Superheat for the umpteenth time

[Mish]

My apologies in advance, shoot me down for being a wally!


I was trying to set up a new system today and was checking my superheat. The system runs on R22. I took the suction pressure at the evaporter outlet and converted to temperature and i got
-16 degrees C. I then took the actual pipe temperature after the T.E.V but before the evaporter and got 10 degrees C.

Now do i subtract 10 from -16 or am i getting this the wrong way round.

Any help will be much appreciated, and i know this subject has been covered at length already, just need a kick in the right direction.

[Peter_1]

You have to measure temperature at the outlet of your evaporator and subtract.
Temperature and pressure must be measured on the same place to know SH on that place.

[Mish]

Thanks Peter.

So im subtracting and looking for a temperature difference of between 4-7?

[TRASH101]

You are supposed to measure the pipe temp at the same place you take the pressure reading then subtract your temp.(from the pt conversion) from the pipe temp.

[Gary]

My apologies in advance, shoot me down for being a wally!


I was trying to set up a new system today and was checking my superheat. The system runs on R22. I took the suction pressure at the evaporter outlet and converted to temperature and i got
-16 degrees C. I then took the actual pipe temperature after the T.E.V but before the evaporter and got 10 degrees C.

The refrigerant is boiling off at -16C. For it's temperature to rise up to 10C, it must rise 16 degrees to reach zero and then another 10 degrees to reach 10C. It rose a total of 26 degrees. Your superheat is 26K.

[TRASH101]

I then took the actual pipe temperature after the T.E.V but before the evaporter and got 10 degrees C.

Sorry Gary but it hasn't had a chance to be superheated by the evap. yet!

I would gladly send some glasses in exchange for a set of your tech books.

[Mish]

Hi Gary i was hoping for a reply from you.
Yeah thats what i made my SH reading at, 26k. I thought that seen as it was so high it was not the correct reading.
I understand i am looking for a reading of between 4-7k.
Iv determined i have the correct charge of refrigerant from taking my subcooling and have achieved a measurement of 3k.
I have good airflow through condenser and evaporator so just need to tweak SH to get optimum efficiency from my system.

[Gary]

Sorry Gary but it hasn't had a chance to be superheated by the evap. yet!

I would gladly send some glasses in exchange for a set of your tech books.

Good catch.

If I had some glasses I might be able to read your second paragraph.

[Gary]

I then took the actual pipe temperature after the T.E.V but before the evaporter...

As Trash101 correctly pointed out, you are measuring flash gas temp (not superheat) and this tells you absolutely nothing useful.

Superheat is measured at the outlet of the evaporator.

[Brian_UK]

Just an aside to this thread but how come you are installing a new R22 system ?

[TRASH101]

Just an aside to this thread but how come you are installing a new R22 system ?

Its obviously taken months to install

[TRASH101]

And for pities sake don't tell Multisync

[Magoo]

Mish.
asper previous, the pipe temp reading is at the thermo buld position.
To check correct setting first establish the evap TD., air on versus evaporationg pressure converted to temp. Super heat should be 60>70% of that calculation, wait 15 mins if adjusting TEV.
Example: air on at -5'C evap press/temp -16'C , Evap TD is 11 degrees, by 65% = nominal 7 degrees superheat. Evaporating -16 suction pipe temp should around -9 'C , can be checked during pulldown or at design.

[Mish]

I have not installed a new R22 system. It was an existing system that wasnt able to cope with a lower temperature we now want to run at.
The system consisted of 1 compressor and 2 evaporators. I split the system and added a new compressor pack running on R404A.
The systems now supply 1 evaporator each.

Thank you all for your help

[chillyblue]

Mish.
To check correct setting first establish the evap TD., air on versus evaporationg pressure converted to temp. Super heat should be 60>70% of that calculation, wait 15 mins if adjusting TEV.
Example: air on at -5'C evap press/temp -16'C , Evap TD is 11 degrees, by 65% = nominal 7 degrees superheat. Evaporating -16 suction pipe temp should around -9 'C , can be checked during pulldown or at design.

Great info mate