# cooling tower in free convection

OK, so the EnergyPlus engineering reference for (variable speed) cooling towers states that "The model will also account for tower performance in the “free convection” regime, when the tower fan is off but the water pump remains on." So, when I look at the performance curve (e.g. CoolTools), where DTA = f(TWB, DTR, fW, fA), with DTA = Approach, TWB = Wet Bulb Temperature, DTR = Range, fW = Water Flow Rate Ratio and fA = Air Flow Rate Ratio, and f() being a 35-coefficient curve, then in my mind, free convection would apply when fA = 0. Is that correct? Or would it be when fA=0.125, or some other Fraction of Tower Capacity in Free Convection Regime?

In addition, the fan power curve coefficients appear to be such that the fan power fraction is negative below fA=0.15, and positive above. But I suppose this has to to with fan VSD turn-own and fan cycling, rather than with free convection.

I am asking because I also tried to follow the logic in function void CalcVariableSpeedTower{} in EnergyPlus-8.7.0/src/CondenserLoopTowers.cc, which seems to complicate matters substantially beyond simple application of the f() curve?

For example, with constant fW=1, I would calculate the required DTR based on chiller operation and then, based on TWB, calculate the DTA for fA=0 to see what TCWS = TWB+DTA in free convection is. If it is less than the TCWSSP (setpoint), I am done. If it is more than TCWS, I would use Newton-Raphson to calculate fA from the f() curve such that DTA=TCWSSP-TWB for said required DTR. Of course, that leaves aside cycling and other finer points but it at least summarizes how I would want to go about free convection vs. forced convection.

Unfortunately, void CalcVariableSpeedTower{} is anything but that straight forward. Any enlightenment would be very much appreciated, especially since Input/Output Reference and Engineering Reference do not help me move on.