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Eliminating Tramp AirIn processes requiring an oxidizing atmosphere—particularly those using a closed system, like a periodic, beehive or shuttle kiln—it’s important to know where your air is coming from and how much you have in any particular part of your firing cycle. The ideal situation is for all of the air to be contained within the combustion system so that it can be controlled with the most efficient amount applied to those stages of the firing cycle where it will do the most good. However, as most of us know, air infiltration into the kiln atmosphere through door leaks, open observation ports, cracked walls, poor seals, gaps between decks, and certain burner control methods all contribute to the overall inefficiency of the burn—translating into dollars needlessly spent.
For example, imagine that someone neglected to plug a single 2-in. (3.14 in.2) observation port in a periodic kiln firing at a maximum 1850?F over a 50-hour cycle with typical ramps and soaks. That small excess load of cold air could consume 40-50 therms over the course of the cycle, costing approximately $20-$25 in excess fuel. This is a minor example, but one that emphasizes the importance of keeping your kiln physically tight and free of tramp air.
The Benefits of Pulse FiringThe previous example is fine for demonstrating what you may do to help control air infiltration, but how do you control a combustion system where thousands of cubic feet of air are being pumped into the kiln every hour? In classic fuel-only control systems, where the air is held constant and fuel is modulated down as heat demand decreases, excess air increases tremendously at any point in the cycle where the kiln is approaching and is at its set point. This method is acceptable for maintaining the high burner velocities necessary for kiln circulation but is somewhat fuel-inefficient due to the increased “load” of excess air.
In modulated control, where total burner output is reduced as heat demand decreases, the burners operate at a preset air/fuel ratio, but the all-important kiln atmosphere entrainment velocity is sacrificed. Circulation in the kiln is significantly reduced, minimizing air penetration into the load; however, convective heat transfer, organic burnout, color development and cycle times are negatively impacted.
Pulse firing and programmable atmospheric control provide an effective alternative. When operated properly, pulse firing has several advantages over fuel-only or modulated control systems. Pulse firing recognizes that high-velocity burners operate most effectively with regard to fuel efficiency and kiln atmosphere entrainment when operated at their maximum rated capacity. A high-velocity burner operating at its maximum capacity provides the maximum thrust, maintaining circulation in the kiln. Since the air/fuel ratio is not changed, excessive air is not introduced. Additionally, as heat demand decreases, the pulse rate decreases, sequencing fewer burners on high fire at any particular time. This action decreases the total heat input with a constant preset air/fuel ratio while maintaining maximum velocity.
Programming for Maximum ControlWith pulse firing, you can set up the burners with a preset excess air percentage necessary for a particular product and use that throughout the cycle, and probably save some fuel over fuel-only or modulated control methods. However, to get the most out of today’s programming flexibility, you should initially set up your burners with a minimum excess air percentage and use some additional regulating equipment to tune the excess air to the portions of the cycle that need it.
To control atmosphere through the burner, you need to control the air/fuel ratio. This is accomplished by using a second gas regulator signaled by a separate impulse line fitted with a bleed valve. In the stages of the cycle where additional excess air from setup is required, the bleed valve is opened at a programmed rate. The pressure signal to the downstream regulator decreases, allowing less fuel to flow relative to the air being delivered to the burner— resulting in an excess air condition.
Programming comes into play when several “modes” or methods of operation are configured to meet specific conditions in the firing cycle. In each mode, variables such as the minimum number of burners on, bleed valve position and timing of these events are all programmable, giving the firing curve creator endless repeatable possibilities specific to the type of product being burned. Using this logic, you could build several firing cycles—each specific to the type of product being fired—using different operating modes for each successive temperature range, including controlled cooling. In the “low-temp” portion of the cycle, you may want to have the bleed at a minimum setting and then open it further when a specific reduced pulse rate is reached, increasing the percentage of excess air introduced into the kiln. During organic burnout, you might want to specify a minimum number of burners to remain on at any one time to promote circulation while opening the bleed valve to increase the excess air provided for the oxidation process. At high temperatures, where excess air should be minimized, you might want the bleed valve to open slightly as a means to cool the flame temperature as you approach set point so as not to overshoot. Numerous other air control possibilities also exist.
Understanding the DataIt is important to know what is happening inside the kiln in order to recognize possible areas for improvement. Figure 1 compares the oxygen levels of a modulated control system to those of a pulse control system on an actual beehive kiln firing building brick. As can be seen in the figure, the modulated control oxygen levels were significantly higher than those of pulse control during two distinct periods. At the beginning of the cycle, former practice at this site dictated that because the ware being set had some residual physical moisture and a relatively high carbon content, excess air should remain high to promote circulation and heat transfer. Programming in the pulse system allowed the user to create an operation mode whereby excess air became a function of heat demand.
Additionally, the burners were set up with the minimum amount of excess air perceived to be adequate to perform the same function as in modulated control. As heat demand decreased due to the kiln approaching the set point, the pulse rate decreased. At a designated point, the bleed valve opened proportionately, eventually reaching equilibrium with the set point and required excess air. Since burners in a modulated system can only operate at the air/fuel ratio that is initially set, full excess air had to be introduced when the heat demand required full heat input, as seen in the second peak in Figure 1.
In each of these cases, substantial data were gathered over several burns prior to making the transition from a modulated control kiln to one that is pulse fired with programmable atmospheric control. The “after” data is representative of several “learning curve” burns. Few parameters were changed during this period, and each burn produced quality ware.