In the interest of energy conversion, process heat can be obtained from a heat recovery unit in which heat is recovered from turbine or reciprocating engine exhaust. In a heat recovery unit, an exhaust gas flows over finned tubes carrying the fluid to be heated. The hot exhaust gas (900°F to 1,200°F) heats the fluid in the tubes in a manner similar to that in which air cools the fluid in an aerial cooler. It is also possible to recover heat from exhausts by routing the exhaust duct directly through a fluid bath. The latter option is relatively inefficient but easy to install and control.
Generally, the following design criteria should be provided to the manufacturers or vendors for sizing an exhaust heat recovery unit.
1. Total heat duty required to heat the fluid
2. Properties of the fluid to be heated
3. The outlet temperature of the heated fluid
4. Operational relationships between heat sources and users (e.g., which users continue to operate when sources shut down?)
5. Exhaust gas flow rates at anticipated ambient and at various loads from maximum to minimum
6. Exhaust gas temperature at anticipated ambient and at various loads
7. Maximum exhaust back pressure
8. Ambient temperature range
The design of heaters and waste heat recovery units is beyond the scope of this book. Sizing and design are best left to manufacturers,
