1
Thermal Expansion Valve
陈江平
上海交通大学制冷研究所
《制冷原理与技术》讲义
Installation, Operation, and
Troubleshooting of TEVs
In a nutshell, the role of the TEV
is to control liquid injection into
an evaporator as a function of the
load. The controlling parameter
is superheat at the evaporator. As
the load on the evaporator
increases, the valve responds to
an increase in superheat and
opens to allow more liquid
refrigerant to flow into the
evaporator.
In so doing, the TEV maximizes
the usable evaporator heat
transfer surface and protects the
compressor by making sure that
only vapor returns to it.
Parts Of a TEV
· The power head assembly, enclosing a
diaphragm;
· The capillary tube and bulb connected
to the power head containing a charge
which, as it expands and contracts, exerts
a varying pressure on the diaphragm;
and
· The valve body, with one or more
pushpins operated by the diaphragm; the
pushpins regulate the opening of an
orifice through which the refrigerant is
metered into the evaporator.
The movement of the pushpins depends
on the pressure on the diaphragm, which
is opposed by the force of a spring.
Spring force, which determines static
superheat, can be fixed or adjustable.
How TEVs Work
The function of a TEV depends on the relationship between three
fundamental pressures.
Bulb charge pressure acts on the upper surface of the diaphragm, moving it
in the valve-opening direction.
Two pressures oppose bulb pressure. Evaporating pressure is introduced
by either internal or external equalization. This equalization pressure acts
on the underside of the diaphragm in the valve closing direction. Note:
Evaporat-ing and equalization pressures should always be the same.
Spring pressure also acts on the underside of the diaphragm in the closing
direction. In a valve with adjustable superheat, the spring pressure can be
adjusted manually.
As the expansion valve regulates, there is balance between bulb pressure
on one side of the diaphragm and equalization pressure plus spring
pressure on the other side. This balance can be upset in either of two ways:
1. When spring force is adjusted manually, there is a proportional change in
the TEV’s static superheat.
2. A change in the cooling load will change the evaporating pressure of the
refrigerant and hence the equalization pressure under the diaphragm. This
change occurs in proportion to the change in temperature at the evaporator
outlet tube where the bulb is strapped. Any change in pressure is
transmitted from the bulb through the capillary tube to the diaphragm.
The balance of forces is disturbed until a new equilibrium is reached as
more refrigerant is injected into the evaporator and the cooling load
demand is met.
Static superheat + opening superheat = operating
superheat.
What is Superheat?
Physically, superheat is the temperature difference between
the external pipe wall temperature and the evaporating
pressure converted to temperature (saturation temperature)
measured in °C. The level of superheat equals the temperature
increase above the saturation temperature at the existing
pressure.
A vapor is superheated when its temperature is higher than
the saturation temperature corresponding to its pressure. For
example, R-22 at 70 psig has a saturation temperature of 41?
and if its temperature actually is 51? it is said to be
superheated by 10?
With respect to valve operation, superheat has two distinct
components:
1. Static superheat is the superheat at which spring force is
met and the valve is ready to open.
2. Opening superheat is the amount of superheat above static
superheat that opens the valve to allow refrigerant flow.
The superheat measured at the outlet of the evaporator is the
sum of the two and is called operating superheat.
On valves with adjustable superheat, we are only changing
spring force, and therefore only the static superheat. By
adjusting the static superheat, however, we are effectively
adjusting operating superheat. The opening superheat cannot
be adjusted and is dependent on the system load or operating
pressures as transmitted from the sensing bulb.
Taking the measurements for
superheat calculation; for the
most accurate readings, place
your gauge and thermometer in
the positions shown.
How to Measure Superheat
1. Measure the suction pressure at the evaporator
outlet (or, if there is no fitting there, at the
compressor inlet service valve).
2. Clean an area of the suction line near the bulb.
3. Tape your thermocouple to the cleaned area
and insulate it; connect the thermocouple to a
calibrated electronic thermometer and read the
temperature.
4. Convert the suction pressure to a temperature
using a refrigerant slide rule or chart, and subtract
the temperature measured near the bulb. The
difference is the superheat.
