PDA

View Full Version : Superheat

nightranger121
05-03-2008, 06:52 AM
Okay, this is kinda bugging me, so I am throwing it out here to you guys.....

What is the correct way to calculate superheat? I was taught one way, which seems to work fine, then someone said yesterday, "no.... you should do it this way, it's better"

Is there more than one method to still give you an accurate number, or was this just a shortcut?

markettech
05-03-2008, 07:12 AM
Okay, this is kinda bugging me, so I am throwing it out here to you guys.....

What is the correct way to calculate superheat? I was taught one way, which seems to work fine, then someone said yesterday, "no.... you should do it this way, it's better"

Is there more than one method to still give you an accurate number, or was this just a shortcut?

How do you currently calculate superheat?

powell
05-03-2008, 07:17 AM

:confused: what is "this"? You gave us no information.

nightranger121
05-03-2008, 07:26 AM
I take suction pressure, convert it to saturation temp, and subtract it from actual temp. He was suggesting a method not involving the gauges. I didn't originally include the method because I didn't want to railroad any replies, but I understand why you ask.

markettech
05-03-2008, 07:36 AM
I take suction pressure, convert it to saturation temp, and subtract it from actual temp. He was suggesting a method not involving the gauges. I didn't originally include the method because I didn't want to railroad any replies, but I understand why you ask.

By taking the suction pressure and converting it to SST, you are essentially calculating the superheat by subtracting the coil inlet temperature from the coil outlet temperature.

Taking an actual temperature reading at the inlet and outlet of the coil is probably the MOST accurate way to calculate superheat...........but it's not always convenient to do so.

nightranger121
05-03-2008, 07:41 AM
Thanks, markettech.... that completely makes sense and answered my question. I just wanted to make sure.

icehouse
05-03-2008, 07:47 AM
Check out http://www.johnstonesupply.com/60/OnlineCatalog/ProductDetails/tabid/1780/Default.aspx?Product_Code=H25-943 :)

icemeister
05-03-2008, 08:22 AM
...Taking an actual temperature reading at the inlet and outlet of the coil is probably the MOST accurate way to calculate superheat...

By definition, the most accurate method of superheat measurement is to obtain an accurate reading of the temperature and pressure of the refrigerant and calculate it using a PT chart.

The two-temperature method doesn't take into account either the pressure drop through the evaporator or (if it's a refrigerant blend) the glide factor.

For example, consider a typical R22 system. The pressure drop through the evaporator, inlet to outlet, is likely in the range of 5 psi or so. That pressure difference will throw off the two temp method's SH by 3.5ºF.

If the refrigerant is a blend like MP39, the glide can be as high as 12ºF, so in that case, the SH as measured by the two-temp approach might indicate 15ºF SH while the temp-pressure measurement might show near zero SH leaving the evaporator.

icehouse
05-03-2008, 08:29 AM

icehouse
05-03-2008, 08:33 AM
Form 10-135

USING THE P-T CARD AS A SERVICE TOOL

Manufacturers of refrigerants, controls, and other suppliers distribute hundreds of thousands of
pressure-temperature charts to the trade every year. It would be rare indeed to find a service
technician who could not put their hands on a pressure-temperature card at a minute’s notice.

In spite of the widespread availability and apparent reference to the pressure-temperature rela
tionship, very few service technicians use the P-T chart properly in diagnosing service problems.

relationship, but to also illustrate how it can be used to thoroughly analyze a refrigeration or air
conditioning system.

®

Page 2 / Form 10-135

REFRIGERANT IN THREE FORMS
Before getting into the proper use of the P-T card, let’s The refrigerant in a refrigeration system will exist in one of
review briefly the refrigeration system and examine exact-the following forms:
ly how the pressure-temperature relationship can be 1. All liquid
applied. 2. All vapor

3. A mixture of liquid and vapor
Figure 1
State of refrigerant in a normally operating refrigeration system.
Figure 1 illustrates the form in which refrigerant is found at
various points in a normal operating refrigeration system.

Notice that the high side contains refrigerant in all of the
three conditions listed above. The discharge line contains
all vapor. The condenser where the vapor condenses into a
liquid contains a mixture of liquid and vapor. The line
between the condenser and the receiver usually contains all
liquid, although it would not be abnormal for this line to
also have some vapor mixed with the liquid. Since the
receiver has a liquid level at some point, it must be thought
of as having a mixture of liquid and vapor. The liquid line

valve should contain all liquid. A sight glass or liquid indicator
is frequently installed in the liquid line to assist in
determining if the liquid refrigerant is completely vapor-
free.

