Dometic 6.7 cf fridge performance - Page 22 - Escape Trailer Owners Community

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Old 07-16-2014, 07:48 PM   #211
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Quote:
Originally Posted by daveandsandyclink View Post
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).
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Old 07-16-2014, 07:53 PM   #212
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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.
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Old 07-16-2014, 08:13 PM   #213
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Originally Posted by rbryan4 View Post
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.
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Old 07-16-2014, 08:50 PM   #214
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Quote:
Originally Posted by Brian B-P View Post
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.
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Old 07-16-2014, 09:45 PM   #215
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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.
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Old 07-17-2014, 01:02 AM   #216
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Originally Posted by daveandsandyclink View Post
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.
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Old 07-17-2014, 11:49 AM   #217
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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.
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Old 07-17-2014, 12:19 PM   #218
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Originally Posted by cpaharley2008 View Post
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.
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Old 07-17-2014, 12:50 PM   #219
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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...
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Old 07-17-2014, 01:07 PM   #220
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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.
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