important topic of rac

1. SUBJECT : REFRIGERATIONANDAIR
CONDITIONING (2161908 )
Gandhinagar Institute of
Technology
Active Learning assignment on:-
“EXPANSION DEVICES”
Prepared by:-Savsani Happy (150123119045)

2.  An expansion device in a refrigeration system normally serves two purposes :
1. The thermodynamic faction of expanding the liquid refrigerant from condenser
pressure to evaporator pressure.
3. The control functions which may involve the supply of liquid to the evaporator
at the rate at which it is evaporated. This determines the efficiency with which
evaporator surface is utilized.
Expansion devices:

3. An expansion device is essentially a restriction offering resistance to flow so that
pressure drops, resulting in throttling process.
Types Of Expansion Devices:
I. Variable flow type
1. Automatic expansion valve
2. Thermostatic expansion valve
III. Constant flow types
1. Capillary tubes

4. 1. Automatic Expansion Valve (AEV):-
The automatic expansion valve modulates the flow of the
refrigerant into the evaporator to keep the evaporator pressure
constant.
Unlike the capillary tube, which cannot adjust the flow of
refrigerant, the automatic expansion valve opens and closes to
either increase or decrease the amount of refrigerant feeding
into the evaporator in response to the pressure of the
refrigerant in the evaporator.

5. The valve operates on a “needled and seat” mechanism that changes the
amount of refrigerant that is able to pass through the valve. The position of
the needle is determined by the difference between two pressures:
I. The spring pressure
II. The evaporator pressure
. The spring pressure is the pressure that opens the valve. This pressure
is adjustable and is set at the desired evaporator pressure.
. The higher the spring pressure, the higher the evaporator pressure will
need to be for refrigerant to flow.

6. Automatic Expansion Valve

7. The evaporator pressure is the pressure that closes the valve. The
spring and evaporator pressures push against each other in opposite
directions. Between the spring pressure and evaporator pressure is
the “needle and seat assembly”
. When there’s higher evaporator pressure than the spring pressure
then the needle will “close in to the seat” allowing less refrigerant
through the system. The lower the evaporator pressure the more
refrigerant will flow.
The automatic expansion valve is ideal for systems that experience a
constant heat load as opposed to systems that operate under a wide
range of conditions.

8. Thermostatic Expansion Valve (TEV):-
which is designed to maintain a constant pressure in the
evaporator, the thermostatic expansion valve is designed to
maintain a constant evaporator superheat.
The thermostatic expansion valve operates on a “seat and needle”
concept that is very similar to the automatic expansion valve. The
main difference is that the TEV closes as the system load is
reduced while the AEV closes as the system load is increased.

9. Thermostatic Expansion Valve

10. The thermostatic expansion valve’s “needle and seat” is
controlled by three pressures
I. Evaporator pressure
II. Spring Pressure
III. Bulb Pressure
Evaporator Pressure
The evaporator pressure is one of the pressures that helps close
the valve. It attempts to push the needle into the seat to reduce the
flow of refrigerant in to the evaporator. The evaporator pressure
can be taken from either the inlet or the outlet of the coil.

11. Spring Pressure :-
 The spring pressure, also known as the superheat spring
pressure, determines how much superheat the evaporator will
open with. The higher the spring pressure, the higher the amount
of superheat.
The spring comes factory set and should only be adjusted by
trained professionals, because improperly adjusted superheat
springs can cause major system damage, including compressor
failure.
 The spring pressure is the other pressure that closes the valve,
reducing the amount of refrigerant flowing into the evaporator.

12. Bulb Pressure:_
The bulb pressure is the only pressure that opens the valve. This
pressure is generated inside a thermal bulb that is mounted at the
outlet of the evaporator. The line that connects the thermal bulb to
the thermostatic expansion valve is called the transmission line.
The thermal bulb is refrigerant filled and for the most part follows
a pressure/temperature relationship. It is the thermal bulb that
senses the evaporator outlet temperature.

13. The refrigerant in the thermal bulb is isolated from that of
the system so no mixing takes place. The refrigerant in the
bulb exerts a specific amount of pressure depending on its
temperature that pushes down on the diaphragm, opposing
the evaporator and spring pressures.

14.  Capillary Tube:-
Capillary tube is one of the most commonly used throttling
devices in the refrigeration and the air conditioning systems.
The capillary tube is a copper tube of very small internal
diameter.
It is of very long length and it is coiled to several turns so that it
would occupy less space. The internal diameter of the capillary
tube used for the refrigeration and air conditioning applications
varies from 0.5 to 2.28 mm .
 Capillary tube used as the throttling device in the domestic
refrigerators, deep freezers, water coolers and air conditioners.

15. Capillary Tube