The low side of the system will usually contain refrigerant
in only two of the three forms that were listed previously.
That is, the low side will contain all vapor in the suction
line, and a mixture of liquid and vapor from the outlet of
the thermostatic expansion valve to nearly the outlet of the
evaporator.

Form 10-135 / Page 3

WHEN REFRIGERANT IS “SATURATED”
The important thing to remember is that the pressure-temperature
relationship as shown by a P-T card is only valid
when there is a mixture of refrigerant liquid and vapor.

Therefore, there are only three places in the normally
operating refrigeration system where the P-T relationship
can be guaranteed with certainty. That is the evaporator,
the condenser, and the receiver — places where a mixture
of refrigerant liquid and vapor are known to exist.
When refrigerant liquid and vapor exist together, the

condition is known as “saturated.”

This means that if we are able to determine the pressure at
any of these points, we can easily determine the “saturation”
temperature by merely finding the pressure on a P-T card
and reading the corresponding temperature. Conversely, if
we can accurately measure the temperature at these three
locations, we can also determine the “saturation” pressure
from the P-T relationship by finding the pressure corresponding
to the temperature that we have measured.

WHEN SUPERHEAT OR SUBCOOLING IS INDICATED
At the points in the system where only vapor is present, the
actual temperature will be above the saturation temperature.
In this case, the difference between the measured temperature
and the saturation temperature at the point in
question is a measure of superheat. The temperature of the
vapor could be the same as the saturation temperature, but
in actual practice, it is always above. If these temperatures
were the same then the amount of superheat would be zero.

Where it is known that only liquid is present such as in the
liquid line, the measured temperature will be somewhere
below the saturation temperature. In this case, the difference
between the measured temperature and the saturation
temperature is a measure of liquid subcooling. Again, it is
possible to find that the actual measured temperature is
equivalent to the saturation temperature, in which case the
amount of subcooling would be indicated as zero.

ANALYZING REFRIGERANT CONDITION
Figure 2 shows some actual pressure-temperature measurements
throughout a normally operating system using R134a
refrigerant. This may give a better insight into the
condition of the refrigerant at the various points. The measured
temperature at the evaporator inlet is 20°F. A gauge
installed at this point indicates a pressure of 18 psig; 18
psig on the P-T card indicates a temperature of 20°F — the
same as was measured. It might also be said that the superheat
is zero and the subcooling is zero. Therefore, the
refrigerant is at saturation, or in other words, at the boiling
point. This is what we should expect since, when refrigerant
liquid and vapor are present together, the P-T relationship
will hold true.

A gauge installed in the suction line measures 16 psig. If
there were a mixture of liquid and vapor at this point, the
measured temperature would be the same as the saturation
temperature or 17°F. However, our actual measured temperature
in this case is 27°F. The amount of superheat in the
vapor is the difference between the measured temperature
of 27°F and the saturation temperature (according to the PT
chart) of 17°F. Therefore, the superheat is 10°F.

If we also measure 16 psig at the compressor inlet with the
measured temperature of 47°F, our superheat in this case
would be 30°F, calculated by subtracting the saturation
temperature equivalent to 16 psig (17°F) from the measured
temperature of 47°F.

Let’s now examine the gauge we have installed midway in
the condenser which reads 158 psig. According to the P-T
card, the saturation temperature will be 115°F. This is the
temperature that we would be able to measure if we placed
a thermocouple in the refrigerant at the point where it is
changing from a vapor to a liquid. At this point, there is no
difference between the measured temperature and the
saturation temperature. It might also be said that the superheat
is zero and the subcooling is zero. Therefore, the
refrigerant is saturated, or in other words, at the boiling
point.

In our example we also measure 158 psig at a discharge
line of the compressor. The measured temperature here is
200°F. Calculating the superheat in the same way as it was
done on the suction line (difference between measured

Page 4 / Form 10-135

VaporLiquidMixture of

vapor and liquidFigure 2
R-134aExample of actual pressure-temperature measurements in a normally operating system.
temperature and saturation temperature), it is determined
that the superheat is 85°F.

