Application of waste heat recovery concepts to generators — Thermal modeling and analysis

The present work concerns a parametric analysis of recovering heat from generator exhaust gases through the use of counter flow concentric tube heat exchanger. The recovered heat is used to heat water. To proceed, many geometrical configurations of heat exchangers are considered and related heat transfer calculations are achieved. These designs consider cases where the exhaust gases are in the inner tube and others where exhaust gases are in the outer tube. The appropriate heat transfer modeling and corresponding iterative procedure are derived and exposed. In each case, the water and exhaust inlet temperatures, the exhaust mass flow rate are fixed and the water outlet temperature is calculated for different water mass flow rates covering real ranges of applications and different diameter ratios. A 500 kVA generator is considered in the parametric calculation. It was shown that water can be heated from 20 to up to 100 °C depending on the mass flow rate. The most efficient configuration is for water flowing in the inner tube with an inner to outer diameter ratio of 0.75 and where an average rate of waste heat recovered of 26 kW is obtained.