A heat exchanger is to be designed to heat a flow of molten bismuth from 377°C to 477°C. The heat exchanger consists of a 50-mm-ID tube with surface temperature maintained uniformly at 500°C by an electrical heater. Find the length of the tube and the power required to heat 4 kg/s and 8 kg/s of bismuth.
GIVENS
FIND
The length (L) tube and power required (q) to heat 4 kg/s and 8 kg/s of bismuth
ASSUMPTIONS
Steady state
Uniform and constant surface temperature
Losses from the heater are negligible
SKETCH
PROPERTIES AND CONSTANTS
for Bismuth at the average bulk temperature of 427°C
At m = 4 kg/s the Reynolds number is
Therefore, can be used. The resulting L/D should be greater than 30
can be used to find the length required
Repeating the analysis for m= 8 kg/s yields the following
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