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Note: Dimensions of steel pipe and heat exchanger tubes are given in App. 3 and 4 of the text,
respectively. Properties of saturated steam are given in App. 7. Viscosities of liquids are given in
App. 9. Thermal conductivities of various materials are given in App. 10 and 11.
1. A³/4 inch (nominal). Schedule 40, mild steel pipe carries 40 psia steam through a room
having an air temperature of 80°F. The pipe is covered with a 0.125 ft thick layer of
fiberglass insulation having a thermal conductivity of 0.037 Btu/(h ft "F). The hi value on the
steam side of the pipe is 1000 Btu/h At' F. The ho value on the outside of the insulation is 2.0
Btu/(h f2 "F).
A
Calculate the thermal resistances of the inner film on the steam side, the pipe wall, the
insulation, and the outer film on the air side in units of h ft2 "F/Btu.
B.
Calculate the fraction of the overall resistance contributed by each of the resistances
determined in Part A. Would you consider any of the resistances to be negligible? Justify
your answer.
C. Calculate the overall heat transfer coefficient based on the inner surface area of the tube
(U) and the corresponding inner surface area (Ai) in units of Btu/(h ft "F).
D. Calculate the total amount of energy lost to the air per day if the pipe were 10 ft long.
2.
Water having an average temperature of 110°F is flowing through a mild-steel, ³/4 inch OD,
16 BWG heat exchanger tube at a velocity of 8 ft/s. The outer heat transfer coefficient (ho) is
300 Btu/(h #2 'F).
A Calculate the thermal resistance of the pipe wall based on the outer tube diameter in units
of h ft² "F/Btu.
B. Calculate the thermal resistance of the outer film based on the outer tube diameter.
C. Neglecting any effect of temperature on the water's viscosity, calculate the inner heat
transfer coefficient (h)
D. Calculate the resistance of the inner film based on the outer tube diameter.
E. Calculate the overall heat transfer coefficient based on the outer tube area (U.).
3. A horizontal, three-inch (nominal), schedule 40 steel pipe containing 212°F steam runs
through a large tank of stagnant 80°F water. The steam-side heat transfer coefficient (h) is
2000 Btu/(h ft °F). Calculate the pipe's outside heat transfer coefficient (ho) and the rate of
heat transfer per foot of pipe.
Hint: an iterative solution is needed for Problem 3, in which in which the outer pipe
temperature is assumed, the he value is calculated based on that assumed temperature, the
overall heat transfer coefficient (U.) is calculated, and then the ho and U. values are used to
calculate the surface temperature. If the calculated surface temperature is different than the
assumed one, then a new surface temperature is assumed (usually the temperature just
calculated), and the iteration is repeated.
4. A condenser having vertical tubes will condense 2,100 kg/h of ethanol at atmospheric
pressure. Ethylene glycol will flow through the tubes at an average temperature of 30°C. The
1.25 inch OD, 12 BWG tubes will be 3 feet long. The inner film heat transfer coefficient will
be about 2,800 W/(m² °C), and the resistance of the tube walls will be negligibly small.
Calculate the outer film coefficient and how many tubes will be needed in the condenser.
(Hint: A trial-and-error solution is required, because a Reynolds' number is needed to use
Fig. 13.2, and a wall temperature is needed to estimate the viscosity of the condensed ethanol
film. As first guesses, assume a condenser tube wall temperature of 45°C and an ethanol
condensate Reynolds number of 600. After calculating the number of tubes, check the
assumptions, and repeat calculations until the iterations have converged to an answer.)
Physical properties of ethanol are given below:
Boiling point = 78.4°C
Heat of vaporization = 8.55x103 J/kg
Liquid density = 770 kg/m³

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