Dometic 6.7 cf fridge performance

[cpaharley2008]

On the 21 there is only one side vent, located below the belt line, about 12-18". This part of the trailer is inclined downward vs the top half which is inclined upward. Thus direct sun would have to occur in late evening which would seem to not be the hottest part of the day.

[J Mac]

My thinking is:
If the refrigerator vents are in direct, unshaded sun weather they are on the roof or on the side of the trailer they get hot. If the vents draw in outside air they will be drawing in hot outside air. Shaded vents are cooler and would draw in cooler air.

[daveandsandyclink]

I am curious about just how much of a difference that makes. Even white plastic lower vents? Are you in a position to test that theory J Mac? My refrigerator side is parked in the shade.

[Brian B-P]

Are people really concerned about sunlight coming in the openings in the vent and directly heating the refrigerator components? I assumed that the concern with sun exposure is that the wall of the trailer exposed to the sun would heat up, indirectly heating the entire compartment at the back of the refrigerator and the air around the trailer, reducing the system's cooling ability.

If indirect heating is the concern, the colour, shape, size, orientation, or position of the vents is not important.

[hotfishtacos]

Originally Posted by J Mac

Good work cpacharley2008, I'm glad the fans are making a difference. I still wonder if the refrigerators temperature would be effected by direct sun on the drivers side refrigerator access panels.

I'm sure it would because the sun heats the chamber (sun shining directly on large, non-insulated fiberglass surfaces) where the evaporation and condensation takes place and a lot of heat I'm sure would affect that whole process. I am thinking about pulling my refrigerator out when Reace shows us how with his baffle instructions and then lining the fiberglass surfaces that the sun shines on (sides and top) with thick fiberglass insulation. If I use the correct insulation thickness then the resulting gap between the insulation and the coils after the refrigerator is installed would be 1" (this is what the factory recommends to direct airflow where it is needed.) I would also replace the two noisy fans there with high volume quiet fans, switched inside, maybe on top. I would also install four thermocouples; 2 on the coils (top and bottom) and 2 in the air stream (top and bottom) of the chamber with a digital temperature display inside the trailer so I can monitor what is going on at any time in that mysterious place...

[Jubal]

Hotfishtacos
The best and most logical strategy proposed yet. If Reace has found the new baffle strategy ineffective, will the procedure be posted? The fridge R&R procedure would be helpful. Creating a good thermal vent with or without fans seems like the most effective approach. Absorption cooling is still limited by ambient air temperatures.

[rbryan4]

Originally Posted by Jubal

Hotfishtacos
The best and most logical strategy proposed yet. If Reace has found the new baffle strategy ineffective, will the procedure be posted? The fridge R&R procedure would be helpful. Creating a good thermal vent with or without fans seems like the most effective approach. Absorption cooling is still limited by ambient air temperatures.

Absorption cooling is limited by ambient temps to be sure, but I suspect that Domestic has released some inferior fridges with these latest designs. Many of the "older" absorption fridges would turn into a block of solid ice if turned all the way up, and had no problem running ice cold in southern climates.

[daveandsandyclink]

Hi Robert,
I wonder why RV refrigerators have no fins on the cooling coils?
Wouldn't that improve the temperature exchange?

[rbryan4]

Originally Posted by daveandsandyclink

Hi Robert,
I wonder why RV refrigerators have no fins on the cooling coils?
Wouldn't that improve the temperature exchange?

Except the condenser fins extend into the fridge, not outside as in conventional compressor cooling. Here's a simple diagram.

[rbryan4]

Or a more complex one from Dometic:

[Brian B-P]

Originally Posted by daveandsandyclink

I wonder why RV refrigerators have no fins on the cooling coils?
Wouldn't that improve the temperature exchange?

Interesting observation.

As I recall, some of the tubing does have fins on typical RV refrigerators; that's the condenser section shown in the diagrams, and those fins are there to improve heat transfer and thus performance. This is a relatively small part, compared to all the other stuff back there. Since it is up top, it is not readily visible from the lower access door and may not be noticed.

Some of the parts which are not shown as tubes in the first diagram are actually built as tubing in RV refrigerators (as you can see in the Dometic diagram), so some of the tubing visible on the back of the unit is not part of the cooling coil. Some of these parts include a heat exchange function (in the gas temperature exchanger in the Dometic diagram) but they move heat between two parts of the coolant mixture inside. In those parts, heat transfer to or from the surrounding air is not the purpose, so fins are not required. The absorber portion (a zig-zag of tubing in the Dometic diagram) isn't for heat exchange at all, so again no fins needed.

The part which transfers heat out of the refrigerator interior is the evaporator - to conduct heat it has a plate (such as on the floor or back of a freezer compartment) or fins (the ones in the interior for which some people are installing fans).

