The atomic mass system is based upon
A. the mass of uranium-238, since it is the heaviest element occurring in nature.
B. the mass of the neutron.
C. the mass of carbon-12, a very common isotope of carbon.
D. the mass of liquid water.
Answer: C
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Photon Momentum: If the frequency of a light beam is doubled, what happens to the momentum of the photons in that beam of light?
A. It stays the same. B. It is halved. C. It is doubled. D. It is reduced to one-fourth of its original value. E. It is increased to four times its original value.
It is proposed to preheat the water for a boiler with flue gases from the boiler stack. The flue gases are available at the rate of 0.25 kg/s at 150°C, with a specific heat of 1000 J/(kg K). The water entering the exchanger at 15°C at the rate of 0.05 kg/s is to be heated at 90°C. The heat exchanger is to be of the reversed current type with one shell pass and 4 tube passes. The water flows inside the tubes which are made of copper (2.5 cm-ID, 3.0 cm-OD). The heat transfer coefficient at the gas side is 115 W/(m2 K), while the heat transfer coefficient on the water side is 1150 W/(m2 K). A scale on the water side offers an additional thermal resistance of 0.002 (m2 K)/W. (a) Determine the overall heat transfer coefficient based on the outer tube diameter. (b) Determine the appropriate
mean temperature difference for the heat exchanger. (c) Estimate the required tube length. (d) What would be the outlet temperature and the effectiveness if the water flow rate is doubled, giving a heat transfer coefficient of 1820 W/(m2 K)?
GIVEN
• Reverse current heat exchanger - 1 shell pass , 4 tube passes
• Water in tubes, flue gases in shell
• Copper tubes
? Inside diameter (Di) = 2.5 cm = 0.025 m
? Outside diameter (Do) = 3.0 cm = 0.03 m
• Specific heat of gases (cpg) = 1000 J/(kg K)
• Gas inlet temperature
? Tg,in = 150°C
• Water temperatures
? Tw,in = 15°C
? Tw,out = 90°C
• Gas flow rate ( m g) = 0.25 kg/s
• Water flow rate ( m w)= 0.05 kg/s
• Tubes are copper
• Gas side heat transfer coefficient ( )ch = 115 W/(m2 K)
• Water side heat transfer coefficient ( )ih = 1150 W/(m2 K)
• Scaling resistance on the water side (Ri) = 0.002 (m2 K)/W
FIND
(a) The overall heat transfer coefficient (Uo) based on the outside tube diameter
(b) The appropriate mean temperature difference (?Tmean)
(c) The required tube length (L)
(d) The outlet temperature and effectiveness if the water flow rate were doubled, making ih = 1820 W/(m2 K)
SKETCH
PROPERTIES AND CONSTANTS
the specific heat of water at the average temperature of 52.5°C (cpw) =4179 J/kg K
the thermal conductivity of copper (k) = 392 W/(m K) at 127°C
Which of the following is an example of a nonconservative force?
a. gravity b. magnetism c. friction d. Both choices A and B are valid.
By how much is the energy stored in a Hooke's law spring increased when its stretch is increased from 4.00 cm to 8.00 cm?
a. 100% c. 300 % b. 200% d. The correct answer is not given.