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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- Spring '10
- Heat Transfer, heat flux, Nucleate Boiling, Boiling Curve, Pool Boiling