[Jubal]

I really think it is all about cost and weight. Adding more surface area to the condenser side of the refrigeration system would require a fan to force the air through the reduced sized air passages such as our AC. Absorption refrigeration absorbes heat on the evaporator side (inside the fridge) only to the extent that the can be displaced on the condenser side. Good air flow displacing the absorbed heat in the condensor is essential for efficiency. Other circulation devices in the fridge may be helpful but dissipating heat on the outside is the limiting factor.
IE. In the automotive industry the trend has been to downsize refrigerant volume, operate at higher pressures and decrease the size of the evaporator and condenser. In spite poor results the cost controllers focus on profit margins even down to cents.

[Brian B-P]

Originally Posted by rbryan4

Except the condenser fins extend into the fridge, not outside as in conventional compressor cooling.

Not quite: the condenser is outside; as a rule of thumb, any condenser is a place where a vapour condenses to a liquid, so it must reject heat to its surroundings (you do that outside the chilled space).
The part extending into the refrigerator is the evaporator; as another rule of thumb, any evaporator is a place where a liquid evaporates to a vapour, so it must absorb heat from its surrounding (you do that inside the chilled space).

Both diagrams show this, although in different ways. The Dometic diagram is realistic (for a single-door unit), so you might recognize the components by their shape and location (fin-covered condenser on the back, plate-style evaporator in the freezer floor). The first diagram is more schematic, and shows the division between outside (on the left) and inside (on the right) as a vertical white wall; unfortunately in this diagram neither the graphic nor the captions explain the evaporator very well.

[rbryan4]

Originally Posted by Brian B-P

Not quite: the condenser is outside; as a rule of thumb, any condenser is a place where a vapour condenses to a liquid, so it must reject heat to its surroundings (you do that outside the chilled space).
The part extending into the refrigerator is the evaporator; as another rule of thumb, any evaporator is a place where a liquid evaporates to a vapour, so it must absorb heat from its surrounding (you do that inside the chilled space).

Both diagrams show this, although in different ways. The Dometic diagram is realistic (for a single-door unit), so you might recognize the components by their shape and location (fin-covered condenser on the back, plate-style evaporator in the freezer floor). The first diagram is more schematic, and shows the division between outside (on the left) and inside (on the right) as a vertical white wall; unfortunately in this diagram neither the graphic nor the captions explain the evaporator very well.

You're right-- they are separate pieces. Thanks for the clarification.

[Parker]

Hey Brian, how much air would you guess, in pounds per hour, pass over the condenser in standard day conditions? It would be fun to play around with the enthalpy of air compared to that of water misted onto the cooling fins. Purely a Gedanken experiment.....I wouldn't want to spray water on the fins, but I bet a little would go a long way compared to the amount of cooling available from 90 degree air.

[J Mac]

Originally Posted by daveandsandyclink

I am curious about just how much of a difference that makes. Even white plastic lower vents? Are you in a position to test that theory J Mac? My refrigerator side is parked in the shade.

No.
I have a 27b and the fridge is on the awning side.
I may never know if direct sun, on the shaded side of the refrigerator vents creates a problem with refrigeration.

[cpaharley2008]

Just checked again, holding at 37 on the door and 2 in the freezer, 28 on top shelf rear inside of refrigerator, 72 outside. Historically, it never got warmer than these numbers over the past 24 hours. I think next time I'll set it on #2 and record results, really impressed with the performance of the kit.

[rbryan4]

Originally Posted by cpaharley2008

Just checked again, holding at 37 on the door and 2 in the freezer, 28 on top shelf rear inside of refrigerator, 72 outside. Historically, it never got warmer than these numbers over the past 24 hours. I think next time I'll set it on #2 and record results, really impressed with the performance of the kit.

That's encouraging Jim. I guess Mother really does know best.

[Mike@SmartRVProducts]

Hello everyone, CPAHARLEY2008 (Jim) invited me to this forum. Just to be 100% clear, I am absolutely not here with the intent of marketing in any way. We participate VERY little at the online forums as we do not want to come across as "trying to drum up business"...

That being said, if anyone has any questions, I would be happy to help. Our entire business is built off of problems with RV refrigerators. We have sold more than 1000 of these units, with great results.

The escape community with the Dometic 8555/8551 units are a new application for us. We understand that folks are having quite a difficult time with getting the fridge cool enough. While we understand the guidance from Dometic is usually towards a rear baffle, or other type of cooling unit issue, it has been our experience that more results can be attained by proper air circulation INSIDE the fridge.

I will address the sun vs cold on the outside of the trailer in my next post...

[paulw]

Hi Mike,
Welcome. I believe my Fridge-Fix is coming today.
I haven't really experienced the problems some of our southern members have, but wanted a fan to improve the air movement and didn't want batteries.