1
Air Conditioning
Metering Devices
2
Metering Devices
• Used in an A/C system to change the
pressure of the refrigerant.
• Is placed between the liquid line and
the evaporator.
• Reduces the high pressure in the
liquid line to low pressure in the
evaporator.
3
Metering Devices
• Controls the flow of refrigerant to the
evaporator coil.
• Maintains the correct superheat.
• Creates the flash gas at the start of the
evaporator coil.
4
Metering Devices
• Types
– Capillary Tube
– Thermostatic Expansion Valve
– Automatic Expansion Valve
– Fixed Orifice (AC Only)
– Electronic Expansion Valve
5
Capillary Tube
• Non-mechanical
• Provides a constant flow (or feed) of
refrigerant.
• Non-Adjustable
• Typical size: .031” diameter (very small) as
in 1/32 inch.
6
Capillary Tube
• Sometimes used to form a heat exchanger
by attaching it (by solder) to the suction line
or by wrapping it around the suction line.
• The best way to cut a capillary tube is to
gently notch it with a file and then snap it at
that point. Do not crush the tube.
7
Capillary Tube
• Installing a cap tube is done by crimp
connections because of its diameter.
• When replacing a cap tube cut the same
length of new tubing as the original one in
the system. Be sure it is the same diameter.
8
Capillary Tube
9
Capillary Tubes
The capillary tube determines the amount of
refrigerant that passes through based on
the diameter and the length.
The combination of the diameter and the
length will cause a pressure drop.
The longer the tube, the smaller the hole to
larger pressure drop.
10
Capillary Tubes
11
Capillary Tubes
12
Thermostatic Expansion Valve
• The TXV is a temperature actuated
metering device.
• The valves mechanisms respond to load
variations.
• The purpose is to keep the superheat of the
evaporator coil constant.
13
Thermostatic Expansion Valve
• The bulb is attached to the suction line after
the evaporator coil (evaporator Outlet). The
bulb must be insulated and mounted on a
horizontal section of line.
• The bulb can NOT be installed at the bottom
of the line.
14
Thermostatic Expansion Valve
15
Thermostatic Expansion Valve
16
Thermostatic Expansion Valve
• The sensing bulb senses the temperature in
the suction line and the force created by the
gas in the bulb will open or close the valve.
• The fluid or gas inside the sensing bulb is
called the “charge”.
17
Thermostatic Expansion Valve
• TXV’s use four types of charges:
– Liquid charge – has a refrigerant that has the
same pressure/temp relationship as the
system.
– Cross liquid charge – has a fluid that does not
follow the temperature/pressure relationship of
the system. Low temp applications.
– Vapor charge
– Cross vapor charge
18
Thermostatic Expansion Valve
• TXV’s use four types of charges:
– Vapor charge – sometimes called critically
charged bulbs. Has a small amount of
liquid, mostly vapor. Also used for
Maximum Operating Pressure bulbs. Can
only exert so much force on the
diaphragm.
– Cross vapor charge – same as vapor but
different temp/pressure relationship than
what is in system.
19
Thermostatic Expansion Valve
Temperature pressure of crossed charged bulb.
20
Thermostatic Expansion Valve
• Inside the valve body there is a needle and
a seat that is usually made of a hard metal,
such as steel. The movement of these two
parts create the valve action.
• The seat is stationary and the valve moves.
21
Thermostatic Expansion Valve
22
Thermostatic Expansion Valve
23
Thermostatic Expansion Valve
• The TXV is adjustable.
– Turning the adjustment counter clockwise
sends more liquid into the coil which
reduces the superheat.
– Turning the adjustment clockwise chokes
off the flow of refrigerant and increases the
superheat.
– Do any adjustments VERY slowly and give
the system time to respond.
24
Thermostatic Expansion Valve
The TXV starts in an equalized setting with 10 degree superheat.
25
Thermostatic Expansion Valve
• As load conditions change and heat is
added to the conditioned space:
– The sensing bulb starts warming up.
– The valve opens.
– Allows more liquid into the evaporator.
– Lowers superheat.
26
Thermostatic Expansion Valve
The load on the evaporator goes up as the load
increases and the valve opens increasing the flow of
refrigerant into the coil.
