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This article takes an in depth look at industrial air compressors.
You will learn more about topics such as:
What is an Industrial Air Compressor?
Types of Industrial Air Compressors
Types of Controls in Air Compressors
Types of Drives in Air Compressors
Performance Specifications in Air Compressors
Picking the Right Industrial Air Compressor
Industries and Applications of Air Compressors
What is an Industrial Air Compressor?
An industrial air compressor is a mechanical device used to generate pressure in compressible fluids or gasses, the most common being air. A variety of compressors are used in the industry to provide functions such as:
instrument or shop air
powering air tools, abrasive blast equipment, and paint sprayers
shifting refrigeration and air conditioning refrigerants
propelling gasses through pipelines
Akin to pumps, compressors can be categorized as either positive-displacement or centrifugal (also known as kinetic or dynamic). Unlike pumps which are predominantly centrifugal, compressors are mostly of the positive-displacement category.
Industrial compressors have varying sizes ranging from fit-in-a-glovebox units, which can be used to inflate tires, to the enormous turbo compressors that can be used in pipeline services.
How Air Can Generate Power
As the air is compressed, it gets under more pressure than that of atmospheric pressure. Energy is used to compress the air. As the compressed air pushes to return to a normal state, energy is released while the air expands, returning to atmospheric pressure. Thus, air compressors compress the air to high pressures, and the potential energy is then harnessed. Contrary to other sources of power, compressed air does not involve converting energy from one form to another at the point of application. Pneumatic or compressed air devices are characterized by a high power-to-volume or power-to-weight ratio.
Compressed air may not be as fast as electricity nor as slow as hydraulics, but it still has broad applications. It is often the preferred option where cost and efficiency are a priority.
The main advantage of compressed air is that owners have more control over it, and it does not pose the risk of shock hazard of electricity or the fire hazard from oils. However, safety codes and regulations for handling compressed air should be followed.
Compressed air can be found in two forms: active air and energy air.
Active Air and Energy Air
Active air is compressed air that comes into contact with a product. It is useful in preserving food and beverages, in electronics, pharmaceutical, and chemical industries.
Energy air is compressed air used to transmit and store energy used in mechanical work. Thus, energy air is used in powering pneumatic tools.
Due to the differences in application, it is critical for active air quality to be of higher purity, unlike energy air. Therefore, contaminants like dust, water or oil should be filtered out. For example, the water used in powering a turbine or flushing a toilet does not necessarily have to be of higher quality than drinking water.
Types of Industrial Air Compressors
Air compressors that are of 30 horsepower size or below include rotary and reciprocating compressors. These all compress air in diverse ways.
Reciprocating compressors or piston compressors make use of the reciprocating action of the piston(s) to compress the gas in the cylinder(s). The compressed air is discharged through valves into receiving tanks at high pressure. The compressor and the tank are usually mounted together in a skid or frame as a packaged unit.
Apart from providing compressed air as a source of energy, piston compressors can be used to transmit natural gas by pipeline operators. The selection of piston compressors are based on the flow rate (scfm) and pressure required (psi). The piston compressors can be oil-lubricated or oil-free.
The major reciprocating compressor types include:
Reciprocating diaphragm compressors
Reciprocating single acting compressors
Reciprocating rocking piston compressors
The major rotary air compressor types include:
Rotary helical screw compressors
Rotary sliding vane compressors
Rotary air compressors
The Reciprocating Single Acting Compressors
The reciprocating single acting compressor can be designed as a one-stage compressor or two-stage compressor. The single-stage compressor operates with air drawn from the atmosphere and then compressed using a single stroke to its final pressure. Such single-stage compressors are mostly used for a range of pressure of 70 psi to 135 psi.
In contrast, the two-stage compressor compresses the air to an intermediate pressure in the first stage. Two stage units can be used to provide higher pressures than single stage compressors provide. The compressed air that goes into the second stage first passes through an intercooler. The intercooler eliminates some heat generated in the first-stage cycle.
Compressors with many pistons operate within a cycle of duty rather than continuously. The cycles permit the heat generated from the operation to dissipate mostly through air-cooled fins.
The single-stage and two-stage reciprocating compressors are typically oil lubricated, but oil-less versions do exist.
