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jan behnke
11-07-2004, 09:30 AM
I have been asked to look at this unit, it’s a dual circuit condensing unit with two compressors on each side.
The problem is on start up in hot weather they bang out on h/p intermittently, discharge go’s to 29 bar straight away and triggers the h/p. another company looked at the unit last summer and suggested non return valves, I think the fault is that on start up both compressors on each circuit are coming on at the same time I think there should be a delay, as usual there is no info on site the unit is nearly 3 years old.

rbartlett
11-07-2004, 09:54 AM
fridge geezer will be here for this one..get model numbers first though


cheers

richard

Peter_1
11-07-2004, 10:50 AM
I have been asked to look at this unit, it’s a dual circuit condensing unit with two compressors on each side.
The problem is on start up in hot weather they bang out on h/p intermittently, discharge go’s to 29 bar straight away and triggers the h/p. another company looked at the unit last summer and suggested non return valves, I think the fault is that on start up both compressors on each circuit are coming on at the same time I think there should be a delay, as usual there is no info on site the unit is nearly 3 years old.

If unit is going to 29 bar, then in my opinion fitting a NRV won't solve your problem at all. I don't see a relation between coming up together and going so high in pressure.
Condensor is selected/made for those two compressors.
And it worked always in the past.

You can easily try it yourself by switching off one compressor (for example push on the button of the overload relay or unscrew a wire of one relay or fit a switch between one relay....)

Is there a liquid receiver?
TEV or capillary?

How fast is it going in HP? Is it in a few seconds or after some minutes?
If it is in minutes, how high is discharge temperature and liquid out of the condensor?

If it happens in a few seconds, you probably have a restriction somewhere.
If it is in a longer time (speaking in terms of 30 seconds to minutes) then possible causes: tech before you overcharged it (probably he charged in the winter till his HP was OK and then overcharged it)

Air entered in the system?
Clogged dryer?
Condensor clean?
Has the other tech charged wrong gas in the system?

Is it that airco model (can't find/remember the type number) with the 2 independent compressors with their own condensor, woven into each other) or is it more a high capacity model with Trane scrolls with the electronic PCB board?

Advice: First check if you have no restriction somewhere (dryer, service valves..) Then evacuate gas, vacuum system and restart complet with gas charging. Then you're sure for yourself that there is no air in the system, proper gas and proper gas filling. You eliminate so many possible causes.

Let this forum know what you found. We all can learn of it.

chemi-cool
11-07-2004, 06:07 PM
Hi Jan.

Had similar problem some time ago. Condensers were of the roof. Installed NRV on each discharge line about 20cm after the discharge king valve and problem gone.

Tha discharge line was full with oil and on start up there was hydraulic pressure and HP tripped.

To make sure, close the king valve and realese some pressure into a bucked and see what comes out - gas or oil.

Chemi

jan behnke
11-07-2004, 09:15 PM
Thanks for your help i think traped oil could be the problem the AHU are about 15m above the unit, there are mag valves fitted to the liquid line's ( i wonder if they are slow to open)
comps are not scrolls and no electronics,

Peter_1
11-07-2004, 10:06 PM
Thanks for your help i think traped oil could be the problem the AHU are about 15m above the unit, there are mag valves fitted to the liquid line's ( i wonder if they are slow to open)
comps are not scrolls and no electronics,

Chemi could be right: trapped oil or even trapped liquid after a longer standstill time.
Is there no siphon fitted in the discharge just after the compressor?
This is then a fault of whom initiallty installed the system.

chemi-cool
12-07-2004, 06:09 AM
Chemi could be right ?????????????? :) ;)


Chemi

Peter_1
12-07-2004, 11:41 AM
?????????????? :) ;)


Chemi

Well... you said :'Tha discharge line was full with oil and on start up there was hydraulic pressure and HP tripped'

chemi-cool
12-07-2004, 12:52 PM
Chemi could be right ??????????????? ;)
Yo peter.

This is what it should look like.
Chemi

FreezerGeezer
13-07-2004, 09:48 PM
Ta Richard! :rolleyes:
Right, Jan, what's the model no. for a start, and please describe the installation - pipe run, pipe size, refrigerant, riser length - any traps fitted?
If it's a 3 year old Trane, and it's got 4 compressors, then it's probably a 3-D scroll or similar. Are the compressors CSHA100k / 150K's? I see that there's no SMM (computerised controller)...

Got to go - I'll try to answer more at work tomorrow. ;)

jan behnke
14-07-2004, 07:27 AM
I have been booked in to return some time this week, i will get all the info, yes it has four scrolls in it, as far as i can see there are no oil traps, from memory liquid 1 1/4 suction 2 1/4 inch 3 bends in the pipe run about 15m away from the cond,s all above the unit

Gary
14-07-2004, 09:11 AM
... the AHU are about 15m above the unit...