A common but inaccurate method for determining
superheat in the field uses evaporator inlet
temperature instead of the saturated suction
temperature equivalent to the evaporator outlet
pressure. The problem with this method is its
inaccuracy, which is most often due to
misplacement of the inlet thermocouple or the
inability to access the inlet at all.
Make sure the sensing bulb is mounted in the corre
t position, according to the tubing size.
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TEV Installation
The TEV must be installed in the liquid line, ahead of the evaporator and as close to it as possible. The bulb is tightly strapped to the
suction line, as close to the evaporator outlet as possible. The bulb will give false signals to the diaphragm if it is installed after a
desuperheater or close to components with large mass, such as large valves or flange connections. Any evaporator with a distributor or with
a significant pressure drop requires an externally equalized TEV. If the valve is externally equalized, the equalizing line must be connected,
otherwise the valve will not operate. The equalization connection is made at a point in the suction line immediately after the bulb, and in a 12
o’clock position on the tube to avoid oil logging the equalizer line. Mount the TEV’s sensing bulb on a horizontal suction line tube at the outlet
of the evaporator in a position between 12 and 4 o’clock. The location depends on the suction line diameter. Tubes smaller than 3/4 in.
should have the bulb located at the 12 or 1 o’clock position; ?- and 7/8- in. tubes require a bulb position at 2 o’clock; for tubes 1 in. and
larger, the correct position is from 3 to 4 o’clock. Never locate the bulb at the bottom of the suction line because of the possibility of a false
signal caused by oil lying there. For the same reason, the bulb must not be mounted in areas where the suction line is bent and may act as
an oil trap, as on a riser. Remember that the optimum location is on a horizontal part of the suction line immediately after the evaporator
outlet. The bulb must have good thermal contact with the suction line. (Danfoss valves with double-contact bulbs improve thermal
conduction.) The bulb mounting strap transfers heat to the area of the bulb that is not in contact with the copper tubing. Never use plastic
straps such as cable ties for bulb mounting. Time and temperature will loosen the plastic material; contact as well as heat transfer will be lost.
The bulb mounting strap supplied by TEV manufacturers is made of heat-conductive material, and should always be tight, but not so tight as
to deform the piping or bulb. Although not a requirement, if heat-conductive paste is available, you can use it on the contact surfaces to
enhance heat transfer. Because it needs to be able to sense the temperature of superheated suction vapor, the bulb must not be located in a
position where external heat or cold will affect it. Insulating the bulb will help, but in cases where the lines operate below 32? the insulating
material must be chosen to seal against moisture that might freeze around the bulb. Insulation of the bulb is also recommended if the bulb is
exposed to a warm air current. On systems where a liquid distributor is used, remember that the TEV must be externally equalized, and the
distributor should be mounted vertically, head outlets downward. It is extremely important that the feeder tubes from the distributor be of the
same diameter and length. It is important to keep pressure drops across the distributor tubes as equal as possible for good liquid distribution.
Avoid liquid traps when routing the distributor tubing. Piping must be carefully designed and executed to prevent any unwanted effects. For
instance, where a circuit has multiple evaporators at different elevations, a higher evaporator can affect the TEV sensing bulb on a lower one.
Also, in multi-fixture circuits, you may find situations where another technician has mislocated a sensing bulb so that it is actually reading the
temperature of the common suction line rather than the evaporator it is meant to serve.
Setting, Adjusting Superheat
All expansion valves are supplied with a factory superheat setting
appropriate for most applications. TEVs with fixed superheat do not allow
readjustment in the field. Other valves, though, are designed to allow field
setting by adjusting the spring force.
To adjust the static superheat, turn the valve’s setting stem. Turning
clockwise increases static superheat and effectively reduces refrigerant
flow through the valve. Turning counterclockwise reduces static superheat
and increases refrigerant flow.
In addition to TEV sizing, correct superheat setting and proper sensing
bulb positioning are two more of the many important determining factors
for proper operation of an evaporator, and for compressor protection.
