To pumpdown or not.....

[incontrol]

I've seen some Trane chillers that close the LLSV and pump down to like 35 psi which is below freezing. I've heard a low charge can freeze the barrel before the freezestat trips due to it's location. Then I've seen a Carrier chiller that just shuts the LLSV and stops the compressor. Would it be a good idea to stop the pumpdown on the Trane?

[RichardL]

Without a doubt in my mind I will state flatly that the coveted "Pumpdown" cycle has destroyed more compressors than any other single reason....PERIOD.
Before anyone starts flaming me with their vast book-learning and quotations from various assinine reasons, please read with an open mind the following and allow a few minutes for it to soak into the never reaches of your frontal lobe...
1- Lets take a large 2E Trane compressor as an example Okay...
Put your gauges on said compressor and have it pumpdown on a normal satisfied pumpdown cycle...
You will notice that as the compressor gets ready to cycle off....whammo...you have "NO" net oil pressure....But then...Wham...The hydraulic loaders expand as the refrigerant expands in the oil actuated loaders and ALL cylinders load up for a few seconds before suction cut-out pressure is reached...
Now, does it seem condusive to long life to fully load a compressor, with Zip for oil pressure and only a foamy mixture of ref/oil left clinging to the bearing surfaces for any period of time....Not to this scribe it does not!!
Now...Lets try another shutdown scheme that has been used by a few that has "NO" downside to it...
If unit is ready to shutdown....Shut it down...Period...
Drop the compressor and LLS at the same time..
Yes you will have an undue and possibly dangerous amount of liquid lieing about in the evaporator & suction side in general....Big deal...It ain't hurt nothing yet..
Now...we get ready to cycle the unit back on..
Cycle the compressor on....
"But how about all that refrigerant thats waiting to slug the compressor"??????
That liquid is not gonna' do anything but vaporize until we open the LLS...It cannot rush to the compressor without the LLS opening and providing the "Motive Force" to move this liquid to the compressor..
Please remember, we started the compressor "ONLY", we have not yet opened the LLS...
Now..we wait until a suction pressure within a few PSI of shutdown has been reached and "Then"...and only "Then" do we open the LLS...
First off...The compressor will always have adequate "Net" oil pressure.(Assuming we have a decent compressor oil heater).
Secondly, The possibility of freezing an evaporator should something be leaking by has been elininated by now having a much warmer evaporator during our Pre-run pumpdown & almost critical suction pressures.
Lastly, as LLS is finally opened, oil lost during previous cooling cycle is mostly returned upon the LLS opening & hi delta-P that would exist...
Actually...the existing pumpdown cycle that is used has made me a fair living and has kept the wolf away from my door for many years. As a mechinic I would prefer the planned destruction of compressors to pay for the bait used on my boat, the gas consumed by it...
But the Gent posting this thread wanted to know about pumpdown.....
We will all make more $$$ using the status-quo than modifieng existing control circuits...
Now...Flame away all that feels a need to...

[wonder_boy]

That was very intresting

[hvacpope]

Richard what kind of stuff are you smoking?Are you from California?

[jayhawker]

I saw a guy take the one time pump-down cycle out of a DX/VAV system. Six compressors later he ask me to show him how to put it back in. Chillers, may be a good Idea to install the pump-out cycle instead of a pump down, but if you have long line sets on a DX cooling aplication I still think the one time pump- down cycle is a good idea. Any low temp application should always have a cont. pump-down cycle due to the temperature the coil is at and the potential for liquid to slug the compressor at start up.
This is a good topic to bring up , I know alot of the mfgs. have done away with any kind of a pump-down or pump-out cycle on their systems and both have a valid place to be used.

[boduke]

Pump down is fine, thats why we invented low pressure controls. As long as you don't go into a vacume, your fine. Period. When refrigeration systems go into defrost, thats what they do! is pump down! so refrigerant doesnt mirgrate back to the compressor, to slug it on start up.

[sonc]

Actually the main idea for pump down before defrost(electric) is so you don't spend the first 10-15 minutes just heating up the refrigerant in the evap instead of clearing ice.

[boduke]

yes very true. but preventing migration is very critical. Which is (I think)is the most important issue for pump down. Theres just a bonus for the defrost issue.

[RichardL]

No where in any responses did anyone mention continious pumpdown or non-re-cycling pumpdowm....
Continious pumpdown has severe drawbacks should any one of many problems exist...Slightly leaking discharge valve(s), LLS not 100% closed, weeping internal relief valve as welll as a few others, which can, and Will result in compressor short-cycling up to every 10 seconds or so...Boy, that sounds good for the system and the mechanics paycheck...
Or as most..a "Non-Recyling pumpdown"...which if "Everything" in not in Perfect, and I mean perfect condition, the system will equalize anyway with a almost complete migration of refrigerant into the evaporator...
"Don't stand beside it Bubba..It's getting ready to crank up"...
I never said or hinted in any way to do away with the pumpdown cycle....
I only state for a fact that the "Pumpdown" should occur on "Start-Up"...as opposed the the destructive pumpdown on shutdown....
And Hell no...I ain't from California.....

[mtnwrench]

you guys might want to listen to Richard, he has been doing this since before many of us were still hanging on the left tit. We had some 40 ton Mcgay units with pumpdown. One with everthing not perfect and this sucker would just keep coming on and off on shutdown. Owners too cheap to repair properly so every five years we get to change an expensive compressor. Most manufacturers are getting away from a pumpdown cycle and just using an ambient liquid solenoid to prevent some of the migration during low ambients. As Richard stated, this doesnt do much for startup. Wire them his way and you will lose far fewer compressors.

[sonc]

I didn't know they still made units with continuous pumpdown with no "pumpout" Any valve leakage and they'll keep spitting the oil out till it pukes.

[incontrol]

The chiller I am thinking about starts the comp & LLSV at the same time. Is that OK? Would you suggest using a timer to delay the LLSV so no other pressure switch has to be added? It has electric unloaders.

[Edited by incontrol on 06-27-2006 at 08:42 PM]