The Reciprocating Rocking Piston Compressors
The rocking piston type of compressor is a variation of the reciprocating piston type of compressors. The rocking piston type generates pressure via a reciprocating action of a piston and a one-piece connecting rod. The setup allows the piston head to rock as it reciprocates. The rocking piston type of compressor uses low friction, non-metallic rings. Lubrication is not necessarily essential. They are typically smaller in size and have lower pressure capabilities.
The Reciprocating Diaphragm Compressors
The reciprocating diaphragm compressor uses an oscillating action of a flexible disc to generate pressure. The action is actuated by an eccentric. A motor-mounted concentric oscillates the disc, which then expands and contracts the compression chamber volume.
Akin to the diaphragm pump, the flexible disc seals the drive from the process fluid, thereby removing the possibility of a lubricant coming into contact with the gas. Thus, due to the non usage of a sliding seal between moving parts, there is no lubrication in such designs.
Diaphragm compressors are the preferred option when contamination of the output air needs to be avoided in laboratory or hospital applications. They have limited output and pressure and thus are mostly used in light-duty applications.
The Rotary Sliding Vane Compressors
The rotary sliding vane compressors have a rotor that is mounted eccentrically in a housing. Centrifugal force slides out the vanes as the rotor turns. This happens until the vanes seal against a film of lubricant coating on the stator wall. The blade tip does not cause metal-to-metal contact because it glides on the lubricant surface. As the rotor turns in the cylinder, the volume of the sliding vanes spaces reduces thereby causing air compression.
Single stage rotary vanes are the most commonly applied in industries where pressure ranges from 60 - 200 psi and are oil injected. Multi-stage rotary vanes are also used in pressure ranges of 60 - 150 psi. They make use of flow-through lubrication, which consumes the lubricant. They are most commonly applied in the movement of bulk material, for example concrete.
As much as rotary vane blowers as well as vacuum pumps can be oil-free, the rotary vane compressors are not free of oil. Thus, rotary vane compressors are not best at providing oil-free air. However, they can provide compressed air free from pulsations. They use bushes instead of bearings and are thus less prone to contaminating the environment when compared to screw compressors.
Rotary vane compressors have advantages such as:
pulse-free and smooth air output
high output volume
prolonged service intervals
The Rotary Helical Screw Compressors
The rotary helical screw compressors have two helical rotors that are intermeshed in a twin bore casing. The single-stage design has an inlet usually located near the drive shaft end at the top of the cylinder. The rotors unmesh at the cylinder air inlet end and draw air into the cavity between the secondary rotor grooves and the main rotor lobes. The rotation continues allowing the rotor tips to pass the inlet ports edges. The air is then trapped in between the cavities of the rotor and the cylinder wall. The main rotor lobes roll into the rotor grooves, the volume is reduced, and cell pressure is raised, causing compression as rotation continues.
Figure 2.12: The Rotary Helical Screw Compressor
After the cell closes, oil is injected to seal clearances and remove the heat of compression. As the compression continues, the rotor tips pass the discharge port. The compressed air and a mixture of oil is then obtained. These can be found as single or multi-stage versions. The rotary helical screw compressors can be water lubricated, oil lubricated, or oil-free. The main advantages of the rotary helical screw compressors are that they have:
A compact size
Pulse-free air output
High output volume
Prolonged service intervals
Operate on 100% duty cycle
They are the preferred option in trailerable applications such as road building or construction.
The Rotary Scroll Compressors
The rotary scroll compressors compress air by interacting a fixed element and a helical element that orbits and progressively compresses the air. The continuous repetition of this process yields the delivery of pulse-free compressed air. The system has fewer moving parts and thus reduced maintenance. The rotary scroll compressors can be lubricated or oil-free.
All compressors require controls to regulate their operation based on compressed air demand. Different types of compressor applications and requirements need different controls. When the bulk of the air attributes are steady, constant speed controls are needed for continuous operation.
Constant speed controls are needed when the air requirement is +75% of the free air delivery or when the motor starts per hour are greater than the motor manufacturer’s recommendations. The constant speed controls examples include inlet valve modulation applicable to rotary compressors and load/unload control applicable to all compressor types.
Variable speed controls are typically used where energy efficiency is critical in air compressor selection. Rotary screw compressors and rotary vane compressors usually have this type of control. The variable speed drive controls the motor speed and provides for the speed to be dynamically adjusted either up or down to meet the air demand of the compressor application.
