The Refrigeration System "An Introduction to refrigeration"
BASIC
REFRIGERATION PRINCIPLES
If you were
to place a hot cup of coffee on a table and leave it for a while, the heat in
the coffee would be transferred to the materials in contact with the coffee,
i.e. the cup, the table and the surrounding air. As the heat is transferred,
the coffee in time cools. Using the same principle, refrigeration works by
removing heat from a product and transferring that heat to the outside air.
REFRIGERATION
SYSTEM COMPONENTS
There are
five basic components of a refrigeration system, these are:
- Evaporator
- Compressor
- Condenser
- Expansion
Valve
-
Refrigerant; to conduct the heat from the product
In order
for the refrigeration cycle to operate successfully each component must be present
within the refrigeration system.
The
Evaporator
The purpose
of the evaporator is to remove unwanted heat from the product, via the liquid
refrigerant. The liquid refrigerant contained within the evaporator is boiling
at a low-pressure. The level of this pressure is determined by two factors:
- The rate
at which the heat is absorbed from the product to the liquid refrigerant in the
evaporator
- The rate
at which the low-pressure vapour is removed from the evaporator by the
compressor
To enable
the transfer of heat, the temperature of the liquid refrigerant must be lower
than the temperature of the product being cooled. Once transferred, the liquid
refrigerant is drawn from the evaporator by the compressor via the suction
line. When leaving the evaporator coil the liquid refrigerant is in vapour form.
The
Compressor
The purpose
of the compressor is to draw the low-temperature, low-pressure vapour from the
evaporator via the suction line. Once drawn, the vapour is compressed. 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. The vapour is then released from the
compressor in to the discharge line.
The
Condenser
The purpose
of the condenser is to extract heat from the refrigerant to the outside air.
The condenser is usually installed on the reinforced roof of the building,
which enables the transfer of heat. Fans mounted above the condenser unit are used
to draw air through the condenser coils. The temperature of the high-pressure
vapour determines the temperature at which the condensation begins. As heat has
to flow from the condenser to the air, the condensation temperature must be
higher than that of the air; usually between - 12°C and -1°C. The high-pressure
vapour within the condenser is then cooled to the point where it becomes a
liquid refrigerant once more, whilst retaining some heat. The liquid
refrigerant then flows from the condenser in to the liquid line.
The
Expansion Valve
Within the
refrigeration system, the expansion valve is located at the end of the liquid line,
before the evaporator. The high-pressure liquid reaches the expansion valve,
having come from the condenser. The valve then reduces the pressure of the
refrigerant as it passes through the orifice, which is located inside the valve.
On reducing the pressure, the temperature of the refrigerant also decreases to
a level below the surrounding air. This low-pressure, low-temperature liquid is
then pumped in to the evaporator.
The
Refrigerant
The type of
refrigerant used will depend on the pressure capabilities of the system and the
temperatures that have to be achieved during refrigeration. The following brief
table shows the relationship between temperature and pressure, given in bara,
for two common refrigerants.
THE
REFRIGERATION CYCLE
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.
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