Screw compressors

[james10]

When i served my apprenticeship i learned from two different engineers now i have to decide which one i listen to,
one told me never to pump down a screw compressor the other did it quite often and said it dosent harm the compressor
i'm not talking industrial comps like halls sabro etc mainly bitzer and carrier hitachi screws on water chillers
wha are your general opions on pumping down screws.
Thanks James

[RANGER1]

Determine what you mean by pump down as in system when all rooms ets down to temp or for compressor serviceing .
I would say have a very short pump down no matter what , screws with differential feed lubrication ie no oil pump .
Industrial screws you could with oil pump application no problem others I'm NOT SURE .
Hanbell say if you have long pump down compressor can overheat & possibly cause severe damage due to very little or no mass flow rate through compressor .

[james10]

Determine what you mean by pump down as in system when all rooms ets down to temp or for compressor serviceing .
I would say have a very short pump down no matter what , screws with differential feed lubrication ie no oil pump .
Industrial screws you could with oil pump application no problem others I'm NOT SURE .
Hanbell say if you have long pump down compressor can overheat & possibly cause severe damage due to very little or no mass flow rate through compressor .

Sorry to clarify, what i mean is a standard pump down for servicing ie change driers,oil etc as in closing the liquid line shut off valve and bridging the lp cutouts

[RANGER1]

I'II also add that semihermetics probably don't like a long pump down as well due to no cooling of windings .

[Eeram]

On Bitzer, Copeland & Daikin screw compressors, the units can be pumped down for servicing procedures. Others, I don't know.

Frequently pump down can upset the thermistor protection, as the motor runs hot without suction cooling and can cut out on thermistor.

Remember, screws oil pressure equals discharge pressure and not suction pressure. So, even in an pumpdown, there is always enough oil pressure and enough lubrication.

Compact Screw Compressors runs in reverse when stopped by safety or normal LP cutout. So when your LP cut is set to zero pressure, the screw will stop on zero pressure, but with the backwards running of the screw when stopped, the discharge pressure before the discharge check valve, will equalize to the suction pressure, which can be up to be approximate 250 kPa both sides.

The only solution is to reclaim this pressure/refrigerant to get the compressor itself to zero pressure to do some work on the compressor itself.

Hope this helps.

[james10]

Thanks for the replies chaps

[NoNickName]

Never pump down, despite all and disregardind brand. It's a barbaric procedure, and it's outdated. It doesn't bring any sensible advantage, and puts the compressor under heavy stress.
More over, with a screw compressor, pumping down the suction increase the differential and lenghtens the counterrotation after stop, and effectively refills the suction, just been pumped down, with negative effects on reliability.
Forget the pump down. It's a thing of the 50s and 60s.

[james10]

Never pump down, despite all and disregardind brand. It's a barbaric procedure, and it's outdated. It doesn't bring any sensible advantage, and puts the compressor under heavy stress.
More over, with a screw compressor, pumping down the suction increase the differential and lenghtens the counterrotation after stop, and effectively refills the suction, just been pumped down, with negative effects on reliability.
Forget the pump down. It's a thing of the 50s and 60s.

i understand what your saying, under normal circumstances i would pull the gas but i have a chiller which suffered a burnout to the comp and had it rewound so i have to keep changing the driers and oil and adding acid flush untill i can get the system acid free

[NoNickName]

Don't pump down. Just close the service valves (if the compressor doesn't have them, install them) and recover the refrigerant in the compressor. Pressurize it with OFN and let the oil out under nitrogen pressure. After refilling, evacuate the compressor and open the service valves.

[james10]

Don't pump down. Just close the service valves (if the compressor doesn't have them, install them) and recover the refrigerant in the compressor. Pressurize it with OFN and let the oil out under nitrogen pressure. After refilling, evacuate the compressor and open the service valves.

no service valves i have suggested this to the client and they aren't interested in this due to cost even still i would need to recover the refrigerant to change the driers

[NoNickName]

Shame on them.

[james10]

Shame on them.

yep they wont even pay for dual prv's which they require changing yearly so every time 2 days labour and pull the gas

[Eeram]

According to some "bodies", you would need to reclaim/recover some 600 kg of refrigerant in a system to replace a drier or expansion valve/s and that for not just pumping down the system?

Crazy I would say! How long would it take you to reclaim 600 kg of refrigerant? Would your Client be willing to pay for that service?

If the Manufacturer approves pumpdown for servicing, then do it!

If pumpdown is a thing of the 50's and 60's, why do we still using it in 2010?

[NoNickName]

why do we still using it in 2010?

We WHO?
I don't and advise my colleagues and clients not to.
In fact, here we have a burnt compressor which has been subject to pump down... a coincidence?

In a unit without service valve, how do you guarantee you're not dispersing refrigerant in the environment, by simply pumping down?

[airmate]

Hi mate,

you will find most of the water chillers software allows chillers to pump down for off cycle when they meet setpoint and cannot unload below minimum capacity. You will find most of the manufacturers in their service literature to recommend pump down if system isolation is necessary for repairs or service. I have pump down carrier06N, mcquay, bitzer, Trane, refcomp and other screws to replace driers, oil filters and strainers but remember not to pump them into a vaccum. Just remember to keep good refrigeration practice when pumping down or opening a system for service.

[NoNickName]

Again, your mileage may vary. I'm not at all assuming that we've gone wrong up to now.
I think we have to move from "doing no harm" to "doing some good".

And no, I don't agree: chiller manufacturers do not set pump down at setpoint. Service engineers do.