Page 98 - Modelling in Transport Phenomena A Conceptual Approach

78 CHAPTER 4. EVALUATION OF TRANSFER COEFFICIENTS

is
where Q(Y) given by
(
0.052 0.007 0.00019
Q(Y) = exp 3.15 + - + - - - (4.3-18)
y1/4
y3/4
The procedure to calculate the particle diameter is as follows:
a) Calculate Y from Eq. (4.3-15),
b) Substitute Y into Eqs. (4.3-17) and (4.3-18) and determine Rep,
c) Once the Reynolds number is determined, the particle diameter can be calculated
from the equation

(4.3-19)

Example 4.5 A gravity settling chamber is one of the diverse range of equipment
used to remove particulate solids from gas streams. In a settling chamber, the
entering gas stream encounters an abrupt and large increase in cross-sectional area
as shown in the figure below. As a result of the sharp decrease in the gas velocity,
the solid particles settle down by gravity. In practice, the gas velocity through the
chamber should be kept below 3m/s to prevent the re-entrainment of the settled
particles.

Gas inlet
Gas outlet

Spherical dust particles having a density of 2200 kg/m3 are to be separated from
an air stream at a temperaturn of 25°C. Determine the diameter of the smallest
particle that can be wmoved in a settling chamber 7 m long, 2 m wide, and 1 m
high.

Solution

Physical properties