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Thread: Superheat
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27-06-2006, 06:45 PM #1
Superheat
Can someone give me a good guide on how to measure suereat and and subcooling when commissioning?
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27-06-2006, 07:45 PM #2
Re: Superheat
Originally Posted by marc5180
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27-06-2006, 07:53 PM #3
Re: Superheat
No i just never beenexpained how to measure superheat, call i poor traning
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27-06-2006, 08:09 PM #4
Re: Superheat
So how have you managed to commission any kit in the past? - I'm not being facitious by the way - just curious
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27-06-2006, 08:37 PM #5
Re: Superheat
Hi frank i am a trainee and due to star college in September can i have a first step guide to superheat measurements.
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27-06-2006, 09:06 PM #6
Re: Superheat
Superheat is the term for the amount of heat added to a fluid above its phase change point.
Water boils at 100C at sea level atmospheric pressure and changes phase to a vapour. If you continue to add heat energy to the vapour, the vapour becomes superheated. So, for water vapour (steam) at 105C you could say that 5K of superheat has been added.
It is easier to understand the term when refering to water if you are not conversant with refrigerants and their boiling points.
Subcooling can be understood as the opposite of superheating. If you have steam at 100C and you cool it so it becomes condensate, then any temperature below 100C (at sea level pressure) can be refered to as subcooling. Water at 95C has been subcooled by 5K.
Superheat - the amount of heat energy added above saturation point (phase change)
Subcool - the amount of heat energy removed below saturation point (phase change)
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27-06-2006, 09:21 PM #7
Re: Superheat
Correct me if i'm wrong but does that mean for me to measure superheat at the evap i have 1 probe on the expansion side of the TEV and 1 probe at the coil exit in to the suction line, and is this with the evap fans on.
always eager to learn from good enginners.
regards
Wizzer
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27-06-2006, 09:25 PM #8
Re: Superheat
Thanks for the quick reply frank but I understand what superheat and subcooing are i just dont know how to measure them
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27-06-2006, 09:48 PM #9
Re: Superheat
To measure the superheat at the evaporator (amount of heat energy added to the refigerant after phase change) you need to determine the refrigerant pressure (using gauges), look this pressure up on a refrigerant properties chart (pressure versus temperature) and then measure the pipe temperature at the same point.
The pipe temperature will be higher than the chart value. Subtract one from the other and this will be the superheat reading.
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27-06-2006, 09:57 PM #10
Re: Superheat
To measure evaporator superheat:
1) Record the actual temperature at the TXV bulb.
2) Record the evaporating pressure at the TXV bulb.
3) Convert the evaporating pressure to temperature by using those handy pocket pressure/temperature cards. These cards/charts show the saturation pressure/temperature relationship for those refrigerants.
4) Subtract the temperature you converted on the pressure/temperature card from the actual temperature you recorded at the TXV bulb.
5) The difference is the actual evaporator superheat.
To measure subcooling:
1) Record the compressor discharge pressure at the condenser or receiver.
2) Convert the discharge pressure to temperature by using the pressure/temperature cards.
3) Record the actual temperature of the liquid leaving the condenser.
4) Subtract the liquid temperature (at the condenser) from the converted temperature (from the discharge pressure using the pressure/temperature card).
5) The difference is the amount of subcooling.
Does that help?
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27-06-2006, 10:05 PM #11
Re: Superheat
Thanks guys
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27-06-2006, 10:15 PM #12
Re: Superheat
Cheers US Iceman ive saved that to my laptop.
Thanks
Wizzer
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27-06-2006, 10:40 PM #13
Re: Superheat
Another free benefit of belonging to this forum.
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27-06-2006, 10:40 PM #14
Re: Superheat
Hi Guys,
Don't be afraid to look around the web for information from manufacturers and suppliers.
