Understanding Compressors

Helical Screw Compressors

Helical-screw compressors are rotary compressor machines known for their capacity to operate on 100% duty cycle, making them good choices for trailerable applications such as construction or road building. Using geared, meshing male and female rotors, these units pull gas in at the drive end, compress it as the rotors form a cell and the gas travels their length axially, and discharge the compressed gas through a discharge port on the non-drive end of the compressor casing. The rotary screw compressor action makes it quieter than a reciprocating compressor owing to reduced vibration. Another advantage of the screw compressor over piston types is the discharge air is free of pulsations. These units can be oil- or water- lubricated, or they can be designed to make oil-free air. These designs can meet the demands of critical oil-free service.

Sliding Vane Compressors

A sliding-vane compressor relies on a series of vanes, mounted in a rotor, which sweep along the inside wall of an eccentric cavity. The vanes, as they rotate from the suction side to the discharge side of the eccentric cavity, reduce the volume of space they are sweeping past, compressing the gas trapped within the space. The vanes glide along on an oil film which forms on the wall of the eccentric cavity, providing a seal. Sliding-vane compressors cannot be made to provide oil-free air, but they are capable of providing compressed air that is free of pulsations. They are also forgiving of contaminants in their environments owing to the use of bushings rather than bearings and their relatively slow-speed operation compared to screw compressors. They are relatively quiet, reliable, and capable of operating at 100% duty cycles. Some sources claim that rotary vane compressors have been largely overtaken by screw compressors in air-compressor applications. They are used in many non-air applications in the oil and gas and other process industries.

Scroll Compressors

Scroll air compressors use stationary and orbiting spirals which decrease the volume of space between them as the orbiting spirals trace the path of the fixed spirals. Intake of gas occurs at the outer edge of the scrolls and discharge of the compressed gas takes place near the center. Because the scrolls do not contact, no lubricating oil is needed, making the compressor intrinsically oil-free. However, because no oil is used in removing the heat of compression as it is with other designs, capacities for scroll compressors are somewhat limited. They are often used in low-end air compressors and home air-conditioning compressors.

Rotary Lobe Compressors

Rotary-lobe compressors are high-volume, low-pressure devices more appropriately classified as blowers. To learn more about blowers, download the free Thomas Blowers Buying Guide.

Centrifugal Compressors

Centrifugal compressors rely on high-speed pump-like impellers to impart velocity to gases to produce an increase in pressure. They are seen mainly in high-volume applications such as commercial refrigeration units in the 100+ hp ranges and in large processing plants where they can get as large as 20,000 hp and deliver volumes in the 200,000 cfm range. Almost identical in construction to centrifugal pumps, centrifugal compressors increase the velocity of gas by throwing it outward by the action of a spinning impeller. The gas expands in a casing volute, where its velocity slows and its pressure rises.

Centrifugal compressors have lower compression ratios than displacement compressors, but they handle vast volumes of gas. Many centrifugal compressors use multiple stages to improve the compression ratio. In these multi-stage compressors, the gas usually passes through intercoolers between stages.

Axial Compressors

The axial compressor achieves the highest volumes of delivered air, ranging from 8000 to 13 million cfm in industrial machines. Jet engines use compressors of this kind to produce volumes over an even wider range. To a greater extent than centrifugal compressors, axial compressors tend toward multi-stage designs, owing to their relatively low compression ratios. As with centrifugal units, axial compressors increase pressure by first increasing the velocity of the gas. Axial compressors then slow the gas down by passing it through curved, fixed blades, which increases its pressure.

Power and Fuel Options

Air compressors may be powered electrically, with common options being 12 volt DC air compressors or 24 volt DC air compressors. Compressors are also available that operate from standard AC voltage levels such as 120V, 220V, or 440V.

Alternative fuel options include air compressors that operate from an engine that is driven off of a combustible fuel source such as gasoline or diesel fuel. Generally, electrically-powered compressors are desirable in cases where it is important to eliminate exhaust fumes or to provide for operation in settings where the use or presence of combustible fuels is not desired. Noise considerations also play a role in the choice of fuel option, as electrically driven air compressors typical exhibit lower acoustical noise levels over their engine-driven counterparts.

Additionally, some air compressors may be powered hydraulically, which also avoids the use of combustible fuel sources and the resulting exhaust gas issues. 

