1 Air Conditioning Metering Devices. 2 Used in an A/C system to change the pressure of the...
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1 Air Conditioning Metering Devices
1 Air Conditioning Metering Devices. 2 Used in an A/C system to change the pressure of the refrigerant. Is placed between the liquid line and the evaporator
2 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. Metering Devices
Slide 3
3 Controls the flow of refrigerant to the evaporator coil.
Maintains the correct superheat. Creates the flash gas at the start
of the evaporator coil.
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.
Slide 6
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.
Slide 7
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.
Slide 8
8 Capillary Tube
Slide 9
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.
Slide 10
10 Capillary Tubes
Slide 11
11 Capillary Tubes
Slide 12
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.
Slide 13
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.
Slide 14
14 Thermostatic Expansion Valve
Slide 15
15 Thermostatic Expansion Valve
Slide 16
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.
Slide 17
17 Thermostatic Expansion Valve TXVs 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
Slide 18
18 Thermostatic Expansion Valve TXVs 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.
Slide 19
19 Thermostatic Expansion Valve Temperature pressure of crossed
charged bulb.
Slide 20
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.
Slide 21
21 Thermostatic Expansion Valve
Slide 22
22 Thermostatic Expansion Valve
Slide 23
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.
Slide 24
24 Thermostatic Expansion Valve The TXV starts in an equalized
setting with 10 degree superheat.
Slide 25
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.
Slide 26
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.
Slide 27
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.
Slide 28
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.
Slide 29
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.
Slide 30
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.
Slide 31
31 Thermostatic Expansion Valve The rate of flow of liquid
through the TXV is directly proportional to the load
conditions.
Slide 32
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.
Slide 33
33 Thermostatic Expansion Valve The TXV is designed to work at
equilibrium.
Slide 34
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.
Slide 35
35 Thermostatic Expansion Valve Adjustments By adjusting the
spring pressure the superheat can be changed. TXVs can be
internally or externally equalized. Internal has two lines, one is
the liquid inlet and the other is the evaporator port outlet.
Slide 36
36 Thermostatic Expansion Valve Externally equalized has three
lines, the liquid line, the evaporator outlet line, and the
equalizer line. With externally equalized TXVs the bulb must be
mounted between the evaporator coil outlet and the equalizer
line.
Slide 37
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 line. Any liquid will cause improper TXV operation.
External equalizers are used on large evaporator coils where there
is a pressure drop.
Slide 38
38 Thermostatic Expansion Valve
Slide 39
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.
Slide 40
40 Thermostatic Expansion Valve Externally equalized TXVs 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.
Slide 41
41 Thermostatic Expansion Valve Superheat Adjustments TXVs 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.
Slide 42
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.
Slide 43
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.
Slide 44
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.
Slide 45
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.
Slide 46
46 Thermostatic Expansion Valve
Slide 47
47 Thermostatic Expansion Valve Some TXVs are considered
maximum operating pressure TXVs. MOP TXVs place a limit on the
evaporators pressure to prevent the compressor from overheating.
They do this by not opening fully under heavy load conditions.
Slide 48
48 Thermostatic Expansion Valve Dual Port TXVs 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.
Slide 49
49 Example of a Dual Port TXV.
Slide 50
50 Thermostatic Expansion Valve Balanced Port TXVs 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.
Slide 51
51 Balanced Port TXVs
Slide 52
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.
Slide 53
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.
Slide 54
54 Automatic Expansion Valve
Slide 55
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.
Slide 56
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.
Slide 57
57 Automatic Expansion Valve AEVs are designed to maintain a
constant pressure in the evaporator. When checking AEVs you rarely
have a pressure port right next to the evaporator and need to add
2psi to your readings to account for pressure drop.
Slide 58
58 Automatic Expansion Valve Systems with AEVs and most systems
with TXVs 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.
Slide 59
59 Automatic Expansion Valve
Slide 60
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 its resistance
based on temperature.
Slide 61
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.
Slide 62
62 Solid State Expansion Valve
Slide 63
63 Solid State Expansion Valve
Slide 64
64 Step Motor Expansion Valve Also known as an Electronic
Expansion Valve. Uses a small motor to control the expansion valves
port. The step motor rotates a fraction of a degree for each signal
sent by a controller.
Slide 65
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.
Slide 66
66 Step Motor Expansion Valve
Slide 67
67 Step Motor Expansion Valve
Slide 68
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.