8. CALCULATION OF COLD STORE REFRIGERATION LOAD


8.1 Cold store capacity


A good deal of experience is required to make a correct calculation of a cold store's refrigeration requirement and this should therefore only be done by a qualified person. The following calculation is not complete but it serves two purposes. It allows the reader to make a similar calculation for his own store and thereby obtain an approximate refrigeration requirement. It also helps the reader to appreciate the number of factors that have to be taken into account in calculating the heat load and also gives him some idea of their relative importance.

One important heat load that has been omitted in the calculation is the heat gain due to solar radiation. This factor depends on a number of conditions which are related to both the location of the store and its method of construction. In some cases, solar heat load may not be significant but in other instances, precautions may be necessary to reduce its effect.

Cold store refrigeration load

Specification

Dimensions 20 m x 10 m 5 m = 1 000m3
Insulation thickness (0.25 m)
External store surface area (771.5m 2)
Maximum ambient temperature (35C)
Store temperature (-30C)

Load calculation

(1) Insulation heat leak through walls, roof and floor

Conductivity of polystyrene 0.033 0.033 kcal/h mC
Temperature difference between ambient and store 35C and -30C = 65 degC
Thickness of polystyrene = 0.25 m
Surface area of store = 771.5 m 2
Heat leak = 771.5 x 65 0.033 0.033 = 7422 kcal/h

(2) Air changes

Average of 2.7 air changes in 24 h
Store volume = 1000m 3
Heat gain (35C and 60% R.H. air) 40 kcal/m 3
Air change heat gain = 1000 2.7 40 24 = 4500 kcal/h

(3) Lights (left on during working day)

1000W = 860 kcal/h

(4) Men working

1 man working at -30C gives off 378 kcal/h
2 men working is equivalent to 756 kcal/h

(5) Product load

5.5 kcal/kg for fish load at an average temperature of -20C
Fish loaded per day 35 000 kg
Product load = 3500 5.5 24 = 8020 kcal/h

(6) Fan load

3 250W = 644 kcal/h

(7) Defrost heat

1 defrost of 8440 W for 1 h (recovered over 6 h) = 1 209 kcal/h
Total calculated refrigeration load (sum of Items 1 to 7) = 23411 kcal/h
Total refrigeration requirement with allowances 23411 24 18 = 31215 kcal/h

If a pump is used to circulate refrigerant, the heat equivalent must be added to the capacity of the refrigeration condensing unit but not to the capacity of the room cooler.

The minimum refrigeration requirement will be when there is only an insulation heat load and the fans are in operation. In this example, the minimum load corresponds to only about 25 percent of the capacity of the installed refrigeration plant. This minimum load factor will vary considerably with the type of store and mode of operation but some account may have to be taken of this difference between the maximum and minimum refrigeration requirements. Large cold stores should be operated with a number of compressors, which can be switched on and off as required. Large compressors may be fitted with off-loading equipment which allows them to work efficiently on partial loads. The reliance on one large compressor for a large cold store could be catastrophic in the event of its failure. In the case of smaller stores it may be that only one compressor is viable. Other arrangements can be made to cater for the variation in refrigeration demand. What must not happen is that a large compressor should operate with a low load and hence operate with a very low suction pressure or stop and start too frequently. The first condition is bad for the compressor and the second for the electrical equipment.

8.1 Cold store capacity

There is no method of defining cold store capacity that satisfies the requirements of everyone concerned with cold storage. Storage capacity based on the weight of produce that can be stored will depend on the storage density of the products and the method of storage.

Therefore, unless only one product is stored under closely defined conditions, this definition is obviously unsuitable. It is generally agreed that it is more appropriate to define storage capacity in terms of the store volume but there are a number of ways in expressing this value.

Gross volume is the volume of the refrigerated space.

Net volume is the volume that can potentially be used for storage and is the gross volume less the volume required for coolers, structural requirements, doorways and other permanent features of the store.

Effective volume is the store space that can actually be utilised for storage and it takes into account the requirements for passageways, stacking equipment etc.

Gross volume and net volume can easily be defined by devising a simple set of rules for making these calculations. These store volumes, however, can only give a rough estimate of storage capacity and their main use may before statistical purposes. The effective volume can only be calculated for each particular case and to achieve any degree of accuracy, a drawing of the store layout would be required together with full details of the storage conditions. Store operators should therefore use general statements of store capacity with care and when placing an order they would give full details of the products and the storage operation to enable the supplier to provide a store to suit the operating requirements with the maximum utilisation of the gross storage volume.