3 way fridge question - Fiberglass RV


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Old 10-14-2009, 10:12 PM   #1
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I have a used 3 way fridge that I am going to install in my Burro to replace the ice box. I know it works because I pulled it from my old popup camper. I am ready to build the platform and connect AC and DC connections. But I am not ready to install the propane (I need to replace all propane lines first) nor am I ready to cut the two needed outside vents.

Any thoughts on running the fridge on either AC or DC without vents? Aren't the vents only needed for propane?

Looking for some guidance before I do something I regret.

Thanks

Mike
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Old 10-14-2009, 10:32 PM   #2
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Quote:
I have a used 3 way fridge that I am going to install in my Burro to replace the ice box. I know it works because I pulled it from my old popup camper. I am ready to build the platform and connect AC and DC connections. But I am not ready to install the propane (I need to replace all propane lines first) nor am I ready to cut the two needed outside vents.

Any thoughts on running the fridge on either AC or DC without vents? Aren't the vents only needed for propane?

Looking for some guidance before I do something I regret.

Thanks

Mike
If you are not going to use propane then maybe you should consider simply buying a 110V dorm fridge or a coleman 12V.
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Old 10-14-2009, 10:50 PM   #3
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Mike,
I repair these, my answer is watered down a little off the technical side.

These are absorption refrigerators, they absorb energy from inside compartments (where the food is) and move it to outside (to where the fins are)...these systems do not make cold...they move heat energy, the heated air coming off the back of the equipment fins/coils is picking up heat/energy being moved out of that inside space to the outside space " the outside fins facilitate this" if they do not breath they will fail to operate.

This is true of any mechanical device that transfers heat energy from point a to point b.

Since in R.V.s we do not use outside fans (generally by design) to exhaust heat away (like under your home refrigerator or your home a/c) must rely on the subtle air currents of convection..."this is cooler air entering thru the bottom part of the appliance at the rear part thru the vented door outside is drawn up across the coils/fins picking up heat. The heated air really being lighter than the cooler incoming air at the bottom will gently exhaust thru the top vent taking the nasty heat energy away outside.

The answer to the question is under no circumstances run this with out venting, you are correct, the gas combustion products also exit thru the venting system BUT that is only one purpose for the vents as outlined above.

This is a great question, your thoughtful to investigate your issues prior to design changes, shows you thinking ahead.

Happy Camping, Safe Trails.

Harry
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Old 10-14-2009, 11:51 PM   #4
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The answer to the question is under no circumstances run this with out venting, you are correct, the gas combustion products also exit thru the venting system BUT that is only one purpose for the vents as outlined above.
Good question Mike, and great response Harry.

My fridge is a rare exception to what Harry says about the combustion gases - only because it has it's own direct intake/exhaust vent separate from the cooling vents.

I modified the cooling vents and the air flow over the rear fins and the fridge efficiency noticably improved. I still have not added the 12V computer fans. Details can be seen here:
http://www.fiberglassrv.com/board/in...howtopic=30778


Proper venting makes a huge difference.
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Old 10-15-2009, 12:10 AM   #5
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Roy and Harry thanks. I had a feeling that I should wait till I was ready to do the whole job, but did not know why. I wont need it for the next planned trip so I can wait till I can do it right.

Floyd I will be using the propane eventually, just not ready to. Over the winter I plan to have all of the propane lines replaced at a local shop, right now they are all original and I can see leaks in the hose. Not willing to gamble with a leak.

As always ask a question here and get an answer and understand why too.

Thanks Mike
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Old 10-15-2009, 11:32 AM   #6
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Harry, nice to have a "go to guy" for the ammonia (?) coolers! I also have one sitting on the shelf and man it is scary to think about putting in. Just being afraid I guess.
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Old 10-15-2009, 11:57 AM   #7
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I agree with you Pat --- it sure is nice that Harry chips in with his vast store of knowledge. Also, it's one thing to have the knowledge, and quite another to be able to pass it on effectively in writing

Isn't it true about the ammonia/propane "scary" angle? I know just what you mean, Pat, as I have had that feeling myself. And it's good to be careful. But what's funny is how blase we all are about electricity. I mean, Hello! 120 VOLTS! I guess it's just that people use it all the time and so they don't think about it as much. Also, wires are so "clean" and they don't rust and whatnot.