When a system employs the use of a liquid receiver, there
can be no subcooling at the surface of the liquid in the
receiver. The reason is that when liquid refrigerant and
vapor exist together, they must obey the P-T relationship or
the refrigerant must be saturated. In our example the measured
pressure in the receiver is 146 psig; the refrigerant in
the receiver must therefore be at 110°F.

Once a solid column of liquid is formed, subcooling of the
refrigerant can take place by lowering its temperature with
the use of liquid-suction heat exchangers, subcoolers, or
from lower ambient temperatures surrounding the line.

Subcooling is a lowering of a temperature below the saturation
point or boiling point. In our illustration in Figure 2,

subcooling of 5°F and 2°F has been determined as illustrated
at two points.

Of course, it is important to maintain some liquid subcooling
in the liquid line to prevent flash gas from forming in
the liquid line and entering the thermostatic expansion
valve.

With the use of a P-T card, we should be able to determine
the condition of the refrigerant at any point in the system
by measuring both the pressure and the temperature and
observing the following rules:

A.
Liquid and vapor are present together when the measured
temperature corresponds to the P-T relationship.
(It is theoretically possible to have “100% saturated liquid”
or “100% saturated vapor” under these conditions,
but practically speaking in an operating system,

Form 10-135 / Page 5

it should be assumed that some liquid and some vapor
are present together under these conditions.)

B.
Superheated vapor is present when the measured temperature
is above the saturation temperature corresponding
to the P-T relationship. The amount of
superheat is indicated by the difference.

C.
Subcooled liquid is present when the measured temperature
is below the saturation temperature corresponding
to the P-T relationship. The amount of
subcooling is represented by the difference.
PRACTICAL LIMITATION TO GAUGE LOCATIONS
In our illustration we have located gauges at points in the
system where it is not always feasible to do so on an actual
installation. Because of this, we must oftentimes make
deductions and assumptions when dealing with an actual
system.

As an example, we would normally assume that the 158
psig read on the gauge installed at the compressor discharge
line is also the pressure that exists in the condenser.
That is, we assume that there is no pressure loss of any consequence
between the compressor discharge and the condenser.
With this reasoning, we arrive at a condensing
temperature of 115°F. If an undersized discharge line or
other restrictions are suspected, we cannot make this
assumption and other pressure taps may be necessary to
locate the troublesome area.

It is also common practice to assume that the pressure
measured at the suction service valve of the compressor is
the same pressure that exists at the outlet of the evaporator
at the expansion valve bulb location. This is particularly
true on close-coupled systems where it has been determined
that the suction line is of the proper size. By making
this assumption, we can determine the expansion valve
superheat without installing an additional pressure tap at
the bulb location. However, to eliminate any doubt as to the

amount of suction line pressure drop and to be absolutely
precise in measuring superheat, a gauge must be installed
in the suction line at the bulb location.

Care must be taken to make a reasonable allowance for
pressure drops within the system. Excessive pressure drops
can be detected by applying the principles of the P-T relationship.
As an example, in Figure 2, with gauges installed
only at the suction and discharge of the compressor and
reading as indicated, a significant pressure drop through
the evaporator would be indicated by a high temperature
of, say, 50°F measured at the evaporator inlet which would
correspond to a pressure at that point of approximately 45
psig. That would mean that there is a pressure drop of 29
psi from the evaporator inlet to the compressor inlet (45
minus 16). While this would be considered excessive on a
single-circuit evaporator, it should be remembered that on
multi-circuit evaporators there will be a pressure drop
through the refrigerant distributor assembly. A pressure
drop through the distributor assembly on R-134a may be
in the vicinity of 25 psi. This means that with the use of a
refrigerant distributor, a measured temperature between
the outlet of the thermostatic expansion valve and the inlet
of the distributor of approximately 50°F would not be
abnormal in the system illustrated in Figure 2.

CHECKING ON NONCONDENSABLES
The proper use of the P-T relationship can be helpful in dis-ured temperature in the condenser or the leaving tempera-
covering the presence of air or other noncondensable ture of the cooling medium being much lower than that
gases in the system. This would be revealed by the meas-indicated by the P-T relationship.

SUMMARY
With an understanding of the refrigerant pressure-temper-system if the refrigerant is saturated, subcooled or superature
relationship, the widely available P-T card is a valu-heated. This is very important in property diagnosing
able tool. A P-T card, along with accurate gauges and system problems.
thermometers, allows us to determine at any point in the

Page 6 / Form 10-135

Figure 3 is an exercise to test your knowledge and use of the P-T relationship. The pressure and temperature are shown at various points in the system.
Check the square that indicates the condition of the refrigerant at each point. In the case of superheated vapor and subcooled liquid, indicate the amount inthe blank shown.

