One of the systems needed in almost every feed mill is compressed air to operate the plant equipment. The proper design and installation of a compressed air system is often overlooked. In this article, we will discuss the uses of compressed air and the components of a properly designed compressed air system.
The most common use of compressed air is to operate pneumatic cylinders that power slide valves under bins and outlets of conveying equipment. Compressed air is also used to open and close the doors under the bottom opening mixers. These operations use air in cyclic action where the cylinder(s) are used to position the gate(s) in the open or closed position. Other uses include continuously running vibrators on equipment or atomizing liquids by being injected into the liquid to atomize it before it is sprayed onto the product.
Other purposes include using hand nozzles at the end of a hose to purge and clean various areas. Blowing generates dust in the atmosphere and should only be carried out when the installation is not in operation to avoid creating an explosive atmosphere. Compressed air can be used to pressurize electrical cabinets to keep dust out or to cool equipment in the cabinet.
A compressed air system has three main components: the machinery that compresses the air; the compressed air distribution system; and equipment that uses compressed air.
Compress the air
To compress the air, outside air is drawn through an intake filter and into a compressor. The compressor is generally 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 which exits to the compressed air system. It’s the same principle an automotive engine cylinder uses to compress fuel at high pressure. This type of compressor can be single piston or twin piston in which the air is compressed in two stages.
A rotary compressor uses tightly fitted twin screws to compress air as it moves through the screw housing. This design works like a volumetric air pump. The screws are constantly rotating and the amount of compressed air is controlled by a modulating valve at the compressor air inlet.
Figure 1 shows the main equipment used to compress air and prepare it for use in the compressed air system. It begins by drawing atmospheric air into the compressor through an air intake filter. This air source should be located outside of hot or dusty areas. It should be located outside a building where the freshest open air is available.
The air is compressed to the pressure required by the plant’s compressed air system. The compressor is equipped with low and high pressure switches. The low pressure switch starts air compression, while the high pressure switch stops air compression. The air is heated in the compression process and is hot when it leaves the compressor. This hot compressed air is then passed through an air dryer to remove any moisture to deliver the driest air possible to the system. The air dryer can be of two types: an adsorption dryer or a chiller/refrigerated air dryer. Removing moisture from the air is important because moisture in the air is detrimental to the piping system and pneumatic equipment.
The third stage of air compression 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 a sufficient quantity of compressed air is available for the system in the event of high demand. It provides a smooth and steady air supply to the system when the pressure drops to the minimum pressure set point where the compressor begins to compress air to the system and the air pressure builds up to the maximum pressure set point . The receiver allows compressed air to be available to operate the equipment for periods of time without the available air pressure falling below the minimum pressure needed in the system.
The receiver size needed for a compressed air system can be determined using the following formula.
V = t C pa / (p1 – p2)
V = receiver tank volume (cubic feet)
t = time required for the receiver to move from upper to lower pressure limits (minutes)
C = amount 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)*
The psig is the pressure indicated on a manometer.
Let’s size a receiver for a compressed air system that requires an average consumption of 900 cubic feet per minute, a maximum tank pressure of 115 psig, a minimum tank pressure of 95 psig, and 10 seconds of time for the receiver to go from the higher pressure to the lower pressure. V = (10 sec) (1/60 min/s) (900 cfm) (14.7 psi)/[(115 psig) – (95 psig)] = 110.25 cubic feet. Receiver volumes are normally listed in gallons. One gallon equals 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 small systems, the compressor, air dryer/cooler, and receiver are often sold as stand-alone units, while larger systems use separate individual components.
Compressed air distribution system
Compressed air piping should be designed to provide adequate supply at the pressure required for each piece of equipment. Runways should be as straight and short as possible without having low points where air humidity could collect. The piping should be away from the compressor so that any entrained moisture is moved with the air. For vertical piping, a tee should be installed so that the horizontal piping enters the side of the tee and the vertical piping rises while a drip valve is located at the bottom outlet of the tee to drain any moisture accumulated.
Air piping should be sized to handle the amount of air in the main line and each branch to supply the amount of compressed air, at the pressure needed, at each point of operation. Secondary lines should be removed from above the network to prevent moisture or other material from entering the network. Using published pressure loss tables for compressed air lines, I found that the pressure loss per 100 feet of length of pipe in lines of different sizes provided the equivalent of 100 cfm of compressed air at 100 psig. The 100 cfm of free air is reduced to a volume of 12.82 cfm at this pressure. For 100 feet of ¾ inch diameter pipe the pressure loss is 7.8 psi, but using 1 inch pipe the pressure loss is only 2.21 psi per 100 feet.
Pipe sizing for the entire system should be sized to supply the proper amount of air needed at each point of use. At each point of use, a pressure adjustment valve, filter and lubricator are installed in the line before the compressed air enters the operation. The air then passes through a solenoid valve which controls the use of air in the equipment. If compressed air is used to supply an air cylinder or recirculation operation, the solenoid controls which port air enters and leaves a cylinder.
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 where the air at the other end is exhausted.
The rod extension force is determined by the pressure of compressed air entering the cylinder against the surface of the piston face (rodless side). The same pressure to retract the rod has less total force because the area of the rod must be deducted from the frontal area of the cylinder.
The cost of energy (PC) to operate a compressed air system can 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 electricity costs $0.06/kw. Power factor = 1.0. What is the electricity cost for this compressor?
PC = [(100)(.746 KW/HP)(8,000 hrs)($0.06/kw)]/[(0.90)(1.0)] = $39,786 per year.
Factors to consider include:
70% of the cost of compressed air is electricity.
A 1/8 inch leak at 100 psig = 8,800,000 cubic feet/year. At $0.06/kwh, that = $1,440/year.
A change in required system operating pressure of +/- 2 psig = +/- 1% energy consumption
A +/- 10 degree change in compressor intake air temperature = +/- 1% energy consumption
Recommendations for use include:
1. Operate the system at the minimum pressure required.
2. Use appropriately sized devices.
3. Regularly check for air leaks.
4. Drain the moisture regularly.
5. Adjust equipment for minimum air consumption.
6. Keep intake air clean and fresh.
7. Watch out for excessive compressor cycling.
A compressed air system should be sized for primary and future use requirements.
Good sources of design information are available on the web. Another good source is Chapter 49 of the AFIA Feed Manufacturing Technology V book available from AFIA.