Sweating fridge pipe

[sailaboutvic]

Our evaporatior plate had a problem and we had to have a new plate made up , unable to get one sent out to us .
Unlike the old plate which had the cap pipe inside the return pipe ,
it now has two separate copper pipe .
The fridge seen to be working of but return pipe is sweating quite a lot with water droplets .
Any suggestion what would cause this . ( over gas /under gas )
Also I should mention the return pipe as a plastic cover over it like a plastic hose , this was put on so when the copper pipe was fed around the fridge it wouldn't kink .
Would this be the cause of the problem ?
The compressor is a danfross 35

 

[vyv_cox]

Try this page, Vic. http://coxeng.co.uk/heating-and-refrigeration/recharging-marine-refrigeration-systems/

 

[sailaboutvic]

Try this page, Vic. http://coxeng.co.uk/heating-and-refrigeration/recharging-marine-refrigeration-systems/

Thanks Vyv may had sorted it by letting a bit of gas out.

 

[superheat6k]

Does the sweating stop before the cold pipe enters the compressor ? If so it is correctly charged, so avoid venting the pressure.

Personally I feel it is now time the excellent advice Vyv provides is modified, in regard to boat fridges, to comply with the Environmental laws that preclude venting of refrigerant gas from fridge systems. Just as we do not expect to dump our used engine oil in the seawater around us, neither should we be venting refrigerant gas deliberately to atmosphere.

 

[vyv_cox]

Does the sweating stop before the cold pipe enters the compressor ? If so it is correctly charged, so avoid venting the pressure.

Personally I feel it is now time the excellent advice Vyv provides is modified, in regard to boat fridges, to comply with the Environmental laws that preclude venting of refrigerant gas from fridge systems. Just as we do not expect to dump our used engine oil in the seawater around us, neither should we be venting refrigerant gas deliberately to atmosphere.

I agree. That particular piece of text was provided by Owen who, despite having owned and run a yachting refrigeration company for several years, has recently paid for himself to do a week-long training course. That entitles him to work on all types of refrigeration equipment, and more importantly to buy and recover the refrigerant. He would not vent gas and I will suggest that the text be modified accordingly.

 

[superheat6k]

I agree. That particular piece of text was provided by Owen who, despite having owned and run a yachting refrigeration company for several years, has recently paid for himself to do a week-long training course. That entitles him to work on all types of refrigeration equipment, and more importantly to buy and recover the refrigerant. He would not vent gas and I will suggest that the text be modified accordingly.

Excellent - C&G 2079-Cat 1 I assume. Nice one Owen.

 

[sailaboutvic]

Does the sweating stop before the cold pipe enters the compressor ? If so it is correctly charged, so avoid venting the pressure.
.

It's not so much sweating more dripping , I through I sorted the problem but not quite there yet , for sure it still not working how it should . And one small area of the plate is not quite frosted up .
I read the info Owen wrote and what penguins says but slightly under or over charge can be very similar , so still no wiser to vent or to charge .
I can buy gas and gauges over the counter here in Tunisia but unless I know what to look for I'm just wasting money , and if so call fridge engineer can get it right , what chance have I got ,
So if any one has any personally experience what to look for not just what written on the internet , I welcome your help .
Chicken bits will freeze in the oval plate but tub of ice cream will be soft to runnie on the highest setting

 

[superheat6k]

You didn't mention frost. How thick is the frost ?

Frost is actually a good heat insulator, so if it builds up on the cold surface of the evaporator plate the refrigerant can't boil properly hence it stays really cold right back to the compressor suction, plus the cabinet won't get properly colld, because the heat flow into the plate is drastically reduced. So defrost the fridge, making sure any ice slab behind the plate also fully melts, and then when it has been running again for an hour or so, observe where the sweat line stops, ideally just near the end of the tube rows of the plate where it exits into the return pipe to the compressor.