Start-stop controls are typically used by compressors with adequate air storage. The requirement of air should be lower than 75% of the free air delivery.
Dual controls are used in switching between stop-start operation and constant speed operation via a switch. Specific air requirements can be selected when using dual controls. They are helpful when the compressor has a variety of applications varying from continuous duty to intermittent.
Sequencing controls are used for alternate operations for each compressor. This is accomplished by providing either individual operating cycles or dual operations in times of peak demand. They are mostly recommended when operating duplex air compressors, when 100% back-up air is needed in critical applications, or operating group efficiency at peak efficiencies.
Types of Drives in Air Compressors
The majority of the air compressors are driven by:
Internal combustion engines
Engine power takeoffs
Gears, V-Belts, or direct drive configurations are used to transmit power in the stated mechanisms. These are further discussed below.
V-Belt drives are typically used with internal combustion engines and electric motors. The V-Belt drives have greater flexibility and can better match the load of the compressor to the load of the power source at minimum cost speed. The belts need to be properly shielded for safety.
Gear drives are typically used with electric motors. They provide an axial load reduction on the compressed air, thereby extending the lifetime in operation. The more the working points in the working range the less the shear force.
Direct drives are compact and require minimum maintenance. The compressors can be direct-coupled or flange mounted to the power source. If couplings are used, they should be properly shielded to ensure safety.
Engine drives that use diesel, gasoline, or power takeoff drives are used due to their portability. A V-Belt, gearbox, or direct drive is then used to transmit power from the source to the compressor.
Performance Specifications in Air Compressors
There are two main air compressor performance specifications, namely:
Delivery: This is the compressed air volume delivered by the compressor as its discharge pressure. This is expressed according to the prevailing atmospheric inlet conditions as actual cubic feet per minute with the acronym acfm. This has a flow rate, which is stated in standard cubic feet per minute with the acronym scfm. The scfm is measured at 14.5 psi, 68 °F (20 °C), and relative humidity of 0%.
Having more than one discharge pressure at varying flow rates can reduce the compressor volumetric efficiency. This occurs when there is increased system pressure expressed in pounds per square in gauge (psig). Therefore, careful consideration should be given when selecting the compressor’s maximum operating pressure.
Displacement: This is measured in cubic feet per minute. Displacement for reciprocating/piston compressors is the product of the volume of the first stage cylinder, and the compressor revolutions in one minute. Displacement for rotary screw compressors produces suction volume per thread and the number of driving rotor lobes.
Picking the Right Industrial Air Compressor
Different compressors have different capabilities and technologies. It is important to select the right air compressor fit for purpose.
1. The first consideration is whether the business needs an oil-free compressor or an oil-injected compressor.
The oil-injected compressor is less expensive and can be used in environments such as the manufacturing environment where there may be no need for immediate oil-free compressed air.
The oil-free compressors are more expensive. However, they can produce the right quality of compressed air fit for the pharmaceutical sector or food production. Thus, oil-free compressors are best for active air, and oil-injected compressors are best for energy air.
2. The second consideration is whether the business needs a screw compressor or a piston compressor.
The piston compressors are less costly and can be maintained with ease. These are applied in environments where a lot of compressed air is needed part of the time e.g in garages. They have the drawback of being noisy and thus making them not fit for environments such as laboratories. They, however, hold well when operated in dirty environments. The piston compressors are prone to passing air into the supply of compressed air, known as carryover. A high amount of heat is generated by piston compressors in operation; they are thus sized based on duty cycle, which has a baseline of 75% run and 25% rest.
The rotary screw compressors can be one of two versions i.e. fixed speed or variable speed. The fixed speed compressors are best for applications that need a steady air flow. The variable speed compressors are best for applications that have fluctuating air demands. They are also energy efficient and therefore, the higher initial investments can be recouped through long term energy savings.
Rotary screw compressors have high initial investments but pose many advantages later on. They can be operated at high speeds producing more compressed air. They are also quieter, more energy efficient, and have a small footprint. They are best suited for continuous operation. An important consideration for rotary screw compressors is that they need to remain at operating temperature to yield effective compression. This is because of the tight tolerances between the rotors. When sizing the rotary screw compressors, attention has to be given to air usage whereas in piston compressors oversizing is not much of a concern.
