Our refrigerated storage area has eight evaporators which are defrosted by closing the liquid line solenoid and allowing the fans to pass air over the fins. There is no hot gas defrost. The temperature is between 2C and 4C (35.6F and 39.2F). Most of the time, having the defrost logic running on a sequencer works fine, but not when it's hot and humid...
Three of the evaporators are in the 'loading dock" end of the area. When it's humid, some outside air enters around dock seals when trailers are loaded. This sometimes causes some (or all) of the evaporators in the area to ice up faster than normal. I've already tried reducing the amount of time between defrosts by monitoring the enthalpy of the outside air and increasing defrost frequency accordingly. This strategy keeps the evaporators free of ice, but it also causes the temperature to increase because there's more time when only two of the three evaporators are actually cooling. Also, if the enthalpy is high but the doors aren't open, there is little need to defost more frequently.
My strategy is to detect frost and trigger defrosting accordingly.
I'm hoping that someone can recommend a method to detect frost. I've been considering a pair of photoelectric send-receive sensors mounted so the beam breaks if a band of ice develops. I've also considered a diffuse-close sensor mounted parallel to air flow behind the evaporator coil to detect frost. Monitoring the current flowing to the fan motors may also work, but I think the ice will have built up too much before a change in current draw will be noticed.
Does anyone have experience with either of the above plans, or can anyone recommend a more efficient method of defrost control? Any help would be appreciated.
Three of the evaporators are in the 'loading dock" end of the area. When it's humid, some outside air enters around dock seals when trailers are loaded. This sometimes causes some (or all) of the evaporators in the area to ice up faster than normal. I've already tried reducing the amount of time between defrosts by monitoring the enthalpy of the outside air and increasing defrost frequency accordingly. This strategy keeps the evaporators free of ice, but it also causes the temperature to increase because there's more time when only two of the three evaporators are actually cooling. Also, if the enthalpy is high but the doors aren't open, there is little need to defost more frequently.
My strategy is to detect frost and trigger defrosting accordingly.
I'm hoping that someone can recommend a method to detect frost. I've been considering a pair of photoelectric send-receive sensors mounted so the beam breaks if a band of ice develops. I've also considered a diffuse-close sensor mounted parallel to air flow behind the evaporator coil to detect frost. Monitoring the current flowing to the fan motors may also work, but I think the ice will have built up too much before a change in current draw will be noticed.
Does anyone have experience with either of the above plans, or can anyone recommend a more efficient method of defrost control? Any help would be appreciated.