3 Air is blown steadily through a heat exchanger at the mass

3. Air is blown steadily through a heat exchanger at the mass flow rate of 2 kg/s. Inlet air pressure is 100 kPa and inlet air temperature is 25 Celsius. Air at the exit of this heat exchange has pressure 400 kPa and temperature 200 Celsius. Calculate how much heat was added to the air in the heat exchanger and how much entropy-per-unit-time has increased during this process.

Solution

>> At, P1 = 100 kPa & T1 = 25 C,

h1 = 104.9281 KJ/Kg

>> At, P2 = 400 kPa & T1 = 200 C,

h2 = 2860.9917 KJ/Kg

>> m = mass flow rate = 2 kg/s

So, Total Heat added = m(h2 - h1) = 2*(2860.9917 - 104.9281) = 5512.13 KJ

>> Entropy Change, ds = m(h2/T2 - h1/T1) {bcz, Q = m*h}

=> ds = 2(2860.9917/473 - 104.9281/298)

=> ds = 11.393 KJ/Kg-K