What does this mean? Evaporator above compressor? Condenser above compressor?

Peter_1
14-07-2004, 09:31 AM
No, the AHU consists only of the evaporator coil, fan and filters and the condensing unit holds the condensor and compressor.

So the evaporator is 15 m. above the condensing unit, a common setup I should say.

Gary
14-07-2004, 02:58 PM
A common problem with this type of setup is migration of liquid to the evaporator during the off cycle. Gravity delivers the migrated liquid down the suction line to the compressor, causing problems at startup.

Looping the suction line up above the evaporator then down to the compressor traps the liquid in the evaporator.

Liquid line solenoid valves can prevent the migration if they are at the evaporator, especially if they pump down before the unit shuts off. If the solenoid valves are at the condensing unit (without pumpdown), then the contents of the liquid line will end up in the evaporator, and possibly the compressor.

FreezerGeezer
26-07-2004, 08:19 AM
Hey Jan, have you got the details yet? If I know what the unit is, I might have an IOM somwhere that will tell us if you need traps, etc. Or is the problem solved?

ATB
FreezerGeezer
:cool:

jan behnke
27-07-2004, 08:37 PM
Sorry have not been back to it yet, due to work load, but there is on thing, there are two scroll,s in each circuit, looking in the panel i think tere was four delay timers for all the comp contactors, but when each circuit came on there was no time delay for the second comp they both came on at the same time, would this cause the head pressure to go to 29bar in a few seconds and trip the h/p, if there was a delay would this lower the head pressure on start up.
thanks Jan

FreezerGeezer
28-07-2004, 02:02 PM
Hello Jan

Yes, it may be possible, especially if the condenser fans aren't kicking in correctly. I seem to remember an IOM from Trane that said that traps were required in the liquid line if the evap. was higher than the condenser, to stop oil & liquid forming too heavy a seal on the condenser. However, I may well be getting mixed up. I need to talk to someone on that. It would really help if we knew the model no. of the unit. I take it that this one is electro-mechanically controlled, rather than with a SMM (computerised controller for Trane 3-D scrolls)? Trane normally have a compressor on-on delay of a couple of minutes on SMM controlled scrolls, if I remember right.

chemi-cool
28-07-2004, 02:26 PM
Hi Jan.

It was not clear from what you have written if the condenser is higher then evaporator or the other way around?

Scrolls take time to build up high pressure not like reciprocating that build it up in no time.

Please, post a clear description of the unit.
It will make it easier to come up with an idea.

Chemi

frank
28-07-2004, 07:22 PM
would this cause the head pressure to go to 29bar in a few seconds and trip the h/p,

it certainly would if you didn't have a complete circuit i.e. a closed solenoid valve or the condenser fans were not working

chemi-cool
28-07-2004, 07:34 PM
Hi Frank.

In the case where SV is closed, there is no suction, therefore, the head pressure will not rise much even if condenser fans not working at start up.

The condenser is designed for tow compressors so I don't think the problem is there.

Gauges should be fitted to see if the pressure is that high at start up or maybe the pressure controll is foulty.

Un fortunatly, Jan doe's not supply enough details we kind of guessing all the time.

Chemi :)

FreezerGeezer
28-07-2004, 10:09 PM
Hi Marc

That's what I was thinking - I'll have to see if I can find the IOM I mentioned earlier.

Jan, any more detail you can add would be good. Is there anyone on site who can supply the model no?

Gary
31-07-2004, 10:47 AM
More likely the IOM referred to trapping the discharge line if the condenser is higher than the compressor.

FreezerGeezer
02-08-2004, 08:25 AM
Hi Gary, Marc. Yes, I think you're right - as I said, I can't remember the thing exactly, and it may well be talking about remote condenser applications. Anyway, I'll see what I can find. If I ever get 5 minutes to look! :D

Leo1967
03-08-2004, 12:46 PM
Traps are for oil,if the condenser is higer than the compressor they should be installed on the discharge line every six metres of vertical pipe.If the compressor is higher traps should be installed on the liquid line but not more than one as there's the risk of flash gas,that's liquid gas that evaporates when the balance between pressure and temperature is lost.To help avoid this you should have a high subcooling.

jan behnke
05-08-2004, 07:31 AM
Sorry have been left out of the loop on this one by my service manager, asked him what was happening, the owners of the kit have looked at there service records and decided to call back the company that installed it apparently it been like this since it was installed. I would like to know the out come so I will try and find out, thank you all for your time and effort, its nice to know there is a lot of great help out there.