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Superheat Pages: 12

Jim Blackwood on Sun July 04, 2010 12:18 PM User is offline

Year: 1980
Make: Triumph
Model: TR7
Engine Size: 231ci
Refrigerant Type: 134

What would it do if we decreased the superheat when we replace the TXV? For instance, going from a 9.9 preheat (4 seasons 38604) to a 5 superheat (38613). Thanks,


Edited: Sun July 04, 2010 at 12:19 PM by Jim Blackwood

Jim Blackwood on Mon July 05, 2010 11:19 AM User is offline

Out of 42 people nobody has any idea? OK granted maybe I haven't waited long enough, but here is my impression, I'm sure some of you, including professionals in the trade know more about it than I do.

So if the superheat (SH) is the number of degrees the core temp has to change before the TXV increases it's flow (I think I saw that somewhere) then reason would seem to dictate that for a 2 ton 10* SH TXV to reach maximum flow the core has to be 10* hotter than the body of the TXV. Maybe that's incorrect but I have to make some assumptions here to get to the result.

If that much is true then it would seem that an identical 5*SH TXV would hold the core temp 5* colder than the above unit, which would translate to a colder air discharge as well as faster response to changes in core temp.

The downside might be a greater tendency to ice up the core, and possibly even some sort of oscillating feedback loop causing the TXV to open and close repeatedly.

Am I getting close?

So in a 2 ton system installed in a convertible English sports car (1980 Triumph TR7) where air leaks a re numerous, heat is plentiful, and insulation is sparse, and icing would seem a rather remote possibility, would there be any downside at all in using the lower superheat TXV?

Thanks for your consideration of this slightly unusual engineering oriented question.


Jim Blackwood on Thu August 26, 2010 10:29 PM User is offline

OK, so maybe I've worn out my welcome on this site?


emsvitil on Thu August 26, 2010 11:47 PM User is offlineView users profile

Originally posted by: Jim Blackwood
OK, so maybe I've worn out my welcome on this site?


More likely no one has a clue of what superheat is.................


bohica2xo on Fri August 27, 2010 2:41 AM User is offline

Wow. 15 posts, and already whizzing on everyone here.

Well Ed, since you know all about it - why not spend the time to answer Jim's question? Perhaps explain in detail the pros & cons of a flooded evaporator?

Maybe post some pics & detailed instructions on how to adjust the superheat on a TXV so Jim does not have to buy another valve?

After all "no one has a clue of what superheat is................."


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

SeppW on Fri August 27, 2010 6:30 PM User is offline

I'm not a thermodynamics engineer nor did go to AC school, so I wouldn't know. personally, I'd return the system to original with British Leyland parts, if they're still around. There's alot of stuff on the WWW about though, superheat that is.

TR7, I haven't seen one of them in years. I had a Spitfire back in the 70's, loved it. Always wanted a TR6 though, rarely see them around anymore either.

But I digress.

Dougflas on Fri August 27, 2010 9:10 PM User is offline

If you lower the superheat, you are allowing more refrigerant to enter the evap. This will allow more of a chance of liquid refrigerant to get to the compressor. Remember, TXV systems use a drier after the condenser. Office systems use an accumulator which is before the compressor to prevent liquid refrigeant from entering the compressor. Now let's ook at airflow. At high speed, the superheat is at 10 as an example. As airflow drops, (lower speed), superheat will decrease. A blocked evap due to dirt and leaves, will also lower superheat. Can you see where this is going? We are losing our safety factor of liquid entering the compressor. This is the reason why automotive systems do not erly on superheat for charging and troubleshooting. Superheat is changing depending upon multiple factors such as low ambient temperature (decrease in SH),dirty condenser or loss of condenser airflow (SH increases).

iceman2555 on Fri August 27, 2010 9:55 PM User is offlineView users profile

I take exception to the statement that no one here knows what SUPERHEAT's the extra heat that SUPERMAN feels when he gazes at Lois Lane with his x-ray vision......dontchya wish you had x-ray vision my would properly result in a distortion of my glasses.....ahhhh what the's hell getting 'more advanced in years'!!!!

