Evaporator Fan Control
Evaporators are sized for the largest anticipated refrigeration load which is some combination of warm summer temperatures and large internal product loads (e.g. cooling fruit from field temperatures to storage temperature). In CA storages, typical refrigeration loads during the holding season are only a small fraction of the design load (less than 25%). In regular storages, the average refrigeration load is typically 50% to 70% of the design load.
The evaporators must reduce their cooling effect to match these reduced loads. There are three primary means of accomplishing the cooling reduction:
- Continuous full-speed operation - Either the flow of refrigerant is cut off from the evaporator, or the refrigerant temperature is modulated with a pressure regulator. This is the least energy efficient form of control. Fan energy is a constant (i.e., 100% fan power at 50% refrigeration load).
- Evaporator fan cycling - Fans are cycled to maintain space temperature at the desired level. This technique is normally employed with a computer control system or manual shedding of fans by plant personnel. This method has an intermediate level of energy efficiency, as fan power is reduced proportional to run time (e.g., time averaged 50% evaporator fan power at 50% refrigeration load).
- Evaporator Fan Speed Control - Fan speed is varied to maintain space temperature. The fan speed reduction is conducted with either variable speed drives or two speed evaporator fans. This method is the most energy efficient alternative, as fan power is reduced as the cube of fan speed (e.g., 15% fan power at 50% refrigeration load).
The fan affinity laws dictate the cubic relationship between fan power and fan speed.
