The boiler system design has to consider these aspects for safety and optimum running cost of Boiler.

Boilers utilizing waste heat contained in the exhaust gases coming from the DRI Kilns, Blast Furnace and Coke Oven are called Heat Recovery Steam Generators (HRSG).

Heat Recovery Steam Generators may be manufactured as units without a supplementary firing, and in this type of boiler, only the heat contained in the exhaust flue gases is recovered, or as so called fired boilers, where a burner (or burners) supplying additional heat are installed in the boiler inlet duct.

The use of finned tubes for heat exchange surfaces is necessary because of considerably lower temperature differential between gas and water/steam side in comparison with conventional boilers. Low temperature differential may also affect operating conditions. The slight change of evaporator pressure results in considerable change in the amount of steam generated thus creates a danger of not achieving the required boiler steaming capacity or a specified steam temperature. This shows the importance of precise thermal calculations and an importance of the heat transfer surface selection.