Seems this post 'got-by' when it was originally presented.
However, seems that the idea of this post....if the SH is decreased...will the cooling efficiency of the evaporator be increased. Seems this is the idea behind the CCOT system. Total removal of SH from the evap and thus reduce the possibility of 'hot spots' in the evaporator. Many will debate which system performs better...the control of the TXV or the full flow of the orifice tube and clutch cycling. It is a given that both system, when working properly, perform to designed characteristics and cool to the customers satisfaction.
Noticed that some of the later model vehicles are changing back to the TXV systems. One of the major changes in these new systems is....guess what...the SH values. We are not seeing SH values in the 3 degree range. An almost flooded evap and the control of a TXV system. Changes in the system, underhood temps, compressor locations (located in heat laden areas) may allow for the decrease in this 'insurance' against accidently allowing liquid refrigerant into the compressor.
From my understanding, the idea of SH in a mobile AC system was to allow sufficient heat absorption in the evaporator to insure that all refrigerant exiting the evap was in a true gaseous form. Of course, I could be so far out in left field that the inning could be over and I would never know....but then that happens already....must be all that exposure to R12 over the years.....
If more efficient cooling is required in this vehicle....why not simply insulate the passenger cabin to the best of your ability and insure that the AC system is totally operational. If the retro is not cooling as expected...and this could easily be the case with this system...why not go back to 12...the system was designed for it and the system will perform better. If I remember correctly, these units were not known for allowing an abundance of air flow thru the condenser/radiator.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

emsvitil on Fri August 27, 2010 11:00 PM User is offlineView users profile

I first saw this thread about a month ago (after it had been posted, and was buried)

At the time, I had just come across the term superheat and searched the forum trying to figure out what superheat was.

Didn't find much on the forum, and this thread was unanswered.

So when the original poster reposted, I made my smart ass answer to see if it would wake up the thread............

Seems to have worked.

and if you're wondering, I don't know enough about superheat to answer the question.


NickD on Sat August 28, 2010 5:42 AM User is offline

Superheat relates to the difference in the temperature within the evaporator based on the theoretical evaporation temperature of the refrigerant based on its corresponding inlet pressure. With R-12 at 28.5 psi, it would be 30*F, varies considerably for different refrigerants and the actual temperature measured on the surface of the evaporator core. That of course would be much warmer due to the thermal resistance between the refrigerant and the core itself. Say if the core temperature was 37*F, the superheat would be 7*F.

The core temperature is also a function of the amount of warmer air flowing through the evaporator, the greater this warm air, the greater will be the temperature of the core, thus the greater the superheat.

Used primarily in HVAC systems as with an oil sump within the compressor, one can play with the inlet pressures as not concerned about recirculating lubricant as in practically all MVAC systems. Another item to play with is the blower speed that is normally running at a fixed speed, but this is becoming history now as greater overall efficiency can maintained by varying both the speed of the compressor that varies pressure and the speed of the blower motor itself. Briefly, no sense in running an AC at its full operation speed on a cooler day as opposed to a much hotter day, just wasting energy.

Really not much you can do with an MVAC system, the key to lowering superheat is to use an evaporator with greater heat transferring surfaces that for all practical purposes requires a larger size evaporator. It won't fit, plus practically all MVAC systems use a variable speed fan. A building AC does remove moisture from the interior allowing lower pressures, can't do that in a vehicle as to keep you from dying several complete air changes per minute are required with a fresh source of moisture always present limiting the degree of low pressure operation, the evaporator core would freeze up. Plus the condenser sees a much wider change in temperatures due to engine heat and traffic conditions, therefore a lot of extra sensors are used in MVAC systems that actually shut down the compressor in the event pressure limits are exceeded, both ways either high or low.

Superheat simply does not apply to MVAC systems, this subject was covered many times before as was the use of HC refrigerants in MVAC systems.

Bottom line is the only variable you really have in a given systems is to affect pressures with charge levels, in a CCOT system you are better off to charge low at high ambient temperatures, but this would result in excessive cycling at lower temperatures where you should be adding more refrigerant, but will pay the price at greater ambients as your system may blow up if your safety devices are not working. Same holds true with either a POA or variable displacement compressor, in theory they operate the same where pressures are continuously variable due to ambient conditions. So you are just better off to charge by weight supplied by the manufacturer of that system, they already played with all of the variables.

Information provide here is brief, but can be found in much greater detail in an AC engineering handbook. AC is over a hundred years old now, you be shocked as the number of different refrigerants developed over that period of time. But today, severely limited by the EPA in application with the advantage of simplifying the process of what you can and cannot do.

Performance vehicles typically suffer from poor AC simply due to lack of space, if that is your concern, roll down the window or select a vehicle noted for good AC. Converting a system to use a refrigerant it wasn't designed for is another mandated problem, easiest solution is to purchase a newer vehicle.

