Evaporator Fan VFD Control Algorithms

Flooded Evaporator Zones

(typical of Controlled Atmosphere applications)

There is typically one VFD per refrigeration zone. Most CA rooms have a single zone and thus have a single VFD.

VFD is normally managed by a computer control system, but can work with manual speed settings in systems without a computer control system.

For CA applications, it is essential that the VFD is maintained at full speed until fruit core temperature is fully pulled down to the target level. This pull down normally takes two to three weeks. Air temperatures will drop more quickly than the core fruit temperatures and should not be completely relied upon as a gauge of core fruit temperature.

The refrigeration control system should be programmed such that there is a minimum speed setting for each VFD. The minimum speed should be set to 100% during the pull-down period. The minimum speed during the holding season is room, variety, and operator preferences, but is usually between 40% and 70%. Normally the VFDs settle down to minimum speed with all further capacity control managed by the back pressure regulator.

Controlled atmosphere rooms normally incorporate flooded evaporator coils. A back pressure regulator (BPR) controls the pressure and temperature in the flooded coil. The BPR is the conventional means of controlling room temperature by controlling coil temperature. The VFD also controls room temperature by varying the flowrate of air over the coil. These two devices, the BPR and VFD must work in concert with one another to avoid an unstable hunting response of the system.

To avoid hunting, the VFD is normally programmed to track the BPR signal until the minimum VFD speed is reached. For example, if the BPR is 100% open, the VFD would be at 100% speed and if the BPR were 50% open, the VFD would be at 50% speed. Once the minimum speed is reached, all further capacity trimming is accomplished with the BPR.

Liquid Overfeed Evaporator Zones

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