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Chapter 3: Problem 160

Consider a house whose attic space is ventilated effectively so that the air temperature in the attic is the same as the ambient air temperature at all times. Will the roof still have any effect on heat transfer through the ceiling? Explain.

Short Answer

Expert verified
Answer: Yes, the roof does have an effect on heat transfer through the ceiling of a house with a well-ventilated attic. The temperature difference between the living area and the attic space, along with the insulation properties of the roof, can impact the rate of heat transfer between the two areas.

Step by step solution

01

Understanding Attic Ventilation and Heat Transfer

Attic ventilation is crucial in maintaining a comfortable temperature in the house. A well-ventilated attic allows for continuous air circulation, which helps in preventing the build-up of heat in the attic space. This means that the air temperature in the attic will be the same as the ambient air temperature outside. Heat transfer occurs when there is a difference in temperature between two adjacent areas.
02

Determine the effect of a well-ventilated attic on heat transfer through the ceiling

In a house with a well-ventilated attic, the air temperature in the attic is the same as the ambient air temperature outside. However, the temperature inside the living space of the house may be different from the ambient air temperature, especially if heating or cooling systems are being used. In this situation, there is still a temperature difference between the living area of the house and the attic. This difference in temperature causes heat to be transferred from the warmer area (living space) to the cooler area (attic).
03

Evaluate the effect of the roof on heat transfer

In a house with a well-ventilated attic, the roof still plays a role in heat transfer through the ceiling. The material and insulation properties of the roof can affect how much heat is transferred. Even though the air temperature in the attic is the same as the ambient air temperature, the roof's insulation can slow down the rate of heat transfer between the living area and the attic. Thus, the roof has an effect on heat transfer through the ceiling.
04

Conclusion

Despite the presence of a well-ventilated attic in a house, the roof still has an effect on heat transfer through the ceiling. This is because there is likely to be a temperature difference between the living area and the attic space, causing heat to be transferred from one area to another. Additionally, the insulation properties of the roof can slow down the rate of heat transfer, further proving that the roof has an impact on heat transfer through the ceiling.

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Most popular questions from this chapter

A row of 3 -ft-long and 1-in-diameter used uranium fuel rods that are still radioactive are buried in the ground parallel to each other with a center-to- center distance of 8 in at a depth of \(15 \mathrm{ft}\) from the ground surface at a location where the thermal conductivity of the soil is \(0.6 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft} \cdot{ }^{\circ} \mathrm{F}\). If the surface temperature of the rods and the ground are \(350^{\circ} \mathrm{F}\) and \(60^{\circ} \mathrm{F}\), respectively, determine the rate of heat transfer from the fuel rods to the atmosphere through the soil.

Determine the winter \(R\)-value and the \(U\)-factor of a masonry wall that consists of the following layers: \(100-\mathrm{mm}\) face bricks, 100 -mm common bricks, \(25-\mathrm{mm}\) urethane rigid foam insulation, and \(13-\mathrm{mm}\) gypsum wallboard.

Chilled water enters a thin-shelled 5-cm-diameter, 150-mlong pipe at \(7^{\circ} \mathrm{C}\) at a rate of \(0.98 \mathrm{~kg} / \mathrm{s}\) and leaves at \(8^{\circ} \mathrm{C}\). The pipe is exposed to ambient air at \(30^{\circ} \mathrm{C}\) with a heat transfer coefficient of \(9 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). If the pipe is to be insulated with glass wool insulation \((k=0.05 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})\) in order to decrease the temperature rise of water to \(0.25^{\circ} \mathrm{C}\), determine the required thickness of the insulation.

Hot water is flowing at an average velocity of \(1.5 \mathrm{~m} / \mathrm{s}\) through a cast iron pipe \((k=52 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})\) whose inner and outer diameters are \(3 \mathrm{~cm}\) and \(3.5 \mathrm{~cm}\), respectively. The pipe passes through a \(15-\mathrm{m}\)-long section of a basement whose temperature is \(15^{\circ} \mathrm{C}\). If the temperature of the water drops from \(70^{\circ} \mathrm{C}\) to \(67^{\circ} \mathrm{C}\) as it passes through the basement and the heat transfer coefficient on the inner surface of the pipe is \(400 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), determine the combined convection and radiation heat transfer coefficient at the outer surface of the pipe. Answer: \(272.5 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\)

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