Flexible operation of refrigeration plant

In order to maximize energy savings, a refrigeration plant requires flexible operation. Meaning that operating set points should be changed from time to time based on fluctuation of ambient conditions and refrigeration loads.

Condensing pressure.

During summer operation, condensing pressure should be kept at optimum level. This pressure should float up and down with the wet bulb temperature.

Example.

Night wet bulb temperature is 60°F. Day wet bulb temperature is 70°F. Optimum wet bulb approach is 15°F.

Condensing pressure should float from 125 psig (condensing temperature is 60°F + 15°F = 75°F) during night operation to 150 psig (condensing temperature is 70°F + 15°F = 85°F) during day operation. Optimum wet bulb approach can be different if wet bulb temperature will be lower than 60°F.

During winter operation condensing pressure should be kept at minimum allowable pressure. Why should we do that? To maximize energy savings, the condensing pressure of the refrigeration plant should be kept as close as possible to the optimum condensing pressure. Less difference between operating condensing pressure and optimum condensing pressure creates better efficiency of the refrigeration plant.

Example.

Optimum wet bulb approach is 15°F. Minimum allowable condensing pressure is 115 psig.

At wet bulb temperature of 55°F, condensing pressure will be 115 psig (condensing temperature is 60°F + 15°F = 70°F). One day wet bulb temperature was 50°F. Optimum condensing pressure for this wet bulb temperature is 100 psig (condensing temperature is 50°F + 15°F = 65°F). However, we can not reach this pressure because minimum allowable pressure is 115 psig. Meaning that as soon as the wet bulb temperature is lower than 55°F (fall, winter, and spring) the refrigeration plant should be operated at minimum allowable condensing pressure of 115 psig.

Suction pressure.

To maximize energy savings, suction pressure of holding freezers and coolers should be kept at optimum level all year round. However, sometimes we can not keep the optimum suction pressure because of limited evaporator surface. During summer operation suction pressure can be lowered to keep the temperature in refrigerated rooms. During winter operation suction pressure should be increased to optimum level.

Example.

The temperature in a refrigerated room is 0°F. Optimum temperature difference is 10°F. Optimum suction pressure is 9 psig. During summer operation suction pressure should be reduced to 3.5 psig (temperature difference is 20°F) to keep 0°F in refrigerated room. As soon as the refrigeration load decreases due to cooler weather, suction pressure should be gradually increased to 9 psig. This should be done if the optimum temperature difference is 10°F. However, if the optimum temperature difference is 20°F, suction pressure should be kept at 3.5 psig all year round.

Determine the optimum temperature difference for your refrigeration plant and try to keep it as long as possible. Though set points can be changed due to fluctuation of ambient conditions and refrigeration loads.