser water flow is specified, the condenser design water flow shall be 2.4 gpm per nominal cooling ton.) |
| Input Restrictions | As designed, but the cooling towers shall meet minimum performance requirements in Table 6.8.1-7 of Standard 90.1-2019 and must be at least 40.2 gpm/hp for an axial fan, open circuit cooling tower and at least 20 gpm/hp for a centrifugal fan open-circuit cooling tower |
| Baseline Building | 38.2 gpm/hp. Defined according to the minimum performance requirements for an axial fan cooling tower in Table 6.8.17 of Standard 90.1-2019 and rated gpm of the autosized tower. |
| 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 Building | 1% wet-bulb design conditions if the proposed building has a cooling tower |
| Cooling Tower Design Leaving Water Temp. | |
|---|---|
| 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 Building | 85°F or 10°F above the design wet-bulb temperature, whichever is lower |
| Cooling Tower Design Entering Water Temp. | |
|---|---|
| Applicability | All cooling towers |
| Definition | The design leaving condenser water 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 Building | Set to 10°F above the cooling tower design leaving water temperature |
| 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: Option 1 (DOE-2 based performance curves)
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 91. Default Capacity Coefficients – Cooling Towers
Option 2: CoolTools performance curve (EnergyPlus)
Where: FRair = Ratio of airflow to airflow at design conditions FRwater = Ratio of water flow to water flow at design conditions Tr = Tower range (°F) Twb = Wet-Bulb temperature Coefficients for this performance curve are provided in COMNET Appendix H (COMNET 2017). |
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| Units | Data structure | |||||||||||||||||||||
| Input Restrictions | User may input curves or use one of the two default curves. If defaults are overridden, the rating software must indicate that supporting documentation is required on the output forms. | |||||||||||||||||||||
| Baseline Building | Use one of the two 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 fans to provide the design condenser water supply temperature at all times when possible. A wet-bulb reset control will reset the condenser water setpoint according to the following control scheme:
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. An nRR 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 control, supporting documentation is required. |
| Baseline Building | Controlled to 70°F when weather permits, floating up to leaving water temperature at design conditions |
| 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: A simple method of capacity control where the tower fan is cycled on and off. This 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 building 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 Building | Variable speed fans |
| Cooling Tower Low-Speed Airflow Ratio | |
|---|---|
| Applicability | All cooling towers with variable speed, 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 | Fraction (between 0 and 1) |
| Input Restrictions | As designed |
| Baseline Building | 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 | Fraction (between 0 and 1) |
| Input Restrictions | Calculated, using the as-designed flow ratio and the cooling tower power adjustment curve below |
| Baseline Building | 0.30 |
| Cooling Tower Fan 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:
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 92. Default Efficiency TWR-FAN-FPLR Coefficients – VSD on Cooling Tower Fan
|
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| Units | Data structure | ||||||||||
| Input Restrictions | User may input curves or use default curves. If defaults are overridden, supporting documentation is required. | ||||||||||
| Baseline Building | Use default curves, 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 | Fraction (between 0 and 1) |
| Input Restrictions | As designed. The default is 0.40. |
| Baseline Building | Not applicable |
| 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. |
| Cooling Tower Type | |
| Applicability | All cooling towers |
|---|---|
| Definition | The type of cooling tower employed. The choices are:
|
| Units | List (see above) |
| Input Restrictions | As designed |
| 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 |
| 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 |
| 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. |
| 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. |
| 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. |
| 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. |
| 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
[table title="Default Capacity Coefficients – Cooling Towers" id="default-capacity-coefficients-–-cooling-towers"]
|
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| 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. | |||||||||||||||||||||||||||||||||||||||
| Cooling Tower Set Point Control | |||||||||||
| Applicability | All cooling towers | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Definition | The type of control for the condenser water supply. The choices are:
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
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. |
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| Units | List (see above) | ||||||||||
| Input Restrictions | As designed. If the user does not use the default curve, supporting documentation is required. | ||||||||||
| Cooling Tower Capacity Control | |
| Applicability | All cooling towers |
|---|---|
| Definition | Describes the modulation control employed in the cooling tower. Choices include:
|
| Units | List (see above) |
| Input Restrictions | As designed. |
| 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. |
| 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. |
| 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
[table title="Default Efficiency TWR-FAN-FPLR Coefficients – VSD on Cooling Tower Fan" id="default-efficiency-TWR-FAN-FPLR-coefficients-–-VSD-on-cooling-tower-fan"]
|
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| Units | Data structure | ||||||||||||||||||||||||
| Input Restrictions | User may input curves or use default curves. If defaults are overridden, supporting documentation is required. | ||||||||||||||||||||||||
| 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. |
| 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. |
| Cooling Tower Type | |
| Applicability | All cooling towers |
|---|---|
| Definition | The type of cooling tower employed. The choices are:
|
| Units | List (see above) |
| Input Restrictions | As designed |
| 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 |
| 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 |
| 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. |
| 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. |
| 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. |
| 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. |
| 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
[table title="Default Capacity Coefficients – Cooling Towers" id="default-capacity-coefficients-–-cooling-towers"]
|
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| 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. | |||||||||||||||||||||||||||||||||||||||
| Cooling Tower Set Point Control | |||||||||||
| Applicability | All cooling towers | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Definition | The type of control for the condenser water supply. The choices are:
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
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. | ||||||||||
| Cooling Tower Capacity Control | |
| Applicability | All cooling towers |
|---|---|
| Definition | Describes the modulation control employed in the cooling tower. Choices include:
|
| Units | List (see above) |
| Input Restrictions | As designed. |
| 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. |
| 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. |
| 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
[table title="Default Efficiency TWR-FAN-FPLR Coefficients – VSD on Cooling Tower Fan" id="default-efficiency-TWR-FAN-FPLR-coefficients-–-VSD-on-cooling-tower-fan"]
|
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| Units | Data structure | ||||||||||||||||||||||||
| Input Restrictions | User may input curves or use default curves. If defaults are overridden, supporting documentation is required. | ||||||||||||||||||||||||
| 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. |