The bulb will give false signals to the diaphragm if it is installed after
a desuperheater or close to components with large mass, such as
large valves or flange connections.
Superheat Problems
Expansion valves are often suspected of causing system problems.
But generally speaking, a TEV is operating properly if it maintains
superheat of 5?to 15?
If superheat is low (lower than 5?, there is a potential for flooding
refrigerant back to the compressor. If superheat is higher than 15°C,
the evaporator is probably operating inefficiently.
Stop Fiddling and Find the Problem
There are countless possible causes
for problems in a system. Superheat
is one of the last things we adjust.
Expansion valves are designed and
set by their manufacturers to serve as
“plug-and-play” devices which, right
out of the box, can operate effectively
in a wide range of applications.
The temptation to adjust them is there
because it is very easy to get to them
before taking time to properly
diagnose the refrigeration system.
Unless there is absolute certainty of
incorrect superheat, leave the TEV
alone.
Here are some problem areas that
can cause low and high superheat.
These areas should be investigated
before adjusting superheat.
Causes of low superheat include:
· An improperly adjusted valve;
· A significantly oversized valve;
· Poor bulb or equalizer location;
· Overcharged system; and
· Excessive oil blocking the evaporator and
acting as an insulator.
Causes of high superheat include:
· Low refrigerant charge, resulting in flash gas in
the liquid line;
· Dirt in the system trapped in the valve;
· A restriction such as a plugged filter-drier in the
liquid line, again causing flash gas;
· A saturated or undersized drier in the system;
unremoved moisture is likely to result in the
formation of ice, restricting the TEV’s orifice; and
· Improper system design, resulting in little or no
liquid subcooling.
If You Need to Adjust
Earlier we talked about the proper way to take a superheat reading. If the valve is adjustable, and if you
determine that the superheat needs to be set (for example, because the system is hunting), prepare for the
adjustment by ensuring that you have operational head pressure and a proper flow of refrigerant to the TEV,
without flash gas.
Next, ensure that there is a nominal (or design) load on the evaporator; use a dummy load if necessary.
Now remove the stem cap to expose the adjustment stem.
Setting superheat is a trial-and-error procedure that will require several changes.
1. Adjust the valve to a point where you get unstable superheat readings, unless you have confirmed that
the system has unstable superheat to begin with. (When superheat is unstable, the system is out of control and
temperature and pressure are randomly fluctuating.)
2. Proceed to adjust the valve by turning the valve stem clockwise to increase superheat until the system is
just stable. Then a further one-quarter to one-half turn clockwise will compensate for system variables during
operation.
3. The valve manufacturer’s instructions give the number of stem turns per degree of superheat. You need
to measure superheat after each adjustment, until the new value results in correct evaporator temperature under
the nominal (or design) load.
4. Recheck the superheat under low-load conditions, too. Now you can be sure that the valve is set correctly.
When there’s a refrigeration problem, don’t start working on a remedy before making a careful diagnosis.
Adjusting superheat without careful observation and measurement is asking for trouble.
The same goes for pumping down a system to replace the expansion valve, only to find out that the system
is still not working. It wastes time and can be rather embarrassing.
But some systematic troubleshooting, examining system pressures and temperatures, will likely lead to a
solid diagnosis, a timely solution, and a satisfied customer.
TEV Hysteresis and
Evaporator
Characteristics
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Superheat Affects Valve Actions1
Friction, which results in resistance to
the movement of the pushpins, causes
hysteresis.
All TEVs are affected by hysteresis. A
certain amount is needed, as we will see,
but more than that is definitely
detrimental to the refrigeration system's
efficiency.
Figure models the action of a typical
TEV. Although valve response is often
represented by a single curve, there are
actually two, one for valve opening and
the other for closing. The area between
the two curves is called the hysteresis
band.
The hysteresis band
Superheat Affects Valve Actions2
A load increase starting from the valve opening curve.
Let's say we're on the valve opening curve and the load on the evaporator increases, in turn requiring an
increase in cooling capacity (Figure 3). The superheat increases, raising the bulb pressure, which opens
the valve. The capacity delivered by the valve changes almost instantly.