If the plate is icing after just an hour with the cabinet still say above 8C then this suggests the system is undercharged. Frost should only form when the cabinet temperature falls below 6C. If the system is undercharged then the suction pressure drops and with it the boiling temperature of the refrigerant. So in a normally charged fridge there is a temperature difference between the plate surface and cabinet temperature approx 4 to 5 C, but as the charge reduces this temperature difference increases and the plate surface falls below 0C thus forming frost. Then as the charge decreases further the frost line moves back along the evaporator pipe away from the outlet, so as there is less and less refrigerant within the plate after the sweat / frost line the evaporator becomes warm, until eventually there is no cooling effect at all.

Then if it is undercharged it must have leaked somewhere, so just pumping in more gas is only a short term fix, but fixing leaks on small boat fridges is notoriously difficult. But a common leak point is the schraeder fill valve. Leaks are normally accompanied by oil stains, which is best indication.

If you do add refrigerant firstly vent the line from the cylinder for a few seconds to purge any air, then only allow a few short bursts to enter each time allowing the fridge to settle down, so eventually with the compressor running the sweat (not frost) line is right at the evaporator outlet. If you buy yourself a simple fridge manifold from eBay the suction pressure back to the compressor when running should be ~2 - 2.1 bar, with the cabinet at ~ 6-8C and certainly less than 1.9 bar at any time (R134a pressures) indicates undercharge. The suction pressure will be higher if the cabinet is warmer, so let the cabinet come to correct temperature before measuring. Then when finally at the correct charge add one more brief puff to account for the small gas escape when the schraeder is removed and unscrew it quickly, but also ensuring the gas escape has stopped before the connector is completely undone. Wear a glove when doing this because frost burns are not pleasant.

If you do buy a gauge manifold then avoid keep putting it on because each time it is used a quantity of gas is lost priming the gauge line and manifold, which on a small boat fridge would have a notable affect within just 4 or 5 separate reconnections.

 

[sailaboutvic]

You didn't mention frost. How thick is the frost ?
the frost is a very thin layer you can just notice it if you run you finger nails Long the plate
Every thing I read so far says there should be a even layer of frost except on one prat of the plate inside the box there no frost But under neath the out side of the box in the same place there is frost , confused

Frost is actually a good heat insulator, so if it builds up on the cold surface of the evaporator plate the refrigerant can't boil properly hence it stays really cold right back to the compressor suction, plus the cabinet won't get properly colld, because the heat flow into the plate is drastically reduced. So defrost the fridge, making sure any ice slab behind the plate also fully melts, and then when it has been running again for an hour or so, observe where the sweat line stops, ideally just near the end of the tube rows of the plate where it exits into the return pipe to the compressor.the sweat goes as far as the connector by the compressor even the connector more wet then sweat

If the plate is icing after just an hour with the cabinet still say above 8C then this suggests the system is undercharged. Frost should only form when the cabinet temperature falls below 6C. If the system is undercharged then the suction pressure drops and with it the boiling temperature of the refrigerant. So in a normally charged fridge there is a temperature difference between the plate surface and cabinet temperature approx 4 to 5 C, but as the charge reduces this temperature difference increases and the plate surface falls below 0C thus forming frost. Then as the charge decreases further the frost line moves back along the evaporator pipe away from the outlet, so as there is less and less refrigerant within the plate after the sweat / frost line the evaporator becomes warm, until eventually there is no cooling effect at all.the cabinet is around 6c the oval plate is about -1.9

Then if it is undercharged it must have leaked somewhere, so just pumping in more gas is only a short term fix, but fixing leaks on small boat fridges is notoriously difficult. But a common leak point is the schraeder fill valve. Leaks are normally accompanied by oil stains, which is best indication.
The guy who fitted the plate had his gauges on for quite a while and said there no leaks
If you do add refrigerant firstly vent the line from the cylinder for a few seconds to purge any air, then only allow a few short bursts to enter each time allowing the fridge to settle down, so eventually with the compressor running the sweat (not frost) line is right at the evaporator outlet. If you buy yourself a simple fridge manifold from eBay the suction pressure back to the compressor when running should be ~2 - 2.1 bar, with the cabinet at ~ 6-8C and certainly less than 1.9 bar at any time (R134a pressures) indicates undercharge. Just to get this right if the pressure is under 1.9 bar it's under charge? What about if it's over 2.1 bar? Do it mean it's over charge? The suction pressure will be higher if the cabinet is warmer, so let the cabinet come to correct temperature before measuring. Then when finally at the correct charge add one more brief puff to account for the small gas escape when the schraeder is removed and unscrew it quickly, but also ensuring the gas escape has stopped before the connector is completely undone. Wear a glove when doing this because frost burns are not pleasant.

