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airconadam
30-05-2007, 05:48 PM
hi fellas i got called to a chiller the other day which was faulting low pressure,so i thought ok, low refrigerant maybe a leak or faulty low pressure switch.This was on r22 and 15kg of it so i thought i would reclaim the refrigerant and start from scratch. Started to reclaim the refrigerant and went for a brew as you do when i got back one bottle full 10 kg, put another one on thinking well ther cant be much left i got another full 10kg:eek: put another bottle on and got a further 5kg iwas gobsmacked in thinking i had got my calculations wrong:confused: but sure enough i got 25kg thats 10kg overchared now does that make the the unit trip on low pressure and if so why:confused: :confused: its prob something quite simple:rolleyes: and why would someone of put 10kg over the correct charge thanks alot

adam

monkey spanners
30-05-2007, 06:58 PM
Hi Adam

Someone has been there before, found the unit tripped on lp and put 10kg in to "fix" the problem.
I'd check for faulty/blocked tev, blocked drier or other restriction. whats the water flow through the evaporator like? Low flow=low load=low sucton pressure. The one (and only, so i'm no expert on water chillers) i worked on had a hot gas bypass solenoid to keep the evaporating temperature above freezing, maybe the coil has gone on that.

Jon

lana
30-05-2007, 07:09 PM
Hi Adam,

Low pressure fault come from the followings :

1- Lack of refrigerant
2- Lack of expansion device capacity
3- Pre-expansion on the liquid line
4- Lack of evaporator capacity

The first three faults will cause the super heat to increase but the fourth on makes the super heat to decrease.

Here you have two problems conflicting each other. So what I suggest is to reclaim the refrigerant and then check for any leaks , evacuate the system and charge again.


Good luck
Cheers

Brian_UK
30-05-2007, 07:44 PM
Not unusual Adam, check the TXV and any other devices that are in the circuit; something is shut/blocked.

We're always glad to hear from you at the end of the day. Some of your tales of woe make our own day not seem too bad ;) :)

airconadam
30-05-2007, 08:07 PM
well the job is done now anyway:D but i done as lana said reclaimed,pressure tested no leak found then vaced out and recharged and unit has run fine no problems at all. Weird why it has tripped on lp before or maybe not but someone somewere as added 10kg amazing but dodgy:eek: if you ask me. Another thing what makes me wonder is the way it has ran fine and no more probs at all but hey ho :confused:
thanks alot

adam

cool#9
30-05-2007, 08:30 PM
Are you sure it was the same refrigerant added ?
Do you have a liquid receiver on this chiller ?
Did you make a strong vaccum and then perfectly remove moisture ?
How did you made your pressure test ? Make sure there's no leak inside the evaporator between gas and water, check first the water pressure: is there abnormal variation ?
Did you have to add water in the indoor side ?
rgds
cool#9

frank
30-05-2007, 08:52 PM
I've had similar situations in the past where the low pressure has caused a trip but when you restart it runs normally.

I got called to a York chiller the other week which had been maintained by York (the installer) for a number of years. We look after a number of other units on the site but the customer had become dispondant with York after paying loads of money for faults found at each service visit.
From the history (service sheets) it would appear that circuit 1 had more than it's fair share of faults, especially a low suction pressure. Each entry in the site log said that circuit 1 had a leak so it must be reclaimed, repaired and recharged.

This seems to have been carried out 3 times previously.

Anyways, I had a look at the beast, and after about 40 minutes (and a coffee later) I got back to the machine and found the lower part of the compressor (scroll) severly frosted as was the suction line.

It seems that the MOP TEV was intermittantly playing up. I've replaced it and it now runs 'as sweet as a nut' :D

I would change the TEV on your unit for peace of mind.

marc5180
30-05-2007, 09:06 PM
Frank... Whats the best way to check if your Expansion device is working properly... is it by checking the superheat? because iv also come across Chillers where the lower part of the compressor is frozen:confused:

frank
30-05-2007, 10:34 PM
Frank... Whats the best way to check if your Expansion device is working properly... is it by checking the superheat? because iv also come across Chillers where the lower part of the compressor is frozen:confused:

You can only check superheat properly when the system is stabalised and close to design temperatures. During pulldown you will measure high superheats that will reduce as product/space/etc. temps drop.

With the severe frosting on the lower part of the compressor, I was reading near normal operating pressures one minute and then very low suction pressures caused by a blockage. The blockage was traced to the TEV.