Date:         Thu, 1 Nov 2001 20:54:47 -0500
Reply-To:     Dennis Haynes <dhaynes@OPTONLINE.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Dennis Haynes <dhaynes@OPTONLINE.NET>
Subject:      Propane pressure facts
Content-type: multipart/alternative;

There has been a lot of talk about the use of propane heaters these past
few days so I though I would clear the air!

The pressure in the propane tank is a function of the propane and
temperature. As long as there is liquid in the tank, the pressure will
be close to that stated in the chart.
http://www.gasplants.com/property.htm  A bulk tank and portable cylinder
will have the same pressure at the same temperature. Note that propane
and butane are great for carrying in small containers because they
compress to a liquid easily. The latent heat of vaporization explains
why a propane tank gets cold as the gas is being used. Yes, heat has to
be absorbed in order to create the vapor you burn to make heat. Yes the
relatively low pressure needed to compress to a liquid also makes it
possible to use as a refrigerant.

Most propane appliances need the vapor pressure reduced to 11" water
column pressure. This is the job of the regulator. Appliances designed
to work with portable cylinders rely on an orifice to limit the flow,
thus reducing the pressure during use to a level appropriate for the
device. The major advantage of propane as compared to other fuels is
it's extremely wide flammability range. Air fuel mixture is not critical
for a clean burn. The down side here is that any propane out of control,
(a leak), presents a real hazard since the vapor is easy to ignite and
reducing the oxygen supply will not extinguish it. You would never see
an electric pump placed inside a propane tank like we frequently see in
gasoline tank. Yes. The pump in the gas tank relies on the fact that
there is little oxygen so there is little chance of ignition. Another
problem with a propane fire is the fact that the tank is already under
pressure.  As the tank is heated in a fire, the vapor pressure will rise
rapidly, especially as the propane tries to boil. This can cause a
BLEVE, (Boiling Liquid Expanding Vapor Explosion).

Using a non vented heater in a Vanagon is not very smart. I did this
many years ago and now consider myself lucky. These heaters need proper
ventilation to ensure a safe oxygen supply for you and to ensure no CO2
accumulates. I believe the recommendation is two 12" X 12" openings for
each 5,000 Btu. One opening high, one low. Not much is going to remain.
Remember, most of these things are designed for tents. If you must use
these, get a good propane alarm and CO2 alarm and mount according to the
directions. You should also make sure the heater has low oxygen cut off.
Do not rely on the battery operated units. Go to an RV supply and get
the permanently installed 12 volt models.

All combustion produces water vapor as a by product. In the winter this
will accumulate behind the interior panels of the van and never dry out.
It will even get inside the square tubes in the sides and roof of the
vans. My 1970 van rusted in places I never though possible in only 2
years of occasional camping this way.

If you really need to add heat your van, you should go with a
permanently installed, vented heater. Do it right. It is your life at
stake here. As for connecting the portable heaters to a bulk cylinder, a
good RV supply can get you the needed fittings and adapters. Marshall
Brass makes a great T that goes between the tank valve and the
regulator. (Extend a Stay).

Last spring, I posted a line of portable heaters that stayed outside and
used a 3" hose to pipe in the heat. I forgot what brand they were. I'll
look some more and repost when I find it.

Hope some of this helpful. Remember, be very careful suing propane and
these portable heaters. It only takes one mistake or unknown situation
to make a disaster.

Dennis


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