Heat recovery steam generators are generally categorized according to the type of circulation system used, which could be natural, forced, or once-through as illustrated in Fig. 2.12. Natural circulation units have vertical tubes and horizontal gas flow orientation, whereas the forced circulation HRSG uses horizontal tubes and gases flow in the vertical direction. Once-through units can have either a horizontal or vertical gas flow path. In natural circulation units, the difference in density between water and steam drives the steam-water mixture through the evaporator tubes and risers and back to the steam drum. In forced circulation units, a pump is used to drive the steam-water mixture through the horizontal evaporator tubes. At the steam drum, steam separates from the steam-water mixture and dry saturated steam flows through the superheater. In once-through designs, there is no circulation system. Water enters at one end and leaves as steam at the other end of the tube bundle.

In Europe, vertical gas flow forced circulation units are common. These require a circulation pump for maintaining flow through the evaporator tubes. A recent design in Belgium has natural circulation with vertical gas flow. The pressure drop through the evaporator tubes is limited by using an adequate number of streams or parallel paths.