Date: Mon, 24 Jul 2006 12:30:44 -0700
Reply-To: Zeitgeist <gruengeist@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Zeitgeist <gruengeist@GMAIL.COM>
Subject: Re: Propane fired airconditioning
In-Reply-To: <44C4F72F.1090108@charter.net>
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Or, you could run propane IN your A/C system. I've presently got one of my
Mercedes running on a mixture of isobutane and propane. So far it's working
ok, but I need to bump up the amount of propane in order to get lower vent
temps (currently 48 degrees). YMMV
Here's a write-up (not mine) on how to run your A/C on isobutane/propane:
[fyi/caveat : If you don't agree with doing this, then my advice is to not
do it -- it's that simple]
"I fully charge the system, using a mix of Isobutane (R600a) and Propane
(R290). This mix (70% Isobutane, 30% Propane) is usually available at
sporting goods stores as high-altitude campstove fuel. An 8-oz (12-oz by
weight) can will run $4-5, you only need one. You want Isobutane (R600a),
not Butane (R600), because Butane has a too-high boiling point. It's
possible for your compressor to ingest liquid if you use it instead of
Isobutane, which can damage its valves. Lots of Butane/Propane mix is
available out there, so beware.
One of the pitfalls of converting an A/C system away from R12 is that there
are a number of things that are dialed in for R12's exact characteristics.
If you use a refrigerant with a different temperature/pressure curve, the
system ends up operating inefficiently. One of the beauties of this
hydrocarbon blend is that you can get a very close match to R12's curve. The
system then operates well.
A usable charge of this hydrocarbon blend is less than 50% of an R12 charge.
In part this is because hydrocarbons are more efficient refrigerants than
hydrochlorofluorocarbons. Also, most R12 systems are actually overcharged to
accommodate slow leaks. If you're doing your own servicing, a lesser charge
is acceptable, because it's pretty easy to re-do. And, having a lesser
charge reduces the fire risk in case of a sudden large leak.
What I find works is to feed in one can of the 70/30 mix, and then augment
this with straight Propane until the temperatures and pressures are right.
(With the system set for maximum cooling, and the engine at 2000 rpm.) What
I do is to monitor the temperature of the pipe next to the high-side fitting
using an infrared thermometer. As you slowly leak in Propane, you watch the
R12 *temperature* scale on the high-side gauge and the actual temperature of
the fitting. When they are close to each other ipso facto you are done. You
also watch the absolute high-side pressure to ensure it's not too high, and
you watch the low-side pressure to make sure it's still in the 20-30 psi
range. You have to make changes slowly enough to let temperatures and
pressures reach equilibrium so that you get accurate readings.
It is my belief that the Propane is doing most of the cooling. The Isobutane
is there to keep the pressures down. It provides enough circulating mass to
transfer both heat and lubricating oil, and has a temperature range that
keeps it from being a liquid in a harmful place. The proportion of the final
charge that is the Propane is an 'undercharge', because if it weren't the
pressures would be too high. But it's still enough to provide sufficient
cooling, and the Isobutane is filling out the ranks. But because the system
is still in a sense 'undercharged', it is very sensitive to leaks. A typical
R12 system is actually 'overcharged' in that it is designed to be able to
lose some portion of the charge (by inevitable slow leakage) without
impacting performance. This blend doesn't have that luxury, so once some
charge is lost the performance will suffer. Fortunately it is cheap and easy
to repair! Because the components of the blend can leak at different rates
you don't know what proportions are left once you've lost some charge, so
you should evacuate the system and start from scratch rather than try to top
it off.
I understand that this blend will work very well until ambient temperatures
get above 100°F. Because Propane has a critical point lower than R12, once
its pressure gets too high it will stop condensing altogether. Then the only
cooling is provided by the Isobutane, which has a higher critical
temperature, and cooling performance is minimal. This can happen when it's
really hot outside. If you live in such a place, stick with R12. (And you
probably wouldn't be happy with R134a's inherently lesser performance if you
were to convert.)
The upshot? Driving down the freeway on a 90°F day yielded 38°F vent
temperatures, which is hard to beat. When stopped in traffic, the auxiliary
fan kicked on. Coolant temperature throughout was just under 100°C.
(Normally 87°C without A/C.) In other words, normal behavior. It works! "
On 7/24/06, John Rodgers <inua@charter.net > wrote:
>
> The current thread dealing with battery power for the refrigerator
> brought the above subject to mind..
>
> The propane/115v refrigerator/freezer I had in my motor home some years
> back was outstanding. When running on propane, it never let me down. Nor
> did it on 115V electricity, either. It was not made to use battery power
> in any way. And that thing would freeze a halibut solid as a block of
> tombstone granite..
>
> So my question is, if there are propane fired refrigeration units that
> work well, can there not be a propane fired AC unit somewhere,
> CERVEL(Cervil?), or some other brand. If not, could one be cobbled
> together with parts from a propane fired fridge. After all, they aren't
> much more than a coil, a fan, and a heating unit. It seems to me if the
> shape was right, it would work great as a cooler for the camper on the
> weekend outing.
>
> Ideas, comments, slams??
>
> Regards,
>
> John Rodgers
> 88 GL Driver.
>
--
Casey
Olympia, WA
Biodiesel: "I drive in a persistent vegetative state"
'87 300TD intercooler #22 (216k)
'84 300D (214k)
Gashuffer:
'89 Vanagon Wolfsburg Edition (187K)
http://users.zhonka.net/zeitgeist/Misc/IMG_0171.JPG
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