Which one of the following IS NOT a characteristic of terrestrial planets?
a. low average density
b. orbits inside the asteroids
c. craters in old surfaces
d. small diameters
e. very few satellites
a
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Exhaust gases from a power plant are used to preheat air in a cross-flow heat exchanger. The exhaust gases enter the heat exchanger at 450°C and leave at 200°C. The air enters the heat exchanger at 70°C, leaves at 250°C, and has a mass flow rate of 10 kg/s. Assume the properties of the exhaust gases can be approximated by those of air. The overall heat transfer coefficient of the heat exchanger is 154 W/(m2 K). Calculate the heat exchanger surface area required if (a) the air is unmixed and the exhaust gases are mixed and (b) both fluids are unmixed.
GIVEN
• Cross-flow heat exchanger – exhaust gas to air
• Exhaust temperatures
? Te,in = 450°C
? Te,out = 200°C
• Air temperatures
? Ta,in = 70°C
? Ta,out = 250°C
• Air flow rate m a= 10 kg/s
• Overall heat transfer coefficient (U) = 154 W/(m2 K)
FIND
The heat exchanger surface area (A) if
(a) Air is unmixed, exhaust is mixed
(b) Both are unmixed
ASSUMPTIONS
• The properties of the exhaust gases can be approximated by those of air
• The air is in the tube
SKETCH
PROPERTIES AND CONSTANTS
the specific heat of air at the mean temperature of 160°C (cpa)
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What will be an ideal response?
In an inelastic collision involving an isolated system, the final total momentum is
A) exactly the same as the initial momentum. B) more than the initial momentum. C) less than the initial momentum.
The R-value of a piece of material goes up if the material is
a. thicker b. a better conductor c. covering a larger area d. thinner