So, I did some research and found lots and lots of sources that spoke about the derating of propane appliances with altitude. However, none of these articles detailed the why ... merely the consequence. For example, an article might state the rule as 2% per 1000 ft and then proceed to discuss the various ways to change orifices and equipment for better performance. But, I can't find the derivation or science that explains the phenomenon.
Propane has about 91,000 BTUs per gallon. That's a fixed property that won't change. Burn a gallon of propane at sea level and you'll get 91,000 BTUs. Burn a gallon of propane at 12,000' and you'll get 91,000 BTUs. So, why then, would using a propane stove at each of these locations produce different results when the fuel itself is unchanged?
Anyone with any basic chemistry background knows that a combustion reaction, in its most basic form, is a reaction of a hydrocarbon with oxygen to produce carbon dioxide and water. This is the easiest way to argue against the idiots in the world that insist that oxygen is combustible. It's not. Try to write a combustion equation for oxygen. You can't, because oxygen isn't combustible.
And, we don't have to make it any more complicated than that, considering that propane is a very simple, straight-chain hydrocarbon: C3H8.
The perfect equation for
complete combustion is: C3H8 + 5 O2 > 3 CO2 + 4 H2O
As altitude changes (or ventilation is terrible in a closed space), oxygen becomes limited ... there isn't 5x amount of O2 as propane. There isn't enough oxygen to react with the supplied propane. We start getting
incomplete combustion, which can actually be written in several forms.
Here are two such equations, but there are others:
2 C3H8 + 7 O2 > 2 CO2 + 2 CO + 2 C + 8 H2O
2 C3H8 + 7 O2 > 6 CO + 8 H2O
First of all, what we see is a dirtier reaction. We're forming soot (C). Second, we see a dangerous reaction. We're forming carbon monoxide. For RV furnaces, the reactants and products are all expelled via the exhaust outside the RV; the fan blows the heat inside the RV. But, portable propane units obviously do not have this separation or an exhaust port/vent. This is why when using a portable propane heater in a tent, for example, you tend to get a lot more condensation -- combustion produces a ton of water vapor. It's also why you have to be careful, since you can potentially be creating carbon monoxide, an extremely lethal gas.
But, we still haven't explained why appliances would put out less heat. Regardless of complete or incomplete reaction, we're burning the same amount of propane which will produce exactly the same amount of energy and heat.
The regulator on the tank should be producing the same 11" WC for propane, regardless of altitude. It works off pressure differential (
gauge pressure, not
atmospheric pressure). Thus, the flow of propane should remain constant, given properly functioning valves, tubes, and such. So, again, we shouldn't see a decrease in energy, since the same amount of fuel is being supplied.
This leaves me with the only logical conclusion is that oxygen becomes so limited that you end up with a reaction that looks like:
a C3H8 + b O2 > c C3H8 + d CO2 + e CO + f C + g H2O
I'll let others work out the stoichiometric coefficients a-g. Note that propane now shows up on both sides of the equation. What this means is that we're not actually burning all of the propane being supplied ... we're dumping propane out the exhaust without burning it. Now, it's not just incomplete combustion, it's
partial combustion. (And, yes, I know that I've conflated a reaction equation with a sort of mass balance ... but I created this "model" to illustrate the point.)
Now you can start changing orifices and other components to get a better fuel mixture. Or, you can blend the propane with other gases to dilute it down for the same effect -- achieving a better ratio. This won't restore the heat output, as you're still limited to burning a finite and fixed amount of propane; you're supplying
less propane now, which will, of course, produce less energy/heat from the appliance.
It will, however, prevent you from wasting fuel and from potentially quenching the flame. At some point, you'll supply so much propane, that it will overwhelm the oxygen and stop the reaction (flame). Sort of like lighting some newspaper and a few pieces of kindling ... then immediately throwing 9 pieces of wood on top. Sure, wood burns, but if you dump all that fuel on a small flame, the whole thing goes out.
Anyway, that's my conclusion.
- At sea level, all propane is burned and heat/energy output is maximized. At altitude, only some propane is burned and heat/energy output is therefore less... OR ...
- At sea level, normal amounts of propane are supplied, all is burned, and heat/energy output is maximized. At altitude, a reduced amount of propane is supplied, all of it is burned, but energy/heat is limited due to the limitation of propane supplied... OR ...
- The excess propane supplied at altitude becomes so great and the oxygen available becomes so restricted, that the combustion reaction is quenched and the appliance fails to function at all. Catastrophic failure.
Happy to hear other points of view, because I don't believe I fully understand everything that's going on that limits efficiency and output of propane appliances at altitude.
Good luck.