Variable speed cooling towers can be modelled by EnergyPlus with user-selectable performance based on the CoolTools correlation, YorkCalc correlation, or user-defined coefficients for either the CoolTools or YorkCalc correlations. The first 2 of these are built into EnergyPlus while the second 2 require additional performance data as explained below.
The empirical CoolTools tower correlation uses a set of 35 coefficients to model the thermal performance (approach temperature) of a cooling tower based on four independent variables. If you specify the Model type as 3-Cool Tools cross flow on the Cooling Tower dialog for a Variable Speed object, then the 35 coefficients derived for the CoolTools simulation model are used and these coefficients are already defined within EnergyPlus. If on the other hand you select the 3-Cool Tools user defined option, then you must also select a Cool tools Cooling Tower Performance component to define the 35 coefficients that will be used by the CoolTools correlation.
To specify the valid range for which the model coefficients were derived, it is necessary to enter the minimum and maximum values for: inlet air wet-bulb temperature, range temperature, approach temperature, and water mass flow rate ratio . For all of these variables, the program issues warnings if the actual values are beyond the minimum/maximum values specified. For inlet air wet-bulb temperature and water mass flow rate ratio, the values of these variables used in the calculation of approach temperature are limited to be within the valid minimum/maximum range specified. For approach and range, the warnings are issued if the values are beyond the specified minimum/maximum range but the actual values are still used.
The CoolTools correlation has four independent variables: inlet air wet-bulb temperature (Twb), tower range temperature (Tr), water flow rate ratio (FRwater), and air flow rate ratio (FRair). Temperatures are in units of ˚C and flow rate ratios are dimensionless (actual flow rate divided by design flow rate). Using these independent variables, tower approach temperature (˚C) is calculated as follows:
Approach = Coeff(1) + Coeff(2)•FRair + Coeff(3)•(FRair)2 +
Coeff(4)•(FRair)3 + Coeff(5)•FRwater +
Coeff(6)•FRair•FRwater + Coeff(7)•(FRair)2•FRwater +
Coeff(8)•(FRwater)2 + Coeff(9)•FRair•(FRwater)2 +
Coeff(10)•(FRwater)3 + Coeff(11)•Twb + Coeff(12)•FRair•Twb +
Coeff(13)•(FRair)2•Twb + Coeff(14)•FRwater•Twb +
Coeff(15)•FRair•FRwater•Twb + Coeff(16)•(FRwater)2•Twb +
Coeff(17)•(Twb)2 + Coeff(18)•FRair•(Twb)2 +
Coeff(19)•FRwater•(Twb)2 + Coeff(20)•(Twb)3 + Coeff(21)•Tr +
Coeff(22)•FRair•Tr + Coeff(23)•FRair•FRair•Tr +
Coeff(24)•FRwater•Tr + Coeff(25)•FRair•FRwater•Tr +
Coeff(26)•(FRwater)2•Tr + Coeff(27)•Twb•Tr +
Coeff(28)•FRair•Twb•Tr + Coeff(29)•FRwater•Twb•Tr +
Coeff(30)•(Twb)2•Tr + Coeff(31)•(Tr)2 + Coeff(32)•FRair•(Tr)2 +
Coeff(33)•FRwater•(Tr)2 + Coeff(34)•Twb•(Tr)2 + Coeff(35)•(Tr)3
It is recommended that a broad set of cooling tower performance data be used to generate these model coefficients. The data set used to create the model coefficients should cover the entire range of water and air flow rate ratios and inlet air wet-bulb, range, and approach temperatures expected during the simulation.
Enter a unique name for the cooling tower performance data.
Enter a description of the source of the data.
This must be Cool tools for entering user-defined Cool tools data.
This numeric field contains the minimum inlet air wet-bulb temperature (in °C or °F) to be used by the model (approach temperature correlation). Inlet air wet-bulb temperatures less than this value will not be used; instead, the minimum inlet air wet-bulb temperature specified here will be used by the correlation and a warning will be issued.
This numeric field contains the maximum inlet air wet-bulb temperature (in °C or °F) to be used by the model (approach temperature correlation). Inlet air wet-bulb temperatures greater than this value will not be used; instead, the maximum inlet air wet-bulb temperature specified here will be used by the correlation and a warning will be issued.
This numeric field contains the minimum range temperature (in Delta °C or Delta °F) (inlet water temperature minus outlet water temperature) to be used by the empirical model. If the range temperature is less than this value the actual range temperature is still passed to the empirical model but a warning will be issued.
This numeric field contains the maximum range temperature (in Delta °C or Delta °F) (inlet water temperature minus outlet water temperature) to be used by the empirical model. If the range temperature is greater than this value the actual range temperature is still passed to the empirical model but a warning will be issued.
This numeric field contains the minimum approach temperature (in Delta °C or Delta °F) (outlet water temperature minus inlet air wet-bulb temperature) to be used by the empirical model. If the calculated approach temperature is less than this value then the calculated value is still used but a warning will be issued.
This numeric field contains the maximum approach temperature (in Delta °C or Delta °F) (outlet water temperature minus inlet air wet-bulb temperature) to be used by the empirical model. If the calculated approach temperature is greater than this value then the calculated value is still used but a warning will be issued.
This numeric field contains the minimum water flow rate ratio (ratio of actual water flow rate to rated water flow rate) to be used by the empirical model. Water flow rate ratios less than this value will not be used; instead, the minimum water flow rate ratio specified here will be used by the model and a warning will be issued.
This numeric field contains the maximum water flow rate ratio (ratio of actual water flow rate to rated water flow rate) to be used by the empirical model. Water flow rate ratios greater than this value will not be used; instead, the maximum water flow rate ratio specified here will be used by the model and a warning will be issued.
