setting LP control on pumpdown system.

[Temprite]

G'day all.

I was recently having a debate with a co worker on pressure control settings for a low temp walk in freezer with pump down.
Refrigerant R404A

I set them to cut out just above atmospheric pressure and cut in about 30PSI (203kPa). -19 deg C

My co worker sets them the same as me except he sets the cut in at about 40 PSI (273 kPa). -14 deg C.

My argument is that if you set the cut in too high when the solenoid opens it will delay the making of the pressure switch and the unit will not cut in until the room warms enough to raise the suction pressure, therefore unit will be taken out of normal operating temps( -18 to -20 deg C)

His argument is that he has set them this way for 36 years and that the suction pressure will not correspond to what the temp is in the controlled space and that the ambient temp will also raise the suction pressure somewhat. He also states that if you get a leak that my settings will cause quick short cycling that could damage the compressor.

I will get a guage and check this but was interested what other members set their pressure controls on low temp pumpdown.

Thanks in advance.

[taz24]

G'day all.


My argument is that if you set the cut in too high when the solenoid opens it will delay the making of the pressure switch and the unit will not cut in until the room warms enough to raise the suction pressure, therefore unit will be taken out of normal operating temps( -18 to -20 deg C)

Thanks in advance.

I agree with you the cut in pressure is in relation to the evap temp. So the pressure temp relationship is constant. I have set systems that run on a LP switch only and they are very efficent.
Cheers taz.

[lana]

Hi there,

If you are setting the low pressure control for Pump-down then you have to set it low so the evaporator is empty when pump-down is activated.
Short cycling is very common in the water chillers because the water pump is always working and basically water brings load to the evaporator therefore if the low pressure control is set high then you will have short cycling.
To avoid all these problems and discussions I suggest the following :
Use two Low pressure controls: one for pump-down and the other for safety. Set the pump-down low pressure control very low and the other one higher for compressor safety. Also you have to add a relay in your electrical box to switch the controls, i.e. when system is pumping-down then the safety low pressure control will be out of the control loop and when the system is working, the safety low pressure control takes over.

I always use this method and there is no short cycling and also there is always low pressure safety in the system.

I hope this helps.
Cheers

[monkey spanners]

Hi Temprite
I'd set the Lp switch the same as you. Setting the cut in pressure above the evaporating pressure may cause none starting problems in cold weather. I'd fit a delay timer if rapid cycling was a problem (6-10 starts an hour depending on the compressor size if memory serves me correctly ).

Cheers Jon

[Brian_UK]

I'll go with settings Temprite, you can always install a non-recycling pumpdown control if someone is worried about it.

[Peter_1]

Temprite, you're right and also for another reason: if your colleague sets them that high, then it will take a long time until the unit will start.
But, in the meantime - while pressures slowly rises - the evaporator will fill completely with liquid refrigerant because this is a very cold spot in comparison with the warm refrigerant under pressure.
The compressor will then start with liquid slugging.

The same phenomena happens with cold plates.

I let it strat +/- 5 K lower then the room temperature.

And what your colleague says is of course not true: temperature has allways a relation with pressure for any refrigerant.

[old gas bottle]

i,am also with you on this one,setting to high and getting temperature problems can take time to track down especily if there are multiple units on the same cold room ,chiller etc,if its a new job there should be a anti short cycle timer in the panel anyway,if not fit one,game over.

[Tony Weston]

I agree with you also my Aussie neighbour. Too high a pressure switch cut in, will raise the freezer temp, and we dont want that eh. Stick to what you are happy with.

[Temprite]

Thanks to all who replied, much appreciated.

Peter this is something that I had not thought of

Temprite, you're right and also for another reason: if your colleague sets them that high, then it will take a long time until the unit will start.
But, in the meantime - while pressures slowly rises - the evaporator will fill completely with liquid refrigerant because this is a very cold spot in comparison with the warm refrigerant under pressure.
The compressor will then start with liquid slugging.

[D. Sergius]

On a pump down system there should be a 15 lb differential between cut out / cut in my experience to avoid short cycling.

I generally set the control at 0 and 15 psig.

Exception to this would be a chiller (which should never be pumped down for fear of freezing.