Refrigeration and refrigerants

Refrigeration

Any process in which heat is absorbed is called refrigeration. According to another definition, refrigeration is a branch of science that deals with reducing and maintaining the temperature of a substance or space at a temperature lower than the ambient temperature. In other words, in a refrigeration process, heat is absorbed from a cooling body and transferred to another body that has a lower temperature than the cooling body. Since in this process the heat absorbed from the cooling body is transferred to another body, there is both heating and cooling in the refrigeration process, and only the way the system is used distinguishes them from each other.

In cryogenic processes, the heat-absorbing substance or the cooling agent is called the refrigerant or the cooling substance. Depending on the effect that the absorbed heat has on the refrigerant, refrigeration changes can be classified as sensible and latent. If the absorption of heat causes an increase in the temperature of the refrigerant, the refrigeration change is called sensible, and if it causes a change in the physical state of the refrigerant (melting or evaporation), it is called latent. In both cases, the temperature of the refrigerant must always be lower than the temperature of the space or the material being cooled.

Refrigerants

In general terms, a refrigerant is a substance that acts as a cooling agent by absorbing heat from a material. In order for a refrigerant to be suitable for use, it must have specific chemical, physical, and thermodynamic properties that make its use safe and economical. It should be noted that there is no ideal refrigerant and, due to the vast differences in the conditions and needs of different applications, there is no refrigerant that can be suitable for all applications. Therefore, a refrigerant will be ideal when it can meet the conditions and needs of a particular application.

When a refrigerant circulates in a system, it passes through a number of changes of state or conditions, each of which is called a transformation. The refrigerant starts from an initial state and returns to its initial state after passing through a series of transformations called a cycle. Various refrigeration cycles are used to create refrigeration.

Properties of Refrigerants

Safety: Safety properties are usually the first consideration in choosing a refrigerant, and for this reason, some substances are less commonly used despite being good refrigerants. The most important of these refrigerants are ammonia and some pure hydrocarbons. If a fluid is non-flammable, non-explosive, and non-toxic in its specific state and when mixed with a portion of the air, and does not react adversely with lubricating oil and other materials commonly used in refrigeration equipment construction and with moisture in the refrigeration system, it can be used as a suitable refrigerant. In addition, the properties of the refrigerant must be such that, if it leaks from the system, it will not contaminate food or other materials in the cold store.

Toxicity:Toxicity is a relative term and only makes sense when the concentration and contact time necessary to cause harmful effects have been determined. The toxicity of common refrigerants has been tested by laboratories. Since the effects caused by some groups are more due to lack of oxygen than to the harmfulness of the refrigerant itself, they are assumed to be non-toxic. Currently, ammonia is the only toxic refrigerant that is widely used, but its use is limited to packaging units, ice factories and cold stores where experienced people work.

Flammability and explosion:Most refrigerants used in refrigeration systems are completely non-flammable and non-explosive. The exceptions are ammonia and pure hydrocarbons. When ammonia is mixed with certain proportions of air, it becomes flammable and explosive to a certain extent, but by taking special precautions, the risk of using ammonia as a refrigerant can be ignored.

Economic considerations

Of course, it is economically desirable that the physical and thermal properties of the refrigerant be such that it absorbs the minimum power for the refrigeration capacity of the unit and the coefficient of performance of the system is high. The most important refrigerant properties that affect system capacity and efficiency are:

1. Latent heat of vaporization
2. Specific vapor volume
3. Compression ratio
4. Specific heat of the refrigerant in both liquid and vapor states

Since the power required per unit of refrigeration capacity is approximately the same for all common refrigerants, and system efficiency and economics are not usually considered in the refrigerant selection decision, properties that reduce the size, weight, and initial cost of the refrigeration equipment and that enable automatic operation with minimal maintenance are important.