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

[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:confused:

[marc5180]

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

[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.

[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.:cool:

[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!!:D

[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?

[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:confused:

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.

[Actinide]

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

[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?:confused:

[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? :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

[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.:confused: :)

[S.K.VARDE]

Hi Frank,

Very nice explaination. Cheer up.

[frank]

Hi Frank,

Very nice explaination. Cheer up.

Well Thanks :D

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

[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 :)

[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.

[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.

[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.

[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.:D:D

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.:D:D

[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".

[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.

[bill1983]

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.:eek: 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. :D
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):cool:;); 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

[nike123]

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

OK, what is your approach to diagnosing refrigerant cycle?

[bill1983]

i know you're not firing the question at me and i'm not saying i know it all.:( Nobody does, we are all here to learn. but the way i usually start if the fault is not immediately obvious, is to listen to the sound the system makes. Next i look at the condenser airflow and the evaporator, because without the correct airflow you cannot achieve the correct transfer. after that it is a question of taking temp/pressure readings for superheat/sub cooling to determine what the system is doing and as an indicator of where i shoud go next to diagnose the fault.:D

[bill1983]

or i could just check the fault code and follow the flow chart

[Peter_1]

Bill1983, I found your post remarkable, I enjoined reading it and I will translate it to Ducth and will show it to my students in class.

[nike123]

Bill1983,
You are right, I was quoting coolhibby1875. Your thoughts are similar to mine.

Cheers

[bill1983]

Bill1983, I found your post remarkable, I enjoined reading it and I will translate it to Ducth and will show it to my students in class.

does this mean i'm getting something right after all these years, or are you going to use my post as an example of how to ramble on and on and on
as my daughter would say lol

[chillyblue]

Hi all

i have one for you!!!
how can you have subcooling unless your reciever is full and the liquid is then backing up into the condensor??? Also if this is the case that the liquid is backed up into the condensor, what happens when the system pumps down ( no space left other than in the condenser), i believe it would trip on HP!!!
the way everybody explains subcooling is that the liquid is further cooled below it's condensation temperature, (correct) the only method i have seen of proper subcooling is by cooling the liquid exiting the reciever not before, by means of further air cooling, water cooling or refrigerant cooling. why would the liquid sit in the condensor to get further cooling (subcooling)?????

CB

[Gary]

Hi all

i have one for you!!!
how can you have subcooling unless your reciever is full and the liquid is then backing up into the condensor??? Also if this is the case that the liquid is backed up into the condensor, what happens when the system pumps down ( no space left other than in the condenser), i believe it would trip on HP!!!
the way everybody explains subcooling is that the liquid is further cooled below it's condensation temperature, (correct) the only method i have seen of proper subcooling is by cooling the liquid exiting the reciever not before, by means of further air cooling, water cooling or refrigerant cooling. why would the liquid sit in the condensor to get further cooling (subcooling)?????

CB

The static gas laws state that you can't have superheat in the presence of liquid and you can't have subcooling in the presence of vapor.

Are the static gas laws wrong? No. The key word here is "static", meaning non-energized. As soon as you hit the start button, you are working on a dynamic (energized) system and the static gas laws no longer apply.

For example, take a jug of refrigerant, place it on a block of ice and put a heating pad on top. You now have subcooled liquid on the bottom, superheated vapor on the top and saturation at the vapor/liquid interface... all in the presence of each other.

The receiver doesn't have to be full of liquid to have subcooling, so long as there is flow through the receiver.

Whatsmore, you can and will have vapor bubbles in a subcooled liquid line and liquid droplets in a superheated suction line. Depending upon velocity, the vapor bubbles in the liquid line disappear at about 10-15F subcooling and the liquid droplets in the suction line disappear at about 5-10F superheat.

[mohamed khamis]

The subcooling effect is mainly occured in the condenser if it is designed to have a portion of area to subcool the refrigerant. The liquid receiver is acted like a collector for the subcooled refrigerant and ishould be designed with a volume of 125% of refrigernat capacity to avoid the flashbacking of the liquid to the condenser.

The subcooling degree depends on the condenser design and the varitions in the operating conditions for the system regardless the recevier is full or not. And sometimes the refrigerant leaves the condenser in subcooling form but it is flashed into the recevier due to excessive pressure drop in the liquid line. That means the refrigerant in the receiver consistes of liquid+gas although it leaved the condenser in subcooling form.

Cheers

[chillyblue]

Hi Gary

correct me if i am wrong, you are saying if the system is in operation or the liquid is moving then it can be subcooled, if the liquid is not moving it can't be subcooled????
Superheat is easy as you can normally come across large superheats, but with subcooling you only normally can measure a few deg C.
I in all my years as a fridge engineer have rarely experienced subcooling, perhaps i'm doing something wrong???
I have never had the pleasure of large deg of subcooling as it always seems to be limitted to high discharge pressures.

