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Our fruit storage industry currently enploys several control strategies for 

 fan cycling. Manual control is still used by some of the smaller growers. Time 

 clock control is used, and a few new systems employ programmable load 

 controllers to sequence fan and refrigeration operations. Presently, however, 

 the most popular technique is to cycle fans and refrigeration with a solid-state 

 thermostat. This thermostat, accurate to +^ 0.25OF, can be set to control only 

 temperature while fans run continuously during loading and pull down. Later 

 the thermostat is switched to control fans and refrigeration together. Remote 

 temperature sensors are installed in the CA rooms where the thermostat is used 

 to control fan cycling. 



Initially we were skeptical of using a single thermostat sensor to control 

 all of the fans in the CA room. Our recommendation still calls for remote 

 tenperature sensors in all rooms whether fans are cycled or not. We encourage 

 growers to turn at least some of the fans on for 30 minutes each 12 hours 

 during very cold weather. We have recorded fan cycles of up to 36 hours off 

 and 1 hour on when the thermostat alone was used to cycle the fans with the 

 refrigeration. 



After 3 years of experience we are not aware of any fruit condition 

 problems resulting from the fan cycling methods described above. Some 

 freezing damage has occurred in open, partially enpty CA rooms, because 

 insufficient respiration heat was available. Our biggest concern is that 

 compressor capacity in older plants now far exceeds the load developed in the 

 CA rooms when fans are cycled. During extended cold periods, machines may 

 sit idle for 24 hours or more, and provisions must be made to keep the 

 conpressor room warm. Heat reclaim from these compressors is no longer 

 possible during the storage season when the fans are cycled. 



Temperature, Relative Humidity, and Atmosphere Variations Due to Fan Cycling 



We find that the tenperature controller, not the fan cycling practice, is 

 the cause of major temperature variations in the CA rooms. We have 

 documented temperature variations in excess of + 20F in rooms with 

 continuously operated fans controlled by mechanical thermostats. Tenperature 

 variations in cycled rooms controlled by the solid state thermostats average + 

 1° or less. 



Few data exist for relative humidity variations. Some very limited data 

 from new storage facilities indicate cycled rooms yield less defrost water than 

 identical continuously operated rooms. In theory, this should be the case, but 

 we do not have sufficient data to confirm it for commercial systems. 



We are equally short of data on atmosphere concentration variations. In 

 one study of 1% oxygen storage in a 30,000 bushel CA room, no variation in O2 

 levels could be detected after the fans were off for 12 hours. This 

 determination was made by inserting sanpling lines into 6 stack locations during 

 loading and monitoring the oxygen level with an electronic analyz.er. 



