text: Heat exchange in a rotary kiln may be by conduction, convection and radiation, in descending order of efficiency.  In low-temperature processes, and in the cooler parts of long kilns lacking preheaters, the kiln is often furnished with internal heat exchangers to encourage heat exchange between the gas and the feed.  These may consist of scoops or "lifters" that cascade the feed through the gas stream, or may be metallic inserts that heat up in the upper part of the kiln, and impart the heat to the feed as they dip below the feed surface as the kiln rotates.  The latter are favoured where lifters would cause excessive dust pick-up.  The most common heat exchanger consists of chains hanging in curtains across the gas stream.