Can a 4ton HVAC compressor pump down?

[JSPPE]

I am an HVAC design engineer, not a certified service tech. Don't worry, they don't let me use the wrenches and gages.

I have a contractor recommending to use a small (4ton) heat pump system compressor to pump the refrigerant into the outdoor unit by closing the liquid line service valve and running the compressor. Don't know if he proposes to bypass the low pressure switch or not. There is no receiver to store refrigerant, only condenser coil tubing.

I understand semi-hermetics are built to do this on a regular basis, and have receivers for storage, but what about the small residential scroll and recip. cans?

Aren't they refrigerant cooled motors?

I would prefer to reclaim thru a filter/drier and either replace or re-use thru another filter/drier instead of stressing the system compressor.

If this was only one system, maybe no big deal, but there are several hundred to do.

[mad fridgie]

If it was a pre-charged unit from new, the I do not see a problem. "the charge you are pumping down" if extra refrigerant has been added then yes can be an issue.
The motor cooling is not an issue for a few seconds.
Some scrolls have an internal pressure relief, which may stop complet pump down (very specific to the system)
If he is using gauges on high and low side, then he should know.
If you want to become a better design engineer, my advice is to go and work with a tech for a while. Learn to swing that hammer!

[nike123]

Is that your product or someone else?

If outdoor unit doesn't have receiver and you have scroll compressor, you should check in compressor technical data sheet what is allowable max charge for that compressor without accumulator/receiver, and then check nameplate if quantity in unit (plus additional charge for extended pipe length) is higher than that amount. If yes, you should recover refrigerant instead of pump down.

[KibGaimb73]

I dont understand how to apply the pressure...i mean in the sonic normal...u apply with the middle pushing down...but in the sonic reverse...the trick thing says to pull the middle toward u and pull the index...HOW DOES THAT APPLY PRESSURE? im very confused...

[NoNickName]

I dont understand how to apply the pressure...i mean in the sonic normal...u apply with the middle pushing down...but in the sonic reverse...the trick thing says to pull the middle toward u and pull the index...HOW DOES THAT APPLY PRESSURE? im very confused...

I beg your pardon?

[chemi-cool]

I recommend not to do it.
Its a heat pump witch means the direction of the liquid is toward the indoor unit in cooling mode and toward the out door unit in the heating mode.

As a design engineer, how do you think it can possibly work?

Scrolls and small recip. don't have any problems to pump down the refrigerant. Using scroll it is advised to set the low pressure cut off not below 1.5 bar [22 psi].

[JSPPE]

This is a service issue, not an every cycle operational pump down.

The unit would obviously have to be in the cooling mode of operation to do this.

My concerns are: extended run time with low refrigerant flow during the pump down. I've heard that service techs do this all the time to do evaporator repair and replacements, but it sounds like the compressor is being highly stressed. Also, since the original post, I've determined that the system doesn't have a low pressure switch, so in my opinion the compressor is at even higher risk for damage.

[NoNickName]

There was warmed discussion on this forum some time ago: http://www.refrigeration-engineer.co...highlight=pump

[Peter_1]

JSSPE, this is only a one time pump down for service issues, no problem at all, practical experience learned us already a long time that this will not harm.
It goes so fast that compressor doesn't have time to overheat.
Highly stressed? No LP switch? If the tech is knowing what he's doing, no problem. As soon you reach atmospheric pressure, shut down the power.

It's even recommended in the Daikin and Mitsubishi manuals. The new Daikins (VRVIII) - correct me if I'm wrong - even perform this on their own when you check liquid level/content via the dip-switch on the PCB.