In the fluid bypass mode, except the free convection, the tower fan runs at full speed for the entire time step. The model determines the fluid bypass fraction by iterations until the mixed water meets the tower exiting water temperature setpoint. In both the free convection cooling when fan is off and normal cooling when fan is on for the entire time step, if the tower exiting water temperature is lower than the setpoint, the tower operates in fluid bypass mode. In fluid bypass mode, portion of the water goes through the tower media and gets cooled while the remaining water flow gets bypassed, two water flows then mix together trying to meet the tower exiting water setpoint temperature. For part-load operation, the model assumes a simple linear interpolation between two steady-state regimes without accounting for any cycling losses.įor single speed cooling towers, the capacity control can be fan cycling or fluid bypass. 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. The model can be used to simulate the performance of both single speed, two speed, and variable speed mechanical-draft cooling towers. Cooling tower performance is modeled using effectiveness-NTU relationships for counterflow heat exchangers. The input objects CoolingTower:SingleSpeed, CoolingTower:TwoSpeed, and CoolingTower:VariableSpeed:Merkel provide models for single-speed, two-speed, and variable-speed cooling towers that are based on Merkel’s theory (Merkel 1925), which is also the basis for the tower model included in ASHRAE’s HVAC1 Toolkit for primary HVAC system energy calculations (ASHRAE 1999, Bourdouxhe et al. Cooling Towers and Evaporative Fluid Coolers One, Two, and Variable Speed Cooling Towers and Evaporative Fluid Coolers Overview
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