This kind of equipment are generally employed downstream different equipment that generates flue gases at a temperature still high to justify additional heat recovery sections to reduce waste heat to the atmosphere.
These units are generally installed downstream Gas Turbines, Incinerators or Existing fired heaters (typically all-radiant heaters or fired heaters with low efficiency).
Heat is recovered either producing steam (with either forced or natural circulation) or heating others heat transfer medium (such as Hot Oils) to be delivered to different users.
The waste heat recovery units are designed as a convection section which can be either under negative or positive pressure on flue gas side.
Flue gases leaving the upstream equipment, flow through insulated ducts to the convection bank and then to the stack. The convection portion of the tube usually has extended surface to increase the convection heat transfer rate. Tubes can be either horizontal or vertical according to available plot area.
Duties for waste heat recovery units can vary from few MW up to more than 100 MW. Sometime a post combustion chamber with inline duct burners is employed upstream the waste heat recovery section to allow the reheat of the combustion products to increase the efficiency of the heat recovery.