Page 470 - Bird R.B. Transport phenomena

450 Chapter 14 Interphase Transport in Nonisothermal Systems

6. In addition to the Nusselt number, we have met up with the Reynolds number Re, the
Prandtl number Pr, the Grashof number Gr, the Peclet number Pe, and the Rayleigh number
Ra. Define each of these and explain their meaning and usefulness.
7. Discuss the concept of wind-chill temperature.

14A.1. Average heat transfer coefficients (Fig. 14A.1). (b) Calculate the total heat flow through the tube wall for
Ten thousand pounds per hour of an oil with a heat capac- each of the oil flow rates in (a).
ity of 0.6 Btu/lb», • F are being heated from 100°F to 200°F 14A.4. Local heat transfer coefficient for turbulent
in the simple heat exchanger shown in the accompanying forced convection in a tube. Water is flowing in a 2-in.
figure. The oil is flowing through the tubes, which are cop- i.d. tube at a mass flow rate w = 15,000 lb,,,/hr. The inner
per, 1 in. in outside diameter, with 0.065-in. walls. The wall temperature at some point along the tube is 160°F,
combined length of the tubes is 300 ft. The required heat is and the bulk fluid temperature at that point is 60°F. What
supplied by condensation of saturated steam at 15.0 psia is the local heat flux q at the pipe wall? Assume that h
r
on the outside of the tubes. Calculate h ]f h , and h ln for the has attained a constant asymptotic value. ]oc
a
oil, assuming that the inside surfaces of the tubes are at the Answer: -6.25 X 10 Btu/hr • ft 2
4
saturation temperature of the steam, 213°F.
Answers: 78,139,190 Btu/hr • ft 2 • F 14A.5. Heat transfer from condensing vapors.
(a) The outer surface of a vertical tube 1 in. in outside di-
ameter and 1 ft long is maintained at 190°F. If this tube is
Steam in
surrounded by saturated steam at 1 atm, what will be the
total rate of heat transfer through the tube wall?
(b) What would the rate of heat transfer be if the tube
were horizontal?
Cold Hot Answers: (a) 8400 Btu/hr; (b) 12,000 Btu/hr
oil in "oil out
14A.6. Forced-convection heat transfer from an isolated
sphere.
(a) A solid sphere 1 in. in diameter is placed in an other-
wise undisturbed air stream, which approaches at a veloc-
Condensate out
ity of 100 ft/s, a pressure of 1 atm, and a temperature of
Fig. 14A.1. A single-pass "shell-and-tube" heat exchanger. 100°F. The sphere surface is maintained at 200°F by means
of an imbedded electric heating coil. What must be the rate
of electrical heating in cal/s to maintain the stated condi-
14A.2. Heat transfer in laminar tube flow. One hundred tions? Neglect radiation, and use Eq. 14.4-5.
pounds per hour of oil at 100°F are flowing through a 1-in. (b) Repeat the problem in (a), but use Eq. 14.4-6.
i.d. copper tube, 20 ft long. The inside surface of the tube is Answer: (a) 3.35 cal/s
maintained at 215°F by condensing steam on the outside
surface. Fully developed flow may be assumed through 14A.7. Free convection heat transfer from an isolated
the length of the tube, and the physical properties of the oil sphere. If the sphere of Problem 14A.6 is suspended in still
may be considered constant at the following values: p = 55 air at 1 atm pressure and 100°F ambient air temperature, and
lb /ft , C p = 0.49 Btu/lb m • F, /u = 1.42 lb /hr • ft, к = if the sphere surface is again maintained at 200°F, what rate
3
w
m
0.0825 Btu/hr • ft • F. of electrical heating would be needed? Neglect radiation.
(a) Calculate Pr. Answer: 0.332 cal/s
(b) Calculate Re. 14A.8. Heat loss by free convection from a horizontal
(c) Calculate the exit temperature of the oil. pipe immersed in a liquid. Estimate the rate of heat loss
Answers: (a) 8.44; (b) 1075; (c) 155°F by free convection from a unit length of a long horizontal
pipe, 6 in. in outside diameter, if the outer surface temper-
14A.3. Effect of flow rate on exit temperature from a ature is 100°F and the surrounding water is at 80°F. Com-
heat exchanger. pare the result with that obtained in Example 14.6-1, in
(a) Repeat parts (b) and (c) of Problem 14A.2 for oil flow which air is the surrounding medium. The properties of
rates of 200,400, 800,1600, and 3200 lb /hr. water at a film temperature of 90°F (or 32.3°C) are /x =
m