by Marc Ward
Clay Times Jan. 1998
Draft is the life's breath of a gas fired kiln. Without it, no fire. No firing. But, it is one of the primary ingredients that is often overlooked or ill planned. Draft is nothing more than the motion of air and combustion gases through the kiln.
Most folks think of gas fired kilns as insulated boxes that have heat added to them. Wrong. Kilns are insulated boxes in which heat transfer takes place. It's a subtle difference, but one, that if misunderstood, can create problems. The only way for heat transfer to take place in a gas kiln is for the burning gas/air mixture to enter the kiln, radiate, conduct, and to a lesser degree convect it's energy to the refractories and ware. Then this mixture has to leave the kiln and make way for more energy to keep the temperature raising. If the cycle is slowed by poor draft conditions, that causes the slowdown of exiting gases and new energy is hard pressed to enter. Here's an analogy; You own a short-term parking garage. It has plenty of entrances (burner ports), but only one exit, a poor layout, and a disgruntled attendant (poor exit conditions). Early in the day (beginning of the firing) the lot fills up easily (good initial temperature rise). The day wears on and people try to leave and others try to find a place to park. But, things are a little screwed up and the lot starts to back up. Some cars are driving around looking for spots while others are waiting in line. New folks see the line and confusion and go elsewhere to park. You realize that if you had better flow in the lot (draft) you could have gotten more cars out (spent combustion gases) which would have let you get more cars in (additional BTU's). If you could have gotten more cars in, you would have made more money (temperature rise).
The draft of the kiln provides the motion through the kiln and the oxygen needed for combustion. The draft is part of the combustion process. Here's another way to think of air flow through the kiln. Many folks make the mistake of trying to limit air flow through the kiln with the idea that air will cool things down and make the firing inefficient. That's like thinking that to conserve your oxygen you're going to hold your nose while you run uphill. It doesn't work that way. You've got to breath and so does the kiln. The better you breath, the easier it is to run up that hill. The better the kiln can use combustion air, the more efficient the firing.
The chimney plays a crucial role in the entrainment of air for the combustion process. This is especially true for kilns that use atmospheric or Venturi burners. These type of burners generally pull in about 30-50% of the primary air they need for combustion through the mixing bell of the burner. The motion of the gas through the burner is mostly responsible for this entrainment, but the remaining air needs to come as secondary air that is pulled into the kiln by the draft. Without this secondary air, combustion would be incomplete and the full BTU potential of the fuel would not be realized.
Next issue, in part two, I'll explain the difference in draft conditions between forced air burners and Venturi or atmospheric burners. I'll also explain how to determine chimney height for downdraft kilns.
Draft Part 2
Last issue I told you about the importance of draft in a gas fired kiln. The examples below are for downdraft kilns only. Updrafts already have the draft moving in an easy direction....up. Downdrafts on the other hand, need some coaxing to move air and gas through the kiln. Now for the difference between forced air and Venturi or atmospheric....
Forced air burners are burners that produce all the draft the kiln needs by the use of blowers. The draft, or motion of fuel and air through the kiln, is pushed into and out of the kiln. Atmospheric burners, on the other hand, depend upon the motion of raising hot air in the chimney to pull the draft through the kiln. This difference between pushing and pulling of draft is no small matter.
Because forced air burners provide their own draft, the height of the chimney is not critical. You need to get the chimney high enough to vent the gases out of the shed or kiln building and that's usually all. Though chimney height doesn't matter with these burners, the size of the exit flue can cause problems. I strongly recommend that you have the same square inches of exit flue as you do with the total of all your entrance ports sizes. Too small an exit flue can cause excessive back pressure in the kiln and show up in a host of problems. Safety valve shutdowns, burned out thermocouples and stalling are the most common problem. This all has to do with the push/pull thing. Think of it this way; it is much easier to pull a piece of string through a small hole than to push it through the same hole. This is why atmospheric burners tend to work better with restricted exit flues, though I still favor a balance between entrance and exit sizes.
Atmospheric burners that have their "breath" pulled into the burner and kiln need a certain amount of chimney height to get things moving. We all know that hot air rises. This simple fact make the atmospheric burner function properly. Without the required chimney height and it's rising column of flue gasses, the kiln couldn't breath. So, what is the required chimney height? Three times the interior height plus 1/3 the linear draw or chamber length. Here's an example;
The kiln is 36 inches deep from the front wall to the back wall. That's part of the linear draw, but wait, there's more. We have to go through the back wall which is 9 inches and to the back of the chimney base....say another 9 inches. So, our total linear draw is 36 plus 9 plus 9 which equals 54 inches. Divide this by three to get 18 inches. Now we need to take our interior height of.....oh say 36" and multiply this by three and we get 108 inches add this to the 18 inches and we have or chimney height of 126 inches or 10.5 feet.
If you have what some people refer to as a "tuned flue" that provides a smaller cross section than the total of your burner ports, your atmospheric burners are probably working fine. But, I get calls from lots of folks that have forced air and a "tuned flue". many of them are experiencing the problems mentioned above. They are having trouble "pushing" the draft through the kiln. In this case you may need to get a little "pull" going to help the situation. Even though forced air systems don't normally require additional chimney height, they may benefit from it with a tuned flue. If that doesn't help, the only recourse is to enlarge the exit flue...and you know what that means; rebuilding the kiln if you didn't leave knock out bricks in the flue area. Draft is the life's breath of a kiln. Remember; "let the big guy breath".