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.
This is the basic theory of baffles...then there is the practice of the theory in real life conditions after the explanation.
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.
To increase the systems efficiency to absorb heat energy we mix the warmer
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.
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.
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.
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 up
with 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?
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 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.
"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.
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).
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.
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
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
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.