bitzer pump test

[shiraz]

Moducel pack with 4 bitzer compressors, each of which have failed pump tests.(possibly)
Each will acheive about -0.6bar in a vacuum but will not hold. lp switch linked out, oil line closed in. As soon as lp is released the pressure will rise to about pack presure within about 10 seconds!.

heres what i do:
link out/hold out lp switch
close in oil line
close in suction valve
let compressor run until vacuum achieved
release lp switch
thats when back pressure will suddenly rise.

there are start unloaders and capacity regulators on each compressor too. no liquid injection. any advice would be greatful. thanks

[fowlie]

take valve plates off one compressor and check gaskets and for worn reeds as these tend to ware out after a while,and can easily be changed

[shiraz]

thanks, but all 4 compressors with the same issue? on that syatem and all 4 like it on the next! am in doing something wrong?

[fowlie]

could be valves passing,how long do you run the compressor on a vac before stopping as its possible that to short a time might be letting leaving some liquid in the sump to boil off giving the impression that the comps passing.firstly shut down all valves and decant all gas from compressor,then check to see if any off the valves are passing then open up the discharge valve and see if anything comes out the suction port.thats the quickest way to be sure.iam guessing by the make its quite an old pack and is quite possible that its simply one or to worn reeds as these do ware down over time as they vibrate off the valve plates and do break up and damage gaskets.hope thie helps:)

[shiraz]

thanks, could you explain in detail the above process? how long shoiuld i leave in running in a vac to allow any potential liquid to boil etc etc? thanks again.

[fowlie]

not to long about thirty to fourty seconds should do,as you dont want to damage the compressor

[shiraz]

ok i will try, and confirm to you my new findings.thanks again.

[coolhibby1875]

set the lp switch for 0 psi shut the suction valve and oil line and switch on if the compressor runs then valve plates passing.
if this is happening on all 4 compressors on every pack your missing something!

[GHAZ]

hi shiraz pump them down twice and they will hold vaccum

[750 Valve]

got a comp model? just interested to know

[NoNickName]

If one of the four compressor has a leaking internal safety valve, all four will not hold vacuum, because the high sides of the four compressors are connected together. Disregarding which one is pumping down, low side will always have a bypass from the high side through the single safety valve.

This happens when the compressor is started mistakenly forgetting the high side shut off valve closed. The burst of pressure forces the SV to release on low side, but it may seize because of the extreme mass flow rate.

[750 Valve]

If one of the four compressor has a leaking internal safety valve, all four will not hold vacuum, because the high sides of the four compressors are connected together. Disregarding which one is pumping down, low side will always have a bypass from the high side through the single safety valve.

This happens when the compressor is started mistakenly forgetting the high side shut off valve closed. The burst of pressure forces the SV to release on low side, but it may seize because of the extreme mass flow rate.

provided you are doing the pump down test proerly then even if one comp has internal relief popped it won't affect the others. They should be checked one at a time by shutting off the suction service valve after isolating oil and any balance lines. Doing it to all 4 comps at once by shutting off suction ball valves at the header is useless, it will not tell you much.

I don't think the poster is doing it this way, I think he appears to be doing it individually which is the correct way.

[NoNickName]

Who knows... let him say.

Is this the same pack http://www.refrigeration-engineer.com/forums/showthread.php?t=3456?

[rnine]

I,m not familiar with Moducel packs, but when working on the older type Safeway (now Morrison) packs. They used to hide a 1/4 " sump equalizing line running beneath the compressor bodies. With the isolating valve tucked neatly away out of sight!!!
This would always catch out the Lift and Shift contractors, and would give the same symptoms as the OP described.
Might be worth a look see.
R9

[750 Valve]

Who knows... let him say.



read his first post mate, he is doing 1 at a time.

and the thread from 2005 you linked to has 5 comps on the rack, this one has 4 - so no its not the same rack. It might pay to read a bit more thoroughly

[NoNickName]

read his first post mate, he is doing 1 at a time.



I can't say he's doing one-at-a-time. Where do you see that? He just says he's closing the suction valve, but it can well be on the common manifold.
Or, really he's got 4 faulty compressors.

[750 Valve]

Moducel pack with 4 bitzer compressors, each of which have failed pump tests.(possibly)
Each will acheive about -0.6bar in a vacuum but will not hold. lp switch linked out, oil line closed in. As soon as lp is released the pressure will rise to about pack presure within about 10 seconds!.

heres what i do:
link out/hold out lp switch
close in oil line
close in suction valve
let compressor run until vacuum achieved
release lp switch
thats when back pressure will suddenly rise.

there are start unloaders and capacity regulators on each compressor too. no liquid injection. any advice would be greatful. thanks

Its in the terminology my friend

- usually to close "in" a valve is to front seat it - like a suction service valve on a bitzer comp, you can't do this to a ball valve

let the compressor (not compressorS) run until vacuum achieved

[NoNickName]

Wow, I'm impressed. In this forum we range from barely understandable mongolian to english on-line translated sentences, to highly specific error-free engineering terminology.
I stand corrected.

[750 Valve]

You only have to read it to know he has a decent grasp on the english language, after all he is english :)