Heat Recovery Steam Generators
Steam boiler drums and systems should be designed to insure that, with a boiler water total solids content as recommended by the American Boiler Manufacturers Association Standards (ABMA), the boiler carryover will not exceed ½ parts per million (ppm) solids by weight in the steam at design rates. A Larson Lane analyzer should monitor the steam quality.
In waste heat recovery, there are both natural and forced circulation steam generators. The weight ratio of water circulation to steam generation in each heat absorbing circuit is:
• Natural: From 5:1 to 20:1, typically 10:1
• Forced: 5:1 to 8:1
For Heat Recovery Steam Generators at the back end of gas turbines, a few design guides are:
1. Use shock tubes with saturated steam production in them, not superheating tubes, immediately after the supplementary-firing.
2. Use stainless steel liners for the inside hot walls of the convection sections.
3. Use only ceramic fiber between the stainless steel liners and the outer walls. (No mineral wool.)
4. Use a coating on all inner cold surfaces of the outer wall.
5. Economic flow velocities of hot exhaust gases are typically in the range of 30 to 70 feet/second. The higher velocities are through the finned and triangular pitched convection sections; the lower velocities are in the ducts.
6. For water stability, thermal design, and erosion prevention, the velocity on the water/steam side of the tubes should normally have a minimum velocity of
3 feet/second and a maximum velocity of 100 feet/second.
7. The gas velocity through the HRSG should not exceed 100 feet/second, and the HRSG should be designed for horizontal gas flow.
8. Figure 3400-3 is a typical heat release profile, plotting the temperature of the unfired gas turbine exhaust versus the water/steam temperature in the Heat Recovery Steam Generator. It highlights that the “Pinch” point is the difference in temperature between the flue gas and the temperature at which boiling of the water starts.
9. Figure 3400-4 shows the temperature advantage of supplementary firing the turbine exhaust. Over 100% of the heat added in supplementary-firing can be recovered because it can be cooled to a lower exhaust temperature while maintaining the same “pinch” point.