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to estimate the load for a chiller room with 9 m^2 area with room temperature 5 degree

celcius wherereas ambient temp is 31 degree celsius i know the formula

Q = AU (T1 - T2)

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'srefrigeration 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 makea similar calculation for his own store and thereby obtain an approximate refrigerationrequirement. It also helps the reader to appreciate the number of factors that have to betaken into account in calculating the heat load and also gives him some idea of their relativeimportance.One important heat load that has been omitted in the calculation is the heat gain due tosolar radiation. This factor depends on a number of conditions which are related to both thelocation of the store and its method of construction. In some cases, solar heat load may notbe 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 000m3Insulation 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 mCTemperature difference between ambient and store 35C and -30C = 65 degCThickness of polystyrene = 0.25 mSurface area of store = 771.5 m 2Heat leak = 771.5 x 65 0.033 0.033 = 7422 kcal/h

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(2) Air changes

Average of 2.7 air changes in 24 hStore 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 -20CFish loaded per day 35 000 kgProduct 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/hTotal calculated refrigeration load (sum of Items 1 to 7) = 23411 kcal/hTotal refrigeration requirement with allowances 23411 24 18 = 31215kcal/h

If a pump is used to circulate refrigerant, the heat equivalent must be added to the capacityof 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 loadand the fans are in operation. In this example, the minimum load corresponds to only about25 percent of the capacity of the installed refrigeration plant. This minimum load factor willvary considerably with the type of store and mode of operation but some account may haveto be taken of this difference between the maximum and minimum refrigerationrequirements. Large cold stores should be operated with a number of compressors, whichcan be switched on and off as required. Large compressors may be fitted with off-loadingequipment which allows them to work efficiently on partial loads. The reliance on one largecompressor for a large cold store could be catastrophic in the event of its failure. In the caseof smaller stores it may be that only one compressor is viable. Other arrangements can bemade to cater for the variation in refrigeration demand. What must not happen is that alarge compressor should operate with a low load and hence operate with a very low suction

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pressure or stop and start too frequently. The first condition is bad for the compressor andthe second for the electrical equipment.

8.1 Cold store capacity

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

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

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 volumeless the volume required for coolers, structural requirements, doorways and otherpermanent features of the store.

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

Gross volume and net volume can easily be defined by devising a simple set of rules formaking these calculations. These store volumes, however, can only give a rough estimateof storage capacity and their main use may before statistical purposes. The effectivevolume 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 thestorage conditions. Store operators should therefore use general statements of storecapacity with care and when placing an order they would give full details of the productsand the storage operation to enable the supplier to provide a store to suit the operatingrequirements with the maximum utilisation of the gross storage volume.