If a metal had a Fermi level of 5.0 eV, what would be the average energy of the electrons at 0 K?
A)
8.0 eV
B)
6.0 eV
C)
3.0 eV
D)
0
E)
5.0 eV
C
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A shell-and-tube heat exchanger with two tube passes and a single shell pass is used to heat water by condensing steam in the shell. The flow rate of the water is 15 kg/s and it is heated from 60 to 80°C. The steam condenses at 140°C and the overall heat transfer coefficient of the heat exchanger is 820 W/(m2 K). If there are 45 tubes with an OD of 2.75 cm, calculate the length of tubes required.
GIVEN
• Shell-and-tube heat exchanger with two tube passes and one shell pass
• Water in tubes, condensing steam in shell
• Water flow rate m w= 15 kg/s
• Water temperatures
? Tw,in = 60°C
? Tw,out = 80°C
• Steam temperature (Ts) = 140°C
• Overall heat transfer coefficient (U) = 820 W/(m2 K)
• Number of tubes (N) = 45
• Tube outside diameter (D) = 2.75 cm = 0.0275 m
FIND
• The length of tubes (L)
ASSUMPTIONS
• Counterflow
SKETCH
PROPERTIES AND CONSTANTS
the specific heat of water at the mean temperature of 70°C (cpw)= 4188 J/(kg K)
Suppose an car engine could be made to burn at a higher temperature, without harming or altering any other part of the engine's operation. At the higher temperature, the engine would
A) have a lower efficiency. B) have the same efficiency but be able to operate at a higher power. C) have a higher efficiency. D) still consume the same amount of gasoline per kilometer traveled. E) consume more gasoline per kilometer traveled.
When a wheel is rolling without slipping, the magnitude of its velocity relative to the ground is greatest at
a. the point in contact with the ground. b. the point at the center of the wheel. c. the point at the top of the wheel opposite to the point in contact with the ground. d. the point farthest forward from the center of mass of the wheel. e. the point farthest behind the center of mass of the wheel.