by Marc Ward
ClayTimes Nov. 1997
Yesterday I was trying to explain the problems of firing a gas kiln at high altitude to a fellow potter who had just moved from the West Coast to New Mexico. He moved his kiln and brought with it a new set of problems.
To begin with, what is high altitude? For a gas kiln, you will begin to see changes in the behavior of a kiln once you get above 5,000 ft. Of course, there are changes between sea level and 4,000 ft., but these are too small to really start affecting the firing. You hear people say; "there's less oxygen the higher you go." This is partly true, but doesn't totally explain what is happening to all the parts of the kiln. At 5,000 ft., 7,000 ft., or even 10,000 ft, there is still the same percentage of oxygen (21%) in the air. This air is just not as dense. In other words, a cubic foot of air at 10,000 ft. has fewer gas molecules than a cubic ft. of air at sea level. So you have to move more cubic feet of air trough the kiln and burners to get the same amount of oxygen for combustion. There are two basic things to do to accomplish this; First, your burner ports and exit flue should be increased by 50% of what they would be at sea level. This will help pull in more secondary air. The next thing you want to consider is a reduction in orifice size. This will cause your gas/air ratio to change in the favor of more air. The rule of thumb is to decrease your orifice by one number (say a 38 to a 39) for every 2000 ft. increase over 5,000 ft.
There's another thing that happens at higher altitudes; your fuel doesn't have quite as much punch. Once it leaves the orifice, it expands more than it would at sea level because the weight of the surrounding air isn't as great and can't "squeeze" the gas as much. So, the BTUs per cubic foot value of your gas is diminished slightly. You've now got a gas that is thinner, with less "punch" and because it isn't as dense, it can't create as much draw to pull in the primary air. Now, all these things are subtle, but the higher you go, the more these things will adversely affect the kiln's ability to reach temperature.
If you are using forced air (blowers), you will need to increase the air available to the burner by 2% for every increment of 1,000 ft. over 5,000 ft. This means if you are at 10,000 ft., you would need to increase the air by 10%. Since you are introducing air automatically through the burner, there is no need to change orifice size with a forced air system. But, you should still increase your port and flue sizes as mentioned above.
So you've moved a kiln up the mountain...can you still use the same burners? Probably. If you kiln was firing marginally or taking a very long time at lower altitudes, you might find you need to upgrade your burner system. If your kiln was firing fine, you might see a slightly longer firing schedule and a tendency to overreduce and possibly stall if you don't make the port and flue modifications. Just as you might find jogging at 7500 ft. more difficult, so to will your kiln find it difficult to breathe unless you expand it's breathing capacity.