Compressor with no pressure difference

[marc5180]

I got called to a site last week with an Airedale condensor that that wasn't running. CUHS3.5 HIEM
Found the system off on LP with a leak on the coil... I fixed the leak and charged the system but when i ran it the suction and discharge pressures stayed the same and didn't alter even though the compressor was running.
The system kept tripping on LP as well for some reason even though the correct charge was in.

I closed the liquid line valve to see if it would pump down but nothing.... i then closed the suction valve but the pressures didn't alter.
The compressor is 3 phase so i swapped two of the phases around thinking it could be running abckwards but again the pressures stayed the same (6 bar R407c)
I spoke to Airedale who agreed that it must be a compressor issue.

I returned today to fit the new compressor and a new LP sensor but when i ran it up it seemed to be doing the same.

Is it possible for a 4way valve to act as a bypass between suction and discharge... i don't think it is but i'm grasping at straws here.

The only things i can think is either the compressor is running backwards or the 4 way valve is discharging straight to the suction.

Has anyone come across anything like this before?

[superswill]

AN old post by Chunk

Hi Guys.

I like these compressors alot.easy to change when they do have a fault.

The main reason for most of the failures is when people forget about the crankcase heater as the majority of these comps are used on outdoor units.
the other thing is these comps have an internal pressure relief valve that sometimes doesnt re-seat properly once comp has cooled down giving engineers reason to suspect comp is not pumping.A good whack with a spanner usually sorts this

[VRVIII]

Hi Mark,
Is it a scroll compressor? If so it would over heat fairly quick and eventually cook if running in reverse.
Why would the unit trip on LP with a suction pressure of 6bar? Are your gauges connected to the suction/discharge local to compressor?
Yes a 4 way valve can bypass if stuck half way but I would still expect to see a slight press difference.

[VRVIII]

.A good whack with a spanner usually sorts this[/quote]

I like your style Paul
Take it it's not a scroll then, do they still use Maneurope comps?

[Brian_UK]

Take it it's not a scroll then, do they still use Maneurope comps?

Should be a scroll, that's what the S stands for in the model number.

[Gary]

the other thing is these comps have an internal pressure relief valve that sometimes doesnt re-seat properly once comp has cooled down giving engineers reason to suspect comp is not pumping.A good whack with a spanner usually sorts this

I find that a rubber hammer leaves less evidence.

A leaking 4-way can indeed bypass, although there should be some small pressure difference... but as we are in the mood for whacking things, what the hell... its worth a try.

[marc5180]

Yes the compressor is a scroll, i doubt that the compressor is stuck now since it was only installed new today.

If the compressor is running backwards can i just swap any of the phases round? I only tried swapping 2 of them round last time.

The gauges are connected to the suction and liquid service ports, there isn't any internal service ports.

[compressormick]

is there a suction strainer at comp end?

[marc5180]

Hi, no there isn't a suction strainer

[VRVIII]

Yes the compressor is a scroll, i doubt that the compressor is stuck now since it was only installed new today.

If the compressor is running backwards can i just swap any of the phases round? I only tried swapping 2 of them round last time.

The gauges are connected to the suction and liquid service ports, there isn't any internal service ports.

Swapping any 2 phases will reverse the motor direction, does the unit not have a reverse phase protection device? This would verify correct direction providing the wiring after the device is as per the wiring diagram.

[NoNickName]

The noise of the compressor is totally different when running the other way around. You will easily realise whether this is the case.
And no, the 4-way valve can't cause what you are saying, because you closed the suction valve and the pressure didn't fall.
Secondly, no LP trip can occur at 6 bar, can it?

Is the shell warm where the motor is supposed to be? If so, then it could be wither a NRV stuck open or a safety valve blowing by.

[TRASH101]

Hello marc

Is there any evidence to support (other than your gauges) that the compressor isn't doing its job. Is there no changes in temp around the associated pipework? A temperature difference between suction and discharge on the compressor?

[monkey spanners]

The noise of the compressor is totally different when running the other way around. You will easily realise whether this is the case.
And no, the 4-way valve can't cause what you are saying, because you closed the suction valve and the pressure didn't fall.
Secondly, no LP trip can occur at 6 bar, can it?

Is the shell warm where the motor is supposed to be? If so, then it could be wither a NRV stuck open or a safety valve blowing by.

NoNickName, I think the service valves are where the interconnecting pipe work connects to the unit, like a split ac system and not on the actual compressor, so having gauges here will not show what the compressor is seeing.

