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Frost Prevention Measures with Energy Recovery Products
During extremely cold winter time conditions, frost formation becomes a possibility and should be considered in the unit design. The stream that is under risk from frost formation is the exhaust air stream. After return air goes through the wheel and gives up heat, the exhaust stream can be well below the freezing point of water (32 F) and below the
dew point. As a result, condensation will occur and frost may build up on the exhaust side of the wheel. A total heat wheel has an advantage over a sensible heat wheel under cold conditions in that it transfers moisture from the exhaust air stream, substantially lessening the opportunity for frost formation. Frost formation on the wheel will basically act to plug or reduce air flow but will not hurt the wheel itself.
In practice several types of frost prevention are employed; variable speed control, heating outdoor or return air, return or outdoor air bypass, or defrost cycling. These methods are discussed below.
Variable Speed Control
Wheel speed control works to limit frost formation by reducing wheel performance to a level where the exhaust air temperature is kept above the dew point. Proper dew point control of exhaust air can be determined using psychometrics by:
- Locating the winter time design return air condition and outdoor air condition and connect the two points on a psych chart
- Determine the dry bulb temperature at which this line intercepts the saturation curve.
- Add 2 degrees and set dew point control at this point and vary wheel speed downward to control at or above this point
- Audit performance of the wheel during actual operation. If frost formation is never evident, it may be because design conditions are never reached in which case it may be possible to gradually work dew point control down.
Measuring exhaust air temperatures should be done with a standard proportional temperature controller The temperature sensor should be located at least 12 inches from the face of the wheel on the exhaust side to allow exiting exhaust air from the wheel to mix. The sensor should also be located near the trailing section of the exhaust half(20 -30 degrees from vertical) as this condition will be nearer saturation than exhaust air first treated by the wheel.
Heating Outdoor or Return Air
Heating outdoor or return air works to lower the RH of the exhaust air stream and thus move it away from the
dew point condition. A small electric resistance heater is most often used, typically installed in the outdoor air. The heater size is dependent on both outdoor and return air design conditions, but a typical recommendation is 1KW/1000 cfm for outdoor temperatures in the range of 0 to -10 degrees F, and 5KW/1000cfm for -10 to -20 degrees F.
Bypass
Bypassing outdoor air works to raise the temperature of the exhaust air stream, also moving it away from the
dew point condition.
Defrost Cycling
Defrost cycling uses the pressure drop across the wheel as an indication of icing. When the pressure drop reaches a trigger point, the wheel is shut down and allowed to defrost. The half of the wheel that is in the return air will defrost quickly, and the other half will not defrost at all. The wheel can be turned back on based on reestablishment of the lower differential pressure or based on a time interval. Several wheel shutdowns may be required to get the wheel adequately defrosted.
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