If the total resistance between the steam and the air (including the pipe wall and scale on the steam side) in Problem 7.36 is 0.05 m2 K/W, calculate the temperature difference between the outer surface of the inner pipe and the air. Show the thermal circuit.

Question

GIVEN

Atmospheric flow through an annulus with steam condensing in inner tube

Diameters Inside

(Di) = 25 cm = 0.25 m

Outside (Do) = 38 cm = 0.38 m

Steam temperature (Ts) = 149°C

Air velocity (V) = 6 m/s

Total resistance between the steam and air (At Rtot) = 0.05 (m2 K)/W

Air bulk temperature (Tb) = 38°C

From Problem 7.36 heat transfer coefficient on the outer surface of the inner pipe (c h) = 25.4 W/(m2 K)

FIND

The temperature difference between the outer surface of the inner pipe and the air (?T)

ASSUMPTIONS

Steady state

Steam temperature is constant and uniform

Heat transfer to the outer surface is negligible

Air temperature given is the average air temperature

Thermal resistance of inner tube wall and condensing steam is negligible (Inner tube wall surface

temperature = Ts)

SKETCH

Answer 1

The thermal circuit for the heat transfer between the steam and the air is shown below



The total rate of heat transfer must equal the rate of convective heat transfer from the pipe wall to the

air



COMMENTS

Note that 79% of the thermal resistance is the convective resistance on the air side.