These numeric fields contain the coefficients to be used by the CoolTools approach temperature correlation shown above.
The empirical YorkCalc tower correlation uses a set of 27 coefficients to model the thermal performance (approach temperature) of a variable speed cooling tower based on three independent variables. If you specify the Model type as 2-York calc on the Cooling Tower dialog for a Variable Speed object, then the 27 coefficients derived for the YorkCalc simulation model are used and these coefficients are already defined within EnergyPlus. If on the other hand you select the Model type as 4-York calc user defined then you must also select a Cooling Tower Performance object from the York calc category to define the 27 coefficients that will be used by the YorkCalc correlation.
To specify the valid range for which the model coefficients were derived, it is necessary to enter the minimum and maximum values for inlet air wet-bulb temperature, range temperature, approach temperature, and water mass flow rate ratio. It is also necessary to specify the maximum valid liquid-to-gas ratio. For all of these variables, the program issues warnings if the actual values are beyond the minimum/maximum values specified. For inlet air wet-bulb temperature and water mass flow rate ratio, the values of these variables used in the calculation of approach temperature are limited to be within the valid minimum/maximum range specified. For approach, range, and liquid-to-gas ratio the warnings are issued if the values are beyond the specified minimum/maximum range but the actual values are still used.
The YorkCalc correlation has three independent variables: inlet air wet-bulb temperature (Twb), tower range temperature (Tr), and the liquid-to-gas ratio (ratio of water flow rate ratio to air flow rate ratio = LGRatio). Temperatures are in units of ˚C and liquid-to-gas ratio is dimensionless. Using these independent variables, an approach temperature (˚C) is calculated as follows:
Approach = Coeff(1) + Coeff(2)•Twb + Coeff(3)•Twb2 + Coeff(4)•Tr +
Coeff(5)•Twb•Tr + Coeff(6)•Twb2•Tr + Coeff(7)•Tr2 +
Coeff(8)•Twb•Tr2 + Coeff(9)•Twb2•Tr2 + Coeff(10)•LGRatio +
Coeff(11)•Twb•LGRatio + Coeff(12)•Twb2•LGRatio +
Coeff(13)•Tr•LGRatio + Coeff(14)•Twb•Tr•LGRatio +
Coeff(15)•Twb2•Tr•LGRatio + Coeff(16)•Tr2•LGRatio +
Coeff(17)•Twb•Tr2•LGRatio + Coeff(18)•Twb2•Tr2•LGRatio +
Coeff(19)•LGRatio2 + Coeff(20)•Twb•LGRatio2 +
Coeff(21)• Twb2•LGRatio2 + Coeff(22)•Tr•LGRatio2 +
Coeff(23)•Twb•Tr•LGRatio2 + Coeff(24)•Twb2•Tr•LGRatio2 +
Coeff(25)•Tr2•LGRatio2 + Coeff(26)•Twb•Tr2•LGRatio2 +
Coeff(27)•Twb2•Tr2•LGRatio2
It is recommended that a broad set of cooling tower performance data be used to generate these model coefficients. The data set used to create the model coefficients should cover the entire range of water and air flow rate ratios and inlet air wet-bulb, range, and approach temperatures expected during the simulation.
Enter a unique name for the cooling tower performance data.
Enter a description of the source of the data.
This must be York calc for entering user-defined York calc data.
This numeric field contains the minimum inlet air wet-bulb temperature (in °C or °F) to be used by the model (approach temperature correlation). Inlet air wet-bulb temperatures less than this value will not be used; instead, the minimum inlet air wet-bulb temperature specified here will be used by the correlation and a warning will be issued.
This numeric field contains the maximum inlet air wet-bulb temperature (in °C or °F) to be used by the model (approach temperature correlation). Inlet air wet-bulb temperatures greater than this value will not be used; instead, the maximum inlet air wet-bulb temperature specified here will be used by the model and a warning will be issued.
This numeric field contains the minimum range temperature (in Delta °C or Delta °F) (inlet water temperature minus outlet water temperature) to be used by the empirical model. If the range temperature is less than this value the actual range temperature is still passed to the empirical model but a warning will be issued.
This numeric field contains the maximum range temperature (in Delta °C or Delta °F) (inlet water temperature minus outlet water temperature) to be used by the empirical model. If the range temperature is greater than this value the actual range temperature is still passed to the empirical model but a warning will be issued.
This numeric field contains the minimum approach temperature (in Delta °C or Delta °F) (outlet water temperature minus inlet air wet-bulb temperature) to be used by the empirical model. If the calculated approach temperature is less than this value then the calculated value is still used but a warning will be issued.
This numeric field contains the maximum approach temperature (in Delta °C or Delta °F) (outlet water temperature minus inlet air wet-bulb temperature) to be used by the empirical model. If the calculated approach temperature is greater than this value then the calculated value is still used but a warning will be issued.
This numeric field contains the minimum water flow rate ratio (ratio of actual water flow rate to rated water flow rate) to be used by the empirical model. Water flow rate ratios less than this value will not be used; instead, the minimum water flow rate ratio specified here will be used by the model and a warning will be issued.
This numeric field contains the maximum water flow rate ratio (ratio of actual water flow rate to rated water flow rate) to be used by the empirical model. Water flow rate ratios greater than this value will not be used; instead, the maximum water flow rate ratio specified here will be used by the model and a warning will be issued.
This numeric field contains the maximum liquid-to-gas ratio (ratio of actual water flow rate ratio [capped to be within the minimum/maximum water flow rate ratio defined above as necessary] to actual air flow rate ratio) to be used by the empirical model. If the liquid-to-gas ratio is greater than this value the actual liquid to gas ratio is still passed to the empirical model but a warning will be issued.
These numeric fields contain the coefficients to be used by the YorkCalc approach temperature correlation shown above.