Page 104 - Chemical Process Equipment - Selection and Design

76 TRANSFER OF SOLIDS
the value off, is 0.58-1.00. The sliding friction in the line is company’s manufacture and a factor for determining the power
requirement. An abbreviated table of about 150 substances appears
w4=f,Lmi (ft lbf/sec), (5.20) in the Chemical Engineers Handbook (1984, p. 7.5). Hudson (1954,
pp. 6-9), describes the characteristics of about 100 substances in
where L is the line length. relation to their behavior in conveyors. Table 5.3 lists bulk
Friction in the curved elbows is enhanced because of densities, angles of respose at rest, and allowable angles of
centrifugal force so that inclination which are angles of repose when a conveyor is in motion;
references to more extensive listings of such data are given in this
u2 2xR table.
w, =La (4)i-n; = 0.0488f,uzmj (ftlbf/sec). (5.21) The angle of repose is a measure of the incline at which
conveyors such as screws or belts can carry the material. The
The total frictional power is tangent of the angle of repose is the coefficient of sliding friction.
This property is a factor in the power needed to transfer the
Wf = w1+ w,+ w, + w, + w,, (5.22) material by pushing or dragging as in pneumatic, screw, flight, and
Redler equipment.
and the total power consumption is Special provisions need to be made for materials that tend to
form bridges; Figure 5.13(a) is an example of a method of breaking
up bridges in a storage bin so as to ensure smooth flow out.
W= (wc + wf) [HP/(ton/hr)], (5.23) Materials that tend to pack need to be fluffed up as they are pushed
55017 (1.8i-n:)
along by a screw; adjustable paddles as in Figure 5.7(d) may be
where 17 is the blower efficiency. Pressure drop in the line is sufficient.
obtained from the frictional power, the total flow rate, and the
density of the mixture: SCREW CONVEYORS
These were invented by Archimedes and assumed essentially their
(5.24) present commercial form a hundred years or so ago. Although the
equipment is simple in concept and relatively inexpensive, a body of
experience has accumulated whereby the loading, speed, diameter,
The specific air rate, or saturation, is and length can be tailored to the characteristics of the materials to
be handled. Table 5.4, for example, recognizes four classes of
saturation = 0.7854(60)D2 materials, ranging from light, freeflowing, and nonabrasive
(cuft/min of air)/(lb/min of solid)], (5.25) materials such as grains, to those that are abrasive and have poor
flowability such as bauxite, cinders, and sand. Only a portion of the
where the velocity of the air is evaluated at atmospheric pressure. available data are reproduced in this table.
Example 5.2 makes the calculations described here for power Lengths of screw conveyors usually are limited to less than
and pressure drop, and compares the result with the guidelines of about 150 ft; when the conveying distance is greater than this, a belt
Table 5.1. or some other kind of machine should be chosen. The limitation of
length is due to structural strength of the shaft and coupling. It is
5.3. MECHANICAL CONVEYORS AND ELEVATORS expressed in terms of the maximum torque that is allowable.
Formulas for torque and power of screw conveyors are given in
Granular solids are transported mechanically by being pushed along Table 5.4 and are applied to selection of a conveyor in Example 5.3.
or dragged along or carried. Movement may be horizontal or Several designs of screws are shown in Figure 5.7. The basic
vertical or both. In the process plant distances may be under a design is one in which the pitch equals the diameter. Closer spacing
hundred feet or several hundred feet. Distances of several miles is needed for carrying up steep inclines, and in fact very fine pitch
may be covered by belts servicing construction sites or mines or screws operating at the relatively high speeds of 350 rpm are used to
power plants. Capacities range up to several hundred tons/hr. The convey vertically. The capacity of a standard pitch screws drops off
principal kinds of mechanical conveyors are illustrated in Figures sharply with the inclination, for example:
5.7-5.13 and will be described. Many construction features of these
machines are arbitrary. Thus manufacturers’ catalogs are the Angle (degrees) <8 20 30 45
ultimate source of information about suitability for particular Percent of capacity 100 55 30 0
services, sizes, capacities, power requirements and auxiliaries.
Much of the equipment has been made in essentially the present Allowable loadings as a percentage of the vertical cross section
form for about 100 years by a number of manufacturers so that a depend on the kind of material being processed; examples are
body of standard practice has developed. shown in Table 5.4.

PROPERTIES OF MATERIALS HANDLED BELT CONVEYORS
The physical properties of granular materials that bear particularly These are high capacity, relatively low power units for primarily
on their conveying characteristics include size distribution, true and horizontal travel and small inclines. The maximum allowable
bulk densities, and angle of repose or coefficient of sliding friction, inclination usually is 5-15‘ less than the angle of repose; it is shown
but other less precisely measured or described properties are also of as “recommended maximum inclination” in Table 5.3 for some
concern. A list of pertinent properties appears in Table 5.2. The substances, and is the effective angle of repose under moving
elaborate classification given there is applied to about 500 materials conditions.
in the FMC Corporation Catalog 100 (1983, pp. B.27-B.35) but is The majority of conveyor belts are constructed of fabric,
too extensive for reproduction here. For each material the table rubber, and wire beads similarly to automobile tires, but they are
also identifies the most suitable design of screw conveyor of this made also of wire screen or even sheet metal for high temperature