Inside refrigerator fans

[Parker]

I can't compete with that clever turn of phrase; won't even try. But I do have the feeling that it's easy to have more fan than needed to move the air around. I mean, when it's working, the fridge does a pretty good job with no fan, just using natural convection of the cool air descending from the fins. I'm going to investigate wiring my Fridge Fix fans in series so they run slower (and quieter). I had two fans on the backside of our Casita fridge wired up so they could be switched between series and parallel. After that experience, and needing so little air movement in the fridge, I think I might just re-wire these in series.

[Parker]

Originally Posted by MyronL

It is very tight quarters back there. I pushed stiff wire through my thermistor hole and was able to push enough through to reach up from behind, grab and pull down. Very anal job, but if you don't want a second hole at the very least, worth the effort.

I removed the fins and fished a piece of lawn trimmer cord through the exising hole to pull the wire through and then put everything back. Actually, I pulled two wires through since neither fin set on our 6.7 cu ft fridge is grounded so I ran a ground wire also.

[tractors1]

Originally Posted by Parker

I can't compete with that clever turn of phrase; won't even try. But I do have the feeling that it's easy to have more fan than needed to move the air around. I mean, when it's working, the fridge does a pretty good job with no fan, just using natural convection of the cool air descending from the fins. I'm going to investigate wiring my Fridge Fix fans in series so they run slower (and quieter). I had two fans on the backside of our Casita fridge wired up so they could be switched between series and parallel. After that experience, and needing so little air movement in the fridge, I think I might just re-wire these in series.

I like that idea; I was planning to add a speed control for my fan experiments but your wiring methods could prove more expedient.

[rscovell]

When I installed my Fridge-Fix fan, I thought I had found the answer to the cooling issue. Now that we have arrived in Texas (100 F), even the Fridge-Fix doesn't help. The temp in the bottom of the Dometic does not get down below 57 F.

[float5]

Originally Posted by rscovell

When I installed my Fridge-Fix fan, I thought I had found the answer to the cooling issue. Now that we have arrived in Texas (100 F), even the Fridge-Fix doesn't help. The temp in the bottom of the Dometic does not get down below 57 F.

You are right where the Groene's book (mentioned on another thread) said, in effect, you would be. At 100 degrees, an RV refrigerator is not expected to go lower than 55 degrees, they said.

In that kind of heat in the day, you can only try to have ice packs freezing overnight to put in the fridge or a cooler in the day. These RV fridges are not built the same and do not work nearly as well as home refrigerators.

[cpaharley2008]

Originally Posted by Parker

I can't compete with that clever turn of phrase; won't even try. But I do have the feeling that it's easy to have more fan than needed to move the air around. I mean, when it's working, the fridge does a pretty good job with no fan, just using natural convection of the cool air descending from the fins. I'm going to investigate wiring my Fridge Fix fans in series so they run slower (and quieter). I had two fans on the backside of our Casita fridge wired up so they could be switched between series and parallel. After that experience, and needing so little air movement in the fridge, I think I might just re-wire these in series.

Please post your progress and pictures for others, thanks.

[Parker]

Originally Posted by cpaharley2008

Please post your progress and pictures for others, thanks.

Will do. In the meantime, here's a view of the fin set in the 6.7 with thermistor wired disappearing behind them. The other pic is with the fins removed, exposing the hole and the high thermal conductivity grease between the fins and cooling tube. I was careful to redistribute the grease to restore good contact as I re-assembled. By the way, the screws require a Torx (star shaped) driver.

[cpaharley2008]

So the cooling tube transmits it's cooling via the grease to the fins? Very interesting.

[MyronL]

I was wondering what that gray goo was for.

[tractors1]

That's REALLY interesting; the heat transfer through the grease (commonly called heat sink compound, used underneath cooling fins and high power transistors in amplifiers, cpu in computers) is key to heat transfer as the total heat flow is directly proportional on the surface area of the compound between mating elements. It's filled with metallic oxides, it's not really dielectric grease like used for light bulb bases. It is commercially available at electronic stores among other places: Cooler Master Thermal Compound: Appliances : Walmart.com

[tractors1]

I'll be adding this myself - silver is by far the most effective heat transfer element used in this stuff.

