hs is called the surface heat transfer coefficient,
Ta is the temperature of the cooling fluid and Ts
is the temperature at the surface of the solid. The surface heat transfer
coefficient can be regarded as the conductance of a hypothetical surface
film of the cooling medium of thickness xf such that
Following on this reasoning, it may be seen that hs can be considered as arising from the presence of another layer, this time at the surface, added to the case of the composite slab considered previously. The heat passes through the surface, then through the various elements of a composite slab and then it may pass through a further surface film. We can at once write the important equation:
where 1/U = (1/hs1) + x1/k1 + x2/k2 +.. + (1/hs2)
EXAMPLE 5.4. Heat transfer in jacketed pan
Sugar solution is being heated in a jacketed pan made from stainless steel, 1.6 mm thick. Heat is supplied by condensing steam at 200 kPa gauge in the jacket. The surface transfer coefficients are, for condensing steam and for the sugar solution, 12,000 and 3000 J m-2 s-1 °C-1 respectively, and the thermal conductivity of stainless steel is 21 J m-1 s-1 °C-1.
Calculate the quantity of steam being condensed per minute if the transfer surface is 1.4 m2 and the temperature of the sugar solution is 83°C.
From steam tables, Appendix 8, the saturation temperature of steam at 200 kPa gauge(300 kPa Absolute) = 134°C, and the latent heat = 2164 kJ kg-1.
For stainless steel x/k = 0.0016/21 = 7.6 x 10-5
and since A = 1.4 m2
Therefore steam required
Heat-Transfer Theory > UNSTEADY STATE HEAT TRANSFER
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