Feed Operations: Compressed air systems

by Fred Fairchild
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Feed facility
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.

Feed Operations

Compressing the air

To compress the air, external air is drawn through an intake filter and into a compressor. The compressor is usually one of two types, a reciprocating-piston design or a rotary screw design. The piston type draws air into a cylinder where it is compressed by the piston into a high-pressure state that exits to the compressed air system. This is the same principle as an automobile engine cylinder uses to compress 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. 

A rotary compressor uses tightly fitting twin screws to 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 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 compressor through an air intake filter. This air source should 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 high-pressure 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.

The third step in compressing air is to install an air receiver tank immediately after the air-drying equipment. The receiver acts as an accumulation tank for the compressed air so that sufficient compressed air is available for the system when there is high demand. It provides a smooth and steady supply of air to the system when the pressure drops to the minimum pressure set point where the compressor begins compressing air to the system and the air pressure rises back up to the maximum pressure set point. The receiver allows for having compressed air available to operate equipment for periods of time without the available air pressure dropping below the minimum pressure needed in the system.

The size of receiver needed for a compressed air system may be determined using the following formula.

V = t C pa / (p1 – p2)
V = volume of the receiver tank (cubic feet)
t = time for the receiver to go from upper to lower pressure limits (minutes)
C = quantity of free air needed to supply the system (scfm)
pa = atmospheric pressure (14.7 psia)
p1 = maximum tank pressure (psig)*
p2 = minimum tank pressure (psig)*
Psig is the pressure shown on a pressure gauge.

Let’s size a receiver for a compressed air system that requires a mean consumption of 900 cubic feet per minute, maximum tank pressure 115 psig, minimum tank pressure 95 psig and 10 seconds time for the receiver to go from upper to lower pressure. V = (10 sec) (1/60 min/sec) (900 cfm) (14.7 psi)/[(115 psig) – (95 psig)] = 110.25 cubic feet.. Receiver volumes are normally designated in gallons. One gallon is 7.48 cubic feet (cf). Therefore, this receiver would be 110.25 cf/7.48 gal per cf. = 14.7 gallons.  
Most receivers are sized in 10-gallon increments. Therefore, it would .be best to use a 20-gallon receiver in this application.

On smaller systems, the compressor, air dryer/cooler and receiver are often sold as self-contained units, while 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 feet of ¾-inch diameter pipe, the pressure loss is 7.8 psi, but using a 1-inch pipe, the pressure loss is only 2.21 psi per 100 feet.

Pipe sizing for the whole system must be sized to supply air at the proper amount needed at each use point. At each use point, a pressure setting valve, filter and lubricator are installed in the line prior to the compressed air entering the operation. The air then passes through a solenoid valve that controls the air use in the equipment. If the compressed air is used to power an air cylinder or recycling operation, the solenoid controls which port the air enters and leaves a cylinder.

Feed operations

Cylinder operations

Figure 2 shows an air cylinder and its components. The piston moves one way or the other depending on where the compressed air enters the cylinder and the air on the other end is discharged.
The force to extend the rod is determined by the compressed air pressure entering the cylinder against the piston face area (non-rod side). The same pressure to retract the rod has less total force as the area of the rod must be deducted from the face area of the cylinder.

The power cost (PC) for operating a compressed air system may be determined using the following formula:

PC = [(HP) (.746 KW/HP) (T) (Power Cost)]/[(Motor Efficiency) (Power Factor)]
Example: An air compressor has a 100 hp motor that operates at 90% efficiency. It operates 8,000 hours per 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 year.

Among the factors to consider are: 
70% of the cost for compressed air is electricity. 
A 1/8-inch leak @ 100 psig = 8,800,000 cubic feet/year. At $0.06/kwh, this = $1,440/year.
A change of the required system operating pressure of  +/- 2 psig = +/- 1% energy use
A change of +/- 10 degrees in inlet air temperature to the compressor = +/- 1% energy use

Operating recommendations include:
1. Operate the system at the minimum required pressure.
2. Use properly sized devices.
3. Check for air leaks regularly.
4. Drain moisture regularly.
5. Set equipment for minimum air usage.
6. Keep inlet air clean and cool.
7. Beware of excessive cycling of the compressor.

A compressed air system must be sized for primary and future use requirements.
Good sources for design information are available on the web. Another good source is Chapter 49 of the AFIA Feed Manufacturing Technology V book available from the AFIA.