Page 169 - Design and Operation of Heat Exchangers and their Networks

Thermal design of evaporators and condensers 157

For small tubes and wires, the constant C is further modified by
1/2
[1+0.34c p, v (T w T s )/Δh v ] .
In Eq. (4.29), α rad is the heat transfer coefficient for the radiation
component
4
σ T T 4
α rad ¼ w s (4.31)
1=ε +1=a 1 T w T s
where ε is the emissivity of the solid surface, a is the absorptivity of the liq-
uid (usually near unity), and σ is the Stefan-Boltzmann constant,
4
2
σ ¼5.6704 10 8 W/m K .
For the case where the value of α rad is small compared with the value of
α cond , an explicit form was proposed by Bromley (1948) as
3
α ¼ α cond + α rad (4.32)
4
For α rad /α cond <10, Bromley (1950) suggested another explicit
approximation:

3 1
α ¼ α cond + α rad 1+ (4.33)
4 3+7:86α cond =α rad
Taking the local variation of film thickness into account, Roetzel (1979)
obtained an implicit equation for the combined heat transfer coefficient and
expressed it in an explicit approximation as

4 1
α ¼ α cond + α rad 1+ (4.34)
5 4+12α cond =α rad
which yields a slightly higher coefficient than Bromley’s approach.

4.1.2 Flow boiling in tubes

Flow boiling has been widely used in power plants, refrigerators, chemical
and nuclear reactors, and evaporators in process industry. In flow boiling, the
nucleate and convective components are superimposed by a very complex
mechanism. The heat transfer characteristics depend not only on the flow
pattern regime but also on the local pressure, that is, pressure drop
characteristics.

4.1.2.1 Flow pattern regimes in upward flow
The flow patterns for upward flow in vertical tubes can be bubble flow, slug
flow, churn flow, annular flow, annular wispy flow, and mist flow. For