Water flows through a horizontal coil heated from the outsid

Water flows through a horizontal coil heated from the outside by high-temperature flue gases. As it passes through the coil the water changes state from liquid at 200 kPa and 80 degree C to vapor at 100 kPa and 125 degree C. Its entering velocity is 3 ms^-1 and its exit velocity is 200 ms^-1. Determine the heat transferred through the coil per unit mass of water. Enthalpies of the inlet and outlet stream are: Inlet: 33.4.9 kJ kg^-1 ; Outlet: 2,726.5 kJ kg^-1

Solution

For the steady flow process, the first law is written as

DH + Du²/2 + gDz = Q + W_s

since there is no shaft work, W_s = 0

and flow is horizontal, Dz = 0

Therefore,

DH + Du²/2 = Q

substituting for the quantities,

(2726.5 - 334.9) x 1000 + (200² - 3²)/2 = Q (in terms of J/kg)

Q = 2591.6 kJ/kg

Heat transferred through the coil per unit mass of water = 2591.6 kJ