A. With
industrial cooling towers, cooling to 90% of the ambient air
saturation level is possible.
B. Relative
tower size is dependent on the water temperature approach to the wet
bulb temperature:
Twater-Twb
Relative Size
5
2.4
15
1.0
25
0.55
C. Water
circulation rates are generally 2-4 GPM/sq. ft (81-162 L/min m2)
and air velocities are usually 5-7 ft/s
(1.5-2.0 m/s)
D.
Countercurrent induced draft towers are the most common. These
towers are capable of cooling to within 2 �F
(1.1 �C) of the wet bulb temperature. A 5-10 �F (2.8-5.5 �C)
approach is more common.
E.
Evaporation losses are about 1% by mass of the circulation rate for
every 10 �F (5.5 �C) of cooling. Drift losses are around 0.25% of
the circulation rate. A blowdown of about 3% of the circulation
rate is needed to prevent salt and chemical treatment buildup.
Conveyors
A. Pneumatic conveyors are best suited for high capacity applications
over distances of up to about 400 ft. Pneumatic conveying is also
appropriate for multiple sources and destinations. Vacuum or low
pressure (6-12 psig or 0.4 to 0.8 bar) is used for generate air
velocities from 35 to 120 ft/s (10.7-36.6 m/s). Air requirements are
usually in the range of 1 to 7 cubic feet of air per cubic foot of
solids (0.03 to 0.5 cubic meters of air per cubic meter of solids).
B. Drag-type conveyors (Redler) are completed enclosed and suited to
short distances. Sizes range from 3 to 19 inches square (75 to 480
mm). Travel velocities can be from 30 to 250 ft/min (10 to 75
meters/min). The power requirements for these conveyors is higher than
other types.
C. Bucket elevators are generally used for the vertical transport of
sticky or abrasive materials. With a bucket measuring 20 in x 20 in
(500 mm x 500 mm), capacities of 1000 cubic feet/hr (28 cubic meters/hr)
can be reached at speeds of 100 ft/min (30 m/min). Speeds up to 300
ft/min (90 m/min) are possible.
D. Belt conveyors can be used for high capacity and long distance
transports. Inclines up to 30� are possible. A 24 in (635 mm) belt can
transport 3000 cu. ft./h (85 cu m/h) at speeds of 100 ft/min (30.5
m/min). Speeds can be as high as 600 ft/min (183 m/min). Power
consumption is relatively low.
E. Screw conveyors can be used for sticky or abrasive solids for
transports up to 150 ft (46 m). Inclines can be up to about 20�. A 12
in (305 mm) diameter screw conveyor can transport 1000-3000 cu. ft./h
(28-85 cu. m/h) at around 40-60 rpm.
Crystallization
A. During
most crystallizations, C/Csat (concentration/saturated
concentration) is kept near 1.02 to 1.05
B. Crystal
growth rates and crystal sizes are controlled by limiting the degree
of supersaturation.
C. During
crystallization by cooling, the temperature of the solution is kept
1-2 �F (0.5-1.2 �C) below the saturation point at the given
concentration.
D. A
generally acceptable crystal growth rate is 0.10 - 0.80 mm/h
Drivers
and Power Recovery
A.
Efficiencies: 85-95% for motors, 40-75% for steam turbines, 28-38%
for gas engines and turbines.
B. Electric
motors are nearly always used for under 100 HP (75 kW). They are
available up to 20,000 HP (14,915 kW).
C. Induction
motors are most popular. Synchronous motors have speeds as low as
150 rpm at ratings above 50 HP (37.3 kW) only. Synchronous motors
are good for low speed reciprocating compressors.
D. Steam
turbines are seldom used below 100 HP (75 kW). Their speeds can be
controlled and they make good spares for motors in case of a power
failure.
E. Gas
expanders may be justified for recovering several hundred
horsepower. At lower recoveries, pressure let down will most likely
be through a throttling valve.
Drying of
Solids
A. Spray dryer have drying times of a few seconds. Rotary dryers have
drying times ranging from a few minutes to up to an hour.
B. Continuous tray and belt dryers have drying times of 10-200 minutes
for granular materials or 3-15 mm pellets.
C. Drum dryers used for highly viscous fluids use contact times of 3-12
seconds and produce flakes 1-3 mm thick. Diameters are generally 1.5-5
ft (0.5 - 1.5 m). Rotation speeds are 2-10 rpm and the maximum
evaporation capacity is around 3000 lb/h (1363 kg/h).
D. Rotary cylindrical dryers operate with air velocities of 5-10 ft/s
(1.5-3 m/s), up to 35 ft/s (10.5 m/s). Residence times range from 5-90
min. For initial design purposes, an 85% free cross sectional area is
used. Countercurrent design should yield an exit gas temperature that
is 18-35 �F (10-20 �C) above the solids temperature. Parallel flow
should yield an exiting solids temperature of 212 �F (100 �C). Rotation
speeds of 4-5 rpm are common. The product of rpm and diameter (in feet)
should be 15-25.
E. Pneumatic conveying dryers are appropriate for particles 1-3 mm in
diameter and in some cases up to 10 mm. Air velocities are usually
33-100 ft/s (10-30 m/s). Single pass residence time is typically near
one minute. Size range from 0.6-1.0 ft (0.2-0.3 m) in diameter by
3.3-125 ft (1-38 m) in length.
F. Fluidized bed dryers work well with particles up to 4.0 mm in
diameter. Designing for a gas velocity that is 1.7-2 times the minimum
fluidization velocity is good practice. Normally, drying times of 1-2
minutes are sufficient in continuous operation.
Drum Type Vessels
A. Liquid drums are
usually horizontal. Gas/Liquid separators are usually vertical
B. Optimum
Length/Diameter ratio is usually 3, range is 2.5 to 5
C. Holdup time is 5
minutes for half full reflux drums and gas/liquid separators
Design for a 5-10
minute holdup for drums feeding another column
D. For drums feeding
a furnace, a holdup of 30 minutes is a good estimate
E. Knockout drum in
front of compressors should be designed for a holdup of
10 times the liquid
volume passing per minute.
F. Liquid/Liquid
separators should be designed for settling velocities of 2-3
inches/min
G. Gas velocities in
gas/liquid separators, velocity = k (liquid density/(vapor
density-1))^0.5,
where k is 0.35 with horizontal mesh
deentrainers and 0.167 with
vertical mesh deentrainers. k
is 0.1 without mesh deentrainers and velocity is in ft/s
H. A six inch mesh
pad thickness is very popular for such vessels
I. For positive pressure
separations, disengagement spaces of 6-18 inches before the mesh pad
and 12 inches after the pad are generally suitable.
