S K Energy Consulting Ltd.

What is the optimum condensing pressure?
To run refrigeration plant at peak efficiency, we have to keep condensing pressure at optimum level.

A wet bulb approach feature should be used to control head pressure. The condenser's job is to maintain the condensing temperature within certain "approach" (temperature difference) above the ambient wet bulb temperature

Tcond.opt. = Twb + T.D.opt.

To keep total power usage (compressors + pumps + fans) at minimum, we have to keep temperature difference at optimum level. For sure, T.D. is a function of the actual system heat rejection loads, but it is a function of the wet bulb temperature as well. Our research shows that optimum T.D. is between 8 degreeF and 25 degreeF for the different condensers at different levels of the wet bulb temperatures.

The reason for that wide range of the optimum T.D. lies in the evaporation process on which the evaporative condenser operates. The major heat-transfer mechanism in the evaporative condenser is due to vaporization of water from the condenser tubes, and the rate of this vaporization is proportional to the difference of water-vapor pressure of the liquid water on the tube and the water vapor pressure in saturated air that surrounds the tube. A psychrometric chart shows that at the low level of temperature, the saturation curve flattens out so that a given difference in the temperature translates to a lower difference in water-vapor pressure.

Every refrigeration plant has its own unique range of the optimum T.D. Right set points for the approach will guarantee the minimum combined compressors and condensers power.