Newbie advice

[costa repairs]

Hi all, new here, had a read and seems a handy place. Had great advice on welding forums so hoping this will be similar.
Been in electro / mechanical engineering for 20 years, the last 10 been repairing catering equipment. 2 years ago I qualified as class 1 F-gas, and been spending the last 2 years actually learning the job.
I reckon I'm at the tip of the iceburg.!
Anyhoo, a question to open with.
How would a seemingly leak free system gain pressure? I don't mean the obvious like blockages etc
but a unit I've been working on has me stumped. Its an external compressor / condensor unit for a shop display fridge. It cuts out on high pressure almost daily. Gas running pressures are good, yet after about 20 hours...POP it's gone again. Never does it when I'm on site of course!
Its on R404 if it matters. I had a similar one last year turned out to be condensor fan, but its pulling correct amps, starts up ok. Been through the motions and everything seems fine. Until about 18 hours after I leave!
Cheers for any words of wisdom.

[chemi-cool]

Replace the 404 with 507 and all will be fine.
404 can act funny at times.

[monkey spanners]

How clean is the condenser? Would be worth either washing it out or blowing out with nitrogen. They can be blocked but appear ok. There would be a higher cooling load and hence condenser heat rejection load after a defrost etc, this could account for delay in tripping.

Is the HP switch operating at the correct pressure? They can go out of adjustment, once they do so they will only get worse so it would want replacing if this is the case.

[charlie patt]

could it be a hubbard unit? or a possibility after df it floods back and brings your suction up and pops hp ie no cpr or mop valve after 18 hrs doubt if df, is there any external influences ie does it trip same time every day? you may have to consider fag brakes etc if all pressures good during testing

[costa repairs]

It does seem to be around the same time each day ( middle of the night )
Although that is also after around 18 hrs run time, so could be run time rather than time.
The hp switch could be at fault, I assumed it was ok because it will run all day with the correct pressures. However it used to last from 4 days to 2 weeks before tripping, now it is almost every night.
Will have another good look tomorrow. Cheers

[RANGER1]

Does it have a defrost, if so what type & time?

If it cycles off on temperature/defrost, does it pump down, if overcharged that may cause it, filling up condenser.

Simulate it to see what happens while you are on site.

Does it have a liquid receiver?

I would also check cut out settings of all controls, high, low pressure, fan cycling switches etc & post them here.
Also normal running pressures & case temperatures, evaporator superheat.

[Brian_UK]

Also check for foreign objects, rubbish bags etc., being tossed against the condenser.

Any neighbours getting annoyed by the noise in the middle of the night?

[costa repairs]

Hi Ranger , the defrost cycle is a simple 240vac timer as used on boilers / plugs its set for 30 mins downtime every 6 hours. It's an Arneg unit ( italian / Portuguese made ) it trips high side with the defrost timer by-passed. It is a bespoke unit for the site, there are 2 side by side. Yes it has a liquid receiver about 4 litre I think, will get more info and pics tomorrow. The internal fridge / evap side are very simple, the problem is in the condensor unit, or at least it thinks it is!
Brian, there are 2 condensor units side by side and 3 display units in shop, no, there are no obvious blockages and its on a large resort, so neighbours aren't an option. Basically, I reset it, it gets down to temp like a good un, no over-frosting, no trouble with fan starting, no problem, gauges all say its happy. But after 12-18 hours she trips. Will post pics of unit tomoz, it may be familiar to some! Arneg stopped supporting it as its over 5 years old. Cheers guys, even without a definite answer, its an education.

Does it have a defrost, if so what type & time?

If it cycles off on temperature/defrost, does it pump down, if overcharged that may cause it, filling up condenser.

Simulate it to see what happens while you are on site.

Does it have a liquid receiver?

I would also check cut out settings of all controls, high, low pressure, fan cycling switches etc & post them here.
Also normal running pressures & case temperatures, evaporator superheat.

[al]

how many units is this condenser feeding? Is it possible that one of the indoors shuts off, unit is slightly overcharged, partially pumping down and then tripping HP? Or condenser fan motor is overheating, does the unit cycle during the day, it shouldn't be running 18 hours continously?

[costa repairs]

how many units is this condenser feeding? Is it possible that one of the indoors shuts off, unit is slightly overcharged, partially pumping down and then tripping HP? Or condenser fan motor is overheating, does the unit cycle during the day, it shouldn't be running 18 hours continously?

Hi, no it is cycling through the control stat, and it is only running one chiller unit.

[monkey spanners]

Check the fan capacitor too, if its weak the fan may not start or run backwards.

[install monkey]

get a temp data logger inthe cabinet and a cctv cam on the condenser- then if it goes off you can see if its a neighbours cat lying infront of the condenser- otherwise camp out!
test the hp switch does trip at its stated pressure