Amazon.com: Arctic Silver 5 Thermal Compound Large Size - 12 Gram Tube: Computers & Accessories

[cpaharley2008]

So you will be removing the original paste?

[tractors1]

Originally Posted by cpaharley2008

So you will be removing the original paste?

I probably will, can't be positive until I get my 21 to play with. It appears to be an oil-based compound as it's really gooey looking, so it could be removed with a plastic putty knife and saved to reapply.

Looks like Dometic only put it along the cooling tube path, I'm curious if the external fins are unevenly warmed only along the tube path. Can't really tell from the pics how thick the attachment blocks are but there appears to be a standoff space between internal and external fin blocks, which makes no sense for heat transfer. Thermally conductive pads are available to fill in the gap if desired - good project for when I get my 21!

[jamman]

There is a LOT of "goo" for something sold in 14gm containers. I would say something like 8 oz on just the lower set of fins.

[Parker]

Dometic only applies the paste to the tubes because that's the only source of cooling. In between the tubes is rigid foam insulation. I looked carefully at the fin construction and doubt seriously that much can be improved. On the backside of the fin assembly is a continuous sheet of aluminum to "spread the cold" across to all the fins attached to the front side. You might do better with some other paste, but again, I think we're probably already transferring all the heat that's going to transfer.

[Parker]

I don't know if these comments belong in this thread or the one about New 4.3 & 6.7 Fridge Info, but here goes. I think we're dabbling in the margins with fans, magic grease, and baffles. After watching lots of things tried, both on the forum and the efforts of a very dedicated friend off the forum, I don't look for a lot more performance improvement with these fridges. Here's why: the boiling/evaporation and condensation points of the ammonia coolant are set by the amount of coolant and the hydrogen pressure loaded during manufacture. We can't change that. I strongly suspect that there is a backside temperature threshold beyond which cooling essentially stops because condensation no longer occurs. I'm using the term condensation loosely here because it's really a fairly complex chemical process. Regardless, about all we can do is delay reaching that threshold by trying to keep the backside cool with insulation and fans so that we don't hit this point sooner than necessary. Heating by the sun and the propane exhaust can get us to this threshold prematurely, so anything we can do to keep the backside cool will probably help. I'd like to always park under a shade tree, but that doesn't help while towing, so I'm going to work on insulation and freezing a large blue ice container overnight to move to the fridge section during the day. And we'll keep packing the cooler. I continue to trust Reace for the best long term solution we're likely to get, but I personally see this as a first world problem that will not diminish our enjoyment of camping in the Escape.

[gbaglo]

So, how many of these refrigerators that used to be covered by warranty, are now not covered because they've been messed with?

[Parker]

I don't think most of us have done anything permanent that can't be restored easily to the original configuration before going in for service. It's pretty easy for me to pull the Fridge Fix fan and wires. Most everything else (external fans and baffles) is done to the camper rather then the fridge.

[tractors1]

Originally Posted by Parker

Dometic only applies the paste to the tubes because that's the only source of cooling. In between the tubes is rigid foam insulation. I looked carefully at the fin construction and doubt seriously that much can be improved. On the backside of the fin assembly is a continuous sheet of aluminum to "spread the cold" across to all the fins attached to the front side. You might do better with some other paste, but again, I think we're probably already transferring all the heat that's going to transfer.

I was wondering what the stuff in between the fins was. I figure there's nothing to lose with a silver paste and completely filling in the void currently around the route of the Dometic paste application. Reace & Dometic working together is our best hope at an "approved" solution, but this is like another mod in my book - any tweak that turns out to be helpful is good!

[MyronL]

"I think we're dabbling in the margins with fans, magic grease, and baffles....etc...

Good stuff, Parker, very good stuff. Now I know better than to get my hopes up too high (not that there's anything wrong with that). Also agree ...do nothing you can't easily reverse.