EnergyPlus CoolTools Airflow Ratio when Modulating Tower Fans
Hello, I’m trying to figure out one of the simulation steps in the CoolToolsCrossFlow model for theCoolingTower:VariableSpeed object (E+ Version 24.1). Specifically determining the airflow fraction of the tower (FRair) when the fans are modulating between maximum and minimum flows. It is the last “step” in the logic as shown in this previous answer.
Per the EngineeringReference, the CoolToolsCrossFlow correlation is numerically solved for FRair at the given range, wet-bulb, and water flow fraction (FRwater)—the range is a constant since the model finds the airflow needed to hit the LWT set point. However, I get much different FRair values when I try to recreate this calculation manually.
The image below shows the EnergyPlus FRair values (dark) and my spreadsheet-calculated FRair values (light) plotted against ambient WB. EnergyPlus modulates the fans down to about 50F wet-bulb while my model reaches minimum fan airflow earlier at ~61F wet-bulb. I’ve spot checked the CoolTools correlation by “hand” and my values satisfy all equations--the shape of the graph is also what I'd expect. Is there an extra step not listed in the EngineeringReference? Are air conditions (wet-bulb, density, etc.) being adjusted behind the scenes? I’ve confirmed that the range and LWT are the same in both models, it’s just EnergyPlus using a lot more air (and consequently fan power) than I was expecting. Note the E+ model is using a LoadProfile:Plant object and also an OutdoorAir:Node object for the tower entering conditions to match trend data.