Compressor Machine Selection in an Industrial Setting

In selecting air compressors for general shop use, the choice will generally come down to a piston compressor or a helical-screw compressor. Piston compressors tend to be less expensive than screw compressors, require less sophisticated maintenance, and hold up well under dirty operating conditions. They are much noisier than screw compressors, however, and are more susceptible to passing oil into the compressed air supply, a phenomenon known as “carryover.” Because piston compressors generate a great deal of heat in operation, they have to be sized according to a duty cycle—a rule of thumb prescribes 25% rest and 75% run. Radial-screw compressors can run 100% of the time and almost prefer it. A potential problem with screw compressors, though, is that oversizing one with the idea of growing into its capacity can lead to trouble as they are not particularly suited to frequent starting and stopping. Close tolerance between rotors means that compressor needs to remain at operating temperature to achieve effective compression. Sizing one takes a little more attention to air usage; a piston compressor may be oversized without similar worries.

An autobody shop which uses air constantly for painting might find a radial-screw compressor with its lower carryover rate and desire to run continuously an asset; a general auto-repair business with more infrequent air use and low concern for the cleanliness of the supplied air might be better served with a piston compressor.

Regardless of the compressor type, compressed air is usually cooled, dried, and filtered before it is distributed through pipes. Specifiers of plant-air systems will need to select these components based on the size of the system they design. In addition, they will need to consider installing filter-regulator-lubricators at the supply drops.

Larger job site compressors mounted on trailers are typically rotary-screw varieties with engine drives. They are intended to run continuously whether the air is used or dumped.

Although dominant in lower-end refrigeration systems and air compressors, scroll compressors are beginning to make inroads into other markets. They are particularly suited to manufacturing processes that demand very clean air (class 0) such as pharmaceutical, food, electronics, etc. and to cleanroom, laboratory, and medical/dental settings. Manufactures offer units up to 40 hp that deliver nearly 100 cfm at up 145 psi. The larger capacity units generally incorporate multiple scroll compressors as the technology does not scale up well once beyond 3-5 hp.

If the application involves compressing hazardous gases, specifiers often consider diaphragm or sliding-vane compressors, or, for very large volumes to compress, kinetic types.

Additional Selection Considerations

Some additional selection factors to which consideration should be given are as follows:

• Oil vs. oil less
• Sizing the compressor
• Air quality
• Controls

Oil vs. Oil less

Oil is important in the operation of any compressor as it serves to remove heat generated by the compression process. In many designs, oil provides a seal as well. In the case of piston compressors, oil lubricates the crank and wrist-pin bearings and the sidewalls of the cylinder. As with piston engines, rings on the piston provide sealing of the compression chamber and control the passage of oil into it. Rotary-screw compressors inject oil into the compressor body to both seal the two non-contacting rotors and, again, to remove some of the heat of the compression process. Rotary-vane compressors rely on oil to seal the minute space between the vane tips and the housing bore. Scroll compressors do not normally use oil, thus are known as oil less but, of course, their capacities are somewhat limited. Centrifugal compressors do not introduce any oil into the compression stream, but these are in a different league than their positive-displacement brethren.

To create oil-free compressors, manufacturers rely on a number of tactics. Piston-compressor makers can employ one-piece piston-crank assemblies that ride the crankshaft on eccentric bearings. As these pistons reciprocate in cylinders, they rock within them. This design eliminates a wrist-pin bearing on the piston. Piston-compressor makers also employ a variety of self-lubricating materials for the sealing rings and cylinder liners. Rotary-screw compressor makers tighten up the clearances between the screws, eliminating the need for oil sealant.

There are tradeoffs, however, with any of these schemes. Increased wear, heat-management issues, reduced capacity, and more frequent maintenance are but a few of the drawbacks associated with oil-less air compressors. Obviously, specific industries put up with these tradeoffs because oil-free air is imperative. But where it is acceptable to filter out oil, or simply live with it, an ordinary oil-based compressor makes sense.

Sizing the Compressor

If you run jackhammers all day, picking a compressor is straightforward: add up the number of operators who will be using the compressor, determine the cfm of their tools, and buy a continuously running helical-screw compressor that can meet the demand and which will run for 8 hours on a single tank of fuel. Of course, it is not really that simple—there may be environmental constraints to consider—but you get the idea.

If you are trying to provide compressed air for a small shop, things get a little more complicated. Air tools can be segregated by use: either intermittent—a ratchet wrench, say—or continuous—a paint sprayer, perhaps. Charts are available to help in estimating the consumption of various shop tools. Once these are determined, and usage based on average and continuous use figured out, a rough determination of the overall air compressor capacity can be made.

