The inside surface of a 1-m-long vertical 5-cm-ID tube is maintained at 120°C. For saturated steam at 350 kPa condensing inside, estimate the average heat transfer coefficient and the condensation rate, assuming the steam velocity is small.

Question

GIVEN

? Steam condensing inside a vertical tube

? Tube length (L) = 1 m

? Tube inside diameter (D) = 5 cm = 0.05 m

? Tube surface temperature (Ts) = 120°C

? Steam pressure (p) = 350 kPa

FIND

? The average heat transfer coefficient (hc)

ASSUMPTIONS

? Steady state

? The steam velocity is small

? Film condensation occurs

SKETCH



PROPERTIES AND CONSTANTS

From Appendix 2, Table 13, for saturated water at 350 kPa

Answer 1

The condensate layer thickness (o) at the bottom of the tube (x = L) can be estimated using Equation (9.17)



Since the condensate layer is much smaller than the tube diameter, Equation (9.21) can be used to estimate the average heat transfer coefficient.



The above analysis assumes the condensate layer is laminar. This assumption can be checked by

checking the Reynolds number at the bottom of the tube. With the aid of Equation (9.14), the Reynolds number can be written as



Substituting Equation (9.17) for S yields



Therefore, the laminar assumption is valid.