Superheat Affects Valve Actions3
If we're on the opening curve and the load decreases, the story is different (Figure 4). Before the valve can start closing,
superheat must decrease by the distance between the opening curve and the valve closing curve. With any smaller
decrease in superheat, we will still be in the hysteresis band, and the valve will not begin to close. Only after reaching the
closing curve is the valve ready to begin closing with any further decrease in superheat.
The relationships between superheat changes and valve actions are similar if we start on the valve closing curve. There, a
decrease in flow through the valve occurs without hysteresis (Figure 5), but since an increase in refrigerant flow requires
crossing the hysteresis band, superheat must increase by the distance between the two curves, overcoming the valve's
hysteresis (Figure 6). Capacity can only increase on the opening curve, and can only decrease on the closing curve.
Figure 4. A load decrease starting from the valve opening curve.
Figure 5. A load decrease starting from the valve closing curve.
Figure 6. A load increase starting from the valve closing curve.
Hysteresis can't be computed. Valve manufacturers make laboratory
measurements over the valve's capacity range to determine the curves
for a given design. At Danfoss, this is done using precision automatic
measuring and recording instruments. The process takes place under
standardized, controlled conditions.
The design of a valve determines its internal friction, and therefore its
hysteresis. Valves with low internal friction have correspondingly low
hysteresis. But if there were no hysteresis at all, the opening and
closing curves would become one, and the valve would react
instantaneously to any change in load, even extremely small changes.
While that might sound great at first, it would cause the valve to be too
sensitive. The system could then become unstable, with the valve
possibly going into a hunting mode.
What Happens inside the Evaporator?
Figure 7 shows the behavior in an evaporator at a given capacity. Given a negligible pressure drop across the system, if we place
temperature probes (T1 and T2) at the inlet and outlet, we can determine the superheat across the entire evaporator (T2-T1).
By moving T2 closer (to T2A), the temperature difference drops as we get closer to the liquid front. Moving T2 even closer (to T2B), we begin to
see temperature fluctuations caused by T2B sensing both liquid droplets and vapor. The point just before the fluctuations can be seen is the minimum stable
superheat (MSS) point. At MSS, the highest efficiency is achieved for the given load condition.
The evaporator is most efficient at the MSS point because this is the point at which all of the refrigerant has finished evaporating and the evaporator is fully
utilized.
Figure 7. Refrigerant behavior in an evaporator, showing
the MSS point and the changes in the quality of refrigerant.
More…
A curve can be graphed by determining the MSS point at
different loads. This is shown by the red curve in Figure
8. For every evaporator, the MSS curve characteristic is
unique. This curve can have many shapes, as it is
primarily a function of evaporating temperature, airflow,
and coil design.
The gray area to the left of the MSS curve represents an
unstable zone where liquid and gas coexist. When a
system operates within this region, liquid refrigerant
escapes at the evaporator outlet and overall system
efficiency falls. The further we go into the unstable zone,
the greater the potential for liquid slugging, which in turn
leads to serious compressor damage.
To the right of the MSS curve there is only superheated
gas. Moving too far into this region also reduces system
efficiency because the evaporator is not being fully
utilized.
In practice, achieving minimum stable superheat for
optimum system performance may involve first
decreasing the TEV's static superheat setting until the
system begins to become unstable, then slightly
increasing the static superheat until stability is just
reached.
Figure 8. A graphical representation of the
action in an evaporator, showing the
evaporator characteristic (the MSS curve).
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Putting It All Together
A TEV is a proportional control device.
That means capacity changes are directly
proportional to changes in superheat.
Figure 9 shows the characteristics of
three TEVs. To simplify the diagram, each
characteristic is shown as a single curve.
The slope of the characteristic is called
its gain. The greater the gain, the steeper
the valve characteristic, and the bigger
the capacity change for a given change in
superheat. Each of the curves in Figure 9
shows the capacity increase for a
1?change in superheat.
