Efficiency of heat exchangers

Thermodynamically thermal resistance of the wall, heat leaving the exchanger with POC influences the thermal efficiency of the heat exchanger.

                             6)

According to the definition of overall thermal efficiency, it appears that the air can be preheated to the temperature above the flue gas temperature since no upper limit is assigned to the temperature of the preheated air temperature. Thermodynamically, in heat exchange between hot flue gas and cold air, air can not be preheated to the temperature above the flue gas temperature.

                              7)

                                                   8)
  
                                9)

Consider a heat exchanger which receives hot flue gas at 1600K and cold air at 298K. The hot flue gases leave the exchanger at 900K and cold air at 1373K. About 15% of the heat in flue gases is lost to the surroundings. The ratio of specific heat of flue gas to air is 1.2. Calculate the various efficiencies.

Heat balance gives    = 0.55

Overall thermal efficiency by equation 8 is 45.4%
Efficiency limit by equation 9 is 55% and relative efficiency by equation 10 or 11 is 82.5%