EASY N SIMPLE PPT FOR GTU STUDENTS. TOPIC: THERMOSTATIC EXPANSION VALVE.

1. 1
Thermostatic Expansion Valve
Subject : Refrigeration and Air conditioning
Subject code : 2161908
For GTU students

2. Index
Introduction to Expansion devices 3
Types of Expansion devices 4
Thermostatic Expansion Valve 5
Main components
Valve Body 8
Diaphragm 10
Needle and seat 12
Spring 17
2

3. An expansion device in a refrigeration system normally serves these purposes
• The thermodynamic faction of expanding the liquid refrigerant from
condenser pressure to evaporator pressure.
• 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.
• An expansion device is essentially a restriction offering resistance to flow
so that pressure drops, resulting in throttling process.
Introduction to Expansion devices
3

4. Types Of Expansion Devices:
I. Variable flow type
1. Automatic expansion valve
2. Thermostatic expansion valve
II. Constant flow types
1. Capillary tubes
4

5. Thermostatic Expansion Valve (TEV)
• Thermostatic Expansion Valve 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.
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6. Thermostatic Expansion Valve
6

7. 7
Thermostatic Expansion Valve

8. Main components of Thermostatic Expansion Valve
8
• Valve body
• Diaphragm
• Needle and seat
• Spring
Valve Body
• Machined brass or stainless steel
• Holds components together
• Provides means to connect valve to the piping circuit
• Fastened by flare, solder, or flange
• Has an inlet screen to stop any small particulate matter from entering
valve

9. 9
Valve Body

10. Diaphragm
10
• Moves the needle in and out of the seat in response to system load changes
• Flexes downward to open the valve
• Flexes upward to close the valve
• Made of thin, flexible stainless steel
• Located at the top of the valve
Diaphragm

11. 11
Evaporator pressure
pushes up to close the
valve
Spring pressure pushes
up to close the valve
Diaphragm
Bulb pressure pushes down to open the valve

12. Needle and Seat
12
• Control refrigerant flow through the valve
• Needle is pushed into the seat to reduce refrigerant flow to the evaporator
• Made of stainless steel
• The greater the pressure difference across the needle and seat, the greater
the amount of flow through the valve
Push Rods
Diaphragm
Seat
Needle

13. 13
Diaphragm pushed up
Needle pushed into the seat,
closing the valve
Diaphragm pushed down
Needle pushed out of the seat, opening
the valve

14. The thermostatic expansion valve’s “needle and seat” is controlled by three
pressures
I. Evaporator pressure
II. Spring Pressure
III. Bulb Pressure
I. 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.
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15. II. 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.
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16. III. 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.
• 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.
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17. THE SPRING
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• One of the valve’s closing forces.
• Acts to push the needle into the seat, causing the valve to close.
• Spring pressure determines the evaporator superheat.
• Spring tension can be field adjusted.
• Only EXPERIENCED field technicians should do adjustments on the
valve.

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The spring pushes up on the push
rods to close the valve

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