understanding superheat.

[The_Apprentice]

I'm a first year apprentice. From what I gather, to measure superheat, you find your suction line pressure, convert that to its temperature using PT charts, find your evaporator temperature directly after the coil, subracts this from first temperature, and thats superheat. This tells me the amount of heat absorbed throughout the evaporator coil.

How do you know the correct superheat rating for any given system? What determines that value? When measuring the suction pressure, most commonly the only access fitting is directly off the compressor. How large a role will that be in getting a correct value of SH?

[mad fridgie]

I'm a first year apprentice. From what I gather, to measure superheat, you find your suction line pressure, convert that to its temperature using PT charts, find your evaporator temperature directly after the coil, subracts this from first temperature, and thats superheat. This tells me the amount of heat absorbed throughout the evaporator coil.

How do you know the correct superheat rating for any given system? What determines that value? When measuring the suction pressure, most commonly the only access fitting is directly off the compressor. How large a role will that be in getting a correct value of SH?
Superheat is the difference between the "actual temperature" (where you measure it with a thermometer) and the temperature it should be at that pressure (using gauges and you PT chart)
The difference is the supheat value (up to this point your are sort of right)
At this stage dot get confused with duty of the evap. (that will come later)
just know that you do need some superheat.
How much "this where it can be difficult to get a straight answer" My advice is you do not want to drop below 6C super at the compressor, (normally it would in many cases (but not all) higher than this.
You should be asking why do we need superheat!
You can not compress a liquid, so we do not want liquid enetering the compressor, how do we know there is no liquid, if the refrigerant is superheated!
To much superheat causes the discharge to become very hot which can damage the oil and the metal within the compressor

[The_Apprentice]

So the purpose of superheat is not to know the amount of heat being transferred into evaporator? But whether there is liquid leaving the evaporator?

[mad fridgie]

So the purpose of superheat is not to know the amount of heat being transferred into evaporator? But whether there is liquid leaving the evaporator?
You can calculate energy absorbed by an evap, by using superheat as part of the calculation. (more advanced than your present level)
Refrigeration is a very big feild, so giving the right answer depends upon which area you are working in.
On DX systems with TEV valves, superheat is used to control refrigerant flow.

[Brian_UK]

I tend to think of compressor suction superheat as a safety measurement.

Get it wrong and the liquid going into the compressor could do serious damage.

Get it right and you can breathe a little easier. ;)

[Quality]

Super heat is one of the very first things I was taught how to measure, then motor current, sub cooling, discharge temp + air volumes & temps

It is quite like a race horse result feed it the wrong stuff and it will not give the proper result

I have nothing to do with horses but I think its quite similar
I think so anyway;)

[The_Apprentice]

Thanks for the replies. Measuring superheat seems pretty straight forward. Where I get lost in translation is just how important it is, beyond liquid returning to the compressor.

Sorry if it seems like I'm not getting it. so what I understand is, superheat is purely a measurement of the temperature increase created by the boiling of the low pressure liquid after your metering device. The importance of superheat is that with too much, your evaporator is not being used efficiently, witch too little you have an excess of liquid which could get in the compressor.

[nike123]

There are more in it.
We have evaporator superheat.
We have suction line superheat.
We have total superheat.
We have discharge superheat

Evaporator superheat should be as low as possible to utilize more of evaporator. How much? That depend on TD of evaporator and it should be like 0.65*TD.


http://www.kueba.de/en-us/Tools/K%C3%BCba-Expansion-Valve-Calculator/Pages/default.aspx

Suction line superheat adds to evaporator superheat. That is heat added in suction lines.

Total superheat is sum of evaporator superheat and all superheat added in suction lines and maybe suction line heat exchanger (if necessary). That superheat value should be as much high as possible to ensure that there is no liquid at compressor entrance in all working conditions of equipment. It is usually stated in compressor manufacturer literature what is preferred total superheat value. Usually, it is 20K.
In same time, that value should be kept low enough to prevent high discharge superheat which tells us how good or bad is compressor motor cooled.
If total superheat is to high compressor will be overheated. Discharge superheat is directly related to suction (total) superheat. And it should not be higher than 30K.

Try search this board with "superheat" key word and you will find lot of educational material and details.

Also you should start with this thread:

http://www.refrigeration-engineer.com/forums/showthread.php?t=19701

[taz24]

Evaporator superheat should be as low as possible to utilize more of evaporator. How much? That depend on TD of evaporator and it should be like 0.65*TD.




I agree with what you say but we have little say in the
designed superheat setting of most TEV's, because they come
preset to between 6 and 8 degs of superheat.

This is a factory setting and we normally do not adjust the factory setting.

So in principle the Superheat out of the evap could be less than one deg K
but normaly it will be nearer 6 to 10 degs K.

All the best

taz

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