Chapter 2: Problem 127
What is a variable? How do you distinguish a dependent variable from an independent one in a problem?
Chapter 2: Problem 127
What is a variable? How do you distinguish a dependent variable from an independent one in a problem?
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Get started for freeLiquid water flows in a tube with the inner surface lined with polyvinylidene chloride (PVDC). The inner diameter of the tube is \(24 \mathrm{~mm}\), and its wall thickness is \(5 \mathrm{~mm}\). The thermal conductivity of the tube wall is \(15 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\). The water flowing in the tube has a temperature of \(20^{\circ} \mathrm{C}\), and the convection heat transfer coefficient with the inner tube surface is $50 \mathrm{~W} / \mathrm{m}^{2}\(. \)\mathrm{K}$. The outer surface of the tube is subjected to a uniform heat flux of \(2300 \mathrm{~W} / \mathrm{m}^{2}\). According to the ASME Code for Process Piping (ASME B31.3-2014, \(\mathrm{A} .323\) ), the recommended maximum temperature for PVDC lining is \(79^{\circ} \mathrm{C}\). Formulate the temperature profile in the tube wall. Use the temperature profile to determine if the tube inner surface is in compliance with the ASME Code for Process Piping.
A heating cable is embedded in a concrete slab for snow melting on a $30 \mathrm{~m}^{2}$ surface area. The heating cable is heated electrically with joule heating. When the surface is covered with snow, the heat generated from the heating cable can melt snow at a rate of \(0.1 \mathrm{~kg} / \mathrm{s}\). According to the National Electrical Code (NFPA 70), the power density for embedded snow-melting equipment should not exceed $1300 \mathrm{~W} / \mathrm{m}^{2}$. Formulate the temperature profile in the concrete slab in terms of the snow melt rate. Determine whether melting snow at $0.1 \mathrm{~kg} / \mathrm{s}$ would be in compliance with the NFPA 70 code.
Consider a large plane wall of thickness \(L=0.8 \mathrm{ft}\) and thermal conductivity $k=1.2 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft} \cdot{ }^{\circ} \mathrm{F}$. The wall is covered with a material that has an emissivity of \(\varepsilon=0.80\) and a solar absorptivity of \(\alpha=0.60\). The inner surface of the wall is maintained at \(T_{1}=520 \mathrm{R}\) at all times, while the outer surface is exposed to solar radiation that is incident at a rate of $\dot{q}_{\text {solar }}=300 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft}^{2}$. The outer surface is also losing heat by radiation to deep space at \(0 \mathrm{~K}\). Determine the temperature of the outer surface of the wall and the rate of heat transfer through the wall when steady operating conditions are reached. Answers: $554 \mathrm{R}, 50.9 \mathrm{Btu} / \mathrm{h}^{\mathrm{ft}}{ }^{2}$
A heating cable is embedded in a concrete slab for snow melting. The heating cable is heated electrically with joule heating to provide the concrete slab with a uniform heat of \(1200 \mathrm{~W} / \mathrm{m}^{2}\). The concrete has a thermal conductivity of \(1.4 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\). To minimize thermal stress in the concrete, the temperature difference between the heater surface \(\left(T_{1}\right)\) and the slab surface \(\left(T_{2}\right)\) should not exceed \(21^{\circ} \mathrm{C}\) (2015 ASHRAE Handbook-HVAC Applications, Chap. 51). Formulate the temperature profile in the concrete slab, and determine the thickness of the concrete slab \((L)\) so that \(T_{1}-\) \(T_{2} \leq 21^{\circ} \mathrm{C}\).
Liquid water flows in a tube with the inner surface lined with polytetrafluoroethylene (PTFE). The inner diameter of the tube is $24 \mathrm{~mm}\(, and its wall thickness is \)5 \mathrm{~mm}$. The thermal conductivity of the tube wall is $15 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\(. The water flowing in the tube has a temperature of \)50^{\circ} \mathrm{C}$, and the convection heat transfer coefficient with the inner tube surface is \(50 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). The outer surface of the tube is exposed to convection with superheated steam at \(600^{\circ} \mathrm{C}\) with a convection heat transfer coefficient of $50 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}$. According to the ASME Code for Process Piping (ASME B31.3-2014, A323), the recommended maximum temperature for PTFE lining is \(260^{\circ} \mathrm{C}\). Formulate the temperature profile in the tube wall. Use the temperature profile to determine if the tube inner surface is in compliance with the ASME Code for Process Piping.
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