27
Thermostatic Expansion Valve
• As the demand for cooling decreases:
– The bulb cools off.
– Takes pressure off of the diaphragm.
– Closes the valve.
– Decreases the flow of refrigerant.
– Raises the superheat.
28
Thermostatic Expansion Valve
The load requirement drops and the evaporator cools
down the valve starts to close and decreases the flow
of refrigerant to the coil.
29
Thermostatic Expansion Valve
• With newer evaporators there is a
pressure drop from the metering device
to the suction line.
• If the pressure drop exceeds 2.5 psi a
TXV with an external equalizer line
should be used.
• The external equalizer is used to
compensate for the pressure drop from
the inlet to the outlet of the evaporator.
30
Thermostatic Expansion Valve
• Distributors are the “octopus” looking things
following the expansion valve on larger
multiple pass evaporators.
• The distributors distribute the refrigerant
through the multiple passes.
31
Thermostatic Expansion Valve
• The rate of flow of liquid through the
TXV is directly proportional to the load
conditions.
32
Thermostatic Expansion Valve
• The forces that control a TXV are:
– Sensing bulb – This is the downward force
that will open the valve.
– Evaporator pressure – Creates an upward
force that will close the valve along with the
spring pressure.
– Spring pressure – creates an upwards
force on the needle and helps close the
TXV. This is the only adjustable part.
33
Thermostatic Expansion Valve
• The TXV is designed to work at equilibrium.
34
Thermostatic Expansion Valve
• There are three factors that affect the
capacity of the TXV:
– Evaporator Temperature
– Pressure drop across the valve
– Temperature of liquid entering the valve.
35
Thermostatic Expansion Valve
• Adjustments
– By adjusting the spring pressure the superheat
can be changed.
• TXV’s can be internally or externally
equalized.
– Internal has two lines, one is the liquid inlet and
the other is the evaporator port outlet.
36
Thermostatic Expansion Valve
– Externally equalized has three lines, the liquid
line, the evaporator outlet line, and the
equalizer line.
• With externally equalized TXV’s the bulb
must be mounted between the evaporator
coil outlet and the equalizer line.
37
Thermostatic Expansion Valve
• The equalizer line must be as close to the
compressor side as possible to ensure that
100% vapor is entering the ¼” line. Any
liquid will cause improper TXV operation.
• External equalizers are used on large
evaporator coils where there is a pressure
drop.
38
Thermostatic Expansion Valve
39
Thermostatic Expansion Valve
• The equalizer line will be connected onto
the suction line to assist the evaporator
pressure (upward force) for proper
operation.
• The sensing bulb must be between the
equalizing line and the evaporator so no
liquid can get to it.
40
Thermostatic Expansion Valve
• Externally equalized TXV’s must be used
when:
– Pressure drop on a air-conditioning system
exceeds 3 psig.
– Pressure drop on a commercial refrigeration
system exceeds 2psig.
– Pressure drop on a low temperature system
exceeds 1psig.
41
Thermostatic Expansion Valve
• Superheat Adjustments
– TXV’s are adjusted at the factory.
– When an improper superheat is suspected first
check the manufacturers recommendations.
– Front setting the valve (turning it in) will starve
the coil or increase the superheat. By front
seating we are turning clockwise.
42
Thermostatic Expansion Valve
• Back seating the stem of the valve (turning
it out) will flood the coil with additional
refrigerant and will lower the superheat.
43
Thermostatic Expansion Valve
• Superheat Measurement
– The best place to get the temperature reading
is at the sensing bulb of the TXV.
– If you can not access this point and the
compressor has a long run to it add 2psi to your
gauge reading.
– Convert the compound (low side) gauge to
temperature.
44
Thermostatic Expansion Valve
– Subtract the saturation temperature (boiling
point temperature from your gauges) from the
suction line temperature (near sensing bulb).
– This is the superheat.
• It is VERY important to realize that it takes a
few minutes for superheats to change.
45
Thermostatic Expansion Valve
• Sensing bulb location
– When mounting the bulb make sure the
suction line area is clean for good heat
transfer. If it is not – sand it.
