Dometic CU 84. Very hot compressor.

[XDC]

Temp inside boat 29 degrees.

Fridge cycling between 3 and 5/6 with sender under evaporator plate.

Very happy with that temp as our food never goes off.

But.

Compressor is REALLY hot.

It is obviously on longer than for our more recent temperatures and the fridge is nowhere full, planning a shop tomorrow but battery not showing signs of stress.

Haven’t timed time on/offbut don’t really have a baseline to check it against.

So how hot should it be?

Very? ... which it is and seems to be staying at.

Medium plus where you could but wouldn’t really want to hold your hand on it?

Medium so that you could hold your hand on it “indefinitely” ?

Only noticed it as had to get something from that locker that we don’t normally use so as I said, no real baseline.

Late thought. Does the compressor have “oil” in it and it’s the hot oil I’m feeling?

 

[Daverw]

Try and makes sure you have air flow over the small condenser near the compressor, yes it’s the oil in the compressor that’s getting hot. The compressor will not get hotter that the small discharge pipe coming from it the the larger suction pipe would normally help with compressor cooling, if the boat inside is at 29 a bet the inside of the unit where the compressor is hotter and this is the rejected heat you need to get rid of.

Nothing more you can really do to help the compressor it’s most likely just having to work very hard.. if the gas charge is slightly low it will also have to work harder and not get the cooling to the compressor but if this was the case normally the fridge temp would be higher than expected.

Also make sure the condenser tubes and fins are clean of dust, it needs good heat transfer as this is where the energy gets rejected

 

[XDC]

Many thanks, that all makes sense.

Still working fine but now the direct heat from the sun isn’t on that locker it has cooled down.

Appreciate your input :encouragement:

 

[coopec]

Refrigerators should never be installed on a boat in a confined space of less than 100cuft so that you get plenty of circulation around the unit.

The bulk of the heat is generated at the compressor discharge port. It's normal for this temperature to be as high as 300 degrees Fahrenheit, but it should never be higher. If the temperature rises to 320 degrees, the refrigerant will start to break down.Apr 10, 2018
https://www.hunker.com/13410081/how-warm-should-a-refrigerator-compressor-be

Wow! That sounds awfully hot. Could you use the waste heat to heat the shower water I wonder?
http://www.cruisersforum.com/forums...use-fridge-condenser-to-heat-water-70234.html

Hot Showers on a 30 Foot Cruising Yacht???? | Sailing Forums, page ...
https://www.seabreeze.com.au/forums/.../Hot-Showers-on-a-30-Foot-Cruising-Yacht?...

Jun 15, 2014 - 31 posts - ‎15 authors
I use an Aqueous 5L 12v Water heater on my fancy forty footer. ... of using the warmed water from the fridge compressor( which I don't own yet) and ... Hot water comes from the fresh water heat exchange connected to a 40lt holding ... Using the waste heat from your engine's heat exchanger is the way to go, ...

https://www.researchgate.net/public..._Heat_From_Condenser_Of_Domestic_Refrigerator

 

[superheat6k]

The idea of heating the water is very good, after all as a heat pump the fridge is rejecting ~ 4-5 times the heat energy it actually consumes as electrical power. However the idea falls apart when the calorifier reaches its intended temperature, when the heat from the fridge still needs to be rejected. This requires a separate condenser, and herein lies a technical problem because it is next to impossible to have twin condensers that will work in balance across the entire load range for both cooling range, outdoor ambient variation range, and hot water demand, which is why heat recovery fridges do not exist at a domestic (or smaller) level.

The discharge condition, pressure and temperature, are controlled by the condenser (not the compressor), or more to the point it is the temperature at which the condensation is occurring within the condenser, which dictates the discharge pressure. The gas will be superheated considerably above the condensing temperature and for a typical piston compressor ~ 40 to 60oC above the condensing temperature.

Now the condensing temperature is directly influenced by how much heat the condenser is getting rid of, which (for air cooled) in turn is largely governed by air flow, condenser external cleanliness and the entry air temperature. So if the fridge has been installed with its condenser effectively trapped within a semi sealed box, relying upon external heat rejection through the boat's bulkheads, then whereas this may be just fine at up to say 22oC, as soon as the temperature goes above this the heat removal becomes less effective, so the local ambient (at the condenser) increases, and with it compressor discharge pressure and temperature.

Most compressors will tolerate a discharge temperature up to 120oC, with a core temperature at the cylinder discharge valve peaking ~ 30oC hotter. So assuming the fridge is using R134a, the air getting to the condenser is at say 40oC in a semi sealed box, there is then a rise to the condensing temperature of ~ 20oC, so the Condensing temperature is at 60oC and the discharge pressure will be at ~ 15.7 Bar. The compressor discharge will be ~ 100 to 120oC and the internals will be peaking at ~ 130 to 150oC.

