The working agent in a refrigerating system that absorbs carries or releases heat from the place to be cooled or refrigerated can be termed as a refrigerant. This heat transfer generally takes place through a phase change of the refrigerant. A more complete definition of a refrigerant could be given as follows:

“Refrigerant is the fluid used for heat transfer in a refrigerating system that absorbs heat during evaporation from the region of low temperature and pressure, and releases heat during condensation at a region of higher temperature and pressure.”

PRIMARY AND SECONDARY REFRIGERANTS

Primary refrigerants are those which can be directly used for the purpose of refrigeration. If the refrigerant is allowed to flow freely into the space to be refrigerated and there is no danger of possible harm to human beings, then primary refrigerants are used. The refrigerants used in home refrigerators like Freon-12 are primary refrigerants.

On the other hand, there may be certain situations in which we cannot allow the refrigerant to come in direct contact with the items being refrigerated, and then the refrigerant used is termed as a secondary refrigerant. As for example, we cannot allow a toxic refrigerant to be used for air conditioning in residential buildings. There are some refrigerants which are highly inflammable and so their direct use is forbidden for safety reasons. Again, it may so happen that if direct refrigeration, such as in cooling a big cold storage, is allowed, then the amount of refrigerant required may be so large that its cost becomes prohibitively high. These are some typical situations for which we favour the use of secondary refrigerants. Water and brine solutions are common examples of secondary refrigerants.

SECONDARY REFRIGERANTS

In some refrigeration systems, the refrigerant is not put into direct use. This is done mainly out of safety considerations. As an example, it is not desirable to use toxic refrigerants like ammonia in home air- conditioning and home refrigeration systems. Also, in some cases the size of the refrigerated space may be so large that direct refrigeration may be uneconomical. In such case, an indirect way is employed. The refrigerants used in this way do not pass through the cyclic process and are referred to as secondary refrigerants. The refrigerants commonly used in this way are water and brine solutions of calcium or sodium. This therefore allows the use of a smaller size refrigerator with a considerably less amount of refrigerant. Also, since the refrigerant does not come in direct contact, cheaper grade materials can be used for the heat exchanger.

Due to its toxicity, ammonia cannot be used as a refrigerant in residential air conditioners. Therefore, the usual practice is to chill brine over the evaporator coil and then air is cooled by passing over the brine coil. This method eliminates the dangers of toxification on account of ammonia leakage into the air streams.

Sodium chloride brine solutions are most common up to – 15°C while calcium chloride brine solutions can be used up to – 50°C. However, these solutions are very corrosive to metals such as brass, copper and aluminium. In place of them, sometimes certain chemicals known as antifreeze are used with water to prevent clogging.