copeland zr scroll

[stevo]

Hi, We fitted a Zr 90 approx 19 months ago, operating on R22 with basic alco tev, the customer wanted next size up! so this comp is oversized (checked with nrs tec, to make sure that this was ok first) trouble now is a loud metalic noise from comp (occasionally). suction pressure dips to nearly o psi, and can trip the lp switch ! some frosting back to comp but not for long! has a a suction accunulator & c/c htr fitted, works well apart from noise! Anyone come across this before ? Has comp had it? Has low pressures caused this, as i believe scrolls do not like low pressures ! perhaps a mop tev would solve the big pressure swing ! the system is not on a pump down cycle, as recommended. any thought on this would be appreciated. thanks. :)

[BESC5240]

If accidental failure in the circuit (filter blocked, ...) or a bad selection of components (expansion valve) causes the compression ratio to be exceeded (10:1 for the ZR series for air conditioning) the floating seal is released from its seat and allows the high pressure gas on the discharge side to flow back to the low pressure side. This flow back comes with some noise, but can be easily monitored on your pressure gauges.

[ernestlin]

It seems that the debries exist in the system(result in blockage) and enter in the scroll plate. You can try vacuum pumping and flush the system(check the filter before).

[Toolman]

Hi, We fitted a ZR90 approx 19 months ago, operating on R22 with basic alco tev, the customer wanted next size up! so this comp is oversized (checked with nrs tec, to make sure that this was ok first)
A ZR90 on R22 at + 7 Suction is 23 KW - what was the system operating on before ?
So why would you make it the " next size up " because the customer wanted it , is he the fridgy or are you ?
My understanding of the KW rating is to stop the wrong compressor being fitted . For example if you have a 5KW compressor and the customer asks for a 10KW do you fit the 10KW to the 5 KW system and expect no trouble ? The nrs tec ( whoever that is ) will say its OK because he just sells the stuff but its not him who will fix it when its stuffed its you :eek:
http://www.frigocommerciale.co.yu/pdf/SCROLL%20Katalog.pdf

[NoNickName]

I don't see what the next size up theory is up to. A bigger compressor will cycle more frequently and this has nothing to do with LP trips at 0 bars.
I don't see what the MOP could do here. It could be useful if the compressor was one size down, but not one size up.

[stevo]

I don't see what the next size up theory is up to. A bigger compressor will cycle more frequently and this has nothing to do with LP trips at 0 bars.
I don't see what the MOP could do here. It could be useful if the compressor was one size down, but not one size up.

Hi, A bit more info i think! compressor is only approx 2 kw bigger than old comp, the application is a maggot room, room temp 0-c , Evap temp - 6 c. (evap duty approx 12kw) Thought an MOP tev would keep suction pressure @ a constant psi thus stopping the drop in pressure ! I have however had a valve selected today = tex2 with a no 5 orifice. it may be that existing tev is to small, hence the hunting !

[Peter_1]

Your TEV will not hunt when it's too small but when it's to large?
A MOP doesn't guarantee you a pressure drop, it's just the opposite what will happen.
If you install a bigger compressor without increasing the evaporator, then your suction will drop.
But going that low that it cuts out at 0 bar???
There's something wrong with another item in the system.
What's the SH which is extremely important to diagnose the system?

[stevo]

[
QUOTE=Peter_1]Your TEV will not hunt when it's too small but when it's to large?


Disagree ! if the valve is to small it cannot allow enough refrigerant into the evaporator, hence the pressure drop & low back pressure, compressor sucking & not enough super heated vapour returning !! also the tev is sited outside the room because of corrosive ammonia !!

[Peter_1]

We have a different explanation then for hunting..hunting is caused by overfeeding.

[US Iceman]

Expansion valves do not hunt when they are undersized, in fact they are more open and control better.

An over sized TXV WILL hunt. I agree with Peter. The valve has too much capacity and cannot reach a stable setting.

A larger compressor has more pumping capacity than the evaporator and TXV can generate. Therefore the suction pressure decreases.

Never oversize a compressor. That's bad!

[stevo]

Expansion valves do not hunt when they are undersized, in fact they are more open and control better.

An over sized TXV WILL hunt. I agree with Peter. The valve has too much capacity and cannot reach a stable setting.

A larger compressor has more pumping capacity than the evaporator and TXV can generate. Therefore the suction pressure decreases.

Never oversize a compressor. That's bad!


Fitted new TEV, system runs a treat even with slightly larger compressor, so as i first thought old tev definetly caused pressure drop ! customer is a happy bunny ! :cool:


Thanks for your input anyway