One area of kiln inefficiency is the energy lost through the shell itself, additional air passing through, ports, and other openings. The energy loss can occur in three fashions
Convective losses occur when hot gases are pushed out of the kiln into the surrounding environment. This represents air that has been heated but does not do any useful work. Actual losses will be based on the temperature of air escaping and the volume. Because most systems are under a slightly negative atmosphere and leaks or opened areas suck air in. When air is introduced into a low-pressure area of the kiln (commonly termed in-leakage) through seal leaks, doors, efficiency is reduced. This air intake is also uncontrolled and susceptible to variations, making overall kiln air and fuel control more difficult.
In-leakage also adds to the load induced on the fans. Air leaks closer to the feed end of the kiln can also affect the measurement of remaining oxygen in the air stream without significantly adding to the combustion process. Normally the air being exhausted is in the range of 350 – 450 deg f. This adds up to heat that cannot be practically recovered.
Radiant losses occur when infrared heat (light) is lost through the kiln shell or any heated surface. The loss is partially dependent on the emitter – in this case the flame, firebrick and glowing lime – and partially on the surrounding surface. Kiln shell temperatures can run as hot as 750 deg f at the burning zone to 300 deg f at the cooler end. Heat is also radiated from all other components in the plant. This loss is minimized by use of refractory lining and insulation. See refractories.