One such site which always has good information is run by Sporlan and a technical file which will answer some of your current questions, with pictures , can be found here..
http://www.sporlan.com/10-135.pdf
Sorry, forgot to say that being from the USA you will find the measurement units in imperial. Something else for you to learn, conversion tables!!Last edited by Brian_UK; 27-06-2006 at 10:42 PM. Reason: Added final paragraph
Brian - Newton Abbot, Devon, UK
Retired March 2015
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28-06-2006, 12:43 AM #15
Re: Superheat
I work mainly with domestic refrigeration but am very interested in understanding the topic under discussion. After determining superheat/subcooling how do you alter the values to desired amount? Indeed, how do you determine desired value? Are adjustments made by refrigerant charge alone once everything else is correctly installed?
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29-06-2006, 11:24 AM #16
Re: Superheat
Originally Posted by wizzer
I prefer to use pressure temperature method as others have explained, as would most people.
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29-06-2006, 12:36 PM #17
Re: Superheat
When taking your superheat temperature from the evap, your TXV superheat reading should come within the 4-7K range... If its under 4K your letting too much refrigerant into the evap, same goes if its over 7K your starving the evap.
With subcooling temperatures at the condensor, "correct me if im wrong" the subcooling should be between 2-5K over the condensor. Any less than 2K start looking at air flow restrictions on the coil, anymore than than 5K, low refrigerant charge, over sized condensor.
Hope this helps
Dale
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29-06-2006, 11:31 PM #18
Re: Superheat
Thanks Actinide. You mention solutions to subcooling readings being outside 2k - 5k, increase gas charge, reduce condensor size or improve air flow over condenser. If superheat readings are outside 4k - 7k how would you reduce/increase refrigerant into the evap. assuming condensor and evap are correctly matched and installed, drier not blocked and tev working correctly?
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30-06-2006, 08:53 AM #19
Re: Superheat
Superheat adjustment on TX Valve. Adjust the screw on the Valve itself. Let the system run and settle for about 3-5 minutes and re-check your superheat.
Good luck
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30-06-2006, 09:59 PM #20
Re: Superheat
Why is this important? Superheat can tell you if there is liquid still at the end of the evaporator. It is important so that you know that no liquid goes to the suction line of the compressor. Subcooling will tell you if you have too much or not enough refrigerant.
Fastco
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02-07-2006, 12:59 AM #21
Re: Superheat
Thanks again Actinide and Fastco. I expected your answer to include gas charge adjustment. Bytheway, if the SH reading was outside the 4 - 7k would you then expect the SC reading to be outside the 2 - 5k.
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02-07-2006, 09:36 AM #22
Re: Superheat
Hmm? Well I guess no not really, checking your subcooling and superheat with give you results of some sort. What they are will determine your next step to resolving what possibilities the fault may be.
All depends on the situation and the faults occouring at the time of being diagonsed. "This is why refrigeration is so fun, Keeps you thinking!!"
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06-07-2006, 02:34 AM #23
Re: Superheat
Thanks for your reply. I'm looking forward to learning and understanding. I agree with your view that refrigeration is both challenging and fun.
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08-07-2006, 03:10 PM #24
Re: Superheat
Hi Frank,
Very nice explaination. Cheer up.
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09-07-2006, 04:42 PM #25
Re: Superheat
Originally Posted by S.K.VARDE
back to normal now
Had a few beers and a couple of good nights sleep
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10-07-2006, 12:12 AM #26
Re: Superheat
Don't forget to allow for glide in R407c systems
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18-05-2007, 11:52 AM #27
Re: Superheat
At site, u can do one thing for calculating the superheat value : Note down the suction pressure & the temperature of suction line at evapprator coil outlet. Note note the evaporation temperature at the noted suction pressure, the superheat will be the difference between the suction line temperature & the temp. of evaporation at suction pressure.
Rgds
Mahesh
+919911299628
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19-05-2007, 11:53 AM #28
Re: Superheat
if you turn the screw on the expansion device half a turn how many degress would you expect it to rise or fall? Whats the general rule of thumb per turn?
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20-05-2007, 06:05 AM #29
Re: Superheat
Hi there,
As far as I have read, there is no rule of thumb for this. Every manufacturer gives the info.
Also this changes with the model of TEV. For example for TE2 (Danfoss) half a turn gives some superheat change and for TE5, TE12.. (Danfoss), half a turn gives another change in superheat.
