Cold water at 15 degree C enters a thinwalled doublepipe cou

Cold water at 15 degree C enters a thin-walled, double-pipe, counter-flow heat exchanger at a rate of 0.25 kg/s and heated to 45 degree C by hot water coming from a water heater at 100 degree C and a rate of 3 kg/s. The heat exchanger is insulated well so the heat transfer between the exchanger and surroundings is negligible. Determine the rate of heat transfer and the rate of entropy generation in the heat exchanger.

Solution

Given: T_ci = 15^oC, T_co = 45^oC, T_h = 100^oC, m_{cold water} = 0.25 kg/s, m_{hot water} = 3 kg/s

a) Cp_water = 4.187 kJ/kgK

So, rate of heat transfer = Q = m_{cold water} * Cp_water *(T_co -  T_ci ) = 31402.5 J/s = 31402.5 W

b) (Rate of entropy generation) + (rate of entropy transfer by heat and mass) = (rate of entropy change of system)

Here, rate of entropy change of system = 0

So, Rate of entropy generation = - ((-Q)/T_h) = 84.189 W/K