Why does a lamp burn out quicker if it is turned on and off many times, rather than one that is left burning?
What will be an ideal response?
The Tungsten filament has a positive temperature coefficient of resistance. Thus when it is first turned on, its current is excessive and the filament for an instant evaporates some of its surface, thereby becoming thinner.
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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
According to the model in which active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of
A) light emitted by hot gas in an accretion disk that swirls around the black hole. B) intense radiation emitted by the black hole itself. C) the combined light of thousands of young, high-mass stars that orbit the black hole. D) radio waves emitted from radio lobes found on either side of the galaxy we see in visible light.
The solar wind is constantly removing mass from the Sun
Indicate whether the statement is true or false
If a force is directed perpendicular to the displacement, no work is done by that force
Indicate whether the statement is true or false