Sustainable Energy, SI Edition
2nd Edition
ISBN: 9781337672092
Author: DUNLAP, Richard A.
Publisher: Cengage Learning
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Chapter 8, Problem 17P
To determine
Calculate the outside temperature for which the heat loss through the wall is
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3.
6.0" wide wooden fence posts are embedded in a concrete driveway a distance 12.125" apart. There is
a 1/16" gap between the post and the concrete (see diagram). The posts are installed when the temperature is
12°C. Determine if the gap will close when the temperature increases to 32°C and if not how big the gap will
be at that temperature. The thermal expansion coefficients of wood and concrete are awood =32 x 105 °C and
aconcrete =13 x 106 °C. 1 inch3D2.54 cm.
6.0"
12.0"
0.0625"
A 1,065 ft², 7 in. thick concrete wall has inside and outside surface temperatures of 25°C and 9°C. A 2 in. insulation batt with a thermal conductivity of k = 0.03 Btu/(h· ft· °F) is added
to the wall. Determine the reduction in heat transfer rate.
What is the thickness of the insulation in meters?
m
What is the thermal conductivity value of the insulation batt in W/(m .K)?
W/(mK)
What is the R-value of the concrete wall (in m². K/W)?
m². K/W
What is the R-value of the insulation batt (in m². K/W)?
m². K/W
What is the heat transfer rate (in W) when the insulation is added?
W
What is the percent reduction in the heat transfer rate?
%
Calculate the thickness of the masonry wall having thermalconductivity of 0.75 W/mK if the heat transfer is to be 80% of theheat transfer through a composite structural wall having athermal conductivity of 0.25 W/mK and a thickness of 100mm.Assume that both walls are subjected to same surfacetemperature.
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- b) Table I shows the design of a composite wall (W1) with its elements and their corresponding thicknesses. Table 2 also shows the design of a wall (W2), its elements and the respective thicknesses. You are required to: i. ii. iii. Choose the wall that will give the lower coefficient of thermal conductivity. Give reasons and show calculation to justify the decision in (i). Comment on the general compliance of the selected wall to recommended thermal values. Table 1 - Properties of Composite Wall (W1): Layers External surface Rendering Block wall (Medium density) Polyurethene foam layer Cavity Block wall (Medium density) Rendering Internal surface Layers External surface Rendering Block wall (Medium density) Rendering Thickness Internal surface - 13 100 15 50 100 13 Thickness 13 Table 2 - Properties of Composite Wall (W2): 150 Units 13 mm mm mm mm mm mm Unit mm mm. mm Thermal Conductivity (W/m K) Thermal Conductivity (W/m K) Thermal Resistance (m² K/W) 0.07 0.17 0.12 Thermal Resistance…arrow_forwardCivilarrow_forwardA 5006 cm long by 4 m high wall is made of 25 cm concrete and 20 cm hollow clay tile with plaster of 1.5 cm thickness on its both sides surfaces. The temperature on inside surface is 19°C while the outside temperature is 45°C. What is the rate of heat transfer through the wall of the hall?arrow_forward
- Question 11 Which statement about thermal conductivity is FALSE? O a. The thermal conductivity of timber is greater in the longitudinal compared to the radial direction. O b. Lower density results in lower thermal conductivity. C. Thermal modification of wood reduces its thermal conductivity. Od. Thermal conductivity of air is higher compared to wood.arrow_forwardA line measured with a 50-m long steel tape was determined to be 645.22 meter when the average temperature during taping was 15.750C. If the tape is of standard length at 200C and the coefficient of thermal expansion of steel is 0.0000116/10C, the correct length of the measured line isarrow_forwardQuestion 5 Which statement about electrical conduction in wood is FALSE? O a. Electrical conduction increases very rapidly with increasing wood moisture content. O b. Electrical conduction is generally the same regardless of wood species. O C. Electrical conduction is greatest in the longitudinal direction of wood. Od. Electrical conduction increases with increased wood temperature.arrow_forward
- A steel tube with 5 cm ID, 7.6 cm OD and k=15W/m °C is covered with an insulative covering of thickness 2 cm and k 0.2 W/m °C. A hot gas at 330° C with h = 400 W/m2oC flows inside the tube. The outer surface of the insulation is exposed to cooler air at 30°C with h = 60 W/m2oC. Calculate the heat loss from the tube to the air for 10 m of the tube and the temperature drops resulting from the thermal resistances of the hot gas flow, the steel tube, the insulation layer and the outside air.arrow_forwardA well-mixed, insulated water storage tank is to be maintained at internal temperature 60°C. The environmental temperature around the tank varies from -25°C to 30°C. The tank's exterior surfaces (total = 125 m²) have four different levels of insulation as indicated in the table. Surface Component Area, Effective RSI, 2 m² 2 m². °C/W Surface A 60.0 Surface B 30.0 Surface C 30.0 Surface D 5.0 Say that just one surface can be selected to have its insulation upgraded such that its effective RSI would be doubled. Which should be selected to achieve the greatest reduction in heat loss? 10.0 15.0 4.0 0.5arrow_forwardDo a quick calculation of the thermal conductance (U value) across a cavity wall consisting of:1. Outer skin 220mm brickwork (North facing, exposed aspect) 2. 20mmcavity 3. Inner skin of 10mm plasterboard Compare this with the worked example of an insulated brick veneer wall in the worked example. Show your working.arrow_forward
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