Explanation Given information: The diameter of tube is 127 mm , the thermal conductivity of concrete bar is 1.7 W / m ⋅ K , the surface temperature of the tube is 120 ° C , the temperature of the air is − 5 ° C , the convective heat transfer coefficient of air is 20 W / m 2 ⋅ K and the temperature difference between the outer surface of concrete bar and the ambient air is 5 ° C therefore surface temperature of concrete bar is 0 ° C . Write the expression for theshape factor for the given configuration. S = 2 π L ln ( 1.08 w / D ) ...... (I) Here the length of concrete bar is L , the width of concrete bar is w and the diameter of the tube is D . Write the expression for the rate of heat loss from the pipe per unit length. Q · = S k ( T 1 − T 2 ) ...... (II) Here the thermal conductivity of bar is k , the surface temperature of the tube is T 1 and the surface temperature of the concrete bar is T 2 . Write the expression for the rate of heat transfer from concrete bar to the surrounding air per unit length. Q · = h A ( T 2 − T ∞ ) ...... (III) Here the heat transfer coefficient is h , the area of the heat transfer is A and the surrounding air temperature is T ∞ . Calculation: Substitute 127 mm for D in Equation (I). S = 2 π ( L ) ln ( 1.08 w / 127 mm ) = 2 π ( L ) ln ( 1.08 w / 127 mm ( 1 m 1000 mm ) ) = 2 π ( L ) ln ( 1.08 w / 0.127 m ) Substitute 2 π ( L ) ln ( 1.08 w / 0.127 m ) for S , 1.7 W / m ⋅ K for k , 120°C for T 1 and 0°C for T 2 in Equation (II). Q · = 2 π ( L ) ln ( 1.08 w / 0.127 m ) × 1.7 W / m ⋅ K × ( 120 ° C − 0 ° C ) = 2 π ( L ) ln ( 1.08 w / 0.127 m ) × 1.7 W / m ⋅ K × ( 393 K − 273 K ) Q · L = 1281

5th Edition

ISBN: 9781307573060

Author: CENGEL

Publisher: MCG/CREATE

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Question

Chapter 3, Problem 194P

To determine

The width of the square concrete bar and the rate of heat loss per meter length.

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Chapter 3, Problem 193P

Chapter 3, Problem 195P

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