NickD on Mon August 30, 2010 6:36 AM User is offline

Did I type all this for nothing with even a bad hand? Not even a "who cares" for a response? OP was quite insistent for a response, but all common knowledge anyway available from many sources, even those old fashion books.

Going for a long trip today to see another specialist to look at my right hand. Even finally after 60 years of this stuff purchased a pair of mechanics gloves. Your hands are your most important tools. Took me this to really realize that.

ACProf on Mon August 30, 2010 8:10 AM User is offline


Sorry to hear about your hand. Hopefully a hand specialist can help you out. I have lived with a "twitchy" right hand myself since I was 16 when I got the palm of my hand stuck between a Chevy 350 engine and its motor mount when the "cherry picker" engine crane operator (another 16 year old) dropped the engine. Have a lot of remaining nerve and tendon damage that nothing can really be done.

Interestingly enough, the wife (of 43 years) says that that hand "has a mind of its own" when I'm sleeping.

I now wear polypropolene (sp?) gloves when working machinery. Keeps the burns, cuts, and minor scrapes down to a minimum.

Hope the specialist has good news for you.

On a side note: To truly understand superheat takes more than a casual knowledge of AC systems. A complete understanding of the causes and results of changes of STATE and latent heat vs sensible heat is required. A firm grasp of the physics of thermodynamics is necessary to apply it.

Although I have taught college level Newtonian physics for 10 years (including thermodynamics), superheat is not a concept that is easily understood. Certainly difficult to explain to satisfaction in a few paragraphs here.

I had an AC "mechanic" once argue with me that if the temperature of the output pipe of the evaporator is the same as the pipe going in, no cooling could possibly happen. He called it "rediculous" to assume that.

I tried to explain to him the physical laws of heat absorbtion to bring about a change of state from liquid to vapor at the SAME temperature (latent heat, of course) but he wasn't having any of that.

I used "common ground" and asked where the additional heat went thats needed to change water at 212 to steam at 212. He said "that's different". End of discussion. He really didn't WANT to know.

I read your response and found it to be right on target, although the response (or lack thereof) pretty well indicates the answer is more complex perhaps for a "want to know" rather than a "need to know" audience. Though I admire your attempt.

Its like fielding the question, "how does a nuclear power plant work" and trying to explain the principles of energy release of nuclear fission in several paragraphs.

HECAT on Mon August 30, 2010 10:10 AM User is offline

I appreciate what you wrote Nick, it confirms my thoughts that although the principles should not be disregarded; such HVAC principles of Superheat, Subcooling, and tonnage ratings have little place in MVAC work.


HECAT: You support the Forum when you consider for your a/c parts.


iceman2555 on Mon August 30, 2010 10:41 AM User is offlineView users profile

Nick good luck with the 'hand doc'...sure hope he is able to help.
After many years working in my family radiator shop...burns that are too numerous to count....acid sores (they hurt like hell) and stains that took years to removed, I too have been wearing gloves. Had a tech laugh at me a few weeks ago when I was in his shop conducting a evaluation for a series of failed units. But then he was a 'young buck' and we all know that nothing bad ever happens to you when you are young. Spent many nites in the dark jungle...all the time thinking I was invincible...yet there were those my age dying every minute. Invincibility is gone is a second.
The correct statement...take care of them...they are the only ones you will receive and for a tech...they are priceless.
We had a very good tech that worked for us...he was a natural mechanic...could do most any repair placed before him. He became 'hooked' on 'crack' and within several months the tips of his fingers had begun to die...rot...turn black...could not feel a thing. Spoke with a doc and he told me that this was not an uncommon issue with certain drugs. Sorry to say, he finally lost his ability or desire for work and simply drifted away.
All this crap about us not responding and when we one appears thankful....canyabelieve it. Damned if you do...damned if you don't.....
Total agreement with Karl....what difference does it make for the average tech....there are more important issues to be concerned with in the repair of a system....first and foremost...getting the darn thing charged correctly.
Ya'll have fun our there today.......

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

NickD on Mon August 30, 2010 5:49 PM User is offline

Realize this is off topic, new doc thinks after my carpal tunnel surgery, instead of freeing up the affected nerve, the second nerve leading into my hand also got blocked. Wants to do more electro tests to verify it. This may drag on.

emsvitil on Mon August 30, 2010 9:29 PM User is offlineView users profile

I liked the responses.........

