World Grain - December 2017 - 40

FEED OPERATIONS

Compressed air systems
Proper design and installation are
essential for effective use in feed mills
by Fred Fairchild

One of the systems needed in nearly every feed mill is
compressed air to operate equipment in the plant. The proper design and installation of a compressed air system often
isn't thought about. In this article we will discuss compressed air uses and the components of a properly designed
compressed air system.
The most common use of compressed air is to actuate
air cylinders powering slide gates under bins and outlets on
conveying equipment. Compressed air also is used to open
and close the doors under drop-bottom mixers. These operations use the air in cyclic action where the cylinder(s) are
used to position the gate(s) in either an opened or closed
position. Other uses are to continuously operate vibrators
on equipment or to atomize liquids by being injected into
the liquid to atomize it before it is sprayed on the product.
Other purposes include using hand-held nozzles on the
end of a hose to blow-down and clean various areas. Blowing down causes dust in the atmosphere and should only
be done when the plant is not in operation to avoid creating an explosive atmosphere. Compressed air may be used
to pressurize electrical cabinets to keep dust out or cool
equipment in the cabinet.
A compressed air system has three major components:
the machinery that compresses the air; the compressed air
distribution system; and the equipment that uses the compressed air.

fuel into high-pressure. This type of compressor may be a
single piston design, or a double piston design where the air
is compressed in two steps.
$ URWDU\ FRPSUHVVRU XVHV WLJKWO\ ¿WWLQJ WZLQ VFUHZV WR
compress the air as it travels through the screw housing.
This design works as a positive displacement air pump. The
screws turn constantly, and the amount of air compressed
is controlled by a modulating valve on the air inlet to the
compressor.
Figure 1 (page 42) shows the main equipment used to
compress the air and prepare it for use in the compressed air
system. It begins with pulling atmospheric air into the comSUHVVRU WKURXJK DQ DLU LQWDNH ¿OWHU 7KLV DLU VRXUFH VKRXOG
be located outside warm or dusty areas. It should be located
outside a building where the coolest free air is available.
The air is compressed to the pressure needed for the plant
compressed air system. The compressor has low and highpressure switches on it. The low-pressure switch starts the
air compression, while the high-pressure switch stops the air
compression. The air is heated in the compression process
and is hot as it leaves the compressor. This hot compressed
air next passes through an air dryer to remove any moisture
to provide the driest air possible to the system. The air dryer
may be one of two types: a desiccant dryer or a refrigerated air cooler/dryer. The removal of moisture from the air is
important as moisture in the air is detrimental to the piping
system and the air operated equipment.
COMPRESSING THE AIR
The third step in compressing air is to install an air reTo compress the air, external air is drawn through an in- ceiver tank immediately after the air-drying equipment.
WDNH¿OWHUDQGLQWRDFRPSUHVVRU7KHFRPSUHVVRULVXVXDOO\ The receiver acts as an accumulation tank for the comone of two types, a reciprocating-piston design or a rota- SUHVVHG DLU VR WKDW VXI¿FLHQW FRPSUHVVHG DLU LV DYDLODEOH
ry screw design. The piston type draws air into a cylinder for the system when there is high demand. It provides a
where it is compressed by the piston into a high-pressure smooth and steady supply of air to the system when the
state that exits to the compressed air system. This is the same pressure drops to the minimum pressure set point where
principle as an automobile engine cylinder uses to compress the compressor begins compressing air to the system and
40

December 2017 / World Grain / www.World-Grain.com

http://www.World-Grain.com

World Grain - December 2017

Table of Contents for the Digital Edition of World Grain - December 2017