Raya
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Old 10-15-2009, 04:01 PM   #8
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Harry, that was a great explaination.
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Old 10-15-2009, 07:08 PM   #9
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Harry, another question for you:

I did a lot of research before I installed my 1.9cuin Dometic 2190 3-way into my Scamp.

I found this info about venting online. In it, there are instructions to keep a baffle between the lower control area from the upper cooling fins. So I did. I also installed two computer fans into the upper venting area (one pulls and one pushes) to help create circulation.

Is this baffle necessary. Or worse, causing a potential problem? I just rechecked the "manual" from Dometic and there's no mention of the need for any baffles.

Can you share any thoughts? Thanks.

RJ
Attached Files
File Type: pdf FridgeVenting.pdf (86.3 KB, 28 views)
File Type: pdf Dometic_Refrig_RM2191_RM2193.pdf (249.3 KB, 22 views)
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Old 10-15-2009, 07:47 PM   #10
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Robert,

I'll be interested to hear Harry's take on this.

When you installed your baffle(s), did you leave the 1/4" space they specified?

The way I understand it it's not so much the baffling itself that is important -- or that the baffle "separate" the control area from the cooling fins -- but more the fact that you are trying to create a chimney effect (sucking air up and out), and if your open space behind the fridge is too wide, you won't get that effect. Like the difference between your large living room (basically still air) and your (relatively) small fireplace chimney (air sucking up).

If I understand it correctly, I also see that they mentioned that one could also make the whole "chimney" area behind the refrigerator 1/4", by filling in between the baffles, if it were safely possible.

Raya

PS: Note that my info is only from a combination of reading that document you linked to, and having experience with wood stoves. With wood stoves, people would often install an 8" diameter chimney on a small stove, thinking a larger chimney would work better; when in fact, a 5" chimney would provide more draft (and the heat would make less creosote form; not that that means anything for a refrigerator).
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Old 10-15-2009, 09:39 PM   #11
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Roy,

What you have could be the French built and designed refrigeration unit, Dometic contracted these in the early years (late 60s thru 70s) when the Canadian Gas Association was trying to grapple with standard design practice for Canadian R.Vs appliance. standards in their country, latter they ceased being built in Europe because they were deemed a shipping hazard, (bogus claim to get it built here) the parts were sent un-assembled to the U.S.A so that final assembly was done here when the gases were installed....The furnaces of this era were somewhat different too being made in the U.S.A. to Canadian rules...Take alot of pics and archive them, to my knowledge I have seen only one other system like yours...Therm-ex built a French cat heater import which used a strange (to me) series of pilot safety's for Canada also specified for the Dominion of Canada.

People do not realize the journey technology took to get here today or the cowboy mentality of the early years.

Harry


Quote:
Good question Mike, and great response Harry.

My fridge is a rare exception to what Harry says about the combustion gases - only because it has it's own direct intake/exhaust vent separate from the cooling vents.

I modified the cooling vents and the air flow over the rear fins and the fridge efficiency noticably improved. I still have not added the 12V computer fans. Details can be seen here:
http://www.fiberglassrv.com/board/in...howtopic=30778


Proper venting makes a huge difference.
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Old 10-16-2009, 02:42 PM   #12
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Harry,

Did you have a chance to see and read my questions about my Dometic, posted here on this thread?
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Old 10-17-2009, 02:11 AM   #13
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Raya and Robert,

I will do my best to answer the issues you guys so cleverly figured out...

First let me say this,

There is the world I live in, the technicians universe, its guided forcibly by insurance company studies and reports, their detailed suggestive information being fed into the R.V. industry resulting in volumes of future changes to the R.V. universe of the engineering...The guru to every problem is the engineer he owns every success or disaster that has ever impacted RV life.

This group is easy to find when the party is going on but hard to reach when the kitchen needs cleaning.

The engineer is the high priest of any design...

I cannot have an opinion contrary to them officially.

