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Specify the pipe diameter total pipe length and the

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Unformatted text preview: e safely without the danger of burning out. Answer: 109.6°C 10–91 A manufacturing facility requires saturated steam at 120°C at a rate of 1.2 kg/min. Design an electric steam boiler for this purpose under these constraints: Computer, Design, and Essay Problems • The boiler will be in cylindrical shape with a height-todiameter ratio of 1.5. The boiler can be horizontal or vertical. 10–86 Design the condenser of a steam power plant that has a thermal efficiency of 40 percent and generates 10 MW of net electric power. Steam enters the condenser as saturated vapor at 10 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°C, and the velocity of the cooling water in the pipes is limited to 6 m/s to keep the pressure drop at an acceptable level. Specify the pipe diameter, total pipe length, and the arrangement of the pipes to minimize the condenser volume. 10–87 The refrigerant in a household refrigerator is condensed as it flows through the coil that is typically placed Boiler FIGURE P10–91 cen58933_ch10.qxd 9/4/2002 12:38 PM Page 560 560 HEAT TRANSFER • 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 cm and 3 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 2 h supply of steam. Also, the boiler will be well insulated. late the surface area of the heater. First, boil water in a pan using the heating element and measure the temperature of the boiling water away from the heating element. Based on your reading, estimate the elevation of your location, and compare it to the actual value. Then glue the tip of the thermocouple wire of the thermometer to the midsection of the heater surface. The temperature reading in this case will give the surface temperature of the heater. Assuming the rated power of the heater to be the actual power consumption during heating (you can check this by measuring the electric current and voltage), calculate the heat transfer coefficients from Newton’s law of cooling. You are to specify the following: (1) The height and inner diameter of the tank, (2) the length, diameter, power rating, and surface temperature of the electric heating element, (3) the maximum rate of steam production during short periods of overload conditions, and how it can be accomplished. 10–92 Repeat Problem 10–91 for a boiler that produces steam at 150°C at a rate of 2.5 kg/min. 10–93 Conduct this experiment to determine the boiling heat transfer coefficient. You will need a portable immersion-type electric heating element, an indoor-outdoor thermometer, and metal glue (all can be purchased for about $15 in a hardware store). You will also need a piece of string and a ruler to calcu- FIGURE P10–93...
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