scrol compressor

hi all new to this site,
has any one had any problem with alco pressure switch and scroll compressor, what happen is that when its pumping down after cut out on LP the back preesure seem to shoot up to say 3 to 4 bar.

Hi Ridwan, welcome.

At what pressure does the compressor stop and why do you think that there is a problem with the pressure switch ?

How far away from the compressor is the solenoid valve and how long is the suction line ?

Hi Brian
well as soon as LP cut out compressor at 2 psi, the back pressure sems to shoot up 3 bar. Solonid valve is metre from TEV, 4 metre suction line run.

What brand scroll is it?

Hello Ridwan,,

if the brand of scroll is Copeland, dont pump down, since pump down is not recommended by the manufacturer it self.

Mahmoud and ridwan

It is OK to pump dowm copeland glacier compliant scrolls (ZS, ZF, ZB). Most new scroll have internal check valve to prevent bleed back.

You gotta determine if problem is compressor or other causing your problem.

If you have suction service valve pump down there and observe for bleed back. if not there problem is not compressor.

Other items to check could be hot gass bypass valves, defrost valves, or any thing that would allow hot gas to low side.

Steve

In the manual of the older scrolls, you can find that it's not allowed to pump down like Mahmoud said. It all depends how old it is.

Hi Ridwan
I`ve designed and install a lot of systems with Copeland scroll compressors (ZF and ZR series) and i always used pumpdown. Don`t hesitate use it.

Hi all
Thanks for replys, both compressors in question are ZB series they are both new install
about 6 months old. The cut out is set to 2psi, LLS valve in line, is there anything else to stop bleeding back.

Check the phase rotation
There was a post here somewhere............where a scroll did run for some considerable time backwards!!
Needless to say..........it did nor provide much cooling!!

I installed a 7 hp scroll and its on pump down
When she shuts down............momentarly there is gas discharge the wrong way.............
Some kind of unloading thingy.............

ridwan

Did you do pump down check?

How does your amp draw compare to what it should be? I have observed on rare occasions plate separation, that can give the your conditions.

Have you had problem with liquid line drier stopping up? Could be pieces of the compressor.

Are both compressors doing the same thing? Are they in parallel or each have it's own circuit?

Do the compressor pump down test and see if it checks out.

Steve

There is a multiply explanation's for this problem but not enough information. You should be looking at the liquid line solenoid valve and make sure its not pumping if you have an in-line service shut off valve try closing that and pumping the system down. If its a twin circuit system on a single evaporator it could be a cross over on the end cover plate so try pumping down 1 circuit but put your gauges across both circuits.