Their realm is sacred to them, their defenses are many, their considerable education worthy of recognition, the societies that they recognize and recognize them tower over me and act as look outs for the profession in general, they are also cannibals and eat technicians for lunch...

They are the guiding light to the service technician forever and ever.

Only in recent decades have engineers consorted with the lowly dirty technicians in the repair field for input into their designs or maintenance issues of the equipment they engineer.

Now much of any engineering is done over seas, by standardizing the rules, any C.A.D. Program in any part of the world can be hired to dream stuff up...Cheaper too.

Point made here is I will explain what I know to be true in as much of a non technical language as I can so the information is less mesmerizing.

Its meant to be down to earth, organic to R.V. only.

That said...

This is the basic theory of baffles...then there is the practice of the theory in real life conditions after the explanation.

Overview...

The baffle system was intended to take incoming cool air at the bottom of a refrigerator inducing the correct drafting effect required to supply the “condenser” hot fins with a supply of cooler air.

Goals...

To increase the systems efficiency to absorb heat energy we mix the warmer and cooler air layers inside the back compartment with baffles.

To get correct air flow across all the hot fins evenly and move the now warmed air in the right direction of flow we continue mixing the air correctly and increase its input velocity at the bottom. If we do this well we transfer the maximum amount of heat energy thru heat transfer of conduction from the hot fins...

Then overcoming any encountered resistance the air exits the system at the physical top of the chimney “the rear space” in the back of the refrigerator, the movement of this air is thru the principle of convection and the expansion of heated air.

Raya,

I love the words you used in your post when you referred to the "chimney effect". It really describes the mechanics being done, though we use terms like drafting as a descriptor I worked it into this explanation.

The theory of baffles is better understood if I describe what the sequence of operation is without baffles.

The first thing happening without baffles...The problem.
As the incoming air being drawn in waifs across the fins up the rear refrigerator cavity (chimney) to the vent at the top, the fins transfer heat from the warmer fins to the cooler air heating it up, the now lighter air moves out the vent we hope...Nice to say this, but it is to simple.

Because...

The air nearest to the fins gets hotter first, since the cooler air mass is moving in a good orderly direction slowly the closer air to the hot fins heats up faster exiting and pulling the cooler air out with it without the rest of the cooler inside the compartment picking up available excess heat from the fins.

Without baffles this is how it works, the air will speed up closer to the fins, but it will not mix well at all...This wastes volumes of cooler air within the compartment that will not come in contact with the hot fins starving for it.

The air needs to mix upwith cooler air, the heated air will not mix up inside by just slipping by the hot fins faster and faster by any real measure...Heat transfer needs to happen with all incoming available air!

The air towards the outer wall without baffles will measure much less in temperature than the air immediately touching the fins, that air is desperately needed...

A law of thermal dynamics states, “heat must move from a warmer to a cooler object”.

When the air touching the fins reaches the same temperature of the fins the energy STOPS moving, when this equalizing of the two temperatures occurs the transfer of energy from INSIDE the box stops too! Ergo hot food.

How do we get cooler air nearer the fins to increase the heat transfer?

Answer: Mix the warmer closer fin heated air layer with cooler non heated air nearer the skin of the trailer which is farther from the hotter fins...By mixing all surrounding air by swirling it around we harness more potential to transfer more heat energy away from the fins.)

(1) Mix How?

We create a closed space open on the top and the bottom at the back of the refrigerator...Then place baffles so the air must roll over the baffle causing the air to mix up by swirling the air current as it goes by the baffle, ([cavitation of air) this air begins to blend all the air warmer air closer to the fins (warm air layer) with the cooler air farther away from the fins (cooler air layer) lowering the over all temp (ambient temperature) of the air mass so the fins farther up the way have a greater Delta T or DT. (temperature difference) to keep transferring heat longer.

The greater the temperature difference of the contact air layer with the actual metal fins of the heat condenser heat exchanger the greater the heat transfer rate out of the system.

(2) The second thing going on...
That said...Lets go to Raya's "chimney effect" a good description because there is another thing going on back there with baffles and the incoming air which her descriptor clues us too.

