Refrigeration Equipment Selection
General
When the hourly BTU load has been determined, equipment can now be selected based on the information obtained in the initial job survey. Some of the factors affecting equipment selection are:
1.Equipment Balance
2.Temperature Difference (T.D.)
3.Capacity Control/Product Safety
4.Type of Operation/Air Flow
1.Equipment Balance
The condensing unit is generally selected first to have capacity greater than the calculated cooling or freezing load. The
unit cooler(s) must be selected to balance the capacity of the condensing unit.
The capacity of the condensing unit should be selected at a suction temperature (after correction for suction line pressure drop) which will balance with the unit cooler(s) at a desirable T.D. between the refrigerant in the unit cooler and the air in the refrigerated storage room. The condensing unit capacity must also be selected at a condensing temperature corresponding to the condensing medium (ambient air or water) temperature available at the job location.
2. Temperature Difference
For Storage Rooms Above 32ºF. (0ºC.)
The nature of the product determines the desirable relative humidity for the storage room. The desirable relative humidity, in turn, dictates the approximate design T.D. between the air in storage room and the refrigerant in the unit cooler.
For the general purpose cooler involving meats, vegetables, and dairy products, it is common procedure to balance the low side to the condensing unit at a 10ºF. to 12ºF. T.D.. It has been learned by experience that if this is done, one may expect to maintain in a cooler 80% to 85% relative humidity, which is a good range for general storage.
Load Calculation Example 2 (page 8) involved the cooling and storage of beef. The table shows that the recommended T.D. is approximately 10ºF. Since the calculated load per hour based on 16 hr. of condensing unit operation was 12696 BTU/hr., the condensing unit to be selected should have a greater capacity than 12696 BTU/hr. based on a suction temperature of +23ºF. (10ºF. T.D. plus 2ºF. allowance for suction line pressure drop).
The unit cooler to be selected should have a minimum base capacity (BTU/º T.D.) of 12696/10º T.D. or 1270 BTU/º T.D./hr. to be sure that the unit cooler is large enough to balance properly with the condensing unit.
Low relative humidity requirements permit higher T.D. which in turn will allow selection of unit coolers with small base ratings (BTU/hr./º T.D.)
For Storage Rooms Below 32ºF. (0ºC.)
In low temperature rooms the amount of dehydration of unwrapped products is proportional to the T.D. Since the prevention of excess dehydration is important and since low temperature condensing unit capacities drop off sharply as the suction temperature reduced, it is considered good practice to use a maximum T.D. of 10ºF.
T.D.’s can be approximated by dividing the unit cooler capacity at a 1º T.D. into the condensing unit capacity at the desired saturated suction temperature (S.S.T.) for example:
Condensing Unit Capacity at S.S.T. = T.D.
Evaporating Capacity at 1º T.D.
Recommended Temperature Differences (T.D.) for Four Classes of Foods (Forced Air Unit Coolers)
Class | T.D. | Approx. RH | Description of Product Classes |
1 | 7º - 9ºF. | 90% | Results in a minimum amount of moisture |
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| evaporation during storage. Includes |
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| vegetables, produce, flowers, |
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| unpackaged ice and chill rooms. |
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2 | 10º - 12ºF. | 80 - 85% | Includes general storage & convenience |
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| store coolers, packaged meats and |
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| vegetables, fruits and similar products. |
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| Products require slightly lower relative |
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| humidity levels than those in Class I. |
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3 | 12º - 16ºF. | 65 - 80% | Includes beer, wine, pharmaceuticals, |
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| potatoes and onions, tough skin fruits |
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| such as melons & short term packaged |
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| products. These products require only |
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| moderate relative humidity. |
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4 | 17º - 22ºF. | 50 - 65% | Includes prep and cutting rooms, beer |
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| warehouses, candy or film storage and |
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| loading docks. These applications need |
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| only low relative humidities or are |
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| unaffected by humidity. |
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3. Product Safety/Capacity Control
In large boxes, it is recommended that the load be divided among multiple units. A load that requires more than a 10 HP unit should be split to provide the customer with some refrigeration level in the event of mechanical failure. In addition, as refrigeration is selected for the 1% worst occurrence of the year, multiple units provide for some capacity control. In low load situations some units can be turned off and the box maintained adequately with a fraction of the horsepower necessary for the summer operation. Multiple units on staged start up also cut the demand charges assessed by the utility company which cut your customer’s electric bill.
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