A light oil flows through a copper tube of 2.6-cm-ID and 3.2-cm-OD. Air flows perpendicular over the exterior of the tube as shown in the following sketch. The convection heat transfer coefficient for the oil is 120 W/(m2 K) and for the air is 35 W/(m2 K). Calculate the overall heat transfer coefficient based on the outside area of the tube (a) considering the thermal resistance of the tube, (b) neglecting the resistance of the tube but assume that fouling factors of 0.0009 (m2 K)/W and 0.0004 (m2K)/W have developed on the inside and on the outside, respectively.
GIVEN
• Air flow over a copper tube with oil flow within the tube
• Tube diameters: Inside (Di) = 2.6 cm = 0.026 m
Outside (Do) = 3.2 cm = 0.032 m
• Convective heat transfer coefficients: Oil h i= 120 W/(m2 K) Air h o= 35 W/(m2 K)
• Fouling factors: Inside (Rdi) = 0.0009 (m2 K)/W
Outside (Rdo) = 0.0004 (m2 K)/W
FIND
• The overall heat transfer coefficient based on the outside tube area (Uo),
(a) considering the thermal resistance of the tube, and
(b) neglecting the tube resistance
ASSUMPTIONS
• Uniform heat transfer coefficients
• Variation of thermal properties is negligible
SKETCH
PROPERTIES AND CONSTANTS
the thermal conductivity of copper (k) = 392 W/(m K) (at 127°C)
From the solution with or without tube wall resistance
(a) The overall heat transfer with fouling can be calculated by rearranging
Based on the outside tube area
(b) The tube wall resistance is negligible as shown in the solution
COMMENTS
The given fouling factors lead to a 4% decrease in the overall heat transfer coefficient based on the outer tube wall area.
You might also like to view...
Free Fall: To determine the height of a flagpole, Abby throws a ball straight up and times it. She sees that the ball goes by the top of the pole after 0.50 s and then reaches the top of the pole again after a total elapsed time of 4.1 s. How high is the pole above the point where the ball was launched? Neglect air resistance.
A. 10 m B. 13 m C. 16 m D. 18 m E. 26 m
The light reflected from a soap bubble (n = 1.40) appears red (? = 640 nm) at its center. What is the minimum thickness (in nm)?
A. 124 B. 104 C. 114 D. 134 E. 234
A 2500-N pile driver ram falls 10 m and drives a post 0.1 m into the ground. The average force on the post is
A) 2500 N. B) 25,000 N. C) 250,000 N. D) 2,500,000 N.
The reaction energy associated with a nuclear reaction is
a. the total change in rest energy as a result of the reaction. b. equivalent to the disintegration energy. c. the minimum energy necessary for such a reaction to occur. d. called the threshold energy. e. the binding energy of the nucleons.