Random high pressure faults, R407C

[petter78]

Hi all,

I am having some trouble with an airconditioner that is failing with a high pressure fault at random when cold outside.

The airconditioner consist of a Liebert Hiross with a Maneurop scroll, a receiver, a solenoid valve and an evaporator. On the roof 5 stories above it has a condenser with a check valve on the exit and a Johnson fan speed controller. The refrigerant is R407C.

The problem is that it has failed twice with a high pressure fault, both times without any traces of trouble afterwards. Both times it has been below 0C outside and the airconditioner has probably been standing still for some time before the fault.

After the first time I mounted a KP5 with manual reset, so I know the pressure has been high.

My theory is that the condenser is filled with liquid at startup and is not emptied fast enough and therefore the airconditioner fails. Is this plausible?

What do you think.

Regards,
Peter

[r.bartlett]

http://www.refrigeration-engineer.com/forums/showthread.php?1275-R-407-C-fractionation&

[The Viking]

The check valve (or NRV for Non Return Valve) should stop the condensor from filling up with liquid, also if liquid had filled the condensor up (assuming the system isn't over charged) the system would normally drop out on LP.

Your Johnson FSC, is it pressure or temperature operated? Is it working as it should?

:cool:

.

[pirestani]

Hi
it seams wondering .is it possible for you to show your schematic flow piping briefly contain the condition of equipments and controllers?

[martinw58]

do you have a check valve in the discharge pipe from the comp to stop liquid runing back from the condenser in the comp 407 can seperate and give hp problems have had this on spilt systems befor

[Magoo]

Possibly the hot gas riser after the check vav is filling with liquid in off cycle and cold ambients creating liquid head problems in riser at start up, may require a balance line between hot gas riser after check vav to top of receiver. Allows free draining in off cycle and gives receiver added start up pressure. and keeps reciever pressure high. Sporlan have a balance / differential vav orit or ord valve some thing like that for balance line

[pirestani]

I agree with Martinw58 and Magoo more than other assumptions.check every stop of your system with pump-down properly. maybe some stops occurred caused of power disconnecting or some faults.

[Magoo]

Hello pirestani.
can you tell us about all the political dramas going on in Iran currently. It would be good to here from someone living there and living with it. If too scary and difficult, don't worry. All is cool.

[install monkey]

condenser fan cutting out on overheat thermistor after prolonged running? increase minimum speed of fan and decrease the set temp value,blast cond coil with nitrogen-may be choked but not blocked fins can collect dirt within the fins although the face looks clean

[petter78]

But the faults has happened with outside temperature at 0C. So as I see it, that wouldn't give any trouble condensing the refrigerant even though the mix isn't right. What do you think?

[petter78]

I do not have a NRV after the compressor before the hot gas riser. The hot gas riser is running inside the building, it is only the last meter of piping that is outside in the cold. I think the refrigerant will end in the condensor rather than in the compressor.
The airconditioner is not set up to run pump down, it is a standard Liebert Hiross with a receiver and a solenoid valve in the liquid line. So the only amount of refrigerant that should stem up in the receiver is from the solenoid to the NRV right after the condensor, that shouldn't be a lot.

Regarding the minimum speed, I will check that, but when I checked it last with low ambient, the fan speed was not to low. The coil is clean. The condensor has to fans, but that of cause is no garantee that both couldn't stop on high motor temperature.

[byrne]

HI Could be faulty pressure transducer in fan speed controller,contracts not closing when set pressure is reached.

[matt p]

Morning gents,

The past couple if weeks I have been having the reverse problem on the same liebert hiross units. They are the 90kw downflows but during the cold spells we have just had and by the looks of it going to have again I'm trying to get my head around it.

I have already repaired 3 circuits from a total of 6. All had various leaks from poor shredder seals and a I had to change out two dsv nylon gaskets. All have been recharged with fresh R407c to the correct charge weights. The fsc works on pressure and not temperature and at the moment set to its minimum set point.

Last week the circuits ran like a dream but as soon as the ambient temp drops off the lp fault comes back. From monitoring the running of them the liquid line drops off to 6 degrees but a few mins of running it rises back up 30 degrees and the system chuggs along happily until its down to temperature.

Now I just get a phone call saying one unit dropped out last night and the onsite engineer is unable to get it going again as it pumps straight down. The only thing that I can
think of is that due to the cold liquid coming back from the condenser on start the oil in
the refrigerant is not passing through the TEV and causing a partial blockage and
causing the LP fault.

I'm off on holiday all this week but any ideas or pointers one what to check for when I go back would be great :)

Regards

Matt

[The Viking]

Condensor fan speed controller?
Short of charge due to colder ambient conditions compared to when charged?
If straight out, manual reset on P/S and site guy just resetting controls?

[Gary]

My theory is that the condenser is filled with liquid at startup and is not emptied fast enough and therefore the airconditioner fails. Is this plausible?


This is entirely plausible and in fact likely. The liquid line temperature being low on start up would seem to confirm this. The question then is how does the liquid get up there during the off cycle? If the liquid line solenoid is closed and there is a check valve at the condenser outlet, the bulk of the refrigerant should be trapped in the receiver. My best guess would be that the check valve is not providing positive shut-off.

The fact that the system is now shutting off on LP fault since charging correctly makes me think that this was the original problem and somebody added a lot of refrigerant to compensate, giving you HP faults.

[martinw58]

try fitting a delay timer to lp cut to so that lp is buy passed on start up to give system time to raise system pressure

[nike123]

I have same problem with few Galleti air-water heat pumps (different models and sizes) working with R407C. When ambient temperature is low enough, at defrost cycle unit goes to high pressure error. Also, same error is happening when unit fans have weakened capacitors and having lowered airflow because of that. That lowers evaporation temperature.
I concluded that is problem with fractionation of R407C at low ambient conditions.

When ambient temperature rises, problem disappears. That is happening usually at about -5°C or lower ambient temperatures.