A scuba diver losing her air supply while far beneath the water surface is advised when ascending to
A) hold air in her lungs.
B) allow air to escape her lungs.
C) panic.
Answer: A
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A horizontal shell-and-tube heat exchanger is used to condense organic vapors. The organic vapors condense on the outside of the tubes, while water is used as the cooling medium on the inside of the tubes. The condenser tubes are 1.9-cm-O.D., 1.6-cm-ID copper tubes, 2.4 m in length. There are a total of 768 tubes. The water makes four passes through the exchanger. Test data obtained when the unit was first placed into service are as follows
Water rate = 3700 l/min
Inlet water temperature = 29°C
Outlet water temperature = 49°C
Organic-vapor condensation temperature = 118°C
After 3 months of operation, another test, made under the same conditions as the first (i.e., same water rate and inlet temperature and same condensation temperature) showed that the exit water temperature was 46°C.
(a) What is the tube-side-fluid (water) velocity?
(b) What is the effectiveness, e-NTU, of the exchanger at the time of the first and second test.
(c) Assuming no changes in either the inside transfer coefficient on the condensing
coefficient, negligible shell-side fouling, and no fouling at the time of the first test,
estimate the tube-side fouling coefficient at the time of the second test.
GIVEN
• A shell-and-tube exchanger, organic vapors condensing in shell, water in copper tubes
• Tube diameters
? Do = 1.9 cm = 0.019 m
? Di = 1.6 cm = 0.016 m
• Tube length (L) = 2.4 m
• Number of tubes (N) = 768
• Number of tube passes (Np) =4
• Water flow rate v w= 3700 1/min = 3.7 m3/min
• Water temperatures
? Tw,in = 29°C
? Tw,out = 49°C
• Organic vapor condensation temperature (Tc) = 118°C
• After 3 months: Tw,out = 46°C
FIND
(a) Water velocity (Vw)
(b) The effectiveness (e) at the time of both tests
(c) Fouling coefficient (1/Ri) at the time of the second test
ASSUMPTIONS
• No fouling at the time of the first test
• No change in the inside and outside heat transfer coefficients
• Negligible shell-side fouling
• Length given is the length of one tube - all four passes
SKETCH
PROPERTIES AND CONSTANTS
for water at 40°C
Density (?) = 992 kg/m3 Specific heat (cpw) = 4175 J/(kg K)
Thermal conductivity (k) = 0.633 W/(m K)
Kinematic viscosity (?) = 0.658 × 10–6 m2/s
Prandtl number (Pr) = 4.3
the thermal conductivity of copper (kc) = 392 W/(m K) at 127°C.
What is this object?
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Energy utilization in living organisms is
A) one of the most basic requirements of life, without which organisms could not maintain order, grow, and reproduce B) only important for organisms like plants which receive their energy directly from the Sun C) neither a necessary nor a sufficient condition for life D) not important for organisms that have adapted to survive extremely low temperatures (psychrophiles)
An unstable particle initially at rest breaks up into two fragments. The mass of one fragment is 1.75 × 10^–28 kg and the mass of the other fragment is 1.25 × 10^–27 kg. The lighter fragment has a speed of speed 0.850c. What is the speed of the heavier fragment?
a. 0.229c b. 0.119c c. 0.127c d. 0.220c