FUEL FIRED CONTINUOUS KILNS
12.1 Fuel Efficiency in a continuous kiln
In general, modern continuous kilns use about half the fuel of modern batch kilns firing the same product on the same cycle. The two most important reasons continuous kilns use less fuel are as follows:
1. The walls of a continuous kiln do not have to be heated every cycle since they remain at temperature sometimes for year. Therefore, all the heat stored in the walls of a continuous kiln is saved when compared to a batch kiln where heat must be replaced every cycle.
2. The most significant reason for the increased efficiency in a continuous kiln is due to the fact that ware moves through a continuous kiln counter – flow to the exhaust gases. As the exhaust gases pass over the incoming ware, they transfer their heat so that the amount of heat lost up the stack is much less in a continuous kiln than in batch kiln.
Illustration 81 shows the heat transfer in a continuous kiln. As can be seen, the flow of exhaust gases from the hot zone moving from right to left is towards the PC fan or products of combustion exhaust fan. The car travel is from left to right. With this system, the exhaust gases are pulled around and through end, thereby transferring their heat to the incoming product. PC fan temperatures as low as 120˚C are common. PC fan temperature in a batch kiln tracks the firing cycle temperature so that when the kiln is at 1000 ˚C the exhaust temperature is approximately 1000 C. When the kiln is at 1300C, the exhaust temperature is approximately 1300C. illustration 81 also shows the flow pattern in the cooling travel. In this diagram, is pulled counter-flow over the fired ware by the WC fan or ware cool exhaust fan. The exhaust from requirements in a dryer or, In some cases, to help heat building. Illustration 81 shows one of three possible situations for the cooling zone of a continuous kiln; -being, where all of the cooling air is pulled out of the tunnel by the WC fan. The other two situations which will be discussed later in this chapter are where part of the cooling air is allowed to continue into the hot zone as preheated combustion air and alternately where all of the cooling air is allowed to proceed into the hot zone to provide all of the combustion air. The time/temperature curve in Illustration 81 shows the temperature of the ware as it moves through the continuous kiln with respect to time. This time/temperature curve is the firing cycle for product being fried. Continuous kilns are usually designed for a specific firing curve, however, the top temperature and throughput rate can be varied.
Illustration 82 show the kiln pressure in a continuous kiln. In this diagram, the horizontal line running through the center of the kiln is the zero pressure point so that below the horizontal line it is a suction. As can be seen, the maximum pressure is at the exit end of the kiln at the cooling air supply fan. The pressure is reduced at the ware cool fan as much of the cooling air is removed from the tunnel. The burner in put in the firing zone tends to hold the pressure positive; however, the strong pull of the PC fan pulls the kiln pressure in to the negative range in the preheat zone. Kiln pressure continues to get more and more negative as it approaches the front end and the PC fan in takes. Since the flow is always from high pressure to low pressure, this diagram shows the flow through the tunnel in a typical continuous kiln.