Mitsi heavy disaster

[Andy AC]

Got a call to a mhi fdc1008 25kw twin cassette system yesterday. The message I got was - we was out the back having a smoke, then suddenly a loud bang and loss of gas leaking out, scared the crap out of us.

Anyway, turned out to site and sure enough, no gas left in system. Traced the leak to the top row of the bottom of the two coils. It's an old R22 system, so I thought I would bypass that section of coil - only 3 or four rows, it'll be alright. Pressure tested it, vacced it, and because I only had half a dumpy of R22R, half filled it, and came back today to finish off.
When I switched it on, machine failed to start because of one of the phases were missing. Eventually found some fuses, put them in, ran in test mode for 3 seconds then a big shower of sparks and stopped. Started testing components, found solenoid coil faulty. Put a new fuse in, and before connecting all the components, pushed the contactor in, compressor ran for a few seconds, then started wobbling about all over the place as it lost a phase again.
Tried another fuse incase the one I put in was weak, pushed the contactor, which resulted in a large bang as the other end of the condenser coil let go, R22 everywhere:rolleyes: - not the same section as was gone before, this was the upper of the two coils this time.

I put an insulation tester on the compressor - each of the windings were around 70Mohm - far too low.
Would this have anything to do with the coils blowing?
If so, can somebody explain why? Could it be something like poor earthing and the fault current finding the weakest part of the system?
I was suprised to find that the system was fed from an old MEM 3 phase switchfuse, with separate fuses, surely not a good idea in itself. Was expecting to find 3 phase breaker, but no.

Your thoughts/explanations welcome.

Andy

[james10]

Like you say a leakage to earth sounds like the likely cause,after all copper does love a bit of the old electrickery

[monkey spanners]

Sounds like it has earthed out through the condenser!

[SteveCass]

I'm working on a site with about 15 Mitsi Heavy VRF condensing units and have found 2 dead comps so far...... i am worried...... ;-p

[nike123]

I would say that unit has serious blockage somewhere in circuit and that compressor build excess pressure until weakest point break. Usually in these units there is no pressure relief valve and only way to release execs pressure is bursting at weakest points.

Humming and circuit breaker burning of compressor is because compressor is doing hard work, which could stall and pull excess amperage, and therefore burn circuit beaker.

70 Mohm with insulation tester is excellent insulation.

Did you done nitrogen blowing thru circuit to see if there is blockage somewhere before pressure testing?

[Andy AC]

I've only managed to have the comp running for a matter of seconds + it only had half it's charge in at the time. When it blew, it blew the moment I pushed the contactor in, the comp didn't have a chance to pump.

Andy

[nike123]

What solenoid valve you are changed?

Still, 70 Mohm tells me that there is no conection between compressor insulation state and these blow-ups.

[Andy AC]

I haven't changed the valve, just disconnected the coil.

[nike123]

I haven't changed the valve, just disconnected the coil.

What is function of that valve?

[Andy AC]

It's the main reversing valve - I ain't fussed whether its in heating or cooling mode. The compressor takes out one of the phase fuses each time.

[nike123]

Disconnect compressor from refrigerant circuit and try it like that.

[Magoo]

Hi Andy ac
Are you using motor rated fuses, have you checked the current sensor module PCB, is the reverse valve jammed midway, what are your gauges doing when compressor starts. Have you checked contactor continuity between phases. Weird how condenser tubes errupt.

[Grizzly]

Andy!
Despite your best efforts surely you are at the point.
Where it would be sensible to admit defeat, given that it is an R22 system.
I am with the short to earth guys, by the way.
Cheers Grizzly

[nike123]

Andy!

I am with the short to earth guys, by the way.


If we presume that his measurements of compressor insulation are OK, then compressor is surely not short to earth.
Even 0,5 Mohm will be enough to say that.

Also, to erupt condenser by passing current (which is sience fiction to me), that current will make jump somewhere and entrance point will be clearly visible and I would expect traces of melting copper on exit point.

[Andy AC]

Hello Grizzly, I have given up on it, doubt we would get all the bits anyway.
Just wondered what you guys thought of the shorting to earth theory - I've never had this sort of spectacular failure before, especially when stood next to it. Seemed the most plausible to me.
There's got to be more to the full story, but I'll never know - could have been running short of gas, blocked, rv jammed etc.
Have been told that it will have to wait until February, most likely get ripped out and have 2 x 12.5 kw single split cassettes fitted instead.
Scrap man be happy:) lots of 1 1/8 copper to come out.

Andy

[momo]

Out of curiosity - what sort of condition was the condenser coil in? The Isle of White probably gets its fair share of salty air. Did you check the state of the connections and the state of the contactor? You could have a higher supply line resistance on one of the phases (dicey connections) that would cause a voltage imbalance on the three phases (compressor wobble and solenoid fireworks). Watch out for protection of inverter electronics in the mentioned environement.

[SteveCass]

If you pressed the contactor in, then no safeties were in use and similarly the solenoids would be closed. Is it not possible the discharge pressure built up and popped the end off? Did you have gauges on the comp when it blew ( and i don't mean at the pipework outlets but inside the unit near the comp)?

[Andy AC]

When it blew, the compressor didn't have time to run, it blew the moment I pushed it.
The coil itself, was in very good condition considering it's not far away from the cliff edge. Is pretty sheltered though.
The phase that kept blowing, was not the same phase as the control circuit, just fed the compressor, so separate from the solenoid.
All 3 phases had pretty even voltage.
The solenoid in question was the main reversing valve, and the only one on the system - so pretty basic compared to more modern systems.
Didn't have the gauges on the system at the time, I was more concerned about getting the system to run electrically before I could add anymore gas.
Had given up on it, just about to put it back together and leave, which is when I pushed the contactor the last time.
I can't see it being a high pressure fault taking the coil out, as when the first time it happened, it was in operation, with all the safties in place.

Andy