In a gaseous mixture of hydrogen and oxygen, the faster molecules on average are
A) hydrogen.
B) oxygen.
C) both the same
D) not enough information
Answer: A
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Is film condensation more effective on vertical tubes or on horizontal tubes?
What will be an ideal response?
Calculate the average heat transfer coefficient for film-type condensation of water at pressures of 10 kPa and 101 kPa for (a) a vertical surface 1.5-m-high (b) the outside surface of a 1.5-cm-OD vertical tube 1.5-m-long (c) the outside surface of a 1.6-cm-OD horizontal tube 1.5-m-long and (d) a 10-tube vertical bank of 1.6-cm-OD horizontal tubes 1.5-m-long. In all cases, assume that the vapor velocity is negligible and that the surface temperatures are constant at 11°C below saturation temperature.
GIVEN
? Film condensation of water
FIND
The average heat transfer coefficient at pressure of 10 kPa and 101 kPa for
(a) A vertical surface of height (H) = 1.5 m
(b) The outside surface of a vertical tube Outside diameter (D) = 1.6 cm = 0.016 m
Height (H) = 1.5 m
(c) The outside surface of a horizontal tube Outside diameter (D) = 1.6 cm = 0.016m
Length (L) = 1.5 m
(d) A 10 tube vertical bank of horizontal tubes Outside diameter (D) = 1.6 cm
Length (L) = 1.5 m
ASSUMPTIONS
? Steady state
? Vapor velocity is negligible
? Surface temperatures (Ts) are constant at 11°C below saturation temperature
? Film thickness is much smaller than the pipe diameter
? Laminar condensate flow
SKETCH
PROPERTIES AND CONSTANTS
From Appendix 2, Table 13, the saturation temperatures for water at
101 kPa (Tsv1) = 100°C, therefore Ts = Tsv – 11°C = 89°C
10 kPa (Tsv2) = 45.3°C, therefore, Ts = 34.3°C
The film temperatures, as given in Section 9.4.1 are
Tfilm1 = Ts + 0.25 (Tsv – Ts) = 89°C + 0.25 (11°C) = 91.8°C
Tfilm2 = 34.3°C+ 0.25 (11°C) = 37.1°C
From Appendix 2, Table 13, for water at the film temperatures
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