1. Section 5: Commercial Refrigeration
Unit 24: Expansion Devices

2. Objectives After studying this unit, you should be able to:
Describe the three most popular types of expansion devices
Describe the operating characteristics of the three most popular expansion devices
Describe how the three expansion devices respond to load changes

3. Objectives (cont’d.)
Describe the operation of a balanced-port expansion valve
Describe the operation of a dual-port expansion valve
Describe how electronic expansion valves and their controllers operate

4. Expansion Devices
Often called metering devices; meters the correct amount of refrigerant to the evaporator
Installed 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

5. Expansion Devices (cont’d.)
Figure 24–2 The complete refrigeration cycle with the four basic components: compressor, condenser, evaporator, and expansion device

6. 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

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. 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

11. 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, but only EXPERIENCED field technicians should do adjustments on the valve

12. The Sensing Bulb and Transmission Tube
Senses temperature at evaporator outlet
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 controls the valve

13. The Sensing Bulb and Transmission Tube (cont’d.)
Figure An illustration of the diaphragm, the bulb, and the transmission tube

14. Types of Bulb Charge
The fluid inside the expansion valve bulb is known as the charge
Four types:
Liquid charge
Cross liquid charge
Vapor charge
Cross vapor charge

15. 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

16. 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

17. The Vapor (Gas) Charge Bulb
Contains same refrigerant as the system
Bulb only contains a small amount
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

18. The Cross Vapor Charge Bulb
Contains a different refrigerant than the system
Bulb only contains a small amount
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

19. Functioning Example of a TXV with Internal Equalizer
Normal load conditions – medium temperature application; R-134a; valve is in equilibrium
Suction pressure 18.4 psig
Suction line temp. 30°F, PBULB= 26.1 psig
PSPRING + PEVAPORATOR = PBULB
Spring pressure psig = 26.1 psig
Spring pressure = 7.7 psig

20. Functioning Example of a TXV with Internal Equalizer (cont’d.)
Figure A TXV under a normal load condition. The valve is said to be in equilibrium. The needle is stationary

21. Functioning Example of a TXV with Internal Equalizer (cont’d.)
Load changes with food added to cooler
Addition of warm food increases evaporator load
Load changes with food removed from the cooler
Removal of food reduces load on the evaporator

22. TXV with External Equalizers
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

23. 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

24. TXV Valve Selection
Each TXV is designed for a particular refrigerant or group of refrigerants
The capacity of the system is very important

25. 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

26. 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

27. 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

28. Servicing the TXV
Care should be taken that the valve is serviceable and will perform correctly
Things to be considered:
Type of fastener (flare, solder, or flange),
Location of valve for service
Expansion valve bulb location
Valve has moving parts that are subject to wear

29. Sensing Element (Bulb) Installation
Procedure:
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

30. 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

31. 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

32. 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
Derivative controller modes
Estimate rate of change of temperature/time curve

33. Automatic Expansion Valve
Maintains constant pressure in the evaporator
When the pressure drops, the valve opens
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

34. Automatic Expansion Valve (cont’d.)
Figure 24–45 The automatic expansion valve uses the diaphragm as the sensing element and maintains a constant pressure in the evaporator but does not control superheat

35. 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

36. Special Considerations for the TXV and AXV
Both are expansion devices that allow more or less refrigerant flow
Both need a storage device (receiver) for refrigerant when it is not needed
The receiver serves both as a storage tank and as a tank into which refrigerant can be pumped when servicing the system

37. 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

38. 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

39. Summary
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

40. Summary (cont’d.)
The thermostatic expansion valve is designed to maintain constant superheat in the evaporator
Three pressures control the operation of the TXV: the bulb pressure, the spring pressure, and the evaporator pressure

41. Summary (cont’d.)
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

42. Summary (cont’d.)
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 pressures control the TXV: the spring pressure and the evaporator pressure

43. Summary (cont’d.) The capillary tube is a fixed bore metering device
The capillary tube meters refrigerant depending on the pressure drop across the tube