In an actual operating system, a TEV's
gain will change with variations in
subcooling and pressure drop across the
valve.
Figure 9. Valve gain for three different TEV
designs. I n an operating system, gain will vary
with changes in subcooling and pressure drop.
More…
In an actual operating system, a TEV's gain will change with
variations in subcooling and pressure drop across the valve.
Figure 10 shows how three valve characteristics with
different slopes "fit" an evaporator characteristic.
Remember that the evaporator characteristic we want the
valve to fit is to the right of the unstable zone, but as close
to it as possible. It's the valve closing curve that's closest to
the evaporator characteristic. The range shown in green is
the system's most-efficient operating range.
It is important to take the hysteresis band into
consideration because the valve opens in the stable area
where there is only gas at the sensing bulb, and closes just
before the system becomes unstable. In the first two
examples in Figure 10, the valve closing curves are
completely outside the unstable zone. Due to the slope of
the characteristics in those examples, there is a
comparatively large area of high efficiency (green). In the
third example, due to a steeper characteristic slope, the
upper portion of the closing curve falls inside the unstable
zone, and the high-efficiency range is much smaller.
Knowing how hysteresis affects valve operation, how
refrigerant quality changes in an evaporator, and how valve
and evaporator characteristics need to match will give a
service technician a base of understanding that will make it
easier to tune refrigeration systems for optimum
performance at minimum stable superheat (MSS).
Where the
valve
characteristi
c's closing
curve fits
most closely
to the
evaporator
characteristi
c (the MSS
line), system
efficiency is
greatest.
Tips for preventing superheat
hunting in TXVs
what is superheat hunting?
Superheat hunting is a cyclical
fluctuation in suction superheat due to
varying refrigerant flow rate in the
system.
Superheat hunting is the result of the
expansion valve (see Figure 1)
excessively opening and closing in an
attempt to maintain a constant operating
condition.
Hunting can be seen indirectly by regular
fluctuations in suction temperature, and
in extremes, suction pressure.
Excessive hunting can reduce the
capacity and efficiency of the system,
resulting in uncomfortable conditions,
loss of product, wasted energy, and
ultimately, customer dissatisfaction.
Figure 1. A conventional balanced port thermostatic
expansion valve and the three forces it responds to. F1:
thermal bulb pressure times the diaphragm effective
area; this force acts on the top of the diaphragm, which
tends to open the valve. F2: evaporator pressure times
the diaphragm effective area; this force acts on the
underside of the diaphragm and tends to close the
valve. This force is transmitted to the diaphragm
through the valve body with internal equalized valves
and through the external connection in external
equalized valves. F3: superheat spring force which
assists in closing the valve.
Why TXVs hunt?
There are several common reasons the service tech should consider when
determining why a TXV hunts.
Oversized valve: The expansion valve may be oversized for the application or
operating condition of the system.
Valve capacity significantly exceeds the requirements of the system and when the
valve attempts to adjust to system load, it overcompensates because it is oversized.
Incorrect charge selection: The charge selected does not have the necessary
control characteristics and/or dampening ability to stabilize operation.
Undercharged system: Intermittent loss of subcooling is causing loss of
expansion valve capacity and resulting intermittent high superheat.
Poor bulb contact: Loss or delay of temperature signal to the bulb, causing
erratic and unpredictable operation.
an imbalanced heat exchanger (multi-circuit coil): An imbalance in the heat
load on each circuit creates a false temperature signal to the expansion valve bulb
and results in erratic operation.
Since this problem is commonly overlooked in the field, a closer examination and a
possible solution are the focus of this article.
TXVs on multi-circuit heat exchangers
TXVs respond to the temperature of the suction line. (They respond to pressure too, but
this is not the concern of this article.)
At the expansion valve outlet, flow is divided into two or more paths (circuits) at the inlet
of the evaporator by the distributor; these paths then recombine as they exit the
evaporator into the suction manifold. (See Figure 2.)
Ideally, each circuit is equally loaded and absorbs an equivalent amount of heat. If one
assumes the refrigerant flow rate and heat load through each circuit is equal, then the
superheat condition exiting each circuit will be equal and when all of the flow streams
recombine, the result is a “true” average condition of the evaporator suction gas.
