Super heat

[knotbuggin]

what should my superheat be for 404a at 38 f box temp

[Tremblay]

For med temp. 6 to 8 Degree of superheat as a general rule. OEM sometimes publish a value.

[Btech]

Hello, I am new to the site, but just wanted to clarify that your superheat is being taken at the evaporator out let and not the compressor inlet??

[Capz]

Hello, I am new to the site, but just wanted to clarify that your superheat is being taken at the evaporator out let and not the compressor inlet??

Yes, that is correct. At the bulb location for the TXV to be exact.

[Btech]

For med temp. 6 to 8 Degree of superheat as a general rule. OEM sometimes publish a value.

I agree, make sure box is within temp range 10 degree max..

[mseng]

On a medium temperature application the 6-8 is correct. The closer you get to the 6 the better the efficient the unit will run saving the costumer energy.

[Tremblay]

The problem with Superheat is that you can only get a range. As technician we tend to have a fix value but there are too many variables: TD, liquid temperature, evaporator's dynamic.

So a superheat amplitude of 6-8 is the target but it may vary with applications.

[Nic88]

I was told from an old tech in trade school that rule of thumb was to go with 5 degree for low temp applications, 10 degree for med temp applications, and 15 for high temp applications.

[.:JJ:.]

The best way to do this is by checking your superheated at the compressor.
Depending on how far the compressor is from the case or box, the condition of the insulation on the suction line and were the pipe is ran ( overhead under the roof or underground).
This is why superheated does not matter. What matters is the proper cooling of the compressor. I had to work with several systems where the superheat needed to be set to 1 df in order to provide proper cooling to the compressor
Check your comp specs. It's usually 60-65 df max suction line temp at compressor on the medium temp and 25df on low temp

[craig1]

This is why superheated does not matter. What matters is the proper cooling of the compressor.

Superheat at the coil is VERY important. Imagine you have a system with a long piping run in a ceiling under a roof. On a sunny hot day, you check compressor superheat and find it too high. To compensate, you open the TXV farther. Now compressor superheat is good. However, you may have just set the valve to 0 degrees superheat and are feeding liquid into the suction line. Under the current operating conditions this is OK because it al vaporizes before getting to the compressor. However, as soon as the temp in the ceiling drops, the load on the line is going to go down and you will have floodback.


Its important to set the TXV at the superheat recommended for the application, then verify the superheat at the compressor is within manufacturers recommendations. If it is too high it should be addressed with additional insulation, a desuperheating valve, or other measures.

[bunny]

The best way to do this is by checking your superheated at the compressor.
Depending on how far the compressor is from the case or box, the condition of the insulation on the suction line and were the pipe is ran ( overhead under the roof or underground).
This is why superheated does not matter. What matters is the proper cooling of the compressor. I had to work with several systems where the superheat needed to be set to 1 df in order to provide proper cooling to the compressor
Check your comp specs. It's usually 60-65 df max suction line temp at compressor on the medium temp and 25df on low temp

I will respectfully disagree with your position here.

The TEV is not designed to regulate superheat at a location that is downstream from the sensing bulb location. It is impossible for any valve requiring temperature and pressure to regulate its adjustment to maintain a condition somewhere other than where it is sensing temperature and pressure at.....that would be the height of lunacy to expect a valve to regulate at some location in the system other than where it is sensing the condition it is attempting to regulate.

In addition, there are so many variables (load, ambient conditions, etc) that will change the refrigerant mass flow requirement necessary to properly feed the evaporator that what might seem to be a decent compressor superheat at one operating condition might be extremely low at another....again, the TEV is not designed to regulate superheat at a location other than the bulb location.

To repeat.....the TEV is designed to maintain superheat at the bulb location only. Once this is set correctly, IF the compressor superheat is too high it must be addressed in another manner such as liquid injection.

[2sac]

I will respectfully disagree with your position here...

.....that would be the height of lunacy...

To repeat.....the TEV is designed to maintain superheat at the bulb location only. Once this is set correctly, IF the compressor superheat is too high it must be addressed in another manner

This

[bunny]

This

Well, don't beat around the bush. If you've got a problem with the content of my post then spell it out.