Form 10-135 / Page 7
VaporLiquidMixtureofvaporandliquidFigure 3
R-22
VaporLiquidMixtureofvaporandliquidFigure 3
R-22

markettech
05-03-2008, 08:58 AM
By definition, the most accurate method of superheat measurement is to obtain an accurate reading of the temperature and pressure of the refrigerant and calculate it using a PT chart.

The two-temperature method doesn't take into account either the pressure drop through the evaporator or (if it's a refrigerant blend) the glide factor.

For example, consider a typical R22 system. The pressure drop through the evaporator, inlet to outlet, is likely in the range of 5 psi or so. That pressure difference will throw off the two temp method's SH by 3.5ºF.

If the refrigerant is a blend like MP39, the glide can be as high as 12ºF, so in that case, the SH as measured by the two-temp approach might indicate 15ºF SH while the temp-pressure measurement might show near zero SH leaving the evaporator.

Thank you for correcting me.

When a person is correct, very rarely is it acknowledged by another..........this is probably why certain threads die VERY quickly.

But, if it is perceived that another is not correct...........there seems to be no limit to the number of people who will not hesitate to "put one in his place" so to speak.

I wonder what it is about human nature that makes us this way?:confused:

Anyway.........thanks again for setting me straight on superheat;)

nightranger121
05-03-2008, 09:34 AM
Thanks for that excerpt icehouse..... it was very informative. I am curious where it came from? I see references to page numbers and such.

So, perhaps I should now re-word my question......

Hypothetical situation:

W/I cooler, condensing unit on the roof, 50ft away. No valves on the evaporator.

Which would be more accurate- using the inlet/outlet temps @ the evaporator, or using the pressure reading at the suction service valve at the compressor and the outlet temp? From what I am seeing here, both have their pros and cons, and neither would be 100&#37; accurate.

itsiceman
05-03-2008, 09:50 AM
Hypothetical situation:

W/I cooler, condensing unit on the roof, 50ft away. No valves on the evaporator.

Which would be more accurate- using the inlet/outlet temps @ the evaporator, or using the pressure reading at the suction service valve at the compressor and the outlet temp? The suction service valve would be most accurate if you were determining superheat @ the compressor :D
.

nightranger121
05-03-2008, 09:56 AM
The suction service valve would be most accurate if you were determining superheat @ the compressor .

I'm not sure if I am misunderstanding you, or you are misunderstanding me..... are you referring to compressor superheat? I was referring to superheat at the evaporator.

icehouse
05-03-2008, 10:03 AM
Contractor
Service Tips
Evaporator Vs. System Superheat
Q: What is the difference between evaporator
superheat and system superheat?
Superheat varies within the system depending on
where it is being measured. The superheat that the
thermal expansion valve is controlling is the
evaporator superheat. This is measured at the outlet
of the evaporator. The refrigerant gains superheat as
it travels through the evaporator, basically starting at
0 as it enters the evaporator and reaching a
maximum at the outlet as the refrigerant travels
though the evaporator absorbing heat.
System superheat refers to the superheat entering
the suction of the compressor. Some people confuse
system superheat with ‘return gas temperature.’ It
should be remembered that superheat varies as the
saturated suction pressure of the refrigerant varies.
Return gas temperature is a temperature value measured
by a thermometer or other temperature-sensing
device. It does not vary because of pressure changes.
Q: How much system superheat should I see at the
compressor inlet?
Compressor manufacturer's like to see a minmum of
about 20 degrees of superheat at the compressor
inlet. This is to assure them that no liquid refrigerant
is entering the compressor.
Tip Card
10
Form No. 2005DS-133 Issued 1/06
Emerson Climate Technologies logo is a trademark of Emerson Electric Co.
© 2006 Emerson Climate Technologies. Printed in the USA.

icemeister
05-03-2008, 10:16 AM
...When a person is correct, very rarely is it acknowledged by another..........this is probably why certain threads die VERY quickly...

