Pressure switch settings?

[Drew]

I have seen pressure switches for small coldrooms being used to control temperature. For a standard 2 degree C room we would normally set our range at 64psig(2 degrees C, R22) and the diff at 32psig, therefore cut out at 32psig (-12 degrees C, R22)

When we design standard coldrooms we normally run at 2 degrees C with an evaporating temperature of -5 degree C ( 7 degree TD). This means that when our room reaches 2 degrees C our suction should then be at -5 ? Why then are we setting our pressure switches to cut out at -12 degrees C. Surley this would force the room to run colder than 2 ?
I realise that using a pressure switch to control temp is not great but still see enough of this practise to wonder why we use these settings?

[taz24]

I have seen pressure switches for small coldrooms being used to control temperature. For a standard 2 degree C room we would normally set our range at 64psig(2 degrees C, R22) and the diff at 32psig, therefore cut out at 32psig (-12 degrees C, R22)

When we design standard coldrooms we normally run at 2 degrees C with an evaporating temperature of -5 degree C ( 7 degree TD). This means that when our room reaches 2 degrees C our suction should then be at -5 ? Why then are we setting our pressure switches to cut out at -12 degrees C. Surley this would force the room to run colder than 2 ?
I realise that using a pressure switch to control temp is not great but still see enough of this practise to wonder why we use these settings?

Using pressure switches to conrol temp is a very accurate and cheap way to control a system temp.

like you say the lp switch needs to be set to the correct settings to get the right opperating temps, so if you see lp switches that are set low are they being used to control temp or are they simply to control pump down or out of gas situations?

In short you can use a lp switch to control temp but it does need to be set acurately to work correctley.

Cheers taz

.

[paul_h]

As above, but you are forgetting pressure drop, which is normally another 2K.
If something is set much below that, it's a poorly designed or set up system. Either excessive presure drop, undersized evap an TD etc.
Or who ever set it started at -10 and when the room got to temp, just wound it up until the compressor cut out and thought that's good enough.

[andyrobbo]

The best way i have found is to wind out the pressure switch. Run the room down to the desired temp then cut the lp out at your setpoint. Then let the room heat up to your desired temp and cut it in. This will over compensate for any poor designs and allow for optimum perfomance.

[paul_h]

That's what I said above, it may not be a correct temps because it's set up you like suggested.
Also I have to add, to be fair we can't verify that the controls are 100% calibrated anyway.

[frank]

It may be that the LP switch is set to maintain a PRODUCT temperature and not the box temperature?

There is a difference.

[Drew]

Thanks for the relpy guys!
I understand that we should allow for a suction pressure drop ,but not up to 7 degrees? This wouldnt be standard.

I have noticed that the room temp is at 2 degrees c when the suction is at -12.

When we design a 2 degree c room we normally (for a standard application) design the suction to run at -5 degree c. This gives us certain humidities and capacity. If our suction runs lower than that we loose capacity and humidity.
Are the compressors delivering more than the capacity charts indicate? ( unlikley) or are the evaporators not delivering what the manufactures say. Either way our suctions seem to run lower than our design specs which is a capacity loss.

The standard of a 32psig R22 cutout seems low even though it works in practise??

Am i missing something??

[Drew]

I think the answer is that when our coldroom compressor runs at -5 degree c suction our head pressure is not normally at 45 degrees, as we designed it, but rather at about 35 degrees, because we have a tendency to overspec the condensor.
The compressor does then deliver more capacity forcing the suction lower????

[Grizzly]

I think the answer is that when our coldroom compressor runs at -5 degree c suction our head pressure is not normally at 45 degrees, as we designed it, but rather at about 35 degrees, because we have a tendency to overspec the condensor.
The compressor does then deliver more capacity forcing the suction lower????

A good point Drew.
A over sized Condenser or even low ambient temps (say overnight during a cold snap) or even poorly sited Condenser where it is subjected to high wind-age through it.
Can all affect the system Suction temp / pressure.

etc etc.
Grizzly

[hendry]

I have seen pressure switches for small coldrooms being used to control temperature. For a standard 2 degree C room we would normally set our range at 64psig(2 degrees C, R22) and the diff at 32psig, therefore cut out at 32psig (-12 degrees C, R22)

When we design standard coldrooms we normally run at 2 degrees C with an evaporating temperature of -5 degree C ( 7 degree TD). This means that when our room reaches 2 degrees C our suction should then be at -5 ? Why then are we setting our pressure switches to cut out at -12 degrees C. Surley this would force the room to run colder than 2 ?
I realise that using a pressure switch to control temp is not great but still see enough of this practise to wonder why we use these settings?

about -12degC setting ....

please remember that there is
1. on coil temperature;
2. off coil temperature;
3. mixed room temperature.

wherever you get on the compressor, it does not really reflects the actual room temperature esp. when the sensor/s is at cooler/s.