PRINCIPLE OF REFRIGERATION
- Just as the natural flow of water is from a high level to a low level, the natural flow of heat too is from a body at high temperature to a body at a low temperature, and just as we would need a pump to reverse the flow or pump water upwards, we need mechanical work to be done or a heat pump to transfer heat from a body at a low temperature and give it to a body at a high temperature.
- In a refrigeration system, gas at a high PR. P¹, low vol. V¹ & high temp T¹ (35°C to 40°C) is obtained from the compressor. It is allowed to expand slightly & cool in the condenser to a liquid at pressure P², vol. V² & SW temp T².
- This cooled liquid gas is suddenly allowed to expand by passing through an expansion valve.
- The expansion of the gas to vol. V³ is accompanied by a slight fall in its pressure P³ and a large fall in its temp. to T³(5°C to 25°C).
- The gas is now kept in contact with the substance to be cooled.
- It absorbs heat from the substance and cools it while in turn, its own temp rises to T⁴ (25° to 35°C) and pressure & volume to P⁴ & V⁴
- It is then compressed in a compressor to its pressure, volume & temperature at the first stage, i.e. P¹, V¹ & T¹.
Refrigerated cargo is divided into 3 categories:
- Frozen cargoes
- Chilled cargoes
- Cooled cargoes
- eg. Meat, butter, poultry and fish.
- These cargoes are carried in a hard frozen state at temperatures around-8°C to -18°C to prevent the growth of bacteria.
- eg. cheese, eggs, fruits and fresh vegetables.
- Beef may also be carried in a chilled state as the tissues get damaged sometimes by freezing.
- Temperatures maintained around 6°C to -2°C.
- It is more critical to maintain right temperatures of chilled cargoes as condensation of moisture due to the variation of temperature encourages bacterial growth.
- eg. fruits and fresh vegetables.
- Temperatures maintained around 2°C to 13°C by air circulation.
The temperature at which the above cargoes are carried may vary beyond the above-mentioned limits depending on:
- the nature of the cargo,
- the ambient temperature at the load port,
- the duration of the voyage
- and the state in which the cargo is to be delivered (whether ripe, frozen, ready for consumption, etc.
There are two types of refrigeration systems:
- in small refrigerated chambers on small ships and provision stores on ships.
- in large installations it is difficult to monitor the pipes for leakages, wastage of expensive gas would results. Due to which an indirect system is used on large ships/compartments.
Indirect method and a second refrigerant
- The primary refrigerant is used to cool a tank of brine and this cooled brine is then circulated through the compartment.
- Brine is chosen because of its low freezing point, -20° to -30°C, depending on its concentration and composition.
- The brine is passed through separate grids surrounding the same compartment.
- If one grid is blocked or chocked, the brine supply can be increased to other grids so that cooling will not be affected.
- Cooling is carried out by a combination of cold brine and cool air circulation.