CB

[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:confused:

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

[Gary]

Hi Gary

correct me if i am wrong, you are saying if the system is in operation or the liquid is moving then it can be subcooled, if the liquid is not moving it can't be subcooled????
Superheat is easy as you can normally come across large superheats, but with subcooling you only normally can measure a few deg C.
I in all my years as a fridge engineer have rarely experienced subcooling, perhaps i'm doing something wrong???
I have never had the pleasure of large deg of subcooling as it always seems to be limitted to high discharge pressures.

CB

Convert high side pressure to temperature. Measure liquid line temperature. The difference between the two temperatures is subcooling.

[Gary]

There always seems to be a lot of confusion about subcooling and superheat.

First to clarify:

Boiling point = boiling temperature = evaporating temperature = condensing temperature = saturation temperature. These are all different ways to say the same thing. These all refer to the temperature at which a change of state occurs, whether liquid to vapor or vapor to liquid. Let's just call it saturation temperature.

Every fluid has a different saturation temperature.

Saturation temperature (for any fluid) rises with a rise in pressure.

Pressure/Temperature charts tell you the saturation temperature for the current pressure of the fluid you are working with.

Any liquid that is below its saturation temperature is subcooled.

Any vapor which is above its saturation temperature is superheated.

[panas]

Hi!
I started in the refrigeration field on my 40 some months before. I try to work properly in spite I have no enough learning-books. I was glad to find this site and was interested this discussion.
I work at domestic fridgers which have the only capillary tube (without receiver and TXV). Could somebody explain if the described upper ( by Gary) theory of using sub cooling\super heating for diagnostic of frigers is the same for capillary tube systems? Any tips for domestic systems in this term - what is there difference for? Thank you and sorry my English.

[mohamed khamis]

Hi Chillyblue




Superheat is easy as you can normally come across large superheats, but with subcooling you only normally can measure a few deg C.


Both the superheat and subcooling has a limitations but the superheat degree is much bigger than that for subcooling, the reasons are:

1- the main target of the superheat is to fully protect the compressor from liquid floodback not to the limit to cause the decrease in compressor refrigernat capacity.

2- The more subcooling results in an enhancement in TXV capacity as a result of the increase in entering liquid density. However, the signifacnce of liquid density is rather small with decreasing in liquid temperture (more subcooling). Becus the nature of liquid density is quiet independent of its temperture not like gases. Alternatively, the more subcooling poses an enlargement into refrigerating effect. Accordingly, the refrigernat velocity inside the evaporator is relatively small results in deficiency in oil back to the compressor. So two competiting effects at a work.

The net outcome is the more subcooling leads to a slight enhancement in TXV capacity with damping in refrigerant velocity inside the evaporator.

3- the specific heat for the vapor is half the value of that for the liquid and this renders the amount of increasing in superheat degree is almost double the amount of subcooling degree for the same heat capacity portion in evaporator and condenser.





I in all my years as a fridge engineer have rarely experienced subcooling, perhaps i'm doing something wrong???
I have never had the pleasure of large deg of subcooling as it always seems to be limitted to high discharge pressures.

CB

The increase in high pressure sometimes comes from incorrect surface area for the condenser or air volume rate for the fans. So this results in boosting in HP and so less amount of subcoling due to the desuperheating takes place a large portion after the condensation with increasing in HP. I wish it could help

cheers

Mohamed

[chillyblue]

Hi

i am going to start a new thread as there are still some issues i have with the subject of subcooling and condenser draining.

thanks for the input
see you on the new thread

CB

[Peter_1]

2- The more subcooling results in an enhancement in TXV capacity as a result of the increase in entering liquid density. However, the signifacnce of liquid density is rather small with decreasing in liquid temperture (more subcooling). Becus the nature of liquid density is quiet independent of its temperture not like gases. Alternatively, the more subcooling poses an enlargement into refrigerating effect. Accordingly, the refrigernat velocity inside the evaporator is relatively small results in deficiency in oil back to the compressor. So two competiting effects at a work.
The biggest advantage is not the increase in density but the increase in available enthalpy of the liquid in the refrigerant which will increase the evaporator capacity.
There will also less flash gas formed direct after the TEV

[mohamed khamis]

The biggest advantage is not the increase in density but the increase in available enthalpy of the liquid in the refrigerant which will increase the evaporator capacity.
There will also less flash gas formed direct after the TEV

The TXV capacity that I meant is the amount of refrigerant mass flow rate not the cooling capacity. The mass flow rate that is metered by TXV functions in the pressure drop across the TXV and refrigerant inlet density. Hence the more subcooling the more amount of flow rate metered by the TXV as a result of the increase in density and free of flashing effect as u mentioned.

If u have a unit with amaximum cooling capaicty of 20 kW at a certain conditions and u implement a mechanical subcooler to improve the subcooling effect, the question may be raised to what extent u can do subcooling if u have facilty to introduce subcooler coolant with different tempertures to the subcooler?

As it is known the more subcooling the more ample in the refrigeranting effect (outlet evaporator enthalpy - inlet evapotaor enthapy) as a result of the decrease (not increase as u mentioned) in liquid refrigerant entering evaporator. if we plot cooling capcity vs the subcooling degree at a specified conditions, i imagine that the trend will go up with increaseing in subcooling degree. Then at a certain value of the subcooling degree the cooling capcity will almost be fixed at a certain value.

It may be explained by that at the begining the more subcooling the more mass flow rate metered by the TXV plus the more ample in refrigerting effect. This accounts for an enhancement in cooling capacity upon some point. Then after that the more subcooling will result in a decrease in evaporator mass flow of refrigernat to offest the big enlargemnt in refrigerating effect to give maximum cooling capacity should it give by the unit. This a drop in mass flow can lead to decelarate in oil velocity and makes the oil to cling on the tube inner surface. So it is certainly a limit for the subcooling plus the enlargemnt in condenser area if the subcooling mainly occurs there.

Cheers

[Peter_1]

As it is known the more subcooling the more ample in the refrigeranting effect (outlet evaporator enthalpy - inlet evapotaor enthapy) as a result of the decrease (not increase as u mentioned) in liquid refrigerant entering evaporator.
This isn't what I said, I was saying that the liquid entering the TEV will have a bigger enthalpy. and this is correct.
If you have a high subcooling, then you have to take this in account because the TEV will be too big for a given nominal capacity

[Gary]

To avoid confusion, it should be stipulated that Peter_1 and mohamed khamis are discussing the benefits of adding subcooling after the receiver outlet by mechanical means, such as a suction/liquid heat exchanger.

The subcooling at the receiver outlet should not exceed 15F/8.5C, because this backs up liquid into the condenser.

[Peter_1]

To avoid confusion, it should be stipulated that Peter_1 and mohamed khamis are discussing the benefits of adding subcooling after the receiver outlet by mechanical means, such as a suction/liquid heat exchanger.

The subcooling at the receiver outlet should not exceed 15F/8.5C, because this backs up liquid into the condenser.

Gary, the last phrase needs some clarification if you want please.

[Gary]

The subcooling at the receiver outlet should not exceed 15F/8.5C, because this backs up liquid into the condenser.

To clarify:

When you are charging a system, the bubbles in the sight glass disappear between 10-15F/5.5-8.5C subcooling. Continuing to add refrigerant at this point backs liquid up into the condenser, unnecessarily raising the head pressure.

[mohamed khamis]

This isn't what I said, I was saying that the liquid entering the TEV will have a bigger enthalpy. and this is correct.


ok simply to solve the problem the example is a good evidence

If the condensation temperture is of 50C and u have subcooling of 5K and 10K of R134a the liquid enthalpy entering or exiting the TEV in both cases are

at T = 50C ====sat.table==== P = 13.17 bar

case1 P = 13.17 bar & T = 45C (5K subcooling) ====== enthalpy = 112.7 kJ/kg

case2 P = 13.17 bar & T = 40C (10K subcooling) ====== enthalpy = 105.4 kJ/kg

So as it may be concluded that the more subcooling the liquid entering the TEV will have a smaller enthalpy




If you have a high subcooling, then you have to take this in account because the TEV will be too big for a given nominal capacity


Of course, the more subcooling the more TEV capacity over the nominal cooling capacity but this comes from the increase in refrigernat flow passing thru TEV not the difference in enthalpy.

becus the designer of the evaporator will take in his account the difference in enthaply due to the subcooling and calculate the refrigerant mass flow rate from dividing the nominal cooling capacity over the difference in enthalpy. However, the mass flow rate metered by TEV is bigger than calculated by evaporator design. This may result in the comprossor to be floodback in case of excessive subcooling becus the TEV meters signifcantly higher mass flow than it is required by the Evaporator.


Gary, could u give me a reason to why the refrigernat liquid backs to the condenser when there is excessive subcooling as u mentioned over 8.5C

Becus the clarification that u mentioned this comes from the system is overchargging problem not excessive subcooling. And the head pressure is ampled due to the overchargging effect not excessive subcooling.

Could u tell me please on a basis of the abovementioned example of two different subcooling of 5K and 10 K what is happened to head pressure and liquid backing to the condenser

Cheers

[Gary]

Gary, could u give me a reason to why the refrigernat liquid backs to the condenser when there is excessive subcooling as u mentioned over 8.5C

I am talking about subcooling at the receiver outlet, not subcooling at the TXV inlet.


Becus the clarification that u mentioned this comes from the system is overchargging problem not excessive subcooling. And the head pressure is ampled due to the overchargging effect not excessive subcooling.

Yes, I am talking about overcharging, which is evidenced by excessive subcooling at the receiver outlet. Overcharging raises the condensing temperature and therefore the receiver outlet subcooling.

[Saturated]

Keep in mind that high subcooling can also be caused by liquid binding, that is, restriction in the liquid line (which can include TEV wound in too far and capillary line blockage). This is usually found when the TD is over 8K and when the TD is taken from the outlet of the condensor.

[Gary]

Keep in mind that high subcooling can also be caused by liquid binding, that is, restriction in the liquid line (which can include TEV wound in too far and capillary line blockage). This is usually found when the TD is over 8K and when the TD is taken from the outlet of the condensor.

The condenser TD is the difference between entering air temp and refrigerant saturation temp and generally runs much higher than 8K... or did you mean something else?

I have never seen a liquid line restriction cause high receiver outlet subcooling, although I would not say that it is impossible... perhaps with an undersized receiver?

Excessive receiver outlet subcooling is almost always caused by overcharge or noncondensables, with overcharge being by far the most common cause.

[Saturated]

Sorry Gary, I should have specified. Most of my work is on A/C's and don't have recievers, and I use quite a few different temperature difference calculations to diagnose and refer to them as TD's. The TD I was referring to was condensor subcooling (SCT- liquid leaving Condensor temp), though I don't see why it would be too much different for a system with a reciever, (perhaps the reciever absorbs the extra refrigerant without causing a bank up at the condensor like a hydraulic accumulator).

In high ambient areas with high discharge temps it's common to find partial to full blockage of capillary lines, high superheat and high subcooling are the results. I've also heard from a guy working on a supermarket system who picked up a partial blockage in a liquid line buried under concrete from a poor TEV feed and high subcooling. He found the blockage was a poorly welded 90 deg bend that had folded over with the flow over time.

Definately agree with you on the most common cause though, I use both superheat and subcooling TD's when charging an older critical charge system where no refrigerant weight can be found.

[Gary]

Sorry Gary, I should have specified. Most of my work is on A/C's and don't have recievers, and I use quite a few different temperature difference calculations to diagnose and refer to them as TD's.

Perhaps some definitions would help to improve communications:

A change in temperature of a single substance or flow of substance is called a delta-T or dT.

A comparison of the temperatures of two different substances or flows of substances is called a TD.

The difference between the condenser entering air and leaving air is the condenser delta-T as it quantifies the change in temperature of a single flow of substance.

Similarly, the air in/air out difference across the evaporator is the evaporator delta-T.

The condenser TD is the difference between entering air temperature and saturated condensing temperature (SCT). This is a TD in that it quantifies the temperature difference between two different substances.

Similarly, the evaporator TD is the difference between entering air temperature and saturated suction temperature (SST).

Subcooling is a specific type of delta-T, namely the difference between SCT and liquid line temperature.

Similarly, superheat is a specific type of delta-T, namely the difference between SST and suction line temperature.

Condenser approach is a specific type of TD, namely the difference between leaving air temperature and liquid line temperature.

Similarly, evaporator approach is a specific type of TD, namely the difference between the leaving air temperature and the suction line temperature.

Hope this helps. :)

[Saturated]

We do seem to have slightly different terminology, at least the equipment stays the same:D I have the same problem discussing fishing!

What method do you use to take a pipe temperature? When I'm getting serious about setting TEV's I'll attach a thermocouple to the pipe with a nylon zip tie and place insulation around it covering about 200mm along the pipe to avoid air temperature affecting the pipe and the TD. Do you know a more effective and fast method of getting an accurate refrigerant temperature through a pipe? I'm not real keen on the lasers or touch probes as the pipework picks up at least a couple of degrees from the air temp around it.

[mohamed khamis]

Hi there,

There is misunderstanding... the subcooling degree is the difference between two tempertures almost have the same pressure (slightly pressure drop).

However, the restrication in liquid pipeline or blockage the outlet of them is not additional subcooling although its temperture is very low compared with the outlet from the condenser. This is due to the restrication acts like TEV it throttles the flow and hence drops the pressure and temperture signifcantly. This throttling process lies approximetly on a constant enthalpy typically like TEV process. And so the outlet flow from restrication can go to saturation condition with lower saturation temperture and pressure than that for condenser. And causes an improper operation of TEV.

Cheers

[mohamed khamis]

The main reason of increasing the subcooling degree in case of overcharge or noncondensable gases is the increase in the head pressure. The more increase in head pressure the more decrease in the condesation latent heat so the other processes (desuperheating and subcooling) take place the part of condenser surface area which has been left by the decrease in latent heat surface area. So the mount of desuperheat and subcooling is increased and noticed by additional subcooling outlet the liquid receiver.

Howver, this is not favourable method to enhance the system subcooling becus what u gain by right hand is lost by left one. Definitely the increase in head pressure burdens on the compressor power and its charcteristices negating the improvement occuring in refrigerting effect.

Cheers

Mohamed

[Springbok]

Hey guys,it seems that every engineer has their own opinion on the superheat theory.Guess it depends if you're in the domestic, commercial or industrial side of refrigeration too.I did my trade test back in South Africa and it is regarded as one of the toughest tests around.(old school)Probably one of the most important tasks/tests of the entire trade test was based on superheat.Not checking your superheat on your evaporator is like not checking the oil level of thecompressor or the liquid level of the system.All components of the system needs to work and maintained to work properly.....Anyway...just a quick word....

[coolblade]

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

Hello.
Superheat, this is equal to the actual temperature of the gas minus the saturated vapour temperature of the gas.
First you need to know the pressure at the point you want to measure the superheat.
Once you have got this pressure you need to find the saturated vapour temperature of the refrigerant at the pressure measured. This can be obtained off guages or a comparitor
Next you need to measure the temperature of the vapour. This is not always easy to obtain accurately as discussed by others. I personally feel the best way is to use a thermocouple and tape it directly onto the pipe or vessel, then cover it with insulation and tape over the top. The thermocouple should be left in position for at leat 20 minutes prior to taking any reading. Once you have an accurate temperature reading superheat = temperature measured minus saturated temperature

[collo]

Message to Gary, My very real thanks Gary for your measured and knowlegible reply's. Being self and experience taught in much of my refrigeration career, I have been so very careful to know and hopefully understand the problems before sticking my fingers into the works,so to speak. Your answers have been a real help in confirming my own understanding the refrigeration process. I was amazed this past week when helping out a fridgie who had just installed a new air-con. He did not have a clue about superheat and was only interested in "what sort of pressures should I have"? My area was experiencing ambients of 22-24c. while he was experiencing 28-30c. My suggestion was, pressures will alter according to temperature, in a capilliary system he would only have SH or SC to check his gas charge against. What do you say?

[Gary]

Contrary to what most people believe, subcooling and superheat are important indicators in both cap tube and TXV systems.

[lexscripta]

Very interesting conversation on a topic I am having a real problem with.

Q. Can I take an ambient temp around the condenser and then measure the air leaving the condenser for a useful measurement?

Q. What does the discharge temp (at the discharge port after PT conversion) tell me in relation to the ambient temperature?

Q. Someone asked about the temp drop across an evap coil - that is simply taking temp readings at the inlet and oulet - no pressure readings. What would I expect to see in say, a freezer?

Q. I assume subcooling to be the difference of PT chart pressure to temperature at the discharge port of the compressor and sensible heat measurement at the outlet pipe of the condenser (Liquid line).

Q.) Would a suction line accumulator figure into any of these measurements?

I am presently having a problem with a 3hp Copeland lala 031a tac, being converted to oneshot (ICOR product)

The ambient room temp fluctuates 90 -105. 6 compressors in a room with an exhaust.

I am running 260-280 PSI in this environment, and 14psi suction. Superheat is rediculous - over 30, subcooling is low - appears to be 2-4 degrees, as measured at the receiver inlet.

I know, those readings sound like classic low refrigerant charge, but with that discharge pressure, makes me leery to put more refrigerant in there. Seems like I have to fudge controls in that hot room so things don't kick off on HP.

Thats the scoop on how things were set up, and I know that the first thing they ought to be doing is getting that ambient down - which in the summer gets even higher, but the owner has had guys out there working on this stuff for years, and their attitude is that if its cooling, it must be working.

What can I do with that attitude?

Anyhow - suggestions? Answers?

LexScripta

[Gary]

Condenser air out minus air in (cond dT) is an indicator of airflow problems. As the airflow decreases, the dT rises.

Condenser pressure converted to temperature on a P/T chart is called saturated condensing temperature or SCT. As a rough rule of thumb, you might expect the SCT to be about 20-35F above ambient (20-35F condenser TD).

Similar to condenser dT, evap dT (air in minus air out) is an indicator of airflow problems. As the airflow decreases, the dT rises. In a freezer, you might expect the evap dT to be less than 20F.

Subcooling is the difference between SCT and liquid line temp at the receiver outlet. If there is no receiver, then at the condenser outlet.

An accumulator catches any refrigerant liquid overflow from the evaporator, so that liquid does not enter the compressor.

Higher ambient = higher SCT = higher operating costs and shorter lifespan.

[Apel]

Hi all

I see a simple question about superheat, when I start reading it to understand better, I found it extremely informative -truely it is not about only the question asked but many others answers also.Now I feel better thinking HVAC engineer without having too much experience.

Thanks

[majo]

95% of every single refr. engineer are not even interested on how all this stuff works precisely and if you talk to them they all have a different explanation and what you are saying is bulls***!

I've worked for 4 yrs in a company.
The oldest technician over there (15yrs) had built a chamber with a bitzer hexagon who was runnig 3/4 full of ice at -20° chamber temp.
I've told him: isn't SH a bit low?
I've got the answer: silly boy, this is very normal at -20°. Comp is getting suction temps of -30° and off course at -30° you got ice.
Chamber didn't cool down as fast as it was supposed to be and didn't reach setpoint of -30° as supposed.
After looking and looking he then added 2 probes to the and started logging SH as I showed him together with chamber's cooling rate.
Adjusted SH a bit higher and whoops... Ice away just until service valve and cooling speed of the chamber whopping fast down... comp was getting liquid.
And suddenly I wasn't as silly as before...

That was when aproxx...8months at labor after school

[nike123]

whopping fast down... comp was getting liquid.



Does compressor getting or doesn't getting liquid? This is question of survival!;)

[handy andy]

just to say this is my frist time on this forum and suberheat and subcool is needed to get the best efficiancy from any refrigeration system in my view and as we engineer are ment to save companys running cost then we need to do this more often. the one thing that all way got me is enthalpy charts :rolleyes:

[El Padre]

Ciao,

Actual suction temperature (measured at compressor inlet) - Saturated suction temperature(read from gauges) = Superheat (system superheat). Saturated condensing temperature(read from gauges) - liquid line temperature = Subcooling. Super and Sub derive from the Latin for above and below.
I wish that somebody would have told me a few years ago not to be preoccupied with pressures and just look at temperatures!!

Cheers

[Gary]

I've been preaching this stuff since the early 1980's, and yet...

The industry still has this obsession with pressures, and the schools just don't get it.

Sometimes I wonder if anyone is listening.

[El Padre]

I've been preaching this stuff since the early 1980's, and yet...

The industry still has this obsession with pressures, and the schools just don't get it.

Sometimes I wonder if anyone is listening.

I have been mate, I have overlooked water flow rates through chillers especially in the past, your comments on air/water flows have fine tuned my thinking!!

Cheers

[cavalieri85]

well u charge a piston by superheat and a txy by subcooling its very simple to do your gauges should have your saturation temperature that corresponds to the pressure with subcooling your liquid line temp minus your ligid saturation temp gives you subcooling and suction line temp minus suction line saturation temp is your superheat this is very important stuff to know you will not get to far in this trade without mastering this skill to a t for example when you have low suction there are alot of possible problems but low suction psig high superheat and good subcool its almost definitely a bad txv i hope this can help i am a terrible teacher and can show youbut cant explain it very well

[chillyblue]

Reguarding evaporator superheat.

I normally set my superheat exiting the evap at about 10degC i.e. gauge reading -10deg c, evap leaving temp 0 degc. thought this was fine have been doing this for years, the other day i thought if i have a evap td of 8deg, i would need to set my superheat lower than 8 deg c to know that i am fully utilizing the evap. If the evap td is less than 10 deg your superheat must be as well.
is my findings correct????

CB

[Gary]

Reguarding evaporator superheat.

I normally set my superheat exiting the evap at about 10degC i.e. gauge reading -10deg c, evap leaving temp 0 degc. thought this was fine have been doing this for years, the other day i thought if i have a evap td of 8deg, i would need to set my superheat lower than 8 deg c to know that i am fully utilizing the evap. If the evap td is less than 10 deg your superheat must be as well.
is my findings correct????

CB

Let's say your refrigerated space temp is -2C and your SST is -10C. That's 8K TD.

Can you have 10K superheat? No... because the suction line is in a space that is -2C.

There is no way for the suction line temp to rise above -2C (the refrigerated space temp) to 0C, which is what it would have to get to for a superheat of 10K.

And you cannot accurately set the superheat to 8K, either. The superheat must be less than the TD.

[chillyblue]

Let's say your refrigerated space temp is -2C and your SST is -10C. That's 8K TD.

Can you have 10K superheat? No... because the suction line is in a space that is -2C.

There is no way for the suction line temp to rise above -2C (the refrigerated space temp) to 0C, which is what it would have to get to for a superheat of 10K.

And you cannot accurately set the superheat to 8K, either. The superheat must be less than the TD.

Thanks thats as i thought, so if you have a td of 6, your superheat must be less than 6. That a low superheat?? I know in theory there should not be any liquid there but it dosen't leave much room for error.
2ndly why do all TEV manufacturers not mention superheat when setting the valves but only mention hunting, surely a system may not hunt but have a dangerously low superheat (or none at all)

CB

[Tony]

Hi Chillyblue,


2ndly why do all TEV manufacturers not mention superheat when setting the valves

Have you never read the data sheet or information on the box that your TEV comes in?

Its all there. :)

[chillyblue]

Hi Chillyblue,



Have you never read the data sheet or information on the box that your TEV comes in?

Its all there. :)

Yes i've seen that, but if you read Danfoss's fitters notes they say to get the system to hunt then shut down by 1/2 a turn, how does that set the superheat???

CB

[Tony]

Have just read them and yes, you are correct in what you say. They state that procedure will ensure that all of the evaporator will be utilised. It seems they are not after a set figure for the superheat at the end of the evaporator.

[chillyblue]

Thanks Tony

I would have thought a system could hunt with a low or high superheat, hunting only indicates valve opening/closing to much or too irratic, rather than small adjustments

CB

[TRASH101]

I would have thought a system could hunt with a low or high superheat, hunting only indicates valve opening/closing to much or too irratic, rather than small adjustments

I believe you are right CB

If you look at "hunting" as a process then it could be described as the rate of change versus the rate of compensation.

i.e. the TEV can only respond to the result of an action that happend previously.

The valves own build characteristics provide a reasonable dampening effect at a specific band of operating parameters.

The important parameters are
1) quality of refrigerant at the entry to the valve
2) Amount of change to the quality of the refrigerant at the point of sensing.

So in effect the evap will flood before it is starved and will starve before it will flood.

The TEV characteristics and the above parameters will change the degree of flooding and starving.

I hope this helps...............otherwise it is an example of too much time on my hands X tendency to waffle about stuff .

[crit71]

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.:eek: 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. :D
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):cool:;); 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

That old guy was probably an engineer who worked for Troldahl.....it was the 1st thing we were tought :)
I find it quite shocking how many engineers these days seem to have grasped a perverted version refrigeration engineering skills.
Just goes to show....there's a certain lack of training and apprentices these days.

[moonman]

Im new to this forum and ive been going through a lot of them and i find them very helpful i would like to say thanx! but i have a question regarding superheat. i got this W/I cooler R22 system i cant get the superheat right my head pressure is 200psi suction 44psi full sightglass TXV bulb is about the 4 O' clock position, i know on coolers superheat should be 8`F to 10`F. i got to 10 but then it just dropped to -18`F the closest ive got it was 2`F. could it be faulty TXV? or oversized orfice? or what else could it be? what do you suggest i check for?

[Gary]

Are you saying that the superheat dropped to -18F?

I don't think so. Negative superheat is not possible.

[Gary]

44psi = 21F
21 + 8 = 29F
21 + 10 = 31F

At 44psi (21F SST) the suction line temp at the TXV bulb should be 29-31F.

[freak]

SUPERHEAT: find your saturated evaporating temperature by converting your low side pressure on your gauges to a temperature using a pressure-temperature chart for that refrigerant.Now take the temperature of the suction line about 100mm after it leaves the evaporator.The difference between these temperatures is you superheat.
EG: evaporating temp= -2 degrees c
suction line at evap= 4 degrees c
Then superheat= 6K
SUBCOOLING: is very similar,you get your saturated condensing temp by converting your high side prssure to a temperature then measure the line exiting the condenser the difference betwwen these is your subcooling
EG condensing temp= 46 degrees c
line exiting cond.= 42 degrees c
subcooling= 4k
good superheat setting is 6 k
good subcooling is 4 k

[dan wong]

Im new to this forum and ive been going through a lot of them and i find them very helpful i would like to say thanx! but i have a question regarding superheat. i got this W/I cooler R22 system i cant get the superheat right my head pressure is 200psi suction 44psi full sightglass TXV bulb is about the 4 O' clock position, i know on coolers superheat should be 8`F to 10`F. i got to 10 but then it just dropped to -18`F the closest ive got it was 2`F. could it be faulty TXV? or oversized orfice? or what else could it be? what do you suggest i check for?

One of my guy call in with almost identical problem as you describe. Negative super heat?? I couldn't beleived my ear.

