What is the typical circulation ratio for a natural circulation HRSG?

The typical circulation ratio for a natural circulation Heat Recovery Steam Generator (HRSG) is generally around 8:1 to 15:1. This range is indicative of how effectively water is circulated through the system relative to the steam generation capacity.

In natural circulation systems, the movement of water is driven by density differences created by heat transfer. When the water is heated in the evaporator section, it becomes less dense and rises to the steam drum, while cooler, denser water descends to the evaporator, establishing a natural loop. A circulation ratio within the range of 8:1 to 15:1 ensures that sufficient mass flow is maintained for effective heat transfer, optimizing performance and efficiency.

A higher circulation ratio could indicate excessive water movement, which may not be necessary for effective operation and can lead to increases in pumping energy requirements. Conversely, a lower circulation ratio may not provide adequate flow to efficiently remove heat from the combustion gases, compromising steam generation efficiency. Hence, the selection of 8:1 to 15:1 as the typical circulation ratio reflects a balance between efficiency, operational effectiveness, and energy consumption in a natural circulation HRSG.