Freezing Technology

3.5.1. Calculation of freezer refrigeration load

The individual items to be taken into account in a refrigeration load calculation depend on the type of freezer. It would be impossible to include all the eventualities in one sample calculation; therefore, a relatively simple one is given below for a HPF and some notes have been added to help with other freezer calculations.

Load calculation

I.  Number of freezer

20 t/day = 20,000 kg/day

20,000 ¸ 10 = 2,000 blocks/day

24 ¸ 2 = 12 cycles/day

2,000 ¸ 12 = 167 blocks/cycle

II.  Fish load

20,000 ¸ 24 = 833 kg/h

Enthalpy at –30^oC = 4.6 kcal/kg

Change in enthalpy = 81.3 kcal/kg

An approximate figure can also be calculated by using the following values:

a) Specific heat of fish above freezing, 0.9 kcal/kg^oC

b) Latent heat of the fish, 60 kcal/k

c) Specific heat of fish below 0^oC, 0.4 kcal/kg^oC

Using these values, the above calculation for fish refrigeration load would be:

Heat to remove on cooling to 0^oC

833 x 0.9 x 10 = 7497 kcal/h

Latent heat to remove 833 x 60 = 49980 kcal/h

Heat to remove on cooling to –30^oC

833 x 0.4 x 30 = 9996 kcal/h

Total heat to remove from fish = 67473kcal/h

Total refrigeration requirement with allowances:

Add 30% = 142681 kcal/h

The calculation of fish load gives refrigeration requirement to freeze the fish only. Depending on the type of freezer used, other heat loads such as fan heat, pump heat from circulating pump, heat leak through freezer insulation, heat load due to pallets, trays, trolleys, defrost procedure, air infiltration,internal lighting etc. have to be taken into account and added to this value to determine the total refrigeration requirement.