Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

11.3 Temperature Difference and Heat Transfer 325

Note again that the 15C temperature difference is equal to a 15 K temperature differ-

ence. We can determine the heat transfer rate from the chip by substituting for the values ofh,

A, and (TsTf) in Equation (11.18), which results in

Example 11.10 Calculate theR-factor (film resistance) for the following situations: (a) wind blowing over a wall,

h5.88 Btu /hFft

2

, and (b) still air inside a room near a wall,h1.47 Btu /hFft

2

.

For the situation where wind is blowing over a wall:

And for still air inside a room, near a wall:

Example 11.11 A typical exterior frame wall ( made up of 24 studs) of a house contains the materials shown

in Table 11.5 and in Figure 11.11. For most residential buildings, the inside room temperature

is kept around 70F. Assuming an outside temperature of 20F and an exposed area of 150 ft

2

,

we are interested in determining the heat loss through the wall.

In general, the heat loss through the walls, windows, doors, or roof of a building occurs

due to conduction heat losses through the building materials — including siding, insulation

R

1

h



1

1.47

Btu

h#ft

(^2) #
°F
0.68
h#ft
(^2) #
°F
Btu
R
1
h

1
5.88
Btu
h#ft
(^2) #
°F
0.17
h#ft
(^2) #
°F
Btu
qconvectionhA 1 TsTf 2  40 a
W
m
(^2) #
K
b10.0009 m
2
21 15 K 2 0.54 W

TABLE 11.5 Thermal Resistance of Wall Materials

Thermal Resistance

Items (hft^2 F/Btu)

1. Outside film resistance 0.17
   (winter, 15 mph wind)


2. Siding, wood (1/28 lapped) 0.81


3. Sheathing (1/2 in. regular) 1.32


4. Insulation batt (3 – 3 in.) 11.0


5. Gypsum wallboard (1/2 in.) 0.45


6. Inside film resistance (winter) 0.68

1

2

■Figure 11.11

Wall layers to accompany Table 11.5.

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