There's an old expression, "Silence is equal to agreement", which by itself would explain this phenomenon.;)

icehouse
05-03-2008, 10:23 AM
There's an old expression, "Silence is equal to agreement", which by itself would explain this phenomenon.;)
While in Japan I met Master Yamaguchi( 10 degree Blackbelt), a small quite man. People would often say that he rarely speaks. To this he would reply. "He who keeps quite is often said to be inept, those who often spoke proved how inept they really are". :)
'.

icehouse
05-03-2008, 11:16 AM
I'm not sure if I am misunderstanding you, or you are misunderstanding me..... are you referring to compressor superheat? I was referring to superheat at the evaporator. Find info from Andy Schoen here http://www.parker.com/portal/site/PARKER/menuitem.75b3c0354ff8851afa93ebde76108a0c/?vgnextoid=528d84aa40701110VgnVCM10000048021dacRCR D&vgnextfmt=default :)

itsiceman
05-03-2008, 01:07 PM
While in Japan I met Master Yamaguchi( 10 degree Blackbelt), a small quite man. People would often say that he rarely speaks. To this he would reply. "He who keeps quite is often said to be inept, those who often spoke proved how inept they really are". :)
'.
Ahhh... It is also said the less you speak the more you hear Grasshopper :D

And you know why they call you Grasshopper? :p

:DTo much ice in one thread makes my lips numb and start to talk funny ;)

itsiceman
05-03-2008, 01:16 PM
I'm not sure if I am misunderstanding you, or you are misunderstanding me..... are you referring to compressor superheat? I was referring to superheat at the evaporator.
so long as you know the difference I would go with the temp difference if you didn't want to put a presure port in at the evaporator.

Rick a testo 905-T2 surface thermometer is awesome for getting that type of measurement at the inlet and outlet of an evap coil :cool:

http://i268.photobucket.com/albums/jj31/itsiceman/0560_9056_01-1.jpg

icehouse
05-03-2008, 01:43 PM
Ahhh... It is also said the less you speak the more you hear Grasshopper :D

And you know why they call you Grasshopper? :p

:DTo much ice in one thread makes my lips numb and start to talk funny ;)
Grasshopper,naw I have been told I look like Captain Crunch and sound like Elmer Fudd. :D:D

itsiceman
05-03-2008, 01:55 PM
Grasshopper,naw I have been told I look like Captain Crunch and sound like Elmer Fudd. :D:DTo funny
Then I'll be varwee varwee qwiet Cap'n Ice :cool: :D

icehouse
05-03-2008, 02:02 PM
To funny
Then I'll be varwee varwee qwiet Cap'n Ice :cool: :D
Sceweey Wabbit :p Twix awe fow kids:D

crackertech
05-03-2008, 02:05 PM
Good info icehouse thanks.:)

nightranger121
05-03-2008, 02:11 PM
Iceman, although that is a generous offer, I think I'll stick with the scope. I actually have a single-lead therm now (fluke), but am shopping around for a 2 (or maybe 3) input therm. Was in the process of checking out the Testo site, along with a few others. They have a few nice ones along those lines, but I wish they had prices on there!

nightranger121
05-03-2008, 02:13 PM
Thanks for all the great info here, guys..... these kinds of threads make it all worthwhile!

icehouse
05-03-2008, 02:17 PM
Thanks for all the great info here, guys..... these kinds of threads make it all worthwhile!
What goes around, comes around. :)

itsiceman
05-03-2008, 02:17 PM
Iceman, although that is a generous offer, I think I'll stick with the scope. I actually have a single-lead therm now (fluke), but am shopping around for a 2 (or maybe 3) input therm. Was in the process of checking out the Testo site, along with a few others. They have a few nice ones along those lines, but I wish they had prices on there!I wasn't being nice :p

Check out www.truetechtools.com

nightranger121
05-03-2008, 03:14 PM
I wasn't being nice :p

Check out www.truetechtools.com

I am pretty excited... I can't wait for it to show up... I'm one of those people who never wins anything, so it was really nice... plus, now I can get that colonoscopy the insurance didn't want to cover...

btw- thanks for the link... I'll check it out.

gas_n_go
05-06-2008, 09:03 PM
My two cents worth:

Obvipously both ways are acceptable techniques to use if done correctly. I generally prefer to tempature delta method if I have a reasonable choice of using either.

Here is my reasoning why:

I use a cooper dual probe termometer, the common ole srh. It actually reads the difference between the two temps for me without me switching back n forth. This is very convenient and it affords me the accurancy of having an instant comparison of the two temps at the same time. In other words if my suction pressure drifts while im taking my readings because of a compressor cycling my meter records it. When your using a gauge your pressures may move a hair without you catching it.