Defining compressor capacities for manufacturing facilities proceeds in roughly the same manner. A packaging line, for instance, will likely use compressed air to actuate cylinders, blow-off devices, etc. Ordinarily, the equipment manufacturer will provide consumption rates for individual machines, but if not, cylinder air-consumption is easily estimated by knowing the bore, stroke, and cycling rate of each air-actuated device.

Very large manufacturing operations and process plants will likely have equally large compressed air demands that might be served by redundant systems. For such operations, having air available at all times justifies the cost of multiple compressed-air systems to avoid costly line stoppages or shutdowns. Even smaller operations can benefit from some level of redundancy. That is a question that must be asked if sizing a small manufacturing air-system: is the operation best served by a single compressor (less maintenance, less complexity) or would multiple, smaller compressors (redundancy, room for growth) provide a better fit?

Air Quality

A compressor takes air in from the atmosphere and by compressing it adds heat and sometimes oil to the mix, and, unless the intake air happens to be very dry, generates a lot of moisture. For some operations, these additional constituents do not affect the end-use and tools run well without performance issues. As air-driven processes become more complex, or more critical, additional thought is usually given to improving the quality of the output air.

Compressed air is usually quite hot, and the first step in reducing this heat is to collect the air in a tank. This step not only allows the air to cool, but it also permits some of the moisture in it to condense. Air-compressor receiving tanks generally have either manual or automatic valves to allow accumulated water to be drained off. Further heat can be removed by running the air through an aftercooler. Refrigerant-based and desiccant dryers can be added into the air-supply piping to increase moisture removal. Finally, filtering can be installed to remove any entrained lubricant from the supply air, as well as any particulates that may have gotten by any intake filtering.

Compressed air will normally be distributed out to several drops. At each drop, the standard best practice is to install FRLs (filter, regulator, lubricator) which adjust the air to the needs of the particular tool and permit lubrication to flow to any tools that require it.

Controls

There are not too many choices when it comes to piston-compressor control. Start/stop control is most common: the compressor feeds a tank with upper and lower thresholds. When the lower setpoint is reached the compressor switches on and runs until the upper setpoint is reached. A variant of this method, dubbed constant speed control, lets the compressor run for some length of time after reaching the upper setpoint, discharging to atmosphere, in case the stored air is being used at a higher-than-normal rate. This process minimizes the number of motor starts during periods of high demand. A selectable dual control system, usually available only on systems in the 10+ hp range, allows a user to toggle between these two control modes.

More options are available for helical-screw compressors. In addition to start/stop and constant-speed control, screw compressors can use load/unload control, inlet-valve modulation, sliding valve, automatic dual control, variable speed drive, and, for multi-unit installations, compressor sequencing. Load/unload control uses a valve on the discharge side and a valve on the intake side which respectively open and close to reduce the flow through the system. (This is a very common system on oil-less screw compressors.) Inlet-valve modulation uses proportional control to regulate the mass-flow of air into the compressor. Sliding-valve control effectively shortens the length of the screws, delaying the start of compression and allowing some intake air to bypass compression to better match demand. Automatic dual-control switches between start/stop and constant-speed control depending on the demand characteristics. Variable-speed drive slows or increases rotor rpm by electronically altering the frequency of the AC waveform that is spinning the motor. Compressor sequencing allows loading to be distributed among multiple compressors, assigning, for example, one unit to run continuously for handling baseload, and varying the starts of two additional units to minimize the restart penalty.

In selecting any of these control schemes, the idea is to strike the best balance between meeting demand and the cost of idling versus the expense of accelerated equipment wear.

Performance Specifications

In selecting compressor machinery, specifiers have three main parameters to consider in addition to the many points outlined above. These air compressor specifications include:

• volumetric capacity
• pressure capability
• the power of the machine

Although compressors are typically rated by horsepower or kilowatts, these measures do not necessarily give any indication of what it will cost to operate the equipment as this is dependent on the efficiency of the machine, its duty cycle, and so forth.

Volumetric Capacity

Volumetric capacity defines how much air the machine can deliver per unit time. Cubic-feet per minute is the most common unit for this measure, although just what this is can vary between manufacturers. An attempt to standardize this measure, a so-called scfm, seems to be dependent on whose standards you follow. The Compressed Air and Gas Institute has adopted the ISO definition of an scfm as being dry air (0% relative humidity) at 14.5 psi and 68°F. Actual cfm, of acfm, is another measure of volumetric capacity. It relates the amount of compressed air delivered to the outlet of the compressor, which will always be less than the displacement of the machine owing to losses from blow-by through the compressor.

