If you were to cut a small permanent bar magnet in half,
A) neither piece would be magnetic.
B) one piece would be a magnetic north pole and the other piece would be a south pole.
C) each piece would in itself be a smaller bar magnet with both north and south poles.
D) None of these statements is true.
C
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Steam having a quality of 98% at a pressure of 1.37 × 105 N/m2 is flowing at a velocity of 1 m/s through a steel pipe of 2.7 cm OD and 2.1 cm ID. The heat transfer coefficient at the inner surface, where condensation occurs, is 567 W/(m2 K). A dirt film at the inner surface adds a unit thermal resistance of 0.18 (m2 K)/W. Estimate the rate of heat loss per meter length of pipe if; (a) the pipe is bare, (b) the pipe is covered with a 5 cm layer of 85% magnesia insulation. For both cases assume that the convective heat transfer coefficient at the outer surface is 11 W/(m2 K) and that the environmental temperature is 21°C. Also estimate the quality of the steam after a 3-m length of pipe in both cases
GIVEN
A steel pipe with steam condensing on the inside Diameters
? Outside (Do) = 2.7 cm = 0.027 m
? Inside (Di) = 2.1 cm = 0.021 m Velocity of the steam (V) = 1 m/s Initial steam quality (Xi) = 98% Steam pressure = 1.37 × 105 N/m2 Heat transfer coefficients
? Inside (hci) = 567 W/(m2 K)
? Outside (hco) = 11 W/(m2 K) Thermal resistance of dirt film on inside surface (Rf) = 0.18 (m2 K)/W Ambient temperature (T?) = 21°C
FIND
• The heat loss per meter (q/L) and the change in the quality of the steam per 3 m length for (a) A bare pipe (b) A pipe insulated with 85% Magnesia: thickness (Li) = 0.05 m ASSUMPTIONS
Steady state conditions exist Constant thermal conductivity Steel is 1% carbon steel Radiative heat transfer from the pipe is negligible Neglect the pressure drop of the steam
SKETCH