Intrestingly one of you mentioned that when your dealing with a blend that this method is less reliable then using the PT method. Another reason that I prefer the delta method is that sometimes the equipment we are working on is charged with what I will call a "field engineered blend" That does not exist on any known PT chart. Tempature is tempature and does not lie.

This brings me to my third point on why I prefer the delta method. I can calibrate my probes with ice water (which for the record is not 100% accurate unles your factoring what elevation your doing it at). I really dont know of anyway to calibrate gauges with any kind of accuracy other then zeroing it out with a screw driver. I find it a little funny that we spend so much time on taking accurate readings but do it with an instrament that we really dont know if it is accurate or not. Point being I trust my probes accurancy over my gauges accuracy.

Those are my three biggest reasons why I prefer the delta method.

Now I also have developed a few rules when using this method that seem to stray from the conventional wisdom on how it is correctly executed.

First I dont strap my inlet probe on the first pass of the evaporator, I prefer to use the center pass of the center circuit if possible. This is to take into account for any tempature glides accuring inside the pipes.

Second; I set all valves to 1 degree of S.H. Low temp and med temp everything gets set to 1 degree at the evap. Your gas cannot return to cold as far as I am concerned.

Regarding The PT method. I think it is more accurate then the method I just outlined above when under certain conditions.

1. When you know your gauge is absolutly accurate

2. When you know you have a pure refrigerant in your sys

3. when you have a contamination free sys

4. when you can simutanously convert pressure to tempature and read the other tempature at the same time.

The problem is how often can you garuntee those 4 things when your working on a system?

This installment of GAS_N_GO's practicle engineering was brought to be the fine sponors of RONCO "Set it, and forget it!"

ogden
05-13-2008, 12:56 PM
i'm getting some conflicting info on SH as well. does it only apply to fixed metering systems? some of the service info doesn't make it so clear.

gas_n_go
05-13-2008, 07:47 PM
i'm getting some conflicting info on SH as well. does it only apply to fixed metering systems? some of the service info doesn't make it so clear.

Superheat is a state of the condition your refrigerant is in. It has nothing to do with what type of mettering device you are using.

The things you want to remember about the importance of SH are:

1. At the evaporator you want some SH to ensure you are getting maxium heat absorbation from your evaporator

2. You want SH at your compressor to ensure you are pumping vapor through your valves. Remember vapor compresses, liquid does not.

ogden
05-14-2008, 12:42 PM
here's where my confusion sprouts from:

REQUIRED SUPERHEAT CALCULATOR
(Cooling,NON-TXV)

and on the flip of the calculator

SUBCOOLING CALCULATOR
(Cooling, TXV)

this was published by carrier and also in one of their service manuals,
somewhat verbatim:

".....TXVs maintains a constant superheat over a wide range of conditions.Because of this, charging using the SH method cannot be done with systems that contain TXVs or similar devices.":confused:

powell
05-14-2008, 03:05 PM
Hypothetical situation:

W/I cooler, condensing unit on the roof, 50ft away. No valves on the evaporator.

Here's my 2c worth in this situation.

Measure the suction pressure at the compressor and assume a 1/4 lb pressure drop per 90 degree elbow in the suction line coming from the evaporator.

Just a SWAG rule of thumb that should get you closer to the actual evap pressure. Also hope that the suction line was sized properly.

gas_n_go
05-15-2008, 08:36 PM
here's where my confusion sprouts from:

REQUIRED SUPERHEAT CALCULATOR
(Cooling,NON-TXV)

and on the flip of the calculator

SUBCOOLING CALCULATOR
(Cooling, TXV)

this was published by carrier and also in one of their service manuals,
somewhat verbatim:

".....TXVs maintains a constant superheat over a wide range of conditions.Because of this, charging using the SH method cannot be done with systems that contain TXVs or similar devices.":confused:

Ahhh ok, thats a different question.

Without getting too technical these are the basics to what your confused about.

In order for a TXV to modulate corectly you need to have a full column of liquid going to the valve. This is why you need to check your subcooling for A txv.

Basically the SH side of the txv is mostly for the purposes of efficency and protection of your compressor.

1 your compressor is a vapor pump and will not compressor liquids.

2 propper sh at your evaporator tells you that your getting maxium heat absorbation

3 propper sh at your compressor is part of giving you good pump efficency

Those are the three main ideas to what I think your asking.