The kiln connects with a material exit hood at the lower end and ducts for waste gases. This requires gas-tight seals at either end of the kiln. The exhaust gas may go to waste, or may enter a preheater, which further exchanges heat with the entering feed. The gases must be drawn through the kiln, and the preheater if fitted, by a fan situated at the exhaust end.  In preheater installations which may have a high pressure-drop, considerable fan power may be needed, and the fan drive is often the largest drive in the kiln system.  Exhaust gases contain dust, and there may be undesirable constituents, such as sulfur dioxide or hydrogen chloride.  Equipment is installed to scrub these from the gas stream before passing to the atmosphere.