1. COMMERCIAL REFRIGERATION
SECTION 5
COMMERCIAL REFRIGERATION
UNIT 24
EXPANSION DEVICES

2. UNIT OBJECTIVES
After studying this unit, the reader should be able to
List and describe the three most popular expansion devices
Explain the operating characteristics of various expansion valves
Explain how various expansion devices respond to load changes
Describe the operation of balanced port, dual port and electronic
expansion valves
Explain how electronic controllers are used to control expansion
valves

3. EXPANSION (METERING) DEVICES
Meters the correct amount of refrigerant to the evaporator
Installed in the liquid line at the inlet of the evaporator
Common devices: Automatic expansion valve, thermostatic expansion valve, fixed bore (capillary tube)
Less common devices: High-side float, low-side float

4. Direction of Refrigerant Flow
Compressor
Condenser
Direction of Refrigerant Flow
Evaporator
Metering device

5. THERMOSTATIC EXPANSION VALVE (TXV)
Maintains a constant evaporator superheat
If the evaporator superheat is high, the valve will open
Superheat ensures that no liquid refrigerant leaves the evaporator
Low superheat increases the net refrigerant effect

6. Thermostatic Expansion Valve
Transmission Line
Evaporator
Thermal Bulb
Thermostatic Expansion Valve
Liquid Line
Direction of Refrigerant Flow

7. TXV COMPONENTS Valve body Diaphragm Needle and seat Spring
Adjustment and packing gland
Sensing bulb and transmission tube

8. THE 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. THE DIAPHRAGM
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

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

11. NEEDLE AND SEAT 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

12. Diaphragm
Seat
Push Rods
Needle

13. Needle pushed into the seat, closing the valve
Diaphragm pushed up
Needle pushed into the seat, closing the valve

14. Needle pushed out of the seat, opening the valve
Diaphragm pushed down
Needle pushed out of the seat, opening the valve

15. THE SPRING 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

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

17. THE SENSING BULB AND TRANSMISSION LINE
Senses temperature at the outlet of the evaporator
This temperature is converted to a pressure and is transmitted to the top of the diaphragm
The fluid in the bulb responds to a pressure / temperature relationship
When the suction line temperature goes up, the bulb pressure goes up
The bulb pressure is the only opening pressure that controls the valve

18. Saturated refrigerant to the evaporator
Transmission Line
Thermal Bulb
Valve body
Saturated refrigerant to the evaporator
Liquid refrigerant from condenser or receiver
Superheat spring adjusting screw

19. TYPES OF BULB CHARGE
Bulb charge is the type and amount of refrigerant contained in the thermal bulb transmission line and the space above the diaphragm
Liquid charge
Vapor charge
Cross liquid charge
Cross vapor charge

20. THE LIQUID CHARGE BULB
Bulb contains the same refrigerant as the refrigeration system
Under all conditions, the bulb will ALWAYS contain some liquid
The refrigerant in the bulb will always follow the pressure/temperature relationship of the system

21. THE CROSS LIQUID CHARGE BULB
Bulb contains a different refrigerant than the system
Under all conditions, the bulb will ALWAYS contain some liquid
The bulb does not follow the pressure/ temperature relationship of the system
Valve closes during the compressor off cycle

22. THE VAPOR CHARGE BULB Bulb contains the same refrigerant as the system
Bulb only contains a small amount of liquid
Also called a critical charge bulb
At some predetermined temperature, all of the liquid in the bulb will boil until only vapor remains
Any further increases in bulb temperature will have no effect on the bulb pressure

23. THE CROSS VAPOR CHARGE BULB
Bulb contains a different refrigerant than the system
Bulb only contains a small amount of liquid
Also called a critical charge bulb
At some predetermined temperature, all of the liquid in the bulb will boil until only vapor remains
Any further increases in bulb temperature will have no effect on the bulb pressure

24. EXAMPLE OF A TXV WITH INTERNAL EQUALIZER – LIQUID-FILLED BULB
Normal load conditions – medium temperature application, R-134a, valve is in equilibrium
Suction pressure 18.4 psig
Suction line temperature 30°F, PBULB= 26.1 psig
PSPRING + PEVAPORATOR = PBULB
Spring pressure psig = 26.1 psig
Spring pressure = 7.7 psig

25. R-134a 30°F 26.1 psig Spring pressure = ?
Evaporator pressure 18.4 psig
26.1 psig = Ps psig
Ps = 7.7 psig
R-134a

26. LOAD CHANGES WITH FOOD ADDED TO COOLER
Addition of warm food increases evaporator load
Refrigerant boils faster and suction pressure rises
Evaporator superheat rises
Valve opens to feed more refrigerant to the evaporator
Increased evaporator superheat causes temperature of remote bulb to rise

27. LOAD CHANGES WITH FOOD REMOVED FROM THE COOLER
Removal of food reduces load on the evaporator
Refrigerant boils slower and suction pressure drops
Evaporator superheat drops
Valve closes to feed less refrigerant to the evaporator

28. TXV WITH EXTERNAL EQUALIZER
Used if an evaporator has more than a 2.5 psig drop from inlet to outlet
The evaporator pressure is sensed at the outlet of the coil instead of the inlet
Used to prevent the coil from starving
Connected to the evaporator outlet after the thermal bulb
Used to compensate for pressure drop in the evaporator

