Post harvest Management of Horticultural Crops (2+1)

Refrigeration cycle

The refrigeration cycle - Principle of refrigeration

The refrigeration cycle (shown in Diagram 1 below) begins with the refrigerant in the evaporator. At this stage the refrigerant in the evaporator is in liquid form and is used to absorb heat from the product. When leaving the evaporator,the refrigerant has absorbed a quantity of heat from the product and is a low-pressure, low-temperature vapour.
This low-pressure, low-temperature vapour is then drawn from the evaporator by the compressor. When vapour is compressed it rises in temperature. Therefore, the compressor transforms the vapour from a low-temperature vapour to a high-temperature vapour, in turn increasing the pressure. This high-temperature, high-pressure vapour is pumped from the compressor to the condenser; where it is cooled by the surrounding air, or in some cases by fan assistance. The vapour within the condenser is cooled only to the point where it becomes a liquid once more. The heat, which has been absorbed, is then conducted to the outside air.
At this stage the liquid refrigerant is passed through the expansion valve. The expansion valve reduces the pressure of the liquid refrigerant and therefore reduces the temperature. The cycle is complete when the refrigerant flows into the evaporator, from the expansion valve, as a low-pressure, low-temperature liquid.

For determination of refrigeration load, the following factors should be quantified (heat inputs)

• Field heat
• Heat of respiration of the produce
• Conductive heat gain – building floor, wall, roof ceiling etc.
• Convective heat gain – air mixing during opening of door
• Equipment load – fans, lights, forklifts and personnel etc.
• Service and defrost factors of the facility – hot weathers