Beyond mixing air temps?

The cooler air is coming in a nice big louvered door slowly...When it must squeeze a little past the first baffle beginning the cavitation or swirl effect cycle mixing the hot and cooler air another thing happens at the baffle.

Principle here...

"When a larger amount of air squeezes thru a smaller opening it must speed up, the tighter space available increases the velocity of the air moving."

Think a wide quiet stream of water transitioning to a narrow rapids)

Those of you who are engineers on FBRV be kind, I am trying to really simplify this.

Review:

The cavitation effect described earlier in (1) is now multiplied by simultaneously increasing the air velocity moving into the place of cavitation itself...Mixing the warmer and cooler air in zones better still, arguably also increasing the total of the air volume passing thru the compartment.

The direction the air wants to go is assisted by the now lighter expanded qualities of heated air (think hot air balloon).

And...

The baffle acts as an air mass back stop or a air check-valve, permitting easily the heated air moving in the desired direction (UP) but making it much harder for the heated air to reverse direction.

Then...
A tiny increase in air pressure via the gentle expansion of air increases ever so slightly above the baffle...This increase in static pressure of the upper refrigerator compartment/chimney ever so slightly lends again to the desired effect of moving air in a good orderly direction.

The new air properties in the upper compartment above the baffle partner together in a concert to encourage the correct direction of the heated, expanded, lighter air exhausted up and out thru a b]correctly sized vent over coming any resistance to it!!!!![/b]

I have NOT included the air currants blowing over the top of the vent system or chimney.

Now I will really shoot myself in the foot here and fall on my sword as a technician. In small systems like most fiberglass trailers these baffles really do not in practice make a hoot of difference in the field with tiny systems like in fiberglass trailers...The temperature swings in the climate zones we all camp in are far more influential in performance outcomes than the itsy bitsy baffles in our tiny spaces...

Except in the big expensive luxury refrigerators.

Not having baffles in new installations is good for voiding warranties...

Having baffles makes SURE manufacturers of R.V.s keep the back fins of the refrigerator off the compartment wall.

What Is Important is...

The size of the top vent and the lower door design must be matched to ensure the correct convective flow of air volume totals...[to often the vent is not the correct one.

I like .25 amp computer fans...even better the solar ones on that subject I want to say this, fans, any size of fan will suck more heat out than it can push, I put them on top of the heated air space and suck the air out. A tiny one is best, always line the space with an infra red reflective barrier, these refrigerators are built to handle sensible heat loads (heat that heats the air thru conduction) Refrigerators DO NOT handle insensible heat loads at all!!! (heat energy that does not raise the temp of air it passes thru BUT raises the temp of any object when it strikes, then the object warms the air) See infrared Heat definitions.

If you have a Burro like trailer, or a 13 foot Scamp-ish trailer with the under counter refrigerators using the side vents consider a solar or battery fan exhausting out the top vent in warmer climates.

If you are using a push pull fan (2 fans) put both of them in the top as exhaust fans, suck the air thru the system and exhaust the air... You can switch the fans independently so you can select low or high volume air exhaust. I do not like a fan blowing on the bottom as it may change the air fuel mix ratio of the burner by pressurizing the compartment or blow the pilot away from the thermocouple flame sensor...

ALSO BY PRESSURIZING THE REAR COMPARTMENT BY BLOWING AIR INTO IT CARBON MONOXIDE CAN BE MOVED INTO THE TRAILER.

Where fans are concerned exhaust only directly thru the vent.

Hope this helps.
Happy Camping, Safe Trails.

Harry
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Old 10-17-2009, 11:34 AM   #14
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Holy crap, Harry! That was awesome!

So, what I've gleaned from what you've written, that's directly applicable to my situation (small, under-counter fridge in a 13' Scamp), is this:

1 Properly matched, and appropriately sized, venting, top and bottom is imperative

2 Baffles, in theory, are okay--but in my configuration, not necessary, and probably ineffective

3 Using a small fan (or two, in a push-pull arrangement) at the top of the airspace (near fins) is a good thing

Do you concur?

RJ
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