When evaporator fans are operated full time after the product cool-down period they produce approximately 70% of the refrigeration load; the balance consists of product respiration and the building envelope loads. Literally all of the energy consumed by the fan motors is converted into heat which the refrigeration system must remove. A typical 2,000 bin CA apple storage room will have (4) three horsepower fan motors which use approximately 10 kilowatts of power. This adds as much heat to the room as would (4) ten foot long electric baseboard heaters. It's no wonder that refrigeration compressors carry a significant load even when ambient temperatures are below freezing!

In addition to the negative impact that full capacity fan operation has on energy efficiency, the product quality can also be adversely affected. Research has shown that the excess evaporator capacity tends to overdrive the temperature which results in less stable product temperature than even intermittent coil operation does. The high air velocity that the fans produce seriously affect the rate of moisture loss, or transpiration, from the product. This equates to less product left to sell (as much as 2-5% weight loss) and lower quality due to shrivel, etc.

Variable speed evaporator fan control is an alternative to fan cycling which offers the same benefits and increased advantages. This technique utilizes AC variable speed drive units to operate the fans over a typical speed range of 40 to 100% of full speed. The control system needs to have setpoints for minimum and maximum fan speed during normal operation. The speed is varied proportionally to the back pressure regulator (BPR) position, within the entered min/max setpoints. A bias factor can be entered such that the fan speed will be offset positively or negatively from the BPR position (e.g.: if the bias is set at +10, and the BPR is 45% open, then the fan speed will be 55% of full speed). When the room temperature is above setpoint, the maximum speed setpoint is overridden to allow the fans to reach 100% speed. If the BPR is wide open and the temperature is high, then the fans are driven to 100% speed regardless of the bias factor.

The advantages offered by variable speed fan control are as follows: