View Full Version : useful & non-useful Superheat

08-05-2003, 12:39 PM
Hi All,

Can anyone pleasec give me some information on useful and non-useful superheat as i have several books that explain superheat but do not mention useful & non-useful.

I am 4 days away from my exams and need some indepth info on this subject.

Any help very much needed and appreciated.


Prof Sporlan
08-05-2003, 04:23 PM
The term "useful superheat" could be construed in a number of ways. The Prof will assume it to mean low side superheated vapor which provides useful cooling. This being the case, then refrigerant vapor in the evaporator that has a superheat less than the operating TD (temperature difference) of the coil will provide useful cooling; TD being defined as the entering air temperature minus the evaporating temperature.

Any superheat the refrigerant vapor picks up in the suction line would not be useful.

If a liquid-suction heat exchanger is used, then the added superheat picked up by the refrigerant vapor to cool the refrigerant liquid could be considered useful. But then we can get into complicated issues as the added superheat can reduce compressor capacity.

08-05-2003, 07:25 PM
Hi John

I was taught that superheat is required to prevent liquid returning to the compressor, i.e. the refrigerant should be a vapour as it enters the compressor (H1 - H2 on the P-H chart). 3 degrees K superheat is sufficient for this. Non-useful superheat is determined as anything above this 3 degrees K (H2 - H3 on the P - H chart). So if you have 6 degrees K superheat, 3K is useful and 3K is non-useful. Anything above useful superheat just adds to the compressor load and therefore reduces the efficiency of the system.

What exams are you taking?


09-05-2003, 07:37 AM
Thanks to you both, the information is very useful.

I have managed to find alittle more info myself that explains as you state that non useful is superheat that can be gained through the pipe work, as the gas flows friction is created and extra heat, which is classed as non useful.


I am taking my first year HNC in refirg & A/C.
At first i was finding it a llitle difficult as i am 31 and going back to college after a long period away has taken a little getting back into, but the other guys at college and the tutor have been a great help, and i now find myself very confident and feel this course was the best move i have made.

I have been working with refrigeration for 11 years, but it has been on non-domestic equipment for the entertainment industry.
smoke machines that produce a similar effect to dry ice but without all of the H&S problems.

Once again thanks for your help:)

09-05-2003, 05:51 PM

Well done on having the conviction to return to studying. Take heart as I took my HNC when I was 45 - just a few years ago!

Good luck with the exams


15-05-2003, 04:02 AM
To put it simply... useful superheat is that which is picked up in the refrigerated space. Non-useful superheat is that which is picked up outside the refrigerated space, reducing system efficiency.

16-05-2003, 09:57 PM
Hi John

I took the HNC a couple of years ago, at about the same age as you are. I was trying to catch up with the knowlege levels displayed by senoir technicians, as I started late in the service side of the industry. Man, that Theory of Refrigeration was hard stuff at times! Air conditioning, especially the pschometrics was worse, though. It definitely helps if you've worked on the type of systems you're studying, as it's incredibly hard to understand equipment you've never seen. What college are you at?

21-05-2003, 12:21 PM
Hi freezer,

I am studying at Grimsby College in lincolnshire.

Like you i have found the theory a little tough at times but i have persisted. I had my A/C exam on Monday, it was tough but i felt good by the end of it.

I have had no experiance with A/C units so trying to understand them without working on them was bewildering. Its the solar gains, U factors and all of that sort of thing that i didnt realise existed.
I though fit it into the area and away you go, how wrong!!!!

Cheers for now!!

JohnM M

02-08-2003, 09:05 PM
I finished the HND at Grimsby last june.

Is Andy Still there? Who's taking the Refrigeration unit now as I heard that the chap who took us left.

How have the exams gone this year, got thermo and fluids to look forward to yet?

If you need a hand with anything give me a shout.

03-08-2003, 11:11 AM
Just finished the H.N.C at Basford hall college, we haven't had to take any final exams as the course is based on constant assessment, this entails finishing a large amount of assignments to prove that you understand what has been supposedly taught to you.
Most often you are given a subject to look up and learn about and then the tutor gets a chance to slag off most of what you write about it:D .
I think i would have liked the chance to have final exams as i feel short changed, is my H.N.C worth as much as yours:confused:

04-08-2003, 08:01 AM
Hi Alex & Jesper

Yes Andy is still there doing his usual pretty useless job. Going round the houses trying to explain systems, but not really succeeding.
He has been abit of a struggle as I and several of the other guys have had to show him how to lay things out in some kind of sequence.
Not really looking forward to thermo or maths but never mind.

Not sure who the new guy is a we never meet him but someone did leave.

Just got my results from Andy, Merit + all round. Pleased with that.:D

Even got a Merit + in design which was surprising as it is a team effort and getting people to work in a team was difficult.

Lots of the younger guys (18/22) have no experience with Fridge or A/C units and the course doesn’t provide any hands on, so I suggested to Andy that next year the college should have a hands on course and it looks like it is going ahead, this will help people get into the ind a little better.

Well if I need any help I will give you a mail.

Cheers for now
John M

19-08-2003, 02:18 PM