Prediction of batch heat transfer coefficients for pseudoplastic fluids ...

40
The s~m
of the resistances,£RR' can thus be found by
plotting the values of l/U for the heating ru.ns versus 1/NR~/3
and extrapolating the curve to zel;'o .. I:RH is the value of the
ordinate vJhen 1/NR~/3
is zero .. i::Rc can be evaluated in a
slifiilar manner by plotting the data for the cooling runs.
Once the resist~nces
are evaluated the batch heat transfer
coefficients may be calculated from equations 2-30 and 2-31.
There are several serious disadvantages using the above
methods for calculating the batch heat transfer coefficients.
1. The temperature of the jacket fluid must be knOl~
accurately. It was found in this work that the
temperature of the steam entering the jacket was
o
often about 108 C and the temperature of the
cdndensate was about 90 0 to 95 0 0.. In this case,
l'11hich temperature should be used?
2 •. In the above case part of the jacket had a condensing
steam film while part had a water fina.
Is one justified in assuming a condensing steam
film for the entire area?
3. The heat transfer coefficient is a function of
the Prandtl number, viscosity ratio, and geometrical
factors.
These could produce much scatter on a
Wilson plot if not held constant.
4. The 2/3 power is not necessarily the proper exponent
for the Reynolds number.
Pursell (150) re-