Huge amount of energy wasted through the flue gas in thermal power station causes great concern in recent years. Discharging hot flue gas in the environment is not only a wastage of energy but also increases the rate of global warming. Efforts are given world -wide to harness the energy for useful purposes. In this project, wasted heat of flue gas approximately of the order of 9 MW in a 500 MW NTPC power plant is utilized in vapor absorption air conditioning plant.

Gas to liquid heat exchangers of tube fin and heat pipe types that have been placed in the existing electrostatic precipitator outlet flue gas duct are designed for this purpose. The dimensions of the finally selected heat exchanger are 3 m x 0.47 m x 9.78 m . The extracted energy from the flue gas is used to heat water to be utilized in the generator of a vapor absorption refrigeration system that has produced a refrigerating capacity of 1584 TR. approximately.

To avoid the use of an additional booster fan, allowable flue gas side pressure drop is limited to a very low value (20 mmH2O). Due to the corrosive nature of flue gas, heat recovery is confined up to the acid dew point temperature of the flue gas.

Software is developed to find out the detailed design parameters of both tube fin and heat pipe heat exchangers. Wickless heat pipe heat exchangers are also designed in this project. Different tube diameters, different wick structures like wrapped screen, sintered metal, rectangular groove, screen covered rectangular grooves are used in the design of a heat pipe heat exchanger. Out of many feasible designs of heat exchangers, the most economic design is selected as the final design. Off design performance of the heat exchanger is also carried out and found that for operating in the off design condition, a water flow control valve has to be adopted to keep the flue gas temperature above the acid dew point.

Keywords: Heat Pipe, Condensing Heat Exchanger, Wick, Working Fluid, Thermo-syphon, Heat Recovery, Flue Gas