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Example 2 [6]
Calculate the cooling time tk in Example 1 if the mold is cooled by a coolant with a heat
transfer coefficient of aa = 2839 W/(m 2 • K).
Solution
The resulting Biot number is
As can be extrapolated from Figure 3.11, the Fourier number does not differ much from
the one in the previous example for &Th = 0.25 and 1/Bi = 0.01342. The resistance due
to convection is therefore negligible and the cooling time remains almost the same.
However, the convection resistance has to be taken into account in the case of a film with
a thickness of 127 jU that is cooling in quiescent air, as shown in the following calculation:
The heat transfer coefficient for this case is approximately
The Biot number Bi
P0 from Figure 3.11
F0 = 95
The cooling time
Example 3 [7]
Cooling of an extruded wire
A polyacetal wire of diameter 3.2 mm is extruded at 190 0 C into a water bath at 20 0 C.
Calculate the length of the water bath to cool the wire from 190 0 C to a centre line
temperature of 140 0 C. The following conditions are given:
haul-off rate of the wire Vn = 0.5 m/s
— = 0.01342
Bi