A well-insulated heat exchanger has one line with 2 kg/s of air at 125 kPa and 1000 K entering, and leaving at 100 kPa and 400 K. The other line has 0.5 kg/s water entering at 200 kPa and 20 °C, and leaving at 200 kPa. Calculate the exit temperature of the water and the total rate of entropy generation?

Explanation:

Step one:

given data

T_{wi} = 20^{\circ}C

T_{Ai}=1000K

T_{Ae}= 400kPa

P_{Wi}=200kPa

P_{Ai}=125kPa

P_{We}=200kPa

P_{Ae}=100kPa

m_A=2kg/s

m_W=0.5kg/s

We know that the energy equation is

making the subject of formula we have

from the saturated water table B.1.1 , corresponding to  

from the ideal gas properties of air table B.7.1 , corresponding to T=1000K

the enthalpy is:

from the ideal gas properties of air table B.7.1 corresponding to T=400K

Step two:

substituting into the equation we have

from saturated water table B.1.2 at  we can obtain the specific enthalpy:

we can see that , hence there are two phases

from saturated water table B.1.2 at