SECTION 5
COMMERCIAL REFRIGERATION
UNIT 24
EXPANSION DEVICES
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
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
Compressor
Condenser
Direction of
Refrigerant Flow
Evaporator
Metering device
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
Transmission Line
Evaporator
Thermal Bulb
Thermostatic
Expansion Valve
Liquid Line
Direction of
Refrigerant Flow
TXV COMPONENTS
•
•
•
•
•
•
Valve body
Diaphragm
Needle and seat
Spring
Adjustment and packing gland
Sensing bulb and transmission tube
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
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
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
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
Diaphragm
Push Rods
Seat
Needle
Diaphragm pushed up
Needle pushed into the seat,
closing the valve
Diaphragm pushed down
Needle pushed out of the
seat, opening the valve
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
The spring pushes up on
the push rods to close the
valve
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
Transmission Line
Thermal Bulb
Valve body
Saturated refrigerant to
the evaporator
Liquid refrigerant
from condenser or
receiver
Superheat spring
adjusting screw
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
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
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
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
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
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 + 18.4 psig = 26.1 psig
• Spring pressure = 7.7 psig
30°F
26.1 psig
Spring pressure = ?
Evaporator pressure 18.4 psig
26.1 psig = Ps + 18.4 psig
Ps = 7.7 psig
R-134a
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
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
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
External equalizer line
connected to the outlet of
the evaporator coil
Diaphragm
Solid brass divider
Evaporator pressure pushing
up on the diaphragm
Liquid refrigerant to the
expansion valve
Saturated refrigerant
to the evaporator
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
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
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
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
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
Thermal bulb
mounted on top
of the line
Use strapping material supplied with the valve
to hold bulb securely to the suction line
Suction line smaller than 3/4”
Thermal bulb located
45° below horizontal
Suction line larger than 3/4”
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
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
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
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
Spring pressure pushes down to open the valve
Diaphragm
Two pressures control
the automatic
expansion valve
Evaporator
pressure pushes up
to close the valve
Diaphragm pushed up
Needle pushed into the seat,
closing the valve
Caused by an increase in evaporator pressure
Diaphragm pushed down
Needle pushed out of the
seat, opening the valve
Caused by a decrease in evaporator pressure
Spring pressure
Spring
Diaphragm
Needle and Seat
Saturated refrigerant to
the evaporator
Liquid refrigerant from
condenser or receiver
Evaporator pressure
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
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
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
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
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
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