When one or more circuits has a lighter heat load, some refrigerant from that circuit
remains unevaporated when it exits the coil. When this unevaporated liquid refrigerant
combines with the other superheated flow streams, the recombined suction flow no
longer represents an average condition.
Excessive hunting can reduce the capacity and efficiency of the system, resulting
in uncomfortable conditions, loss of product, wasted energy, and ultimately,
customer dissatisfaction.
The suction temperature where the bulb is mounted will be lower than the “true” average
of the circuits if they were all properly superheated.
Sensing a “cold” suction condition will cause the valve to close down because it is
sensing a condition that is not superheated enough; when the valve closes down, it
restricts flow to all circuits and eventually “dries out” the circuits which are flooding.
By this time, the remaining circuits have become highly superheated due to the reduced
flow rate. At the point the “flooding” circuit(s) begin to be superheated, the suction
temperature rises rapidly because there is no more liquid present to falsely reduce the
suction temperature.
Sensing a now “warm” suction condition, the valve opens to decrease superheat and the
lightly loaded circuit begins to flood into the suction manifold again. Suction temperature
drops rapidly again, the valve closes down again, and the whole sequence repeats in a
cyclical fashion.
Figure 2. At the expansion valve outlet, flow
is divided into two or more paths (circuits) at
the inlet of the evaporator by the distributor.
These paths recombine as they exit the
evaporator into the suction manifold.
5
Why circuits get loaded unevenly
Again, the ideal situation is to assume each circuit is equally loaded
and absorbs an equivalent amount of heat; however, this situation
does not always occur.
There are several reasons why circuits can become unevenly
loaded.
Poor heat exchanger design: In this case, each circuit is not of
equal length and loading.
Poor refrigerant distribution: This problem occurs due to the
wrong choice of distributor or feeder tubes, partially blocked
passageways of feeder tubes, unequal feeder tube lengths, and/or
kinked feeder tubes.
Uneven airflow: Airflow across the evaporator is reduced in
some areas while increased in other areas. Dirty coils or damaged
coil fins can have a similar effect on airflow.
Diagnosing a hunting problem
Diagnosing a hunting problem due to an imbalanced heat exchanger requires
measuring the exit temperature of each circuit upstream of the suction manifold.
To perform this process, average the temperatures of all of the circuits upstream of
the suction manifold and compare this average temperature to the actual
temperature of the suction manifold close to where the bulb is mounted.
If the average value of the circuit exit temperatures exceeds the actual suction
temperature value by more than 2°C, then there is probably one or more circuit(s)
which are not completely superheated (flooding).
A closer examination of the individual circuit temperatures and the associated
suction pressure should reveal which circuit(s) are causing the problem.
One simple rule to remember is that the valve’s response will favor the circuit that is
flooding. Because of this favorable response, a heat exchanger can be operating at
a reasonable exit superheat but still have a significant loss in capacity, because the
expansion valve is responding to one or more flooding circuits while the other
circuits remain highly superheated, and thus highly inefficient.
Correcting the problem
This can be a difficult task. First, the service tech must recognize the cause of the
problem. If not, the problem can only be compensated for and this could mean a
reduction in system performance.
Here are some tips for correcting or compensating for an imbalanced heat
exchanger:
If possible, examine and correct any problems with airflow, coil circuitry, and
distribution so that the circuits are more evenly fed and loaded. The goal is a
more consistent circuit exit temperature on all circuits. One lightly loaded circuit
may be tolerable if there are, for example, eight circuits. However, this is probably
not the case if there are only three.
Adjust the superheat of the valve to a slightly higher value. Attempting to control
an evaporator near to or lower than 5癋operating superheat can exceed the
sensing capability of most expansion valves and result in hunting and subsequent
intermittent flooding.
If practical, move the bulb farther downstream on the suction line.Better mixing of
the refrigerant prior to the bulb can smooth out the valve response, although
capacity and efficiency may not improve significantly.