Is there anyway you could connect to where the lp switch is? if its one of those small round jobbies with a 1/4" piece of pipe sticking out, could you fit a line tap valve or piercing pliers and then put your gauges on here? (assuming the LP is between the comp and the reversing valve)

[NoNickName]

http://www.modine.com/web_server_con...12_CFCU_UK.pdf

quoting:

SERVICE VALVES
CU./CUH. 1 - 7.5 Units are provided with suction and liquid line shut-off valves fitted with Schrader
connectors to facilitate connection of a service gauge manifold (except the CUS 5 - 7.5,
which have no Schrader connection on the suction valve).

So please recheck it with suction valve closed, if you didn't yet.
It also says that there may be an optional hot gas bypass valve fitted. I would bet a fiver on it.

[acnerd]

I think you guys have covered just about everything. Knowing my luck, if it was my job, I would probably have been supplied with a duff new compresor!!

[marc5180]

Thanks for all the replies guys, i was back there today this morning just finishing off the maintenance that i had started.

I used a magnet on the 4way valve and could hear the magnet being pulled across, i even tapped it with a rubber hammer.

I ran the system up and on heating the compressor would run but the suction and discharge pressures stayed the same at 8 bar each ( theres nothing wrong with my gauges since i checked them on another unit). Eventually the compressor would trip on internal overload.

I then ran the system up in cooling by placing a magnet on the 4way valve and the discharge stayed at around 8bar but the suction kept dropping and eventually cut off on LP.

The discharge pipe temp going into the 4 way valve got up to 43°C and coming out of the valve was around 35°C. The suction entering and leaving was around 17°C each but the compressor didn't run long enough to take a proper reading.

The compressor doesn't have sduction and discharge service valves just liquid and suction service ports on the side of the condensor.
The only other things that could restrict the flow are TXV's but if the cooling TXV was faulty then i would still expect the system to work in heating and vise versa.

There is no HGB on this system just 2 TXV's with small non return valves, a drier (which i changed) a sight glass, compressor and a 4 way valve.

If it was just a problem with the system pumping itself down then it should be a relatively easy task but since on heating the compressor doesn't seem to pump has me stumped.

[Gary]

Sounds like a 4-way valve problem to me.

[VRVIII]

Mark,

When energizing the 4 way with your magnet you would hear the solenoid clicking but this only opens a small pilot valve allowing gas through to push the plunger across, but the valve also requires a 4 bar pressure difference to push it across.

Did you charge liquid in the suction line after evacuating? Most scroll compressors have a discharge reed valve that can be damaged by liquid pumping.

You would really need check suction/discharge pressures at compressor to verify what's happening.

[GHAZ]

i had a similar problem about 2 years ago on a airedale heat pump it had 2 circuits with scrolls and 1 circuit every time it ran it went out on lp and it only had 1 drier ,2 txv and a four way valve, the drier is special it goes both ways and the problem was 1 off the txv on cooling mode it was not openning,so change for new one and it was fine and the only reason i knew it was going out on lp was by checking the break on switch, in heat mode it was fine

[marc5180]

Mark,

When energizing the 4 way with your magnet you would hear the solenoid clicking but this only opens a small pilot valve allowing gas through to push the plunger across, but the valve also requires a 4 bar pressure difference to push it across.

Did you charge liquid in the suction line after evacuating? Most scroll compressors have a discharge reed valve that can be damaged by liquid pumping.

You would really need check suction/discharge pressures at compressor to verify what's happening.

Yes i charged liquid into the suction and the liquid but i left the crankcase on for a few hours to boil the liquid off.

[marc5180]

i had a similar problem about 2 years ago on a airedale heat pump it had 2 circuits with scrolls and 1 circuit every time it ran it went out on lp and it only had 1 drier ,2 txv and a four way valve, the drier is special it goes both ways and the problem was 1 off the txv on cooling mode it was not openning,so change for new one and it was fine and the only reason i knew it was going out on lp was by checking the break on switch, in heat mode it was fine

Thats the thing, if it was only doing it in heating or cooling then the expansion device would be the suspect part but it is doing it in both heating and cooling

[VRVIII]

Yes i charged liquid into the suction and the liquid but i left the crankcase on for a few hours to boil the liquid off.

Did you get to the bottom of this fault or did you have to replace the compressor?

Leaving the crank heater on would only boil off liquid from the compressor, if the suction line was full of liquid this could have damaged the discharge reed valve due to liquid pumping when started.