Turning off the evaporator fans after the compressor is switched off does not waste the thermal inertia (the “cold”) inside the evaporator coils. Once the refrigerant inside the evaporator coils is evaporated and returned to the compressor, which takes just a few seconds, the cold remains in the evaporator. The walk-in cooler itself is as cold as you want it, as evidenced by the thermostat having shut down the refrigeration process for the time being. If the fans are not moving air through the evaporators and continuing to drive down the temperature of the air inside the space, the cold is simply stored in the evaporator and available for cooling the air once the thermostat switches on the compressor and the evaporator fans turn on again. If the evaporator fans have been off for only 20 minutes or so, they are likely to be still cold when the compressor begins to operate again. They will then more quickly get cold enough to begin to cool down the space again.
Keeping the evaporator fans on after the compressor is off and the “pumpdown” solenoid ensures that there is no more evaporating refrigerant actively creating cold within the evaporator coil means that blowing air across the coils continues to lower the air temperature inside the cooler and to raise the temperature of the evaporator. Operating the evaporator fans until the coils have warmed to the ambient temperature inside the cooler, or even worse, never switching them off (still the most common practice in commercial refrigeration), means that there is less and less cold being transferred to the air passing through the evaporator. The temperature difference, or “delta T” (∆T), is just too small. However, if the motors continue to draw the same amount of electricity, all of which turns to heat inside the cooler, their cooling efficiency steadily drops and eventually become negative when they, in effect, become greatly overpowered circulating fans.
There are 3 good, but different, ways to efficiently reduce the evaporator fan operation during the compressor off-cycle, while supplying sufficient circulation around the room:
- reducing the speed, and therefore the airflow and wattage of the evaporator fan motors;
- switching the evaporator fans on intermittently every few minutes; or
- turning on a low-wattage, high flow circulation fan the entire time the evaporator fans are off. This can be aimed at the floor to effectively “de-stratify” the air, so warm air does not rise to the top and cold air does not sit at the bottom of the cooler. This is what Freeaire’s Circ fan, shown here, does.
At Freeaire, we take advantage of techniques 2 and 3, choosing the one best suited for the air flow needs of the cooler. For example, a Circ fan may be a better fit for a cooler that is oddly shaped or where there could be dead pockets.
Each of these strategies ensures that the evaporator fans only operate when they can efficiently provide cooling, but there is enough circulation to maintain an even temperature throughout the space. All products stay cold. Nothing is wasted.