Vapor refrigeration is divided into vapor compression refrigeration, vapor absorption refrigeration. Among these refrigeration systems, vapor compression is most widely used. Vapor compression refrigeration system has four main components. They are compressor, condenser, evaporator coils, and expansion valve or throttle value.
Introduction
- Vapor
compression refrigeration system is a type of mechanical refrigeration.
- The
vapor compression system employs a liquid refrigerant which evaporates
and condenses readily.
- A
refrigerant readily evaporates and condenses depending upon the pressure
and temperature during the cycle. Therefore, refrigerant undergoes a
change of phase alternately between liquid and vapor phases without
leaving the system.
- During
evaporation, it absorbs latent heat from the cold chamber (freezer box)
and the liquid refrigerant changes into vapor refrigerant hence creates
cooling effect in the cold chamber.
- During
condensation, refrigerant rejects heat to an external system like cooling
water or to the atmosphere and the refrigerant gets converted from vapor
to liquid.
Construction
- Compressor: It is motor-driven equipment. Used to pump the refrigerant through the system and circulates it again and again in cycles. To create a pressure difference in the system and to make a flow of the refrigerant from one part of the system to the other. It sucks vapor refrigerant from the evaporator and compresses.
- Condenser: The
condenser is a device which acts as a heat exchanger in the refrigeration system, which rejects heat energy of the refrigerant gas gained in the evaporator and the compressor. Atmospheric air and water are the two most convenient heat sinks to which the heat can be rejected.
- Expansion
valve: Expansion device is also known as a metering device or throttling device. The rate of flow of refrigerant is metered. Used to expand the liquid refrigerant from the condenser pressure to the evaporator pressure(to maintain a pressure difference).
- Evaporator: Evaporator is a
device which absorbs heat from its surroundings or space to be cooled or products to be cooled using a refrigerant. The temperature of the boiling refrigerant in the evaporator must always be less than that of the surrounding medium so that the heat flows to the refrigerant. The evaporator becomes cold. The evaporator is installed on the low-pressure side of the refrigeration system. The evaporator is also known as a cooling coil, a
chilling coil, or a freezing coil.
- The
working substance is a refrigerant like NH3, R11, R12, etc.
- A
receiver tank is used to provide storage for a condensed liquid.
Working:
- When
the compressor is started, it draws the low-pressure vapor
from the evaporator at state 1 and compresses
it isentropically to a sufficiently high pressure up to state 2.
- Since
the compression work is done on the vapor, its temperature also
increases.
- Hot
vapor from the compressor under pressure is discharged into the condenser
where it is cooled at constant pressure by rejecting heat to the condenser
cooling medium usually water or surrounding air.
- This
converts the hot vapor into liquid and the liquid is collected in the
liquid receiver at state 3.
- The
liquid from the liquid receiver at high pressure is then piped to a
refrigerant control valve which regulates the flow of liquid into the evaporator.
- This
control valve, while restricting the flow, also reduces the
pressure of the liquid with the result the liquid change into a vapor of
low dryness fraction represented by state 4.
- During
the process the temperature of the refrigerant reduces corresponding to
its pressure.
- Finally,
the low-pressure, low-temperature refrigerant passes through the
evaporator coil where it absorbs its latent heat from the cold chamber or
from the brine solution at constant pressure and converts into vapor at
state 1.
- It
is again supplied to the compressor. Thus the cycle is completed.
Applications
- Used
in refrigeration and air conditioning both for industrial and domestic applications.
- Used in food processing units.
Advantages
- C.O.P
is quite high.
- Low
evaporative temperature.
- Ability
to transfer a large amount of heat.
- For
the same refrigerating effect the size of the evaporator is smaller.
Disadvantages
- High initial cost.
- High
Toxicity.
- Easy
leakage of vapors.
- Inflammability of refrigerant vapors.
Important terms
Throttling: It is the process in which the flow of fluid is restricted by a small opening. So that
high-pressure fluid is converted into low pressure. In throttling process, enthalpy remains constant, and work done is
zero.
Isentropic process: It
is a thermodynamic process, in which the entropy of the fluid remains constant. It
is a reversible adiabatic process.
Tags:
Refrigeration