Superheat

[marc5180]

Can someone give me a good guide on how to measure suereat and and subcooling when commissioning?

[frank]

Can someone give me a good guide on how to measure suereat and and subcooling when commissioning?

Are you a trainee?

[marc5180]

No i just never beenexpained how to measure superheat, call i poor traning

[frank]

So how have you managed to commission any kit in the past? - I'm not being facitious by the way - just curious

[dunny123]

So how have you managed to commission any kit in the past? - I'm not being facitious by the way - just curious

Just answer the question you tit!!
If you carnt then dont ask a quetion back!!!

[taz24]

Just answer the question you tit!!
If you carnt then dont ask a quetion back!!!

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.

[wizzer]

Hi frank i am a trainee and due to star college in September can i have a first step guide to superheat measurements.

[frank]

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)

[wizzer]

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

[frank]

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.

[Temprite]

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

That's the two temperature method.Some people use this method.

I prefer to use pressure temperature method as others have explained, as would most people.

[crit71]

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

Yes Wizzer...you are correct. Forget about all the science behind it for now, aim for 6 degrees SH and your laughing.

[martyone]

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)

It is absolutely right the way you explained the question about superheat.

[ismail.mch]

Hi Frank,
I have installed Cold Room its set point in -22 C but the compressor does not off except defrosting time means remains on and some times temperature reaches to -18 C I measured its super heat which details are below.
Suction line pressure 20 PSI convert to PT chart R-22 temperature is -5 F
Suction line temperature is 71 F so that according to calculation super heat is (71 - (-5) ) = 76 F ? (71-5) = 66 F ?
In both condition the super heat is too high so my question is that in cold room what super heat is ideal at evaporator and compressor side as in A/C
Compressor side Super heat 20~25 is ideal kindly reply I am really confused.

[marc5180]

Thanks for the quick reply frank but I understand what superheat and subcooing are i just dont know how to measure them

[US Iceman]

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?

[marc5180]

Thanks guys

[wizzer]

Cheers US Iceman ive saved that to my laptop.

Thanks

Wizzer

[US Iceman]

Another free benefit of belonging to this forum.

[Brian_UK]

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!!

[forry]

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?

[forry]

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?

[Actinide]

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

[Fastco]

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

[forry]

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.

[Actinide]

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!!"

[forry]

Thanks for your reply. I'm looking forward to learning and understanding. I agree with your view that refrigeration is both challenging and fun.

[S.K.VARDE]

Hi Frank,

Very nice explaination. Cheer up.

[frank]

Hi Frank,

Very nice explaination. Cheer up.

Well Thanks

back to normal now

Had a few beers and a couple of good nights sleep

[John Wood]

Don't forget to allow for glide in R407c systems

[engineer_mahesh]

Can someone give me a good guide on how to measure suereat and and subcooling when commissioning?

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

[marc5180]

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?

[lana]

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

[marc5180]

Ok cheers lana

[Gary]

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?

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".

[rajesh]

what about centrifugal machine? Does superheat or subcooling matters?

[Josip]

Hi, rajesh

what about centrifugal machine? Does superheat or subcooling matters?

....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

[william_wye]

Can the size of the reciever affect subcooling?

[nike123]

Can the size of the reciever affect subcooling?

Yes, but marginally.

[stiz]

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

[nike123]

You then read pressure where you can and take in consideration pressure drop from evaporator to measuring point.

[stiz]

are there charts used to calculate this pressure drop?

[nike123]

I don't know, I use CoolPack software and equivalent lengths for fittings and experience.

[Gary]

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.

[nike123]

None of the above is accurate unless you have sufficient airflow through both the evaporator and the condenser, so always check the airflow first.

I just have sensed sudden urge to color this statement in red, because I think that is most important thing that someone need to check before any other action on RAC system.

Gary, these few sentences should became first law of troubleshooting of RAC systems.

[Gary]

I just have sensed sudden urge to color this statement in red, because I think that is most important thing that someone need to check before any other action on RAC system.

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.

[nike123]

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.

[dan wong]

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.


None of the above is accurate unless you have sufficient airflow through both the evaporator and the condenser, so always check the airflow first.

Hi Gary,
How does a tech know if he has proper air flow?. For discussion purpose: I will use this example; a Walk in freezer, approximately twenty years old, unknown brand - label is long gone.

Technician notice: On evaporator; one fan motor is 1/15 HP 1550RPM, the other fan motor is 1/20HP 1500RPM. both fan blade is 14", 4 blade, approx. 23 pitch. Question; how do you determine if you have proper air flow?
Further: On condensing unit: one fan motor is 1/10 HP, 1600RPM, one motor is 1/8 Hp 1600 RPM. How do you know if you have proper air flow?

[coolhibby1875]

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

[Peter_1]

Waw, what a long sentence.