Often a simple computer cooling fan will provide adequate airflow to the condenser chamber and dramatically improve the fridge's performance. Also keeping the fridge full reduces the thermal load once the content are at the desired temperature.

 

[coopec]

Thanks for that: it appears very well explained. I'll have another look at it the morning as I'm "watching" the F1 GP tight now!

 

[Neeves]

I saw this when I was on shore leave but I might comment.

Try to find a way of getting the coolest air in the yacht, bilges, anchor locker - into the locker than holds the compressor. Use common ducting - anything that fits (you might need to use your imagination!). Cut a hole in the locker, housing the compressor of the appropriate size.

Buy two small computer fans, or recycle from 2 old computers (100mm square should be fine). Use one to such cool air into the locker use the other to directly force the air in the locker out. Ideally you want incoming air over the compressor, maybe on one side, and duct the air to the other fan that is expelling the warm air.

There should already be a fan attached to the compressor - that is wired to come on with the compressor. Wire the 2 new fans with the existing fan - so all three come on together.

I think this is one way of increasing air circulation - which we use to increase air flow over both the fridge and separate deep freeze.

Jonathan

 

[superheat6k]

I saw this when I was on shore leave but I might comment.

Try to find a way of getting the coolest air in the yacht, bilges, anchor locker - into the locker than holds the compressor. Use common ducting - anything that fits (you might need to use your imagination!). Cut a hole in the locker, housing the compressor of the appropriate size.

Buy two small computer fans, or recycle from 2 old computers (100mm square should be fine). Use one to such cool air into the locker use the other to directly force the air in the locker out. Ideally you want incoming air over the compressor, maybe on one side, and duct the air to the other fan that is expelling the warm air.

There should already be a fan attached to the compressor - that is wired to come on with the compressor. Wire the 2 new fans with the existing fan - so all three come on together.

I think this is one way of increasing air circulation - which we use to increase air flow over both the fridge and separate deep freeze.

Jonathan

Can I suggest it is the condenser that needs the air far more than the compressor, but the idea is sound.

 

[Neeves]

SUPERHEAT agree - in our arrangement the compressor and condenser are all bolted together to become one unit. Its actually impossible, in our case, not to attempt to cool one without the other.

Jonathan

 

[gregcope]

Buy two small computer fans, or recycle from 2 old computers (100mm square should be fine). Use one to such cool air into the locker use the other to directly force the air in the locker out. Ideally you want incoming air over the compressor, maybe on one side, and duct the air to the other fan that is expelling the warm air.

There should already be a fan attached to the compressor - that is wired to come on with the compressor. Wire the 2 new fans with the existing fan - so all three come on together.
Read more at http://www.ybw.com/forums/showthrea...CU-84-Very-hot-compressor#K59Vw28tUAOhGe1y.99

+1 Simple, Cheap. You may need to drill holes for the in/extract fan. I just have a extract fan high in the locker, which barely keeps up with the temp in the locker. I may change this for a larger fan. Fit an input fan as Jonathan suggests or change how they are wired (presently in series to reduce speed, may go parallel), the later being the easiest to do.

 

[Leighb]

When I built a fridge for my last boat I fitted the compressor at the aft end of a locker below the chart table. I cut a hole through the bulkhead into the cockpit locker and fitted a computer fan to extract the warm air to the cockpit locker. The fridge performed very well and I had no worries about overheating.

 

[Neeves]

Drilling a hole in the furniture and leaving a fan in full view is unsightly, if not downright ugly. Adding perforated ventilation screen will reduce the output from the fan.

It is recommended to leave an air gap round the fridge itself - we have used this as part of our ventilation system You can source tiny little fans, used for heat sinks in computers, and we have a couple in use in our recent fridge installation (where the compressor/condensor are located in a recess at the top back of the fridge). We used ducting and bigger fans in the earlier iteration where we had a remote condenser/compressor unit.

You need to be imaginative as each installation will be different - but computer fans, or little 12v fans, come in anything from about 30mm square (maybe smaller) and upto 150mm square (maybe bigger). The 100mm versions draw very little power (from memory, which may be flawed - around 0.1 amp and the very little ones 0.05 or 0.065 amp)

Jonathan

 

[superheat6k]

SUPERHEAT agree - in our arrangement the compressor and condenser are all bolted together to become one unit. Its actually impossible, in our case, not to attempt to cool one without the other.

Jonathan

The reason the compressor is hot is because of the higher discharge pressure a less functional condenser creates, but this is a function of the condenser, rather than the compressor, which, when all is considered, is simply a gas pump. If the condenser is well ventilated the saturated condensing temperature reduces and with this the discharge pressure also reduces and with it the compressor temperature. Thus cooling of the compressor itself in a correctly set up system is not actually necessary, although yes a small amount of heat will be taken away if it is present in the cooling airstream to the condenser.