Problem 138
Saturated water vapor at \(40^{\circ} \mathrm{C}\) is to be condensed as it flows through a tube at a rate of \(0.2 \mathrm{~kg} / \mathrm{s}\). The condensate leaves the tube as a saturated liquid at \(40^{\circ} \mathrm{C}\). The rate of heat transfer from the tube is (a) \(34 \mathrm{~kJ} / \mathrm{s}\) (b) \(268 \mathrm{~kJ} / \mathrm{s}\) (c) \(453 \mathrm{~kJ} / \mathrm{s}\) (d) \(481 \mathrm{~kJ} / \mathrm{s}\) (e) \(515 \mathrm{~kJ} / \mathrm{s}\)
Problem 139
Steam condenses at \(50^{\circ} \mathrm{C}\) on the outer surface of a horizontal tube with an outer diameter of \(6 \mathrm{~cm}\). The outer surface of the tube is maintained at \(30^{\circ} \mathrm{C}\). The condensation heat transfer coefficient is (a) \(5493 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (b) \(5921 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (c) \(6796 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (d) \(7040 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (e) \(7350 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (For water, use $\rho_{l}=992.1 \mathrm{~kg} / \mathrm{m}^{3}, \mu_{l}=0.653 \times 10^{-3} \mathrm{~kg} / \mathrm{m} \cdot \mathrm{s}\(, \)\left.k_{l}=0.631 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, c_{p l}=4179 \mathrm{~J} / \mathrm{kg} \cdot{ }^{\circ} \mathrm{C}, h_{f g} \oplus T_{\omega}=2383 \mathrm{~kJ} / \mathrm{kg}\right)$
Problem 140
Steam condenses at \(50^{\circ} \mathrm{C}\) on the tube bank consisting of 20 tubes arranged in a rectangular array of 4 tubes high and 5 tubes wide. Each tube has a diameter of \(3 \mathrm{~cm}\) and a length of \(5 \mathrm{~m}\), and the outer surfaces of the tubes are maintained at \(30^{\circ} \mathrm{C}\). The rate of condensation of steam is (a) \(0.12 \mathrm{~kg} / \mathrm{s}\) (b) \(0.28 \mathrm{~kg} / \mathrm{s}\) (c) \(0.31 \mathrm{~kg} / \mathrm{s}\) (d) \(0.45 \mathrm{~kg} / \mathrm{s}\) (e) \(0.62 \mathrm{~kg} / \mathrm{s}\) (For water, use $\rho_{l}=992.1 \mathrm{~kg} / \mathrm{m}^{3}, \mu_{l}=0.653 \times 10^{-3} \mathrm{~kg} / \mathrm{m} \cdot \mathrm{s}\(, \)\left.k_{t}=0.631 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, c_{p l}=4179 \mathrm{~J} / \mathrm{kg} \cdot{ }^{\circ} \mathrm{C}, h_{f g} \oplus T_{\omega}=2383 \mathrm{~kJ} / \mathrm{kg}\right)$
Problem 141
Design the condenser of a steam power plant that has a thermal efficiency of 40 percent and generates \(10 \mathrm{MW}\) of net electric power. Steam enters the condenser as saturated vapor at \(10 \mathrm{kPa}\), and it is to be condensed outside horizontal tubes through which cooling water from a nearby river flows. The temperature rise of the cooling water is limited to \(8^{\circ} \mathrm{C}\), and the velocity of the cooling water in the pipes is limited to \(6 \mathrm{~m} / \mathrm{s}\) to keep the pressure drop at an acceptable level. Specify the pipe diameter, the total pipe length, and the arrangement of the pipes to minimize the condenser volume.
Problem 144
A manufacturing facility requires saturated steam at \(120^{\circ} \mathrm{C}\) at a rate of \(1.2 \mathrm{~kg} / \mathrm{min}\). Design an electric steam boiler for this purpose under these constraints: \- The boiler will be cylindrical with a height-to-diameter ratio of \(1.5\). The boiler can be horizontal or vertical. \- The boiler will operate in the nucleate boiling regime, and the design heat flux will not exceed 60 percent of the critical heat flux to provide an adequate safety margin. \- A commercially available plug-in-type electrical heating element made of mechanically polished stainless steel will be used. The diameter of the heater cannot be between \(0.5 \mathrm{~cm}\) and \(3 \mathrm{~cm}\). \- Half of the volume of the boiler should be occupied by steam, and the boiler should be large enough to hold enough water for a \(2-\mathrm{h}\) supply of steam. Also, the boiler will be well insulated. You are to specify the following: (a) The height and inner diameter of the tank, \((b)\) the length, diameter, power rating, and surface temperature of the electric heating element, (c) the maximum rate of steam production during short periods of overload conditions, and how it can be accomplished.
