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.
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 isused 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 vapor.
This low-pressure, low-temperature vapor is then drawn from the evaporator by the compressor.
When vapor is compressed it rises in temperature.Therefore, the compressor transforms the vapor
from a low-temperature vapor to a high-temperature vapor, 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.
At this stage the liquid refrigerant is passed through the expansion valve. The expansion valve
reduces the pressure of the liquid refrigerant and therefore reduces the temperature. The cycle is
complete when the refrigerant flows into the evaporator, fromthe expansion valve,as a low-pressure,
low-temperature liquid.
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 vapouris 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 vapor 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 vapor 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.
Refrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a
reversible phase change from a gas to a liquid.Traditionally, fluorocarbons, especially
chlorofluorocarbons were used as refrigerants, but they are being phased out because of their ozone
depletion effects. Other common refrigerants used in various applications are ammonia, sulfur
dioxide, and non-halogenated hydrocarbons such as methane.
Examples: R22, R134a, R600a,R717 , HCFC22, HFC134a etc
Properties of Refrigerant: They are;
1) The refrigerant should have low boiling point and low freezing point.
2) It must have low specific heat and high latent heat. Because high specific heat decreases the
refrigerating effect per kg of refrigerant and high latent heat at low temperature increases the
refrigerating effect per-kg of refrigerant.
3) The pressures required to be maintained in the evaporator and condenser should be low enough to
reduce the material cost and must be positive to avoid leakage of air into the system.
4) It must have high critical pressure and temperature to avoid large power requirements.
5) It should have low specific volume to reduce the size of the compressor.
6) It must have high thermal conductivity to reduce the area of heat transfer in evaporator and
condenser.
7) It should be non-flammable, non-explosive, non-toxic and non-corrosive.
8) It should not have any bad effects on the stored material or food, when any leak develops in the
system.
9) It must have high miscibility with lubricating oil and it should not have reacting properly with
lubricating oil in the temperature range of the system.
10) It should give high COP in the working temperature range. Thesis necessary to reduce the
running cost of the system.
11) It must be readily available and it must be cheap also.
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.
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 isused 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 vapor.
This low-pressure, low-temperature vapor is then drawn from the evaporator by the compressor.
When vapor is compressed it rises in temperature.Therefore, the compressor transforms the vapor
from a low-temperature vapor to a high-temperature vapor, 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.
At this stage the liquid refrigerant is passed through the expansion valve. The expansion valve
reduces the pressure of the liquid refrigerant and therefore reduces the temperature. The cycle is
complete when the refrigerant flows into the evaporator, fromthe expansion valve,as a low-pressure,
low-temperature liquid.
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 vapouris 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 vapor 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 vapor 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.
Refrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a
reversible phase change from a gas to a liquid.Traditionally, fluorocarbons, especially
chlorofluorocarbons were used as refrigerants, but they are being phased out because of their ozone
depletion effects. Other common refrigerants used in various applications are ammonia, sulfur
dioxide, and non-halogenated hydrocarbons such as methane.
Examples: R22, R134a, R600a,R717 , HCFC22, HFC134a etc
Properties of Refrigerant: They are;
1) The refrigerant should have low boiling point and low freezing point.
2) It must have low specific heat and high latent heat. Because high specific heat decreases the
refrigerating effect per kg of refrigerant and high latent heat at low temperature increases the
refrigerating effect per-kg of refrigerant.
3) The pressures required to be maintained in the evaporator and condenser should be low enough to
reduce the material cost and must be positive to avoid leakage of air into the system.
4) It must have high critical pressure and temperature to avoid large power requirements.
5) It should have low specific volume to reduce the size of the compressor.
6) It must have high thermal conductivity to reduce the area of heat transfer in evaporator and
condenser.
7) It should be non-flammable, non-explosive, non-toxic and non-corrosive.
8) It should not have any bad effects on the stored material or food, when any leak develops in the
system.
9) It must have high miscibility with lubricating oil and it should not have reacting properly with
lubricating oil in the temperature range of the system.
10) It should give high COP in the working temperature range. Thesis necessary to reduce the
running cost of the system.
11) It must be readily available and it must be cheap also.
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