Determine the moisture migration through the wall shown in Figure 8-32.
The moisture migration through the composite wall is best analyzed by first doing a heat
transfer analysis. Referring to the sketch for station point numbers we can generate a
steady state conduction through the wall.
The heat transfer analysis
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In an industrial plant a shell-and-tube heat exchanger is heating pressurized dirty water at the rate of 38 kg/s from 60 to 110°C by means of steam condensing at 115°C on the outside of the tubes. The heat exchanger has 500 steel tubes (ID = 1.6 cm, OD = 2.1 cm) in a tube bundle which is 9-m-long. The water flows through the tubes while the steam condenses in the shell. If it may be assumed that the thermal resistance of the scale on the inside pipe wall is unaltered when the mass rate of flow is increased and that changes in water properties with temperature are negligible, estimate (a) the heat transfer coefficient on the water side and (b) the exit temperature of the dirty water if its mass rate of flow is doubled.
GIVEN
? Shell-and-tube heat exchanger - dirty water in steel tubes, steam condensing in shell
? Water flow rate
( mw ) = 38 kg/s
? Water temperatures
? Tw,in = 60°C
? Tw,out = 110°C
? Steam temperature (Ts) = 115°C
? Number of tubes (N) = 500
? Tube diameters
? Di = 1.6 cm = 0.016 m
? Do = 2.1 cm = 0.021 m
? Tube bundle length (L) = 9 m
FIND
(a) The heat transfer coefficient on the water side
( hi )
(b) The exit temperature of the dirty water (Tw,out) if the mass flow rate
( mw FIND
(a) The heat transfer coefficient on the water side
( ) hi
(b) The exit temperature of the dirty water (Tw,out) if the mass flow rate
( mw ) is double
ASSUMPTIONS
? The thermal resistance of the scale in the pipe is unaltered when the mass flow rate is increased
? Changes in water properties with temperature are negligible
? Two, or a multiple of two, passes
? The dirty water has the same thermal properties as clean water
SKETCH
PROPERTIES AND CONSTANTS
From Appendix 2, Table 13, for water at the average temperature of 85°C
From Appendix 2, Table 10, the thermal conductivity of 1% carbon steel (ks) = 43 W/(m K) (at 20°C)
A Chernobyl-type accident would not be possible for a Western-style reactor
What will be an ideal response?
A metal sample of mass M requires a power input P to just remain molten. When the heater is turned off, the metal solidifies in a time T. The specific latent heat of fusion of this metal is:
A. P/MT B. T /PM C. PM/T D. PMT E. P T /M
You and Al are both floating freely in your spaceships. Suppose that Al is moving away from you at 85 km/hr. You throw a ball in his direction at a speed of 75 km/hr. According to Al, which of the following is going on?
A) He sees you moving toward him at 85 km/hr and the ball moving toward him at 75 km/hr. B) He sees you moving away from him at 85 km/hr and the ball moving toward him at 10 km/hr. C) He agrees that he is moving at 85 km/hr and the ball is moving at 75 km/hr while you are standing still. D) He sees you moving away from him at 85 km/hr and the ball moving away from him at 10 km/hr.