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Engineering and Tech / Fundamentals of Heat and Mass Transfer 7 / Chapter 7 / Problem 7.93

Fundamentals of Heat and Mass Transfer | 7th Edition | ISBN: 9780470501979 | Authors: Theodore L. Bergman; Adrienne S. Lavine; Frank P. Incropera; David P. DeWitt

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An air duct heater consists of an aligned array of

Chapter 7, Problem 7.93

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QUESTION:

An air duct heater consists of an aligned array of elec-trical heating elements in which the longitudinal andtransverse pitches are SL?ST?24 mm. There are 3rows of elements in the flow direction (NL?3) and 4elements per row (NT?4). Atmospheric air with anupstream velocity of 12 m/s and a temperature of 25?Cmoves in cross flow over the elements, which have adiameter of 12 mm, a length of 250 mm, and are main-tained at a surface temperature of 350?C.(a) Determine the total heat transfer to the air and thetemperature of the air leaving the duct heater.(b) Determine the pressure drop across the elementbank and the fan power requirement.(c) Compare the average convection coefficient obtainedin your analysis with the value for an isolated (single) element. Explain the difference betweenthe results.(d) What effect would increasing the longitudinal andtransverse pitches to 30 mm have on the exit tem-perature of the air, the total heat rate, and the pres-sure drop?

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QUESTION:

An air duct heater consists of an aligned array of elec-trical heating elements in which the longitudinal andtransverse pitches are SL?ST?24 mm. There are 3rows of elements in the flow direction (NL?3) and 4elements per row (NT?4). Atmospheric air with anupstream velocity of 12 m/s and a temperature of 25?Cmoves in cross flow over the elements, which have adiameter of 12 mm, a length of 250 mm, and are main-tained at a surface temperature of 350?C.(a) Determine the total heat transfer to the air and thetemperature of the air leaving the duct heater.(b) Determine the pressure drop across the elementbank and the fan power requirement.(c) Compare the average convection coefficient obtainedin your analysis with the value for an isolated (single) element. Explain the difference betweenthe results.(d) What effect would increasing the longitudinal andtransverse pitches to 30 mm have on the exit tem-perature of the air, the total heat rate, and the pres-sure drop?

ANSWER:

Problem 7.93

Step by Step Solution

Step 1 of 6

Known: An air duct heater consists of an aligned arrangement of electrical heating elements with , and . Atmospheric air with an upstream velocity of and temperature of moves in cross flow over the elements with a diameter of 12 mm and length of 250 mm maintained at a surface temperature of .

Assumptions: (1) Steady-state conditions, (2) Negligible radiation effects, (3) Negligible effect of change in air temperature across tube banks on air properties.

Properties: Table A-4 , Air :

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