I went to site. it turned out, Suction valve was broken - split in the middle, sometime it will pump down to negative, sometime not. (it was a semi-hermatic compressor, we take the head apart and replace the valve. problem solved).

Do a pump down test, see if you have the same problem.

[marc5180]

As Gary says, you can't have negative superheat, more like subcooling which suggests that you have liquid floodback to your compressor.

[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?

[Gary]

Delta-T (dT) is the difference between coil entering and leaving air temperatures. As airflow decreases, dT increases. On a freezer evaporator coil, the dT should be no more than 20F/11K. On a condenser coil, the dT should be no more than 30F/16K. Excessive dT means insufficient airflow.

[Rafael valenzue]

interesting wise and nice people thanks for letting me be in the forum

[yacoub]

Hello all
how do you masuer to superheat foor sabro compresso
thak you

[nike123]

Hello all
how do you masuer to superheat foor sabro compresso
thak you

Same as any other compressor.:eek:

[jumper]

here's a controversial superheat question. As an evaporator coil frosts up does superheat increase or decrease? There's less heat being absorbed but the compressor may pull to a lower pressure.

[nike123]

here's a controversial superheat question. As an evaporator coil frosts up does superheat increase or decrease? There's less heat being absorbed but the compressor may pull to a lower pressure.

That depend on expansion device.

[DrFreezeMD]

On Starting new Equipment, Only check superheat when you are close to Thermostat Setpoint. Under High load and of course your superheat will higher than target range. Do not change Superheat Setting just to change it unless it is really needed. Of course as Gary as stated, Consider Air Flow and Dirty Coils first but than again Dirty Coils on new Equipment shouldn't be happening. But most Air Conditioning Manufacturers are more concerned with Subcooling on charging their systems correctly. If you notice most installation Manuals will have Subcooling Chart but not a Superheat Chart. But simply making a note of Superheat on new Equipment is probably good practice. Are you going to need to change setting on new Equipment with proper sized TXV. Probably not.;)

[venkatesan]

hi,iam new to this site,i want to know the what will be the correct super heat for evaporator and compressor in f or c

[monkey spanners]

hi,iam new to this site,i want to know the what will be the correct super heat for evaporator and compressor in f or c


It depends on the system and compressor, what system are you working on?

Jon :)

[venkatesan]

its a cold room temp is -22'c

[venkatesan]

what are basic tips of an installation engineer before instaling the cold storage panels and pipe routing

[GOODLOYAL]

I learn from you guys, thank you

[VIKAS_AHLAWAT87]

hey sir can u tell me v can measure the super heat with out the gauge as u said earlier so whtz the thing which v can come to find the superheat with out gauge.

[Brian_UK]

You cannot measure superheat without a gauge only temperatures.

[Peter_1]

You could do it with 2 temperature meters (TM): The 1st TM measures the temperature of the first bend where there's no SH realized yet and the other TM measures the temperature on the outlet of the evaporator.

[devlin maguire]

Why are temps expressed in C and superheat in K, I assumed it was Celcius and Kelvin but thats not right is it, and quite apart from How we measure and where what ,in simple terms are the desired figures as far as I can make out it's 6C superrheat and 5 to 8 c subcooling is that right, it,s very near to what I learned
Devlin

[nike123]

http://en.wikipedia.org/wiki/Kelvin

[Magoo]

Hi devlin,
'K used in superheat refers to the delta/ temperature difference. Check the kueba.de web site for super TXV super heat checks and settings.

[Josip]

Hi, devlin maguire :)


Why are temps expressed in C and superheat in K, I assumed it was Celcius and Kelvin but thats not right is it, and quite apart from How we measure and where what ,in simple terms are the desired figures as far as I can make out it's 6C superrheat and 5 to 8 c subcooling is that right, it,s very near to what I learned
Devlin

see this ....

http://www.refrigeration-engineer.com/forums/showthread.php?22316-K-Definition-reqd

http://www.refrigeration-engineer.com/forums/showthread.php?23821-Negative-superheat

you can search RE forums too .... for Kelvin, SC&SH ... a lot of posts ..

Best regards, Josip :)

[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.

[Tradewinds]

Hi Ismail,

I try to get the superheat between 4-8K when working in Celcius. This will let you know that the evaporator is fully flooded with refrigerant but you have enough superheat to make sure the refrigerant is in a vapor state when it enters the compressor.

For a freezer room you should be evaporating at least -25'C. This could just be short of gas but you would need to take more readings and find out what your sub cooling value is. That should also be between 4-8K.

Low subcooling + High Superheat = low gas charge

Also check for clean condenser with correct fan speed. Did you remove all the air with a vacuum pump before charging with refrigerant?