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Air
Knife
Blower sizing for air knife
Sizing a blower for an air knife application is not a
simple process. The velocity of air required depends on how
fast the material to be dried passes under the air knife.
With a blower as an air source, the higher pressure developed
by the blower will increase the air temperature, resulting
in a better drying process. Designing of equipment should
include work to minimized air friction losses in plumbing
connecting the air knife to the blower. It is best to use
a larger pipe size than the air source discharge. The best
method to determine the air consumption of an air knife is
to test it. To increase the airflow exiting an air knife,
the pressure must be increased four times.
Air Knife Facts:
A. To get equal distribution of air across the length
of the air knife, the opening area must be 1 1/2 to 2 times
smaller than the cross sectional area of the knife housing.
B. Different air velocities are required depending on the
surface to be dried. Higher velocities are necessary if liquids
tend to cling to the surface. For proper drying, the velocity
may vary from 5000 to 20,000 feet per minute.
C. Higher blower pressure capability may be required to overcome
restriction resulting from plumbing between the blower and
the air knife.
D. The air knife should be located as close to the surface
to be dried as practical.
E. Air Velocity,V(FPM)=3204 times times the square root of
pressure (pressure, IN-H2O), this is an approximation since
the knife edge radius or sharpness can change these values
considerably
F. Air Velocity, V=Q(air flow in cfm) divided by A(area of
opening in square feet)
Hot Tub/Tank Aeration
Size of blower to be used depends on the water
depth and the surface area:
Blower Model
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Tank Pump Down Times
An approximation of time can be determined as follows:
t=V/Q (N)
where
t = time in minutes
V= volume to be evacuated in cubic ft
Q= average capacity*
N= 1 for vacuums to 15 in-Hg
2 for vacuums greater than 15 but less than 22.5 in-Hg
3 for vacuums greater than 22.5 but less than 26 in-Hg
* using advertised vacuum curves use the
above formula to calculate time in 5 -hg increments, then
add each time increment to get the total time
note: add a 25% safety factor for system
leakage
Tank Pump Up Time
An estimated pump up time
can be calculated as follows:
T=V(P2-P1)/P0(Acfm)
Where:
T= time in minutes
V= tank volume in cubic feet (cuft=gallons/7.48)
P0= atmospheric pressure in psia
P1= initial tank pressure in psia
P2= final tank pressure in psia
Acfm= average cfm during pump up
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To use chart above:
1. determine pressure range___________
2. determine tank size_________________
3. amount of air required by application__________________
4. cubic feet of air(in pressure range) from chart____________________
5. air flow (in pressure range) of pump of choice______________
average compressor flow between starting pressure and final
pressure)
6. "4"/("5"-"3")= minutes ON____________________
7.("5"/"3")/"3" = minutes OFF_________________
Vacuum at different
elevations
From catalog performance curves for a specific
vacuum pump, determine the pump's maximum vacuum capability.
Use this to determine what % it is of perfect vacuum at sea
level. (Example: if a pump is capable of 25 in-Hg vacuum at
sea level, it's capability is 83.4% of perfect vacuum [25/29.92]).
Example:
The barometer reading at 5000 ft is 24.9. The maximum vacuum
capability of the 25 in-Hg pump at that altitude is 83.4%
of 24.9 in-Hg or 20.8 in-Hg.
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Performance at different elevations
A 10% decrease in air density reduces performance by 10%.
For example: If a blower produces 110 cfm at 40 in of water
pressure, with a 10% decrease in air density, it will produce
the same air flow at 90% of 40, or 36 in of water.
Vacuum Lifting
Theoretical lifting force of a vacuum cup
W=C(P)(14.7)/F(29.92)
Where:
W = force in lbs
C = area of cup (sq in)
P = vacuum level in in-Hg
F = safety factor
Better than choosing the smallest cup that will work, use
the largest possible cup to ease requirements on the vacuum
pump. It is better to use a larger cup than to overwork the
vacuum pump. Use a safety factor of 4 for vertical movement
and a safety factor of 2 for horizontal movement.
Application Notes:
Porous material -- use a smaller size cup unless a very high
flow of air is available
Metal Parts - use six or eight evenly
distributed cups instead of two or three smaller cups which
calculations show should be able to lift the weight of the
plate.
Vacuum Hold Down
The maximum hold down force on an object,
at standard atmospheric conditions with a perfect vacuum,
is 14.7 psi. The actual force depends on the vacuum level
capability of the vacuum pump, the barometer in your area,
and the surface area of the object that is in contact with
the vacuum. For example, if you were to use a Gast 0323 oilless
model (capable of 25 in-Hg) as the vacuum source in an area
with a standard atmospheric pressure (14.7 psi), the maximum
hold down force would be 12.3 psi (25in-Hg¸2.03). If the hold
down table has four (4) ½" diameter holes, the maximum hold
down pressure would be .196 sq-in x 4(or.79 square inches)
x 12.3 psi or 9.7 psi.
Additional hold down force is required if
an object is being held for fabricating purposes, which may
produce side, or twisting loads. If the hold down table surface
is smooth, the object may tend to move no matter the hold
down force. Therefore, it is recommended that a non-skid hold
down table surface be used if possible. Hold down of a small
part may not be possible unless a limited amount of fabrication
is being done to it.
To select the best product for an application,
be aware that a vacuum pump, such as the 0323, is best for
use with materials that air will not easily be drawn through.
Blowers, on the other hand, are best to use with porous materials,
since blowers pull large volumes of air.
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