In the field of industrial refrigeration, the freezing and conservation of food are most probably the major applications, and as a result a great deal of attention is focused on the safety and reliability of these systems.
In all applications, certain conditions must be controlled and maintained:
temperature,
humidity,
air velocity.
In addition to these, the refrigerant charge must be calculated precisely, based on the product being stored.
This sector differs from air-conditioning and climate control due to the cooling loads and capacities involved, however the procedure applied to produce the cooling effect is the same.
The operating principle of the refrigeration process involves the combined action of a number of mechanisms, which exploit the properties of a substance (refrigerant) that can absorb the heat from the environment in the evaporation phase, and dissipate it in the condensation phase.
In fact, the action of the refrigerant inside the circuit can be divided into two phases:
the refrigerant changes from the liquid to the gaseous state by absorbing a certain amount of heat from the surrounding environment (corresponding to the latent heat of vaporisation), thus cooling the environment. This occurs at low temperatures and pressures.
the refrigerant, in the gaseous state, gives up the absorbed heat, and returns to the liquid state; the cycle then starts again.
The main components in a mechanical refrigeration system are:
Compressor: its function
is to draw in the superheated gas refrigerant arriving from the evaporator,
compress it, and then deliver it to the condenser, where it will become
liquid again. The mechanical operation of the compressor implies an
increase in the heat contained in the vapour. The compression increases
the pressure of the vapour and consequently its temperature.
Condenser: this absorbs the heat from the gas that evaporates in the evaporator (plus the heat of compression), causing it to condense. In refrigeration systems, the condenser is made up of finned tubes or a coil containing a refrigerant, and may be cooled: by air, by water, or through a tube bundle.
Expansion valve: this has the function of lowering the pressure of the gas arriving from the condenser, so as to decrease its boiling point and consequently its evaporation temperature.
Evaporator: this allows the liquid refrigerant that arrives from the condenser through the expansion device to accumulate. By removing heat from the air that flows through the evaporator, the liquid refrigerant evaporates, while maintaining a constant pressure.
Fans: these are used for ventilation both inside and outside the environment. Inside, the fans draw in air from the environment and force it through the evaporator, where it is cooled before returning to the environment. Outside, the fans have the purpose of cooling the refrigerant, by blowing the air through the condenser.
In a refrigeration system, the refrigerant is used to absorb heat from one area and transfer it to another. As a result, its is important to optimise the rate of heat transfer. To ensure this, the materials used must feature good thermal conductivity, such as copper and aluminium. Another way of improving the transfer of heat is to enlarge the surfaces of the condenser and the evaporator.
