Pump Down or NOT to Pump Down

[lana]

Dear all,

I always connect my systems with pump down cycle(systems which pump down can be done), i.e. thermostat closes the solenoid valve and low pressure cuts off the compressor.
Some people I met, have this opinion that pump down is not good for the compressor because at start up evaporating and condensing pressures are not close together.

I always disagree with them because pumping down is the best way to protect the compressor from the refrigerant when idle.

I would like to here your expert opinion on this issue.

Thanks

[abet_meneses]

lana,

i dont think it will harm the compresor,instead it will lengthen the useful life of it because it will re start on minimal pressure of 15 psi as i have always done it.

that's is my opinion although others may disagree.

regards

[martinw58]

i think pump down is best as long as pressure swith is set corectly

[Brian_UK]

If, on start up, the compressor has to wait for the suction pressure to rise above the LP switch after the solenoid valve has opened then I don't see any problems.

The method has been used for years without serious problems that I know of.

As long as the pump down system is a one shot process and the compressor is not switched on and off by LP switch movement due to leakage or temperature changes.

[Quality]

http://www.refrigeration-engineer.co...essor+envelope

Try this sure enough pump doewn is ok

[mad fridgie]

For evaps with electric defrost, it is better for defrosting not to pump down!
If pump down is used the quality of the LP switch setting is important, we do not want short cycling

[lowcool]

this has been covered a couple of times here from memory.
me-pump down all the time regardless of defrost technique

[lana]

Hi Guys,
Thanks for the input.

I agree about th Low pressure switch setting.

In applications where working pressure and safety low pressure setting are not close like, water chillers and cold rooms with temperature above zero, I use two low pressure switches. One for safety which is set lower than working pressure and the other for pump down which is set to 0bar.
This complicates the electric board but works perfectly. When system is working normally, safety low pressure switch is controlling and pump down low pressure switch is by-passed.
When system goes into pump down cycle then safety low pressure switch is by-passed and pump down low pressure switch takes control. This ensures that all refrigerant is completely evacuated from the evaporator and suction line before low pressure cuts out.

Also to eliminate cycling pump down a relay is used to lockout the compressor until the thermostat calls for cooling .

My electrical engineer did the board and it works very well.

About the electric defrost, I say before defrost elements are energized, the system must be pumped down. Otherwise there will be pressure build up in the suction line.

Apart from this issue, when the pipe run is long then pumping down takes a few minutes. This worries me about safety of the compressor.


Any comments?

Cheers

[D.D.KORANNE]

Pump down is a method is proven one for a long many years to safeguard compressor. But however , a non-recycling relay to prevent compressor start up as the suction pressure build up due to leaky valves would need to be incorporated to limit compressor start/stop ; particularly in semi-hermetic units . In these motor start up per hour are restricted to say six cycles or so .

[Sandro Baptista]

But however , a non-recycling relay to prevent compressor start up as the suction pressure build up due to leaky valves would need to be incorporated to limit compressor start/stop....

I think prevent the compressor start up as the suction pressure build up it could be very dangerous. I would not do that.

[nando]

they invended pumpdown when they invended centrales, then they started to use it with freezers
pumpdown is not bad for the compressor
with electric defrost it is indeed better to use not pumpdown, with a startregulator en liqiudrecervoir
when there is still gas in the evaporator, the pressure increase, the ice will melt all the way, everywhere
from 1980 they begin to use pumpdown, that is not so long time

[Sandro Baptista]

they invended pumpdown when they invended centrales, then they started to use it with freezers
pumpdown is not bad for the compressor
with electric defrost it is indeed better to use not pumpdown, with a startregulator en liqiudrecervoir
when there is still gas in the evaporator, the pressure increase, the ice will melt all the way, everywhere
from 1980 they begin to use pumpdown, that is not so long time

My opinion is with electric defrost it should be used even more the pumpdown.

[nando]

the melting is better when there is still gas inside, to protect de compressor use a startregulator.
like this they did from the beginning in the cooling till they start to use magnetic valves.
ir agancies recommanded this system together with a regulator called QKL

[mad fridgie]

My opinion is with electric defrost it should be used even more the pumpdown.

Defrost with electric is much better with refrigerant in the coil, less time to defrost, no so excessive temperatures, less steam from the coil.
The heat transfer is more evenly spread due to the natural flow of the refrigerant
You protect the compressor from short term overloading by useing a CPR (crankcase pressure regulator)

[Sandro Baptista]

the melting is better when there is still gas inside, to protect de compressor use a startregulator.
like this they did from the beginning in the cooling till they start to use magnetic valves.
ir agancies recommanded this system together with a regulator called QKL

Nando I don't understand why the melting is better. Also i that way you must also te heat the liquid-vapor refrigerant inside (that will absorbs heat). Also the pressure will increase and migration/condensation of liquid to the cold parts (suction pipe) can occur. Don't you think so?

Well, I know that at rise of pressure the crankcase of the compressor the suction valve will be totally close...but even so with the age it can let pass some vapor and also is expensive and the system get one more pressure drop.

[Sledge]

My rule of thumb: When in doubt, pump it out.

As stated by many others, I have installed countless pump down circuits over the last 30 years, main issue being to protect the compressor from short cycling on LP during off cycle due to leaky valves/solenoid valves...either by using a start time delay...or fix the leaky component