Can someone give me a good guide on how to measure suereat and and subcooling when commissioning?
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Can someone give me a good guide on how to measure suereat and and subcooling when commissioning?
Are you a trainee?Quote:
Originally Posted by marc5180
No i just never beenexpained how to measure superheat, call i poor traning
So how have you managed to commission any kit in the past? - I'm not being facitious by the way - just curious
Hi frank i am a trainee and due to star college in September can i have a first step guide to superheat measurements.
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)
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:confused:
Thanks for the quick reply frank but I understand what superheat and subcooing are i just dont know how to measure them
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.
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?
Thanks guys
Cheers US Iceman ive saved that to my laptop.
Thanks
Wizzer:)
Another free benefit of belonging to this forum.:cool:
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!!:D
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?
That's the two temperature method.Some people use this method.Quote:
Originally Posted by wizzer
I prefer to use pressure temperature method as others have explained, as would most people.
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
:) 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?:confused:
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 :)
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
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.
Hmm? :rolleyes: 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!!" :D
Thanks for your reply. I'm looking forward to learning and understanding. I agree with your view that refrigeration is both challenging and fun.:confused: :)
Hi Frank,
Very nice explaination. Cheer up.
Well Thanks :DQuote:
Originally Posted by S.K.VARDE
back to normal now :)
Had a few beers and a couple of good nights sleep ;)
Don't forget to allow for glide in R407c systems
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
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?
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.
Cheers
Ok cheers lana
what about centrifugal machine? Does superheat or subcooling matters?
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 :)
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.
Can the size of the reciever affect subcooling?
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
You then read pressure where you can and take in consideration pressure drop from evaporator to measuring point.
are there charts used to calculate this pressure drop?
I don't know, I use CoolPack software and equivalent lengths for fittings and experience.