– Should be secured tightly (at least by two
straps).
– Should be insulated.
– Should not be mounted under the pipe as
liquid refrigerant and oil can sit on it and
cause incorrect readings.
46
Thermostatic Expansion Valve
47
Thermostatic Expansion Valve
– Some TXV’s are considered maximum
operating pressure TXV’s.
– MOP TXV’s place a limit on the
evaporator’s pressure to prevent the
compressor from overheating.
– They do this by not opening fully under
heavy load conditions.
48
Thermostatic Expansion Valve
• Dual Port TXV’s
– Have two needles and seats that are in
series.
– Are used in cases where a single TXV
does not fit the specifications.
– Example of this is a beer cooler where you
may need a large amount of cooling for a
short time when a new shipment comes in.
Then the cooler cools off and you need
less of a load.
49
Example of a Dual
Port TXV.
50
Thermostatic Expansion Valve
• Balanced Port TXV’s
– Are designed to operate in low ambient
conditions when the head pressure (liquid)
drops.
– The liquid pressure that opens the TXV is
canceled out across a large needle.
– This TXV can also handle small amounts
of vapor and flash gas.
51
Balanced Port TXV’s
52
Automatic Expansion Valve
• Also known as an AEV or a constant
pressure valve.
• The AEV does the same thing as a capillary
tube does – it acts like a water valve.
• It is not seen as much as the TXV.
53
Automatic Expansion Valve
• The AEV responds to a load change exactly
opposite from the TXV.
• As the load increases the AEV will start to
starve the evaporator, thus maintaining
pressure and boiling point.
• As the load drops the AEV will begin to
open and allow the pressure to remain
constant as well as boiling point.
54
Automatic Expansion Valve
55
Automatic Expansion Valve
• The force that operates an AEV is the
evaporator pressure. This is the upward
force on the bottom of the diaphragm that
tends to close the valve
• When you front seat the valve on the AEV
(clockwise) you are opening the valve which
puts more liquid into the coil and lowers the
superheat.
56
Automatic Expansion Valve
• When you backseat the valve on the AEV
(counter clockwise) you are starving the coil
which raises the superheat.
• Atmospheric and adjustable spring pressure
exert a downward force that will open the
valve.
57
Automatic Expansion Valve
• AEV’s are designed to maintain a constant
pressure in the evaporator.
• When checking AEV’s you rarely have a
pressure port right next to the evaporator
and need to add 2psi to your readings to
account for pressure drop.
58
Automatic Expansion Valve
• Systems with AEV’s and most systems with
TXV’s should have a receiver to ensure a
proper refrigerant flow to the valve.
• The systems with a capillary tube will never
(or rarely) have a receiver.
• The receiver is a type of storage tank to
hold extra refrigerant.
59
Automatic Expansion Valve
60
Solid State Expansion Valve
• Solid state expansion valves replace the
sensing bulb with a thermistor and the
spring/diaphragm combination with a heat
motor.
• The thermistor varies it’s resistance based
on temperature.
61
Solid State Expansion Valve
• As this resistance varies the voltage that is
sent to the heat motor varies, thus opening
and closing the valve.
• The thermistor is inserted INTO the vapor
stream at the outlet of the evaporator.
• It will sense liquid in the line and CLOSE the
metering device.
62
Solid State Expansion Valve
63
Solid State Expansion Valve
64
Step Motor Expansion Valve
• Also known as an Electronic Expansion
Valve.
• Uses a small motor to control the expansion
valve’s port.
• The step motor rotates a fraction of a
degree for each signal sent by a controller.
65
Step Motor Expansion Valve
• The controller remembers how many steps
taken and can return to a prior step at any
time.
• The controller includes a microprocessor
that is programmed with an algorithm that
controls the motor based on a feedback
loop with a thermistor.
66
Step Motor Expansion Valve
67
Step Motor Expansion Valve
68
Step Motor Expansion Valve
• The communication between the
microprocessor (computer), the step motor
(valve), the algorithm, and the sensor is
considered a feedback loop.
69
Step Motor Expansion Valve
70
Step Motor Expansion Valve
71
Step Motor Expansion Valve
72
Step Motor Expansion Valve
73