Question 11.20: If instead of water, atmosphere pressure air was flowing in ...
If instead of water, atmosphere pressure air was flowing in the tube of Illustrative Problem 11.19, determine the inside film coefficient.
We first check the Reynolds number and note that G is the same as for Illustrative Problem 11.19. The viscosity of air at these conditions is obtained from Figure 11.16 as 0.062 lb _{ m }/ft.·h. The Reynolds number, DG/μ, therefore is
\underline{D G}=\frac{(0.87 / 12) \times 290680}{0.062}=339908which places the flow in the turbulent regime. Because W/1000 is the same as for Illustrative Problem 11.19 and equals 1.2, we now enter Figure 11.18 at 1.2 and 400°F to obtain h_{1} = 135. Because the inside tube diameter is the same as before, F = 1.25. Therefore,
h = 135 × 1.25 = 169 Btu/(h·ft.2 ·°F)
It is interesting to note that for equal mass flow rates, water yields a heat-transfer coefficient that is almost five times greater than air.
