X-Source: The Hotrod Mailing list
Subject: Refrigerant
Date: Thursday, Jul 21 1994 11:49:25
X-Sequence: 66
From: John De Armond

>There was discussion of recharging an A/C system with propane some
>time ago.  Has anyone actually *done* this?  I am considering it,
>and would like pointers on exactly what type of propane I need,
>where to get it, etc.  I have a set of gauges, do I use the pressures
>etc. specified in the service manual or something else?

Propane by itself won't work.  The vapor pressure is too high.  In order
to duplicate the vapor pressure curve of R-12, one must use the
78% propane, 22% isobutane mix previously discussed.  A short discussion
of why.

Refrigeration occurs when a liquified gas is sprayed into the evaporator
where the pressure is low enough that it boils.  The boiling refrigerant
carries off the heat in the form of latent heat of vaporization of the
refrigerant.  This heat is released in the condenser when the gas is
compressed to a pressure sufficient to liquify it again.  Any substance
that boils at a reasonable pressure in the 30-40 deg f range, and
condenses at a reasonable pressure in the 100-140 deg range, and whose
critical temperature (the temperature above which the substance will not
liquify regardless of the pressure) is sufficiently high will work as
a refrigerant.  That includes water, most light hydrocarbons, ammonia,
carbon dioxide, sulfur dioxide and so on.  Freons are used because they
are non-toxic, have a reasonably high latent heat of evaporation, reasonably
low enthalpy, reasonably non-flammable, reasonably easy to make, chemically
inert and dissolves oil.  Pretty neat stuff, neat enough that it was almost
inevitable that the neo-luddite econazis would discover or invent a reason
to ban it.  The light hydrocarbons have almost the same characteristics
with the one exception that they are more flammable.  Any of them could
be used under the proper conditions.

The complication is the expansion valve, the device that meters
liquid refrigerant into the evaporator.  The expansion valve is
an active control element that meters refrigerant according to the
heat load in order to maintain a given superheat.  Superheat is simply
the amount of extra heat added to the refrigerant in the evaporator
after it has boiled. The reason one wants superheat is that it is
most desirable to have all the refrigerant evaporate in the evaporator.
Any liquid refrigerant that escapes the evaporator is wasted as far
as cooling is concerned.  The expansion valve has a thermal bulb
that measures the evaporator outlet temperature.  The force generated
by the thermal bulb is balanced across a diaphram by the pressure in
the evaporator and by a spring which supplies the superheat bias.
The valve functions such that at a given temperature, the valve opens
to admit more refrigerant as the pressure drops.  The valve closes
to restrict the refrigerant as the temperature drops.  The relationship
between the temperature and the vapor pressure of the refrigerant is
non-linear so the way to enable the valve to maintain the superheat
constant over the whole temperature range of interest is to fill the
thermal bulb with the refrigerant of interest.  The thermal bulb on
your car A/C is filled with R-12.  This is why any substitute refrigerant
must duplicate the characteristics of R-12.  Unless one refills the
thermal bulb with another refrigerant, of course.

I have experimentally done just that.  I have removed the R-12 from
an expansion valve and filled it with ordinary butane, as is used in
lighters.  This is NOT isobutane and has a much lower vapor pressure,
low enough that the evaporator runs near zero psi during normal operation.
Even though the pressures are lower, this substance works just fine.

Refitting an expansion valve with new refrigerant is pretty easy.  Most
automotive valves have nothing more than a capillary tube for a thermal
bulb.  Simply cut the very tip of the tube off to release the old
refrigerant.  then silver-solder a shraeder valve to the end so that the
capillary tube can be connected to a set of gauges and a vacuum pump.
Pull a good vacuum on the tube, fill it with gaseous new refrigerant,
evacuate again and bring the pressure back up to atmospheric with
refrigerant.  The entire expansion valve and most of the capillary tube
must next be refrigerated to below that temperature where the new
refrigerant is a liquid at ambient pressure.  This is so the refrigerant
will condense in the valve and tube and completely fill it.  For butane,
one can use dry ice and alcohol or ice and salt.  Gaseous refrigerant is
admitted to the tube until the tube fills with refrigerant.  One can
connect the tube to the refrigerant source with a short chunk of tygon
tubing so one can observe when the whole thing is full of liquid
refrigerant.  When the tube is full, use crimp on lug pliars or
something similar to crimp the tip of the tube tightly shut.  Score the
tube upstream of the crimp, break the tube and quickly silver-solder the
end shut.  That does it.

The superheat must be set.  This is normally done by an adjustment
accessable through the outlet port of the valve.  To set the superheat,
one needs a source of pressure, a gauge and a thermometer.  Connect the
gauge and pressure source to the outlet port (flow backward.) Measure
the temperature of the capillary tube.  Subtract 10 degrees from that
value.  This is the superheat.  Look that temperature up on the
refrigerant's vapor pressure table and note the PSI.  Turn on the
pressure source regulated to a low flow and adjust the superheat screw
to achieve this pressure.  For example, if the temperature is 70
degrees, subtract 10 to get 60.  If the vapor pressure is 30 psi, set
the pressure to that value.  (these are made-up numbers - consult the
vapor pressure table for your refrigerant of interest.  Put the system
back together, charge it with the new refrigerant and enjoy.

If your system has a POA valve or other similar anti-frost
suction line pressure regulator (most modern vehicles) the situation is
more complicated.  Most POA valves are non-adjustable so if a refrigerant
far removed from R-12 is used, the valve will have to be removed and
frost-up protection implemented with a pressure switch that turns the
compressor off when the evaporator pressure gets low enough to
cause frosting.

In case anyone is thinking of using R-22, forget it.  Not only are the
pressures too high, the critical temperature is too low, low enough that
it can easily be reached in the summer.  Ditto for propane.  Ordinary
butane is the best refrigerant commonly available without econazi
restrictions.

Lastly, if you do such a conversion, please, pretty please, permanently
label the system so the next owner won't get an unpleasant surprise.

John