An environment where air is constantly used, e.g an auto body shop for painting, can have lower carryover rates on its rotary screw compressor and running continuously may be desirable. On the contrary, an environment with infrequent air use and less concern on supplied air cleanliness, e.g a general auto repair business, can make use of a piston compressor.
Both the rotary and piston compressors can have oil-free and oil-injected versions. This provides a wider array of options for businesses to choose the right industrial compressor for a specific application.
Compressed air is typically cooled, then dried, and then filtered before it gets distributed via pipes regardless of the type of compressor. Plant-air systems and its components should be selected based on the size of the system being designed as well as incorporating filter-regulator-lubricators at the points of supply drops.
Rotary screw varieties with engine drives are often used in larger site compressors and are mounted on trailers. They are meant to run continuously even if the air is being dumped.
Scroll compressors are increasingly becoming popular in the lower end air compressors and refrigeration systems. They are best suited for manufacturing processes that need clean air of class 0. Examples of such manufacturing processes include food, pharmaceutical, electronics, etc. They can also be used in laboratories, cleanrooms, and medical settings.
In scenarios where hazardous gases need to be compressed, sliding-vane or diaphragm compressors are recommended, or in cases where large volumes need to be compressed, kinetic types are recommended.
Parameters in Selecting the Right Air Compressor
In addition to the considerations already outlined in the previous sections, the parameters to consider in specifying an air compressor are:
The pressure capability, measured in psi, is based on the equipment needs with which the compressed air will be operating. The bulk of air tools operate at typical air pressures. However, some applications, for example, starting an engine needs high pressures. Therefore, a single-stage unit may be sufficient in pressure of up to 135 psi in daily tools, but a two-stage unit would be needed in higher-pressure special applications.
The volumetric capacity is the quantity of air that the compressor can deliver per unit time. This is commonly measured in cubic feet per minute (cfm), which can be subjective depending on the manufacturers and the standardized version is the standard cubic feet per minute (scfm). The volumetric capacity can also be measured in actual cubic feet per minute (acfm). This is the quantity of compressed air that is delivered to the compressor outlet. This is always less than the compressor displacement due to blow-by losses in the compressor.
This is the power needed to drive the air compressor and is determined by the pressure and volume considerations. In determining the compressor capacity, it is also vital to consider the system losses such as pressure drops through filters and dryers, and piping losses etc. Considerations of the drive also need to be made, such as motor direct drive or belt, diesel drive or engine gas etc.
Additional Considerations for Air Compressor Selection
Some of the additional factors to consider in the selection of an air compressor are:
Quality of air
Oil vs oilless
Oil vs Oilless
The main purpose of oil is to remove heat that is generated in the process of compression. Oil can also act as a seal in some designs.
In piston compressors, oil is used to lubricate the wrist pin bearings, the crank, and the cylinder sidewalls. The piston rings seal the compression chamber and control oil movement into it.
In rotary screw compressors, oil is injected into the body of the compressor to remove heat generated from the compression process and seal the non-contacting rotors.
In rotary-vane compressors, oil seals the small space between the housing bore and the vane tips.
Scroll compressors do not typically use oil and are thus oilless compressors.
Centrifugal compressors also do not make use of oil in the compression stream.
There are a number of tactics that manufacturers use in creating oil-free compressors. Piston compressors can be made to have one-piece assemblies of piston-crank that use eccentric bearings to ride the crankshaft. The reciprocating pistons in the cylinders rock within the assemblies. Thus, in such a design, a wrist-pin bearing is not used on the piston. Other self-lubricating materials can be used in piston compressors to seal rings and liners of the cylinder(s). Manufacturers of rotary screw compressors can tighten up in between screw clearances to remove the need to use oil sealant.
Using these tactics to make oilless air compressors can come with tradeoffs such as issues with heat management, increased wear, more frequent maintenance, and reduced capacity.
Defining compressor capacities in a setup that runs jackhammers all day, consideration should be taken on the number of operators using the compressor, the cfm of the tools, and environmental constraints. In such circumstances, a helical screw compressor that runs continuously can be recommended to meet the demand. This can run on a single tank of fuel for 8 hours.
