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iceman007
07-04-2006, 10:43 PM
I had a look at a chiller this morning, a York screw chiller. The thing has tripped out on the fault code low current/mp/hp. Manual Hp switch hasn't tripped, HP wasn't that high anyway. Low current I don't think was to blame. I noticed though that the discharge temperature was a bit on the high side at 85 degrees c or so. Bear in mind that the ambient outdoor temp is still a bit low, so the condenser has probably got a bit of excess capacity (in the summer I think that high discharge temp might have tripped out the chiller). On the black box/CT/phase protector (whatever you want to call it) the number 5 is displayed, relating to high temperature, so I suppose a high current draw might have caused this. I think taking everything into account, the thing has an overcharge, even though the manufacturers specified weight had been charged into the unit by the service company. I have isolated the problem circuit to avoid it running and advised the customer.
Any other ideas?? A second opinion I suppose is what I need.
Thanks

NoNickName
07-04-2006, 11:08 PM
Is it a millennium chiller? If yes, then it is the dreaded starting contactor bouncing and kriwan protection tripping.

US Iceman
07-04-2006, 11:10 PM
Does the black box also contain a device to measure the motor temperature with thermistors?

It could be the motor overheating due to lack of suction gas flow. If the discharge temperature is a little higher than normal, the suction superheat may also be high.

If the ambient temperature is still fairly moderate I would tend to look on the suction side of the compressor for the problem.

NoNickName
07-04-2006, 11:14 PM
Kriwan INT69 normally controls also the discharge temperature, but is set on 130°C (or 120°C depending on the model and year of production).
And Iceman is correct: this time of the year, chillers are unloading frequently, with the result of a low flow through the motor.

iceman007
08-04-2006, 09:53 AM
Its a millenium chiller. The first thing I did was swap the auxilliary contactors with the ones on the other circuit that was running OK, because I had heard that they hada tendency to play up and bounce on startup, tripping the chiller on low current. I think that the problem might (I only say might) be the motor protectors overheating, as a 5 on the black box klixon display relates to an over temperature. Suction pressure is good, discharge within limits, but as I mentioned before at this time of year you won't always see high temps and pressures due to unloading and excess condenser capacity, in the summer then the problem i would expect to be more obvious. I felt that an overcharge might be the problem, but maybe it's a less obvious fault (I once had a TEV problem that caused this). The plate has a chrge of 65kg, which is what was weighed back in after an earlier repair. Everything else seems fine apart from as I said higher than usual discharge temperature(88 degrees, where usually in this ambient it should be 65-75 degrees). The other thing that leads me to this fault is a low suction superheat (read from the panel) of no more than 2K at most-at one point read at -0.5K (hence why I shut down the circuit).

Many Thanks
James

PS To US Iceman-yes there are thermistors to measure on each phase.

NoNickName
08-04-2006, 12:16 PM
We've got two icemen in one thread.
No: high temp is more obvious at low load and not in summer.
And, I would not trust the klixon diagnostic.
Low superheat may be TEV related as well.

Andy
08-04-2006, 01:47 PM
Hi Iceman 007:)

I would take a look at the flow thru the evaporator. On low flow the suction superheat would be low, the head would be higher than seasonal and the evaporation would be low. The low evaporation will cause the discharge temperature to rise, if there is a thermistor in the discharge line this will explain the tripping.

Usual log would be require to confirm

Evaporation
Condensation
Suction temp
discharge temp
liquid subcooling


Kind Regards Andy:)

Andy
08-04-2006, 01:49 PM
Hi Iceman 007:)

An overcharge would show higher evaporation than normal and a hunting of the TEV may also be common.


Kind Regards. Andy.:)

US Iceman
08-04-2006, 02:35 PM
We've got two icemen in one thread.

You cannot have too many icemen. :D

There used to be a lot more of us, except no one wants ice delivered to their homes anymore. :( A sad turn of events I say.

US Iceman
08-04-2006, 02:48 PM
During low load operation the mass flow is significantly decreased. At the same time the motor input power can be disproportionately higher than you might expect.

The part load performance of screws is not that good. When you combine the effects of low load with the higher amount of motor energy per kg of refrigerant flowing into the compressor, the actual suction temperature entering the compressor can be elevated. This occurs in the compressor, so you cannot measure it.

As an old man once said, what goes in, must come out.

The higher suction temperature (more superheat) is directly related to higher discharge temperatures.

When the compressor loads up, the amount of refrigerant mass flow increases. This provides more cooling for the motor (I'm assuming semi-hermetic or hermetic of course), so you will see the discharge temperature decrease.

For a confirmation... What does the manual say the over temperature is for? High discharge temps or motor temp?

The fact that you are seeing low suction superheat confirms the low load situation. I would expect to see slightly higher superheat at full load conditions.