Steam is flowing in a 0.62-inch vertical tube at a rate of
50 pounds per hour. Calculate the
heat-transfer coefficient if condensation is Nusselt-type. Density and viscosity values are provided
for both condensate and steam. In
addition condensate specific heat and thermal conductivity values are
provided.
Solution:
For a Nusselt-type condensation, condensing coefficient can
be found to be 0.925 times k times (rhoL2 g over mu
alpha), where alpha is condensate rate per unit periphery, and g is
gravitational constant. Plugging in the
values, we get heat-transfer coefficient value of 692 BTU per hour per square
foot per degree Fahrenheit.
If we do not assume, Nusselt type condensation, then we need
to find three parameters, as recommended by Dukler, to establish the value of
the average heat-transfer coefficient.
These parameters are terminal Reynolds number (4 alpha over mu), the
Prandtl number (c mu over k), and a dimensionless group designated Ad
and defined by an involved expression.
One can calculate, average condensing coefficient from the
Figure 7-17 by using terminal Reynolds number and Prandtl number. This figure is based on Ad equal
to zero [no interfacial shear]. Now
this value can be corrected by multiplying with a correction factor, h/ho. This correction factor is a function of
terminal Reynolds number and Ad.
[Use Table given on page 7-50, Chopey].