606 COOLING COIL
The Type 52 cooling coil model in the TRNSYS 14 Library is only valid for
cooling coils having more than about four rows. This is because the model
approximates the multipass cross flow geometry characteristic of standard
cooling coils with a counterflow geometry. The model can then apply a
combined wet and dry analysis using modified definitions for the number of
transfer units and the capacitance rate ratio. However, many inlet cooling
system designs call for cooling coils with four rows or less.
By defining an effectiveness for each tube pass, it is possible to write an
exact expression for the total effectiveness of a multipass overall
counterflow geometry with the fluids mixed between passes. Assuming that the
water inside the cooling coil tubes is the maximum fluid, the effectiveness
for a single tube pass with a dry outer surface is given by:
epspass = 1/Cstar*[1 - e-(gamma*Cstar)] (3.6.2)
where "Cstar" is the ratio of the minimum to the maximum capacitance rate,
"gamma" is given by:
gamma = 1 - e-Ntup (3.6.3)
and "Ntup" is the number of transfer units per pass, given by the overall
number of transfer units divided by the number of tube passes. The total
cooling coil effectiveness (still assuming that the exterior surfaces of the
tubes are dry) is given by
eps = (deltan - 1)/(deltan - Cstar) (3.6.4)
where
delta = (1 - epspass*Cstar)/(1 - epspass) (3.6.5)
(Kays and London 1964).
The total effectiveness of wet tubes can be calculated in a similar manner.
The capacitance rate ratio and the number of transfer units per pass are
replaced with modified quantities based on the change in enthalpy of
saturated air with respect to the change in temperature over the temperature
range of interest. The remainder of the modified cooling coil model used in
this project is identical to that in the TRNSYS 14 Library.
References:
Cross, Kevin, An Evaluation of Ice and Chilled Water As Thermal
Storage Media for Combustion Turbine Inlet Air Cooling Systems, M.S. Thesis,
p 29-30, University of Wisconsin, Madison, 1994.
Kays, W.M. and A.L. London, Compact Heat Exchangers, pp 19-20, Second
Edition, McGraw Hill, New York, 1964.