Refrigerator Temperature, Humidity and Air Motion in Food Storage

It is important that the temperature difference (TD) between the product being stored and the refrigerant is correct for that product. Too wide a TD will result in excessive dehydration of the product. Too close a TD can result in rapid deterioration of the product; fresh meat, for example, will soon become discoloured and slimy to the touch. This section deals with storage conditions and temperature control settings, thereby giving an indication of evaporating temperatures and pressures to be expected during system operation.

Food products may be divided into four classes to provide proper storage conditions:
1. Foods which dehydrate quickly: fruits, vegetables, eggs and cheese.

2. Foods subject to sweating and some dehydration: fresh cut meats and provisions.
3. Carcase meats, chilled meats, and products not subject to excessive dehydration.
4. Products not subject to dehydration: dried fruits, tinned goods and canned/bottled beverages.

Table 2 gives the recommended TDs for these classes of product. Slight variations may be necessary to obtain ideal conditions. For better understanding, a few examples of the simple calculations needed to determine various factors are now given. These calculations are based on a system charged with refrigerant R12. See Figure 13.

To determine the TD

1. Subtract the TD from the product temperature to obtain the average evaporating temperature.
2. Convert this temperature to pressure, which will be the average evaporating pressure.
3. The cut-out pressure will then be 3 psig or 0.2 bar below the average evaporating pressure.

Example
For chilled meat (product class 3), at a storage temperature of 32 to 36^o F (0 to 2.2^o C and using a forced air evaporator, the recommended TD is 12 to 15^o F (6.0 to 8.3^o C ) The average product temperature is therefore 34^o F (1.1^o C)  The cut-out pressure is 22 psig (1.5 bar), and so the average suction pressure is 22 + 3 = 25 psig (1.5 + 0.2 = 1.7 bar). This converts to an average suction temperature of 22^o F or -5^o C The TD is therefore 34 – 22 = 12^o F (1.1 – (-5) = 6.1^o C ) It can be seen that the TD is within the recommended range.

To establish the low pressure cut-out point
1. Subtract the TD from the product temperature to obtain the average evaporating temperature.
2. Convert this temperature to pressure, which will be the average evaporating pressure.
3. The cut-out pressure will then be 3 psig or 0.2 bar below the average evaporating pressure.

Example (imperial)

For a class 3 product with a storage temperature of 30^o F the recommended TD is 20 to 25^o F for a gravity coil. The average TD is 23^o F The average evaporating temperature is therefore 30- 23 = 7^o F Converted to pressure, this is 13 psig. The pressure control cut-out point will then be 13 – 3 = 10 psig. For an off-cycle defrost with a low pressure control, set the cut-in to 35 psig. This will provide an approximate coil temperature of 38^o F when the coil will be completely free of frost and ice.

Example (SI)

For a class 3 product with a storage temperature of-2^o C the recommended TD is 8^o C for a forced air evaporator. Then the average evaporating temperature is -2- 8 =-10^o C Converted to pressure, this is 1.2 bar. The cut-out pressure is established as 1.2- 0.2 = 1 bar. Temperature is -2- 8 =-10^o C  Converted to pressure, this is 1.2 bar. The cut-out pressure is established as 1.2- 0.2 = 1 bar.

When selecting cut-in and cut-out pressures to control fixture temperatures, the following information is required:
1. Dry bulb temperature of refrigerated space.
2. Relative humidity of refrigerated space.
3. Type of refrigerant used.
4. Type of evaporator.