Question
We have a cold storage overseas to store fruits and vegetables. Could you help me with the required C.F.M. (Cubic Feet Per Minute) of air required to store: 1) onions at 0°C temperature and 70% R.H., per ton; 2) carrots at 0°C temperature and 98% to 100% R.H., per ton; and 3) cabbage at 0°C temperature and 98% to 100% R.H., per ton?
Once the required temperature is reached the compressor switches off and switches on again when the temperature goes beyond a setting. Do the fans of the cooling coils have to run 24 hours or do they need to be run only when the compressor is running for the above mentioned vegetables? (K.S.)
Answer
The general rule for refrigerated storage is to use 100 cfm per ton of product until the product reaches storage temperature and after that the air volume can be reduced to 20 to 40 cfm per ton. Use the lowest possible air volume that maintains uniform product temperature in the storage. In practice this means that product or air temperature must be periodically measured in the facility and the air volume sequentially reduced until there is an unacceptable variation in temperature. The air volume would then be increased to the lowest level that produced an acceptable temperature variation. At the low air volume the fans will be run continuously.
Another option is to use fan cycling instead of reducing air volume of the fans. With this system the fans are sized to produce 100 cfm per ton and run continuously until the product reaches design temperature. But instead of reducing fan volume the fans are cycled off for perhaps 20 minutes every hour. The storage is run for several days and product or air temperature is checked while the fans are operating. If the temperature is adequately uniform, the off cycle time is increased and temperatures are checked again. The cycle is repeated until you find the longest fan off time that produces acceptably uniform temperatures. Many distribution systems depend on the fans 'throwing' the air far enough to get good air distribution in the storage. The fan cycling system always operates at maximum capacity when on and maintains good air distribution in the storage. --Jim Thompson