29. External equalizer line connected to the outlet of the evaporator coil
Diaphragm
Solid brass divider
Evaporator pressure pushing up on the diaphragm
Saturated refrigerant to the evaporator
Liquid refrigerant to the expansion valve

30. TXV RESPONSES TO LOAD CHANGES
When load increases
Refrigerant boils faster and the suction line temperature increases
Valve opens to feed more refrigerant to the evaporator
When load decreases
Refrigerant takes longer to boil
Valve closes to feed less refrigerant to the evaporator

31. BALANCED PORT TXV Designed to operate in low ambient conditions
Used if any of the following conditions exist
- Large varying head pressures
- Large varying pressure drops across the TXV
- Widely varying evaporator loads
- Very low liquid line temperatures
Have larger-than-normal orifices

32. DUAL PORT TXV
Used when systems need a larger TXV for short periods of time
Dual-port valves have two independent capacities
- Larger port for periods of high load
- Smaller port for periods of normal load
- TXV capacity is doubled when larger port is open all the way

33. PRESSURE LIMITING TXV
Allows evaporator pressure to only reach a predetermined pressure
If the evaporator pressure exceeds this pressure, the valve will close
Desirable on low-temperature applications

34. SENSING ELEMENT (BULB) INSTALLATION
Bulb should be mounted on the suction line as close to the evaporator as possible
Suction line should be clean and straight
Bulb should be mounted securely
Follow manufacturer’s instructions
For small suction lines, the bulb is usually secured to the top of the line

35. Thermal bulb mounted on top of the line
Use strapping material supplied with the valve to hold bulb securely to the suction line
Thermal bulb located 45° below horizontal
Suction line smaller than 3/4”
Suction line larger than 3/4”

36. THE SOLID-STATE CONTROLLED EXPANSION VALVE
Uses a thermistor as a sensing element
Electrically controlled
When coil is energized, the valve opens
Responds very quickly to temperature changes
Suitable for heat pump applications

37. STEP MOTOR EXPANSION VALVES
Uses a small motor to control the valve port
Valve port controls evaporator superheat
Temperature sensor sends a signal to the controller
The controller sends a signal to the motor
The motor turns a fraction of a rotation for each controller signal

38. ALGORITHMS AND PID CONTROLLERS
Proportional Controllers
- Generate an analog output signal
- Difference between actual superheat and superheat set point is the “offset” or “error”
Integral Controller Modes
- Helps reduce the “error” or “offset”
- Calculates error size and the length of time the error exists
Derivative Controller Modes
- Estimate rate of change of temperature/time curve

39. AUTOMATIC EXPANSION VALVE
Maintains constant pressure in the evaporator
When the evaporator pressure drops, the valve opens
The spring pressure pushes to open the valve
The evaporator pressure pushes to close the valve
Turning the adjustment screw into the valve increases the spring pressure

40. Two pressures control the automatic expansion valve
Spring pressure pushes down to open the valve
Diaphragm
Two pressures control the automatic expansion valve
Evaporator pressure pushes up to close the valve

41. Caused by an increase in evaporator pressure
Diaphragm pushed up
Needle pushed into the seat, closing the valve
Caused by an increase in evaporator pressure

42. Caused by a decrease in evaporator pressure
Diaphragm pushed down
Needle pushed out of the seat, opening the valve
Caused by a decrease in evaporator pressure

43. Saturated refrigerant to the evaporator
Spring pressure
Spring
Diaphragm
Needle and Seat
Saturated refrigerant to the evaporator
Liquid refrigerant from condenser or receiver
Evaporator pressure

44. AUTOMATIC EXPANSION VALVE RESPONSE TO LOAD CHANGES
Responds in reverse to load changes
If the load increases
Refrigerant boils faster in the evaporator
The evaporator pressure increases
The valve closes
Used where the load is fairly constant

45. THE CAPILLARY TUBE METERING DEVICE
Controls refrigerant flow by the pressure drop across it
Diameter and length of the tube determine flow at a given pressure
Does not maintain evaporator pressure or superheat
Used when the load is relatively constant
No moving parts to wear out

46. OPERATING CHARGE FOR THE CAPILLARY TUBE SYSTEM
Capillary tube systems are critically charged
All refrigerant in the system circulates at all times when the system is running
Capillary tube sometimes fastened to the suction line for heat exchange
Responds very slowly to system load changes

47. UNIT SUMMARY - 1
Expansion devices meter the correct amount of refrigerant to the evaporator according to system operating conditions
Common expansion valves include the capillary tube, automatic expansion valve and the thermostatic expansion valve
The thermostatic expansion valve is designed to maintain constant superheat in the evaporator

48. UNIT SUMMARY - 2
Three pressures control the operation of the TXV: the bulb pressure, the spring pressure and the evaporator pressure
Thermal bulb can be liquid-charged, vapor-charged, cross liquid-charged, or cross vapor-charged
Internally equalized TXVs get the evaporator pressure from the inlet of the coil, while externally equalized TXVs get the evaporator pressure from the outlet of the coil

49. UNIT SUMMARY - 3
Special TXVs include the balanced port TXV, the dual port TXV and the electronic TXV
The automatic expansion valve maintains a constant evaporator pressure
Two pressure control the AXV: the spring pressure and the evaporator pressure
The capillary tube is a fixed bore metering device
The capillary tube meters refrigerant depending on the pressure drop across the tube