Pressure Capacity

Pressure capability in psi is largely based around the needs of the equipment the compressed air will be operating. While many air tools are designed to operate at normal shop air pressures, special applications, such as engine starting, require higher pressures. Thus, in specifying a piston compressor, for instance, a buyer would find a single-stage unit that delivers pressure up to 135 psi adequate for powering everyday tools but would want to consider a two-stage unit for special, higher-pressure applications.

Power of the Machine

The power required to drive the compressor will be determined by these volume and pressure considerations. A specifier will also want to think about system losses in determining compressor capacity: piping losses, pressure drops through dryers and filters, etc. Compressor buyers also have drive decisions to make such as motor belt- or direct-drive, engine gas- or diesel-drive, etc.

Compressor makers will often publish compressor-performance curves to enable specifiers to evaluate compressor performance over a range of operating conditions. This is especially true for centrifugal compressors which, like centrifugal pumps, can be sized to deliver different volumes and pressures depending on shaft speed and impeller sizing.

The Dept. of Energy is adopting energy standards for compressors against which some compressors makers are publishing data sheets. As more manufacturers publish these data, compressor buyers should have an easier time sorting through the energy usage of comparable compressors.

Applications and Industries

Compressors find application in different industries and are also prevalent in settings that are familiar to everyday consumers. For example, the portable 12V DC electric air compressor that is often carried in the glove compartment or trunk of a car is a common example of a simple version of air compressor that finds use among consumers to inflate tires to the correct pressure.

Some of the common applications and industries that utilize compressors include the following:

• Truck and vehicle-mounted compressors
• Medical and Dental applications
• Laboratory and specialty gas compression
• Food and beverage processing applications
• Oil & Gas applications

Truck and Vehicle-Mounted Compressors

Vehicle-related use of air compressors and common vehicle applications involve truck mounted electric air compressors, truck mounted diesel air compressors, or other vehicle-mounted air compressors. For example, air brake systems on trucks involve the use of compressed air to operate, thus requiring an air compressor onboard to recharge the braking system. Service vehicles may require onboard air compressors to perform needed functions or to permit the compressor to be mobile and able to be deployed as needed to various job sites or locations. For example, fire trucks may include breathable air compressors onboard to provide air tank filler capability to replenish breathable air tanks for firefighters and first responders.

Medical and Dental Applications

Compressors find use in the medical and dental fields, as well.

Dental air compressors provide a source of clean compressed air to assist in the performance of dental procedures as well as to power pneumatically operated dental instruments such as drills or toothbrushes. Choosing the right dental air compressor, requires several considerations, including the horsepower and pressure required.

Medical air compressor uses include the generation of a supply of breathing air that is independent of other gases stored in gas cylinders and that can be used as an option for patients who may be sensitive to oxygen toxicity, for example. The medical breathing air compressors may be portable or stationary systems within a hospital or medical facility. Other medical air compressor uses may involve providing air to specialized patient equipment, such as compression cuffs, where compressed air is needed to provide pressure on a patient’s limbs to prevent fluid buildup in the extremities as a result of a weakened cardiac function.

Laboratory and Specialty Gas Compression

Laboratory air compressors and air compressors for other specialized industrial applications are used to process and generate supplies of specialized gases, such as hydrogen, oxygen, argon, helium, nitrogen, or gas mixtures (for example, ammonia compressors) or carbon dioxide, where it may be used in the food and beverage industry. Helium compressors would supply the gas to storage tanks for use in laboratory applications such as fine leak detection, while other gas compressors, such as oxygen compressors, might serve the need to store tanks of oxygen for use in hospitals and medical facilities.

Food and Beverage Processing Applications

Food-grade air compressors serve a critical role in the food and beverage processing industries. Finding application throughout the entire production cycle, these compressors can be used to facilitate processing operations, such as sorting, preparation, distribution, packaging, and preservation. Additionally, compressed air can be used to maintain the sanitary environment necessary when manufacturing consumable products.

Oil & Gas Applications

The use of compressors is also prevalent in the oil and gas industry, where natural gas compressors are used to generate compressed natural gas for storage and transport. Some of these gas compression operations require the use of high-pressure compressors, where discharge pressures may be anywhere from 1,000 to 3,000 psi and upwards, with 10,000 – 60,000 psi range possible, depending on the application.