Date: Fri, 30 Oct 1998 07:52:54 8
Reply-To: Wes Neuenschwander <wesn@ESKIMO.COM>
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: Wes Neuenschwander <wesn@ESKIMO.COM>
Subject: Re: Fridge ?
A 1.5V fan would actually draw much *more* current than an equivalent 12V
rated fan - Ohm's law (I = V/R). Though only briefly - my guess is about 2
seconds - since the 9x overcurrent condition would very quickly overheat
the windings and burn the unit to a crisp. Using an 18V or 24V rated fan
would reduce the current draw (relative to nominal), but would also
substantially reduce the airflow since airflow is related to fan
horsepower, which varies with the square of the current. Operating the fan
under such 'derated' conditions would also likely decrease the overall fan
efficiency, further decreasing airflow. Bottom line is that you basically
get out what you put in (variations in fan efficiency factors aside).
I think the most popular modification is installing one of the small, 12V,
"muffin" style fans, commonly used in electronic/computer units. These are
designed for continuous use, are extremely high quality, and have
relatively high efficiencies, due to the airfoil blade and cowling designs.
They are also much quieter than the open blade units factory installed on
the Dometic fridge (which, btw, is related to the improved efficiency.
Noise energy = wasted energy).
These can be used either to replace or augment the existing condensor fan
unit (the one on the back of the fridge) or can be installed inside the
cabinet to improve heat exchange at the evaporator (the fins near the top
of the cabinet interior). Another, simpler, method for improving
evaporator efficiency is to buy one of those small battery powered units
commonly sold in RV stores. 2 D-size cells last for several weeks and the
fan dramatically improves performance, especially with fairly full fridges.
The exhaust flue insulation suggestion has considerable merit. Indeed, it
was recommended to me by the Dometic tech service folks. It not only
improves the draw, but also reduces condensation, which is a major
contributor to the crud that accumulates in the burner assembly
(necessitating pulling the unit for cleaning). Their suggestion was to use
fiberglass batt insulation (Note: Temp rating is an issue, due to
proximity to the burner; foam or even paper lined batt would probably not
be suitable). I wouldn't overdue the draft enhancements though, since the
Dometic 182 series were designed for this flue configuration. Extensive
mod's will probably be detrimental.
-Wes
> Date: Wed, 28 Oct 1998 15:08:02 -0600
> From: Blue Eyes <lvlearn@MCI2000.COM>
> Subject: Re: Fridge ?
> I recall reading a description of what seemed a wonderful improvement to
> the Westy Dometic refrigerator's performance. It suggested installing a
> tiny electric powered fan to blow air through the cold internal heat
> exchanger. While it's been a long time since reading that, it seems they
> were suggesting use of a 1.5 volt continuously powered fan. They said
> this was justified because the energy draw was trivial. At least a
> trivial energy draw won't add much heat load to the fridge.
>
> I'd like to use the excellent suggestion of forcing air over the
> internal heat exchanger. But even if the energy draw is trivial, life
> expectancy of such low cost motors probably isn't suitable for any
> continuous duty or long term use installation. Besides, I'd rather use
> my 12 volt house current.
>
> Does anyone know of any suitable small super low current draw 12 volt DC
> fans? If so, please post contact information, current draw, price, etc.
> Also, what would be a nice way to power this ONLY when the main burner
> heat is driving the refrigeration cycle? That feels like the right
> approach to me. Doesn't this appeal to the inventive spirit of you
> creative guys like Dave B.? No used three pound boiler controllers
> please. Keep saying this: Simple, compact, reliable, cheap, elegant.
> Say it, and designs will come.
>
> On a completely different refrigerator design consideration, I don't like
> the fact that in even tepid weather, my refrigerator's external heat
> exchanger fan keeps cycling on and off. It's distracting, irritating and
> calls attention to itself when I prefer to sleep. I always wonder if it
> might drain the battery enough to cause a starting problem if it were
> left on for a few days without starting the motor to recharge. Remember,
> old battery marginal performance failures always occur at their margins,
> by definition.
>
> Wouldn't it be good to find (or make from two sizes) some double
> walled insulated ducting to better retain flue gas heat? This would
> induce more flow through the chimney so the fan wouldn't come on so
> often. I even considered trying to move the exhaust location up near the
> top of the van. I think it might be possible to snake a duct between the
> kitchen windows inside the wall.
>
> Static chimney effect pressure is affected by average
> internal/external gas density difference per foot times the number of
> feet. So the hotter you can keep it before it exits at the top, the
> better it flows. Also the taller it is, the more static pressure it can
> develop. But flow is a dynamic state effect that's also affected by
> resistance. The smaller the chimney's cross sectional area and the
> longer it is, the higher the resistance is. So we have a balancing of
> effects. The designers' thinking shown in other RV absorption cycle
> installations I've seen usually caused them to create chimney runs as
> tall as possible, unlike the Westy design. I think the Westy's short
> vertical chimney length was a design error. A fully adequate full
> height chimney might require a small vertical interior intrusion rising
> between the kitchen windows. But to me, having a well functioning
> refrigerator that rarely needs additional air flow induced by an external
> fan would make that trade-off preferable.
>
> My overheated mind begins to create a Rube Goldburg cartoon-like
> modification wherein this former liability is turned into an asset.
> Picture a Westy parked in warm arid environment. An 8 foot chimney
> extension is attached to the roof. Inside, the refrigerator is dialed to
> it's coldest setting. This is causing air to flow through a small hidden
> swamp cooler cast into normally wasted space. A little electric fan
> silently blows air from the swamp cooler to the lower sleeping area or
> the back seat, depending on how the duct's outlet louver is directed.
>
> I prefer this kind of distraction to spelling humor, sandwich spreads,
> etc. John
>
Wes Neuenschwander
Seattle, WA
wesn@eskimo.com
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