CheersEven Einstein Asked Questions
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20-05-2007, 09:44 AM #30
Re: Superheat
Ok cheers lana
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08-01-2008, 05:41 PM #31
Re: Superheat
what about centrifugal machine? Does superheat or subcooling matters?
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08-01-2008, 09:39 PM #32
Re: Superheat
Hi, rajesh
....always, but thermostatic expansion valve is a superheat control and some subcooling would be expected at the condenser outlet....thus have nothing with the type of compressor
...of course you can damage your compressor either with liquid on inlet or running it with high discharge temperature...
....maybe there can be some other problems related to centrifugal compressors, but that I do not know...
Best regards, Josip
It's impossible to make anything foolproof because fools are so ingenious...
Don't ever underestimate the power of stupid people when they are in large groups.
Please, don't teach me how to be stupid....
No job is as important as to jeopardize the safety of you or those that you work with.
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08-01-2008, 11:10 PM #33
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09-01-2008, 03:54 AM #34
Re: Superheat
Are you being rude for the sake of it or have you got a point to prove?
Have you actualy read the posts?
If you have then you would of noticed that the post was back in June 2006 and Frank went on the to give very good advice.
I don't normally bite when people like you post, I tend to think its better to let the little boys play on their own.
But I can't quite make my mind up about you,
You either have a misguided point to prove or you are just a little boy playing games.
Frank needs no help from me or anyone but quite frankly there is no need for rudeness.
taz.
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16-01-2008, 03:47 AM #35
Re: Superheat
Can the size of the reciever affect subcooling?
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16-01-2008, 06:57 PM #36
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17-01-2008, 12:31 AM #37
Re: Superheat
I have question about this procedure of measuring the super heat at the evaporator outlet side . I see where some will instruct to check the temperature on the surface of the copper tube at this point to determine refrigerant pressure. They will also instruct to check the refrigerant pressure at this point. Now I have observed a number of installations of a/c units in homes, and don't see a place coming out of the evaporator to hook up the gauge? I am looking at mostly factory pre-cased coils. So where or how do you take a low side reading at this point when the manufacturer does not provide a gauge port? Thanks,stiz
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17-01-2008, 12:46 AM #38
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Re: Superheat
You then read pressure where you can and take in consideration pressure drop from evaporator to measuring point.
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17-01-2008, 01:27 AM #39
Re: Superheat
are there charts used to calculate this pressure drop?
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17-01-2008, 01:45 PM #40
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Re: Superheat
I don't know, I use CoolPack software and equivalent lengths for fittings and experience.
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21-01-2008, 05:01 AM #41
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21-01-2008, 05:19 PM #42
Re: Superheat
The subcooling tells you if there is not enough refrigerant (low subcooling) or too much refrigerant (high subcooling) in the high side of the system.
The superheat tells you if there is not enough refrigerant (high superheat) or too much refrigerant (low superheat) in the low side of the system.
If there is not enough refrigerant in the high side (low subcooling) and not enough refrigerant in the low side (high superheat), then there is not enough refrigerant in the system (undercharged).
If there is too much in the high side (high subcooling) and just enough or too much in the low side (normal to low superheat), then the system is overcharged.
If there is not enough or just enough in the high side (normal to low subcooling) and too much in the low side (low superheat), then the metering device is overfeeding or the compressor is inefficient.
If there is just enough or too much in the high side (normal to high subcooling) and not enough in the low side (high superheat), then the system is restricted/underfeeding.
None of the above is accurate unless you have sufficient airflow through both the evaporator and the condenser, so always check the airflow first.Last edited by Gary; 21-01-2008 at 05:54 PM.
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21-01-2008, 05:38 PM #43
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Re: Superheat
Last edited by nike123; 21-01-2008 at 05:41 PM.
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21-01-2008, 05:59 PM #44
Re: Superheat
Indeed, if I were teaching a course on system trouble shooting, I would not allow my students to use gauges until somewhere near the end of the course.
They would become airflow experts first.
They would be taught how to measure saturation temperatures without gauges.
And then they would be taught to measure saturation temperatures with gauges.
And heaven help the student who tells me what the pressures are. I want to know what the saturation temperatures are. Trouble shooting is about temperatures, not pressures.Last edited by Gary; 21-01-2008 at 06:20 PM.
