Is there a formula for setting the low pressure control for a pumpdown system?

[kengineering]

I am looking for a specific way to set the control for a pumpdown system that would work with all refrigeratnts. I was once told to subtract 10psi from the observed pressure when the t-stat is satisfied for the cut out pressure. Then observe the pressure 30 seconds after the t-stat calls for the cut in. I'm not sure this is correct. Any feed back would certainly help. Ken

[US Iceman]

I feel a pumpdown cycle should ensure the evaporator is pumped down completely, not just to lower the pressure some.

After the solenoid closes the suction pressure will begin to decrease. I would suggest allowing the compressor to run until the suction pressure reaches 5 psi or thereabouts.

The cut-in pressure would be determined by the normal evaporating pressure you want to run. This is determined by the application. If it's a freezer you do not want the pressure too high. Otherwise the controlled space temperature will go too high before the system starts again.

[Pooh]

No disrespect but have you beem shown how to use a comparator, this piece of equipment will give you the answer you are looking for. take the room temperature that the stat cuts out at allow the system to pumpdown to just above zero and stop then on the stat closing allow the pressure to the equivalent cut in temperature ie. the saturated temperature of the refrigerant at that point and set the LP switch to make at that point.
Why do people have a thing about pressure, as fridge guys we do not need to know about pressure, temperature rules OK.

Ian
PS. sorry for going on but been to the pub.

[Dan]

If you are using a thermostat for pumpdown, Iceman's advice is solid, except I recall hearing from compressor manufacturers that you really shouldn't pump a compressor down unnecessarily below the point where it remains off. I believe this advice pertains to the frequent operation of a compressor below it's published low pressure rating criteria... for example, it is not necessary to pump down a 410A compressor to 5 psig, since it can be effectively pumped out at a higher pressure. I would use the highest cutout setting that does not result in frequent short cycles after the pump down. Putting the solenoid close to the TEV is important no matter what setting you choose.

[LRAC]

Putting the solenoid close to the TEV is important no matter what setting you choose.

The above is the most important thing to observe on a pumpdown system, some installations have the sol valve on the condensing unit and can cause the unit too short cycle as it trys to reclaim the refrigerent in the liquid line before the TEV.

As for setting the L/P switch follow US icemans advice.

Regards
Lrac

[US Iceman]

I would use the highest cutout setting that does not result in frequent short cycles after the pump down.

This is a good point to discuss which type of pumpdown cycle should be used. There are two types I am aware of; recycling and non-recycling.

A "recycling pumpdown" method is very simple. With this type the compressor starts and stops on a pressure switch. The solenoid valve is simply controlled by a thermostat.

Therefore, if the cut-out pressure is not low enough during pump down the compressor can eventually be started when the gas pressure rises to the cut-in pressure.

This type of pump down cycle is more prone to frequent starts/stops if the low pressure switch is not correctly set.

On the other hand you can use a "non-recycling pumpdown" method. In this case, the thermostat controls the liquid line solenoid and auxilary relay.

When the temperature is satisfied, the solenoid valve is de-energized and the pump down cycle starts. After the pressure is sufficiently reduced, the compressor stops and is locked out by the auxliary relay contacts.

The compressor cannot start again until the thermostat AND low pressure switch are both proved closed. This keeps the compressor from short cycling on a poorly set low pressure switch.

[Gary]

If the solenoid valve is near the TEV and the pressure setting is somewhere in the ballpark, the compressor isn't going to cycle more than once or twice... unless the solenoid is leaking through.

If the solenoid is leaking through, then cycling will be frequent and ongoing.

If the pumpdown is non-recycling, (aside from shutting down the compressor) a leaking solenoid will negate the benefits of a pumpdown.

[US Iceman]

If the solenoid valve is near the TEV and the pressure setting is somewhere in the ballpark, the compressor isn't going to cycle more than once or twice... unless the solenoid is leaking through.

If the solenoid is leaking through, then cycling will be frequent and ongoing.

If the pumpdown is non-recycling, (aside from shutting down the compressor) a leaking solenoid will negate the benefits of a pumpdown.

I can agree with this, since it's based on two suppositions. One, that the pressure switch is not properly set, and two, the solenoid valve might be leaking.

If the solenoid valve is leaking it should be repaired or replaced, as this is not desired. Secondly, the pressure switch should be properly set, so I don't believe this is really a point of argument to determine the pros' or cons of either pumpdown method.

Most applications where I have seen non-recycling pumpdown used are on large chillers (positive displacement compressors).

On larger systems (or chillers) you do not want the compressor to be cycling unless it is due to load requirements. If an adequate pumpdown is performed on a large system, it should be shut down until the requirements dictate so.

Allowing a large compressor to cycle one or two times on each shutdown creates undue wear on the equipment (and electrical switchgear) and increases the kW demand when unnecassary.

On small systems a recycling pumpdown works quite well, if the two conditions above are met.

[750 Valve]

Why do people have a thing about pressure, as fridge guys we do not need to know about pressure, temperature rules OK.

Ian
PS. sorry for going on but been to the pub.