Superheat measurement

[Zilvinas]

Hello guys,

I am young cooling engineer and now I have faced one problem. I will try to explain where is the problem. Earlier we used differential thermometer to measure superheat in evaporator. Now we are measuring by suction pressure on compressor suction valve and temperature out of evaporator.
It looks like we are doing right. It works perfect when we have one evaporator. But how about measuring two different evaporators SH (both has TXV) by using the same suction pressure? I mean one evaporator is slightly bigger than other, so there is correct to measure suction pressure on compressor valve and two temperatures out of evaporators?

[Glenn Moore]

Using a differential thermometer to measure superheat can be very inaccurate unless you measure the evaporator coil temperature in the correct place, the suction pipe temperature is a simple measurement but from the expansion valve outlet through a distributor , through distributor tubes all create pressure drops (P/D's).
The most accurate way is using the suction pressure/ temperature and actual suction pipe temperature.
When you have more than one evaporator, depending how the suction pipe work has been designed , and if it has been designed and installed correctly then the suction line pressure should be the same at the outlet at both evaporators or very close. If the suction pipe runs are very long then there maybe a too large P/D in the suction line which could result in an inaccurate superheat measurement.
To make the install measurement correct a tee piece installed in the equalisation lines of the expansion valves with a schraeder access port where a gauge can be installed during the measurement process will make the superheat measurement as accurate as possible for both evaporators
The sizes of the different evaporators is of no consequence to the superheat

[Rob White]

.

I agree with Glenn.

Remember there are two different measurements for superheat.

There is evaporator outlet superheat which is the most important
to us because that is the one we need to set the evaporator up correctly
and that needs to be measured as close to the evaporator outlet as possible,
with both gauges (for evaporating temperature) and with thermometer.

The other method is total superheat and that is measured at the compressor
and it allows for added heat gained through the suction pipe back to the compressor.

If there is more than one evaporator then they must be measured as close to the
evaporator and individually set.

Regards

Rob

.

[Zilvinas]

Using a differential thermometer to measure superheat can be very inaccurate unless you measure the evaporator coil temperature in the correct place, the suction pipe temperature is a simple measurement but from the expansion valve outlet through a distributor , through distributor tubes all create pressure drops (P/D's).
The most accurate way is using the suction pressure/ temperature and actual suction pipe temperature.
When you have more than one evaporator, depending how the suction pipe work has been designed , and if it has been designed and installed correctly then the suction line pressure should be the same at the outlet at both evaporators or very close. If the suction pipe runs are very long then there maybe a too large P/D in the suction line which could result in an inaccurate superheat measurement.
To make the install measurement correct a tee piece installed in the equalisation lines of the expansion valves with a schraeder access port where a gauge can be installed during the measurement process will make the superheat measurement as accurate as possible for both evaporators
The sizes of the different evaporators is of no consequence to the superheat

Thank you for the answer,

What actually makes me concerned, we have heat exchanger at the suction line (liquid and suction pipes are soldered side by side), heat exchanger length is almost equal to suction line length. So I am worry we are getting not accurate pressure for SH measuring at the compressor inlet.

Here is principal scheme of refrigeration system: http://imgur.com/iIAMZX5

[Glenn Moore]

The suction line H/Ex should make no difference to the suction pressure as the suction line shows a large 22mm pipe which should cause little if any pressure drop in the line . The drawing shows internally equalised expansion valves which may not be the best valves to use with the evaporators piped in series as they will most likely have a large P/D through the evaporators , where externally equalised expansion valves maybe a better solution if the superheat your measuring is to high

[Gary]

Where liquid and vapor are in physical contact with each other (in the coils), if you raise the temperature you raise the pressure and vice versa. Where there is only vapor, if you raise the temperature the pressure remains constant. A heat exchanger does not alter the pressure because the suction side of the heat exchanger is (or should be) filled entirely with vapor.

That said, friction in the lines can alter the pressures, so it is more accurate to measure pressures at the coil outlets.