Measuring superheat with no access at evap

[HiltonMeatHead]

Morning all

As title, i'm trying to measure the superheat on 3 seperate systems that feed 1 common AHU. Howeve the install engineers did no put in any access ports at the evap outlet. I have access immediately before and after the TXV. The compressors are approx 20mtrs from evap with bends and each system has a suction accumulator.

What do you guys use as a rule of thumb given the pressure drop at the compressor suction service valve?

[Yuri B.]

The SH is always further increased in such cases due to at one hand pressure decrease due to pressure drops in the pipe and at another hand temperature increase in them. The only reliable way to measure the latter is when the sensor is placed in direct contact with refrigerant.

[shooter]

measure the temp just after the TXV valve and on the suction header. the difference is your SH.
roughly but managable.

[Rob White]

.

There is no real rule of thumb that I'm aware of but you
seem to realise that there will be a discrepensy between
what you read at the comp and the actual.

For the ideal measurement you need to access both pressure
and temp at the outlet of the evap but if you can't then measure
your pressure at the comp knowing that the errors will fall to the
safe side becase of the pressure drops and distances between the
evap and comp.

If you need to do it and you need to be as accurate as possible the
only way would be to conect some schradder fittings in each evap and
then you will know that you are measuring at the correct spot.

Regards

Rob

.

[HiltonMeatHead]

Thanks for the replies Gents. I have always just used common sense when measuring SH when i have no access immediately after the evap and taking my pressure reading at the compressor.

I'm just getting a little confused with 3 separate systems supplying 1 evap and if i need to take the other systems in to account when taking reading from 1 system

[sterl]

The TXV's do not have an external equalizer or are we talking some other form of liquid feed?

Unless the temp difference between air and refrigerant suction is so high that the coils don't have to be anywhere near balanced to keep them all loaded: you will never get all the capacity out of all 3-condensing units....the one operating at lowest SH will always pull more moisture than the others and thus work hardest. or longest, depending on the nature of the rest of your controls...

[HiltonMeatHead]

The TXV's do not have an external equalizer or are we talking some other form of liquid feed?

Unless the temp difference between air and refrigerant suction is so high that the coils don't have to be anywhere near balanced to keep them all loaded: you will never get all the capacity out of all 3-condensing units....the one operating at lowest SH will always pull more moisture than the others and thus work hardest. or longest, depending on the nature of the rest of your controls...

All 3 systems have their own external equalizer lines.

Can you please elaborate on the pulling moisture part as the reason i actually posted this thread was because the AHU keep icing up. I can't find any pattern or correlation between weather, ambient temp, washdown of production area etc that is pointing me in any particular direction. I have recently brought the Humidifier coil back in to use and this did seem to make a difference by pulling some of the moisture out the air before it is drawn over the main coil.

If you need any measurements etc give me a shout

[r.bartlett]

beer can cold as they say

[Done it all]

One way is to measure and set the superheat at the compressor suctions, go for about 12° there instead of 6° at the evap. As long as your suction line insulation is good. Not ideal but better than guessing. At least you will get them all about the same.
Icing up is caused by evaporating temp too low, excessive run time with no off cycle, or airflow too low (among other things).

[THE DUDE]

measure the temp just after the TXV valve and on the suction header. the difference is your SH.
roughly but managable.
Shooters on the money. The temp on the TXV outlet is your Saturated Temp. You may have a slight pressure diff on the outlet but it should get you very close.