Ward Burner Systems

Customized Combustion Equipment

Pressure & Volume: How They Relate



by Marc Ward
Clay Times March 2003


    Almost all of us did it as children. If you spend time around little kids, you’ll see them do it sooner or later. We were all curious about the physics of the natural world when we are little and this manifested itself, not by delving into thick volumes of math textbooks, but by something far simpler. It was something that caused our parents and today's current parents much consternation…we played with water. We spilled it, we carried it from place to place in buckets and glasses, and we spit it out of our mouths in sheer delight. But, through all this fun and the wet mayhem that it usually caused, our brains were grasping lessons about the forces that shape our universe. Because of our childhood fun with fluids, it’s easier to describe one of the least understood relationships that effect gas kilns in terms that call on these same childhood lessons.

    This confusing relationship is the dance between pressure and volume.

    The pressure of gas is the force that the gas is pushed down the line. That’s easy…we all get it on a basic level. The part that sometimes gets confusing is how volume affects pressure. Most of you have heard that you need larger pipes when using natural gas. That’s because of volume issues. So, here’s the water analogy; we’re going to substitute BTU’s (British Thermal Units) for gallons of water.

    Say, your kiln needs 10 gallons of water per hour and you can choose between two reservoirs of water that are the same size. One reservoir is on the same level as your kiln and the other one is 200 feet in the air above your kiln. The water simply drains into your kiln. You instinctly know which tank of water is going to produce the hardest flow of water (pressure). So, how are you going to get the same number of gallons of water out of the tank on the same level with the kiln? You have to have a bigger hole coming out of the lower tank to get the same amount of water. This is why natural gas piping is so important. It is almost always low pressure (the lower tank of water), while propane is not only the higher reservoir, it is a reservoir that can be raised and lowered to different heights (you can change pressure by using different regulators). Sounds better to have propane, huh? Well, the water in that tank is usually more expensive and you have to buy it and store it before you use it. Also, you have to have enough water on hand to keep the flow going. It’s a give and take thing… propane has the limits mentioned, but you can raise and lower the reservoir to create different amounts of pressure. With natural gas, you almost always get the reservoir of water that’s on the same level as your kiln. All of this doesn’t mean you can’t have volume issues with propane.

    Remember, our gallons issue results from the height of the tank and the size of the hole. So, what happens when we put a wall, with a hole in it, in the way of our flow of water? If the hole is the same size as our previous hole, nothing happens…we get the same amount of water. But, suppose our second hole is much smaller…. the force of the water is the same coming out but, we won’t get the same amount (gallons) of water. This is why propane users can get into trouble by putting regulators in places they don’t belong. You can reduce your volume by putting a small hole in the way of the flow. Or, you can have two holes that are way too small (you can’t use a BBQ grill regulator to fire a 50 cubic ft. kiln…the right force is coming out [pressure] just not enough volume). So, here are some easy rules to remember regarding this stuff;…. If you’re unsure of how many gallons of water you need (BTU’s), find out! If you know how much volume BTU’s) you need, be sure to pass this information along to the folks at the gas company. If the folks putting in your lines (especially natural gas) don’t ask about your volume (BTU’s), don’t let them start. To be blunt, if they don’t ask, they are negligent at best, incompetent at worst. If the folks at the gas company never ask you or don’t seem to care, find another gas supplier or if that’s not possible, alert them to the fact you may need more volume than they are used to providing. You’re spending too much time, money, and energy not to get it right the first time.