World Grain - December 2017 - 44

FEED OPERATIONS

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Example: An air compressor has a
Air direction
100 hp motor that operates at 90% ef¿FLHQF\ ,W RSHUDWHV KRXUV SHU
Rod movement
year and the electricity costs $0.06/kw.
Power factor = 1.0. What is the power
cost for this compressor?
PC = [(100)(.746 KW/HP)(8,000 hrs)
($0.06/kw)]/[(0.90)(1.0)] = $39,786 per
Air direction
year.
Among the factors to consider are:
Rod movement
x 70% of the cost for compressed air
is electricity.
x A 1/8-inch leak @ 100 psig =
8,800,000 cubic feet/year. At
$0.06/kwh, this = $1,440/year.
feet of ¾-inch diameter pipe, the presx A change of the required system
sure loss is 7.8 psi, but using a 1-inch
operating pressure of +/- 2 psig =
pipe, the pressure loss is only 2.21 psi
+/- 1% energy use
per 100 feet.
A change of +/- 10 degrees in inlet air
Pipe sizing for the whole system temperature to the compressor = +/- 1%
must be sized to supply air at the prop- energy use
er amount needed at each use point.
Operating recommendations include:
At each use point, a pressure setting
1. Operate the system at the minimum
YDOYH¿OWHUDQGOXEULFDWRUDUHLQVWDOOHG
required pressure.
in the line prior to the compressed air
2. Use properly sized devices.
entering the operation. The air then
3. Check for air leaks regularly.
passes through a solenoid valve that
4. Drain moisture regularly.
controls the air use in the equipment.
5. Set equipment for minimum air
If the compressed air is used to power
usage.
an air cylinder or recycling operation,
6. Keep inlet air clean and cool.
the solenoid controls which port the
7. Beware of excessive cycling of the
air enters and leaves a cylinder.
compressor.
A compressed air system must be
CYLINDER OPERATIONS
sized for primary and future use requireFigure 2 (above) shows an air cyl- ments.
inder and its components. The piston
Good sources for design information
moves one way or the other depending are available on the web. Another good
on where the compressed air enters the source is Chapter 49 of the AFIA Feed
cylinder and the air on the other end is Manufacturing Technology V book
discharged.
available from the AFIA.
The force to extend the rod is deterFred Fairchild is feed science professor
mined by the compressed air pressure
emeritus
in the Department of Grain Science
entering the cylinder against the piston
at Kansas State University. Prior to coming
face area (non-rod side). The same presto Kansas State in 1994, he worked in
sure to retract the rod has less total force
the industry designing, constructing and
as the area of the rod must be deducted
commissioning numerous mill facilities. He is
from the face area of the cylinder.
a licensed professional engineer. He may be
reached by e-mail at fjf@k-state.edu.
The power cost (PC) for operating a
compressed air system may be deterWe want to hear from you - Send comments and
mined using the following formula:
inquiries to worldgrain@sosland.com. For reprints of
WG articles, e-mail reprints@sosland.com.
PC = [(HP) (.746 KW/HP) (T) (Power

Figure 2. Air cylinder parts and rod movement

Air inlet/outlet ports

Piston

Rod
Cylinder housing

larger systems use individual separate
components.
COMPRESSED AIR DISTRIBUTION SYSTEM
Compressed air piping must be designed to ensure an adequate supply at
the required pressure needed for each
piece of equipment. Runs should be as
straight and short as possible without
having any low spots where any moisture in the air might concentrate. Piping should slope away from the compressor so that any entrained moisture
is moved along with the air. For vertical piping, a tee should be installed
so the horizontal piping enters the side
of the tee and the vertical pipe goes
up while a drip valve is located at the
bottom outlet of the tee to drain any
accumulated moisture.
The air piping must be sized to handle the amount of air in the main and
each branch to deliver the amount of
compressed air, at the needed pressure, to each operating point. Branch
lines should be taken off the top of the
mains to keep out moisture or any other materials in the main. Using published tables on compressed air piping
pressure losses, I found that the pressure loss per 100 feet of pipe length
in different sized piping to deliver the
equivalent of 100 cfm of free air that
is compressed to 100 psig. The 100
cfm of free air is reduced to a volume
12.82 cfm at this pressure. For 100
44

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