Compressors with no valves

[lana]

Hi everybody,

In compressors where there is no suction and discharge valves - like screw or scroll - how will be the pump down done? when the suction pressure is decreased below the LOW pressure setting then the compressor will be shut down and because there is no valves then, the pressure will increase unles; there is a check valve on the discharge line.
Any comments?
Cheers

[NoNickName]

Pump down and shut off valves have no relation.
It may not be possible to pump down a scroll with or without shut off valves. Infact, Sanyo scrolls have a equalisation valve between discharge and suction. At stand still, the two pressures are the same.
Screws, on the other hand, turn back when shut off, nullifying the effect of a pump down. Take care, pumping down a screw maybe dangerous for suction side bearings.

[lana]

Pump down and shut off valves have no relation.
It may not be possible to pump down a scroll with or without shut off valves. Infact, Sanyo scrolls have a equalisation valve between discharge and suction. At stand still, the two pressures are the same.
Screws, on the other hand, turn back when shut off, nullifying the effect of a pump down. Take care, pumping down a screw maybe dangerous for suction side bearings.

Thanks for reply.
I don't mean shut off valves. My question is about the valve reeds like in the piston compressors. There is no suction and discharge valves in scrolls and screws. It will make disfference for pump down for sure.
Any comments?
Cheers

[NoNickName]

As I wrote, it may or may not be possible to pump down depending on the type of compressor. Copeland scrolls can be pumped down, LG and Sanyo cannot because of equalisation valve.
Screws comps are supplied with non-return valves on the discharge, although the built-in oil separator is at discharge pressure and will refill the suction side while compressor is stopping. On the other hand, screw compressors with external oil separator can be pumped down.

[Renato RR]

Which is type off compresor?

[monkey spanners]

All the copeland scrolls ive worked on have had a check valve built in. All the bristol scrolls ive worked on havn't and have had a check valve fitted in the dicharge line before the condenser. Have seen one fail and it kept on cycling until it tripped the overloads.

Cheers Jon

[ernestlin]

Copeland scrolls also have the inner pressure protection equipement. When the pressure difference between H and L reaches the max acceptable value, this equip. will act and connect the H and L areas to protect the compressor. You can find the detail in copeland's catalogue.