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21-01-2008, 06:13 PM #45
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Re: Superheat
My first approach to system is "face in wind and hands on pipes", than "sniffing, hearing, touching" and than gauges.
When we modify some of terms, we could say "approach to system as you would approach to adored woman". First some foreplay, and than penetration! With right foreplay, maybe you don't need penetration at all.Last edited by nike123; 21-01-2008 at 06:19 PM.
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21-01-2008, 06:51 PM #46
Re: Superheat
The general rule is: TXV's almost never go out of adjustment. Assuming it was adjusted properly to begin with:
If the superheat is high then:
a. The evap air in temp is high or...
b. The refrigerant charge is low or...
c. The inlet screen is plugged or...
d. The TXV is undersized or...
e. Somebody has been screwing around with the adjustment.
If the superheat is low then:
a. The airflow through the coil is insufficient or...
b. The system is grossly overcharged or...
c. The compressor is inefficient (broken valves) or...
d. The TXV is oversized or...
e. Somebody has been screwing around with the adjustment.
Sadly, the most common reason I have found for adjusting TXV's is "e".
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21-01-2008, 10:10 PM #47
Re: Superheat
hi guys please do not shoot me down in flames but i have been a member off this forum now for best part off 2 years and every week it seems that the question off superheat appears.its almost that people think that superheat is the be all and end all off refrigeration i have been in the trade 18 years 14 as an engineer and have never once worrried about superheat now i am not having a go at anyone for asking the question but it seems people worry about superheat ask the forum then get bamboozeled with the science when people try to explain. not trying to upset any one but dont worry about it
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22-01-2008, 09:05 AM #48
Re: Superheat
Waw, what a long sentence.
It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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22-01-2008, 10:43 PM #49
Re: Superheat
i have been in the trade for 25 years but didn't start thinking about superheat until 9 or 10 years ago. i don't know how i managed to fix anything and make the repair last. In commercial refrigeration the emphasis wasn't(and still isnt) on the scientific side of the business, but on how many calls you can do in a day. When i was younger i used to pride myself on doing as many calls as i could. Now i tend to concentrate on properly fixing the 3 or 4 calls i now attend each day.
What made me change?
Well gather round and i'll tell you.
It was a chance meeting with a retired engineer who was shopping in the store where i was working. I was having a problem setting a tev in a freezer case. Despite working through a 5 year apprenticeship i couldn't recall ever using eithe a guage or thermometer to set up a tev. I was always tought how to do it with the frost test; that is if the suction line is frosting back to the compressor, then shut the valve in until it stops frosting, then you would be in the right area. and this teaching came from a well respected engineer. The retired engineer (i never found out his name) told me how to measure superheat properly (and what is more impportant why); when i saw how simple and obvious it was i was amazed, and began to start questioning all i had been tought. i got as far as i thought i could with logical thought then started buying books. eventually i found this forum and others like it.When you see the kind of questions posted and the depth of knowledge and experience in some of the answers, it must make a lot of engineers realise how little a lot of us know. My advice to any members of this forum is not just to use it to ask questions for your own benefit, but every once in a while flick through the forums to see if anything catched your eye. If it doesn't, select a question at random you never know, something you think you know all about may just be being questioned, and it might just make you think. For those who like me thought they knew enough to do a good job try it somebody elses way for a change; you never know, it might just be better than the way you've been doing it for years.
oh and the question is, if you've never bothered about superheat then how can you tell if the system is efficient, if you have a full refrigerant charge, if the tev is correctly set, if the compressor is receiving the correct amount of superheat to maintain it's cooling, if there is no liquid returning to the compressor, if the evaporator is defrosting correctly, if the controls are correctly set for the systems best and most efficient operation. in these times of energy efficiency drives, anybody who doesn't understand superheat really should ask the question: what kind of engineer do i want to be?
i'm not shooting you down coolhibby because i used to be just like you, but it is time now to start thinking for yourself, use this forum and any other sources that are available to you to begin to become an engineer.
sorry about the length everybody but i thought the point needed to be made
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22-01-2008, 11:08 PM #50
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