LP pump down

[R1976]

Hi,

Could someone tell me the benefit of LP pumpdown control on a coldroom. As I believe, if you pump down near to 0psi then there will be hardly any refrigerant in the low side that can migrate to the compressor as a liquid on start up.

I see systems which have a simple control feed to the compressor relay and no LP pump down so surely it's not that bad on a smaller system?

I have a small system with a few meters of pipe on it and I don't want to over complicate it. Also I will have to modify the condensing unit to fit a new LP switch for control. This brings me to another thought of why can't you use the safety LP which is already installed in the condensing unit for LP pump down control?

Appreciate any thoughts on this matter.

[nike123]

http://www.refrigeration-engineer.com/forums/showthread.php?t=18406

[lowcool]

gday 76
never set your safety or pump down cut out that low,unless system is badly designed

[coolhibby1875]

hi you should be using the lp switch thats on the unit for the pump down, pump downs are good if you are using heaters for defrost as if the unit just cuts out and brings on the heaters, the presure rises in the system and put extra load on the compressor during start up.

[fridge doctor]

To be honest I think Nike's link says it all!

[Mark II]

R1976,
Lowcool is spot on, @ 0 suction your compresspor wont have much oil in it. I'm a pumpdown man myself. The liquid's where it should be & you get secondary protection aganst icing up the evap coil if the cut is set right..

[chemi-cool]

One of the more important thing about pump down is the location of the LSV.

It is recommended to install it as close as possible to the TXV. In that case, the pump down will empty the evaporator.

If the LSV located just after the liquid receiver, it will take a long time for the compressor to pull in all the liquid as there is no pressure to push the liquid and the TXV could be almost closed.
this can cause the compressor to heat up and loose oil.

Care should be taken when the evaporator is a flooded one.

Never let the pressure to drop below 5 psi. And if your suction pressure is low, you do not need LSV in the system, liquid separator will do defend the compressor from flood back.

[coolhibby1875]

what is a liquid separator?

[chemi-cool]

Also called liquid accumulator.

[R1976]

Thanks for the replies.

This system has 3/8" liquid and 1/2" suction and pipes are less than 5 meters each.

Simple condensing unit with compressor, liquid receiver, drier and sight glass although does have fan speed control and relay for compressor. I will have to fit an extra LP switch for the pump down. It seems that the pump down setting should be the pressure at which the refrigerant cannot remain in liquid state for the given ambient temp whilst in it's off cycle. Is this correct?

After reading that lengthy link I still feel a bit confused although we will probably go down the pump down route.

What's the score with all the fridge that I see with no pump down and simple control to comressor relay?

[lowcool]

What's the score with all the fridge that I see with no pump down and simple control to comressor relay?[/quote]

gday 76
i reckon there has been other discussions on this also,to me it seems it has to do with personal choice,cost of instal and design application