607 CENTRIFUGAL CHILLER
Although a TRNSYS chiller model (Type 53) has been in existence for a number
of years, a new chiller model was written that is somewhat easier to use.
The established TRNSYS model requires an external performance data file, and
one of the parameters required (the ratio of the temperature difference
between the condenser water outlet and the evaporator water outlet relative
to a design temperature difference) must be derived from manufacturer-supplied
performance data. This centrifugal chiller model does not require an
external data file. It is based on a five parameter equation relating the
dimensionless power requirement to the dimensionless load and deviations from
the design entering condenser and leaving evaporator water temperatures.
That equation, which is used by the Trane Company in its simulation program
"TRACE", is:
Preq/Pdes = [0.140 + 0.544*(Qload/Qdes) + 0.316*(Qload/Qdes)2 ]
*[ 1 + 0.012*(ECWT - DECWT) - 0.015*(LEWT - DLEWT)]
(3.2.1)
where "Preq" and "Pdes" are the actual and design power requirements, "Qload"
and "Qdes" are the actual and design loads, "ECWT" and "DECWT" are the actual
and design entering condenser water temperatures, and "LEWT" and "DLWET" are
the actual and design leaving evaporator water temperatures (Pawelski 1994)
All temperatures are in degrees Fahrenheit. The design entering condenser
water temperature and the design leaving evaporator water temperature are
85 F and 44 F, respectively.
Type 68 inputs, parameters, and outputs include:
xin(1) chiller set point temperature (DLEWT) [C]
xin(2) entering evaporator water temperature [C]
xin(3) evaporator water mass flow rate [kg/hr]
xin(4) entering condensor water temperature [C]
xin(5) condenser water mass flow rate [kg/hr]
xin(6) on/off control variable
par(1) maximum chiller load [kJ/hr]
par(2) minimum chiller load [kJ/hr]
par(3) design chiller load [kJ/hr]
par(4) design power requirement [kW]
par(5) "a" in equation 3.2.1 above (0.140)
par(6) "b" in equation 3.2.1 above (0.544)
par(7) "c" in equation 3.2.1 above (0.316)
par(8) "d" in equation 3.2.1 above (0.012)
par(9) "e" in equation 3.2.1 above (-0.015)
out(1) leaving evaporator water water temperature [C]
out(2) evaporator water mass flow rate [kg/hr]
out(3) leaving condenser water temperature [C]
out(4) condenser water mass flow rate [kg/hr]
out(5) actual load [kJ/hr]
out(6) actual power requirement [kW]
out(7) heat rejection at condenser [kJ/hr]
out(8) coefficient of performance
Reference:
Cross, Kevin, An Evaluation of Ice and Chilled Water As Thermal
Storage Media for Combustion Turbine Inlet Air Cooling Systems, M.S. Thesis,
p 22, University of Wisconsin, Madison, 1994.