Compressor capacities to provide a small shop with compressed air can use air tools categorized as either continuous or intermittent. Intermittent air tools can be a ratchet wrench, and a continuous air tool can be a paint sprayer. Charts can be used to estimate consumption of the different tools and thus, usage based on continuous or intermittent can be figured out. The overall capacity of the air compressor can be determined.
Compressor capacities in manufacturing facilities use the same logic. As an example, a packaging line can use compressed air to blow-off devices, actuate cylinders etc. Equipment manufacturers provide consumption rates for each machine, or alternatively air consumption of the cylinder can be calculated for each air actuated device from the bore size, stroke, rate of cycling.
Larger process plants and manufacturing operations will have larger demands of compressed air. In such cases, continuous availability of compressed air outweighs the cost of multiple systems of compressed air. This mitigates costly line stoppages.
Quality of Air
In the compression process, air taken from the atmosphere can have heat and sometimes oil mixed with the air. This generates moisture unless the intake air is dry. These additional constituents may or may not affect the end-use and performance depending on the operations.
As the air is compressed, heat is generated and needs to be removed by collecting the air in a tank. This allows the air to cool and some of the moisture condenses. Thus, receiving tanks for compressed air have valves to facilitate the draining off of accumulated water. These valves can be manual or automatic. An aftercooler can also be used to remove heat further. Moisture can be further removed by adding desiccant and refrigerant-based dryers to the air supply piping. Filters can also be used to remove entrained lubricant in the supply air. Any other particles that may have come in at the intake are also filtered.
The compressed air is then distributed to several drops at which a filter, regulator and lubricator (FRL) should be installed as per best practices. The FRLs adjust the air according to the particular tool’s needs and allow lubrication to any tool in need.
With piston-compressor control, the most common choice is the start/stop control. This is used to feed the tank with lower and upper thresholds. As the pressure reduces to the lower setpoint the compressor switches on. It then runs until the upper setpoint is reached.
An alternate approach known as constant speed control allows the compressor to run for a certain period after it reaches its upper setpoint and discharges to the atmosphere lest the stored air is in usage higher than normal. The number of motor starts are thus minimized during peak demand.
Systems with higher than 10hp can have a selectable dual system that allows for control and toggle between the two modes.
Compressors like the helical screw can have other controls in addition to the start/stop control and the constant speed control. These can be inlet valve modulation, load/unload control, sliding valve, variable speed drive, and automatic dual control. Compressor sequencing can be found in multi-unit installations.
The right choice of a control scheme is based on the best balance between servicing the demand and the cost of idling vs. the cost of accelerated wear of equipment.
Industries and Applications of Air Compressors
Nearly every type of business makes use of an air compressor. They are used to power pneumatic tools, inflate tires, apply paint, and clean surfaces. Any process that involves metal fabrication, woodworking, painting, coating, or construction uses an air compressor.
The uses for air compressors fall into six main categories, which are air tools, blasting, spraying, pumping, inflating, and breaking.
Air Tools - Air tools are easier to use than electric-powered tools because they are lightweight and do not have cords or batteries. Compressed air flows through a tube into the tool, producing the necessary force to nail or staple.
Blasting - Blasting removes dirt, dust, grime, and residue from surfaces. The various types of blasting tools operate quickly and do not need to hold power. Surfaces are immediately cleaned in preparation for other applications.
Spraying - Spraying is more complex than other air compressor functions since the air pressure has to vary in accordance with the different types of spraying jobs. Air compressor sprayers are used to apply coatings, paint, and surface protection. They evenly coat a surface without buildup.
Pumping - Air compressors store air under high pressure to be used for various purposes. The amount of pressure under which the air is held differentiates an air compressor from an air pump. Air compressors are incapable of pumping fluids.
Inflating - One of the common uses for air compressor pumped air is as a means to inflate a wide variety of things from boats, tires and balls to rafts and large inflatables such as water slides.
Breaking - Air compressors are used to break up pave materials, concrete, and hard surfaces such that they can be removed and replaced.
Machine shops and repair shops have two sets of receptacles. One set is for electrical tools, while the other set is for tools that operate by compressed air. The number of kinds and uses for air compressors is endless and includes nearly every industry. The convenience and power of air compressors makes the completion of various vital tasks easier and more efficient.
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