Wasn't quite over my head and understood part of it....

(water was at eye level and I had to bob up and down to breath......)


HECAT on Tue August 31, 2010 6:20 AM User is offline

A carpal tunnel surgery comeback; not good. Sorry to hear this problem still lingers.


HECAT: You support the Forum when you consider for your a/c parts.


NickD on Tue August 31, 2010 6:56 AM User is offline

Originally posted by: HECAT
A carpal tunnel surgery comeback; not good. Sorry to hear this problem still lingers.

Encouraging part of this doctor, his final words to me were, we are going to find the problem and fix it. Discouraging part is he is going to start at ground zero so I will have to live with this for another four months. In his opinion, the problem is repairable, so that gives hope.

300zxer on Tue August 31, 2010 7:36 AM User is offline

Well, I didn't think I would learn anything out of this thread, but I did. I am more familiar with superheat in the high pressure steam in power plants. For anyone who has been around steam turbines, they know you must have dry steam going into the high pressure turbine or moisture will erode the blades. So, typically we would shoot for 50 degrees of super heat. Sounds like the same issue with the evaporators, so, go in with enough "dry refrigerant" or superheat to keep from sending any moisture to the compressor. Thanks for the lesson. It also made me think more about expansion valve systems versus orifice tube systems.

Logan on Sat September 11, 2010 2:58 AM User is offline

The expansion valve basically controls the volume of refrigerant entering the evaporator coil. This is accomplished by controlling the superheat leaving the evaporator. a properly sized and adjusted expansion valve will adjust to the varying evaporator load and frosting conditions, allowing the evaporator to function at maximum capacity. The expansion valve should be selected for the compressor capacity and not the evaporator capacity.

NickD on Sat September 11, 2010 5:49 AM User is offline

Would actually prefer the term, differential than super heat, more accurate in function. For a long time using that super adjective was quite common, everything was "super", even Superman. A more common word I kept on hearing today is awesome, could equally call it awesome heat.

Actually, the prime function of the expansion valve as oppose to a fixed orifice was to decrease the cycling rate of the compressor. Nothing more than constantly switching the compressor on and off continuously beat the hell both out of its drive train and the compressor itself.

If they left in the POA regulator and bypass valve in along with an oil sump in the compressor, wouldn't have to cycle the compressor period. But why b!tch about it, more maintenance work, unless you are the consumer paying for it.

Jim Blackwood on Mon October 03, 2011 1:56 PM User is offline

How about Epic Heat? Nick, I hope you have made good progress with your hand. Sorry to be so tardy, sometimes I just lose track of things. But I still appreciate the posts above and the effort to type them. I enjoyed reading them and may have even gotten an idea or two through this thick noggin. (As words go, is "noggin" better than "Epic"?)

Anyhoo, I did get that system working and it does pretty well. I thoroughly cleaned everything out, don't recall what I used for the TXV but it was probably the stock replacement.

I was intrigued by the statement that the TXV is sized for the compressor output. Never would 'a thunk it.


ice-n-tropics on Mon October 03, 2011 7:56 PM User is offline

I did A/C repair on a TR8 with the 215 in3 Buick alum engine after everyone else gave up on it. Just accessed the TXV inlet and removed the plugged inlet screen and recharged.
The exit of a txv is a liquid spray with maybe less than 10% liquid (by volume compared to vapor/gas).
@10 deg F SH:
Evap SH only begins after the liquid refrigerant is all basically evaporated. For example, if a evap has 10 tube passes in a circuit, the liquid may be boiled off after 8 passes with a 10 deg SH txv.
@5 deg F SH:
With a 5 deg txv, the liquid mass flow would increase so that the liquid was boiled off after 9 passes.
This means that w/ 5 deg SH, very effective latent heat exchange would extend through the 9th tube and the low effectiveness, sensible heat, would be reduced to just the 10th exit tube.
Evap BTU delta:
In other words, decreasing the SH has little to no heat exchange effect on the first 80% of the evap. (in this example). The increase in heat exchange is only basically due to liquid mist in the 9th tube or about 1 to 7% more BTUs (assuming the compressor and condenser capacity is greater than the evap BTU capacity) . This is explained and illustrated in the book, "How to Air Condition Your Hot Rod".
If you see white frost at the compressor inlet (due to liquid exiting the evap), you have too little SH and can damage the compressor.

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

Edited: Mon October 03, 2011 at 7:58 PM by ice-n-tropics

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