Happy Friday All!

I'm having an issue in a model where I have an open cooling tower which is isolated from a water-source heat pump loop by a heat exchanger. I'm modeling an efficiency measure wherein I'm changing the mechanism of changing cooling tower capacity in response to heat rejection load. The baseline condition is an arrangement where each cell (of a two-cell tower) has a dual-speed fan, and capacity is reduced by first reducing fan speed, then staging cells. The proposed condition is changing capacity through VFD-driven fans and running both cells simultaneously at reduced fan speeds (until, of course, the minimum speed threshold is reached, and one cell shuts off). We get savings through reduced fan power.

However, when I apply the measure, I get negative savings. I did a little investigation and found that the pump work on the cooling tower side of the heat exchanger increases considerably when I change the CELL-CTRL keyword from MIN-CELLS to MAX-CELLS. A little more investigation into the PS-H report shows that in the baseline, the pump flow is reduced between 40-50% of full flow for most if its operating hours, while in the proposed case, the pump flow remains at full flow. I presume that this is because the pumps are delivering a fixed flow per cell, regardless of whether the cell's fan is ramping up or down in response to required capacity. In the proposed case, both cells might be running at minimum fan speed, but the pump is still providing full flow to each cell. I've tried various things to get the water flow to vary, but so far nothing has worked. Anyone have a solution or insight?

Thanks,

Drew Morrison |Energy Engineer
Seventhwave
20 N Upper Wacker Dr, Ste 1642
Chicago, IL 60606
608-210-7151