by Marc Ward
Clay Times Sept. 2002
I guess it’s like a missing tooth or a scab… some folks just can’t seem to let it alone. They gotta’ keep messing with it. I’ve seen it several times just this week alone. I’ve witnessed this incessant behavior from folks that should know better and from those that don’t realize the consequences of their actions. It is, of course, messing around with your orifices.
Now, I’m not talking about your nostrils or ears, I’m referring to the orifices in your burners. There is a fine dance going on in a Venturi or atmospheric burner. There is a relationship amongst the size of the orifice, the length of the flame tube, the diameter of the Venturi throat, the type of gas, and the pressure of the gas. All these things affect each other and place limits on how far away from optimum these variables can stray before things get outa’ whack. Here’s one of the cases I saw this week.
A guy comes into my place fit to be tied. He has a glass furnace that he fires on high-pressure propane, but has trouble with back burning (the flame jumping back onto the orifice deep inside the burner instead of burning on the tip). He’s been given this big ‘ole burner so, of course, he’s using it. Problem is; it’s way too big for the furnace. So, to resolve the problem, he takes out the orifice and replaces it with a much smaller one. Now, he’s got the right amount of BTU’s for the furnace, but….and it’s a big but… he now has the wrong size orifice for the burner. Remember the fine dance I mentioned? Well, now there are toes getting stepped on.
The shape of a Venturi tube, with its trumpet like resemblance, is designed to match the cone of gas emerging from the orifice. As the gas emerges from the orifice it expands. The flaring out of the Venturi (getting larger towards the head) hopefully matches the expanding gas. If this happens as it is supposed to, it creates a slight vacuum behind the orifice and pulls in air to mix with the gas. This is the important primary air. Turn up the gas, and more vacuum is created and more air is pulled in. If the orifice is too large or too small, it upsets this balance.
Too large an orifice causes the gas to expand too much and not match the shape of the tube…. the result; not enough vacuum created and therefore not enough air pulled in. It can work OK at low pressure, but once you try to turn it up or increase pressure you have a lot of excess fuel and poor combustion. Now our glassblower friend had the opposite problem. His orifice was too small for his burner. As long as he was running at high pressure (faster gas motion), the burner was able to pull in enough air because it was creating a greater vacuum and things worked adequately. When he went to turn the burner down, it all fell apart. Now his cone of gas exiting his orifice was not able to expand to match the burner flare, couldn’t create a vacuum, so it couldn’t pull in air. The burner lost its Venturi action and he had a long stream of yellow flame coming out of the burner…. In his case, it was coming out of the back of the burner because his burner was shooting down into a furnace and the rising heat pushed it back when the Venturi action was lost. The solution was a much smaller burner that matched the orifice he was using. Sound’s bassackwards don’t it? Again, the point is, everything is interrelated and needs to match up.
OK,… be brave… I’m going to briefly talk math at you. The math that deals with BTU output has two parts; orifice size and gas pressure. The gas pressure part is a quadratic equation and the orifice size is a linear equation. In really simple terms, this means that if you double the pressure it doesn’t double the output. But, if you double the area of the orifice you will double the output. I said area, not diameter…. If you double the diameter you will almost quadruple the output!! Here’s one we can all relate to; a 16 inch pizza is only 33% bigger in diameter than a 12” pizza but, the 16 inch pizza has almost 100% more pizza than the 12” one. Small change in diameter, produce far greater changes in area. Little changes in orifice size have large consequences… some of them not real pretty. Think before you drastically change your orifices. It usually isn’t the answer to your problem.
So, this guy calls me the other day because he can't figure out why the new burner he just bought isn't working the way he thought it would. I then go through my usual litany of questions to try and find out what's the matter. I don't get very far down the list before it becomes obvious to me the nature of his problem. He's saved a few dollars and bought a burner from a ceramic supply house that doesn't understand the relationship between pressure and orifice size.
Many clay suppliers buy burners from the burner manufacturer that are for propane or natural gas. What they, and their customers, don't understand is, that many times, the propane/natural gas designation refers to those two gases being delivered at LOW pressure. Low pressure is generally 7 inches water column (1/4 PSI) for natural gas and 11 inches water column (2/5 PSI) for propane. Say the well meaning supplier then provides you with a burner that they got from the manufacturer that's for propane and you proceed to hook it up to 10 PSI. Guess what? It very well may not work. Especially if it's a lower quality, single piece cast burner.
An orifice is a hole. Plain and simple. Your car has orifices in the carburetor (if you still have an old car) or in the fuel injectors that let gas into the cylinders. Now, you pretty well know that if these holes are way too big in your car, it will get too much gas, flood, and fail to run. The same thing can happen to burners. The low pressure orifices are, many times, too large to run high pressure through. Or, if you're running low pressure, an orifice that is bigger still, will cause the same kind of problem.
The size of an orifice relates to the size of the burner tube and the pressure behind the gas running through the orifice. As the pressure increases, the cone of gas exiting the orifice becomes wider and faster. The faster part is usually fine but the wider part causes the pull of primary air into the burner to be lessened. This disrupts the normal flow of gas and air within the burner and causes it to run rich, producing less BTU output. You're flooding it and the kiln won't run right. In many cases,. this is what happens when folks turn down the burner and the kiln fires faster.
This relationship between pressure and orifice size is not an even thing. You can double the pressure, but not double the BTU potential. Double the area of the orifice though, and you will double the BTU potential. From a mathematical standpoint, the part of the equation that involves the orifice area is linear. The part that involves pressure is quadratic. Whoa!... who thought you'd be reading stuff about algebra in an art mag? In plain speak, this means you shouldn't mess around with the orifice size unless you know what you're doing. Messing around with the pressure isn't as big a deal. Pressure increases involve, in the mathematical sense, square roots. To give you an example; say your pressure is 5 (five anything, it doesn't matter). Well the square root of 5 is about 2.2. Now double 5 to 10. The square root of 10 is about 3.2. You doubled the pressure (from 5 to 10), but the real change in output is from 2.2 to 3.2. That's less than a 50% increase. What happens when you start changing the orifice?
Well, if your orifice is 1/16 of an inch and you increase it to 1/8 have you doubled the output? No way!! The orifice function is about the AREA of the hole. When you double the DIAMETER (1/16 to 1/8), you make the area about four times the previous size. A 12 inch pizza is half the size of a 16 inch pizza even though the 16 inch pizza's diameter is only 30% larger. So, before you grab that drill and ream out that orifice, remember two things; Little changes can have big effects (not always good) and it's easy to make a small hole bigger, but it's 'bout impossible to make that hole that's now too big ....smaller.