Question

The wall has dimensions of 100 ft x 10 ft including window and door areas. The wall has a total window area of 80 ft2 and a door (8 ft x 2.5ft). The heat-transfer coefficients are 0.2 Btu/hr-ft2 –F for wall, 0.1 Btu/hr-ft2 –F for window, and 0.1 Btu/hr-ft2 –F for door. Determine the effective overall heat-transfer coefficient for the combination, U

Answer 1

The effective overall heat transfer coefficient for the combination,
`U = 0.19 Btu/hr-ft^2 -F`

Step-by-step explanation:

According to the question, the total area includes the areas of the wall, window and door.

Total area, A = 100 * 10 = 1000 ft²

Area of the window,
`A_(wind) = 80 ft^2`

Area of the door,
`A_(d) = 8 * 2.5 = 20 ft^2`

Area of the wall,
`A_(wall) = A - (A_(d) + A_(wind) )`

`A_(wall) = 1000 - ( 80 + 20)\\A_(wall) = 900 ft^2`

Heat transfer coefficients:

For wall,
`h_(wall) = 0.2Btu/hr-ft^2-F`

For window,
`h_(wind) = 0.1Btu/hr-ft^2-F`

For door,
`h_(d) = 0.1Btu/hr-ft^2-F`

To get the overall heat - transfer coefficient for the combination:

`AU = A_(wall) h_(wall) + A_(wind) h_(wind) + A_(d) h_(d)\\1000U = (900*0.2) + (80*0.1) + (20*0.1)\\1000U = 190\\U = 190/1000\\U = 0.19 Btu/hr-ft^2 -F`