If you do buy a gauge manifold then avoid keep putting it on because each time it is used a quantity of gas is lost priming the gauge line and manifold, which on a small boat fridge would have a notable affect within just 4 or 5 separate reconnections.

 

[superheat6k]

Why did the evaporator plate need changing ?

If it is overcharged the pressure will increase but not by very much, in which case I would not expect to see frost on the plate at 6C cabinet temperature.

Did you notice if the chap fitting it put a vacuum pump on after he checked for leaks and before he charged the system, if so for how long, or did he perhaps just give it a good blast through with the refrigerant to purge the air before finally charging ?

If the latter then possibly some air and even worse moisture from the air freezing in the capillary tube from the condenser, restricting the flow, and causing the pressure to drop. Or possibly a poor brazed joint restricting the capillary pipe at the evaporator inlet. The capillary tube is very fine so getting the brazed joint right is critical. No easy fix for either of these. If it wasn't vacced out properly then the system needs degassing, properly purging through, evacuating with a decent vac pump for at least two hours and ideally overnight because this is the only way to get any free moisture out.

If you have to go to this effort then have the capillary brazed connection to the plate redone anyway to eliminate a partial blockage.

 

[sailaboutvic]

Why did the evaporator plate need changing ?

If it is overcharged the pressure will increase but not by very much, in which case I would not expect to see frost on the plate at 6C cabinet temperature.

Did you notice if the chap fitting it put a vacuum pump on after he checked for leaks and before he charged the system, if so for how long, or did he perhaps just give it a good blast through with the refrigerant to purge the air before finally charging ?

If the latter then possibly some air and even worse moisture from the air freezing in the capillary tube from the condenser, restricting the flow, and causing the pressure to drop. Or possibly a poor brazed joint restricting the capillary pipe at the evaporator inlet. The capillary tube is very fine so getting the brazed joint right is critical. No easy fix for either of these. If it wasn't vacced out properly then the system needs degassing, properly purging through, evacuating with a decent vac pump for at least two hours and ideally overnight because this is the only way to get any free moisture out.

If you have to go to this effort then have the capillary brazed connection to the plate redone anyway to eliminate a partial blockage.

Thanks for all the info , I given it two five second blasé with Gas only to find first the temp inside the fridge when up then the pipe started to frost by the compressor , no doubt thats a sign of over charge , I then release about the same amount of gas and the temp inside the fridge when back down and the pipe defrost , so if any thing the fridge was over charge , leaving it to to morrow before going further as I don't want to let too much gas out .

 

[Heckler]

Thanks for all the info , I given it two five second blasé with Gas only to find first the temp inside the fridge when up then the pipe started to frost by the compressor , no doubt thats a sign of over charge , I then release about the same amount of gas and the temp inside the fridge when back down and the pipe defrost , so if any thing the fridge was over charge , leaving it to to morrow before going further as I don't want to let too much gas out .

Vic
just be aware that the pressure for a given charge will change with the temperature of the evap, condensor and ambient! The guages, iirc do have a temperature rings on them to give an idea of what is going on. As the fridge gets colder then the frost line COULD creep along towards the compressor. So experiment, get the fridge to temperature THEN observe where the frost line is and charge accordingly

 

[superheat6k]

Vic
just be aware that the pressure for a given charge will change with the temperature of the evap, condensor and ambient! The guages, iirc do have a temperature rings on them to give an idea of what is going on. As the fridge gets colder then the frost line COULD creep along towards the compressor. So experiment, get the fridge to temperature THEN observe where the frost line is and charge accordingly

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