Air Compressor Terms

Single Stage Compression | Two Stage Compression | Displacement | Free Air or Actual Capacity | ASME Receivers | None-Code Receivers |

Types of Air Compressors | Reciprocating Units | Rotary Screw Compressors | Centrifugal Compressors | Air Systems | Receiver Tanks |

Air Dryers | Filters | Piping Distribution System |


Single Stage Compression:

Compression from initial to final pressure is completed in a single step or piston stroke. Single stage compressors
are normally used in pressure ranges up to 100 psi for continuous service and 150 psi for intermittent service .

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Two Stage Compression:

Compression from initial to final pressure is accomplished in two steps: air compressed in a first stage
low pressure cylinder passing through a cooling coil to a second stage high pressure cylinder for final
compression. Two stage compressors are normally used for pressure ranges of 100 psi and greater.

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Volume displaced by the compressor pump pistons in cubic feet multiplied by rpm of the compressor pump and expressed
in standard cubic feet per minute (scfm). In two stage, only the first stage is considered in determining displacement.

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Free Air, or Actual Capacity:

The quanity of air compressed and delivered measured in terms of its volume
at atmospheric pressure, expressed in standard cubic feet per minute (scfm).

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ASME Receivers:

Tanks made and inspected to meet standards pf the American Society of Mechanical Engineers (ASME). An ASME certificate
is on file for each receiver, listing registered serial numbers and certifying that the receiver has been inspected by an
independent agency in accordance with ASME standards. ASME receivers must be used where OSHA compliance is required.

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None-Code Receivers:

Compressed air holding tanks which do NOT meet ASME standards. Restricted to use
in locations where compliance with OSHA, state and local standards are NOT required.

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Types of Air Compressors:

There are three basic types of air compressors:
• Reciprocating (Recip)
• Rotary Screw (Screw)
• Rotary Centrifugal (Centrifugal)

These types are further defined by:
• the number of compression stages
• method of cooling (air, water, oil)
• drive method (motor, engine, steam, other)
• how they are lubricated (oil, oil-free)
• packaged or custom-built

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Reciprocating Units:

Reciprocating air compressors are positive displacement compressors. This means they are taking in successive volumes of air which is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by using a piston within a cylinder as the compressing and displacing element.

The reciprocating air compressor is considered single acting when the compressing is accomplished using only one side of the piston. A compressor using both sides of the piston is considered double acting.

The reciprocating air compressor uses a number of automatic spring loaded valves in each cylinder that open only when the proper differential pressure exists across the valve.

Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure in the cylinder is slightly above the discharge pressure.

A compressor is considered to be single stage when the entire compression is accomplished with a single cylinder or a group of cylinders in parallel. Many applications involve conditions beyond the practical capability of a single compression stage. Too great a compression ration (absolute discharge pressure/absolute intake pressure) may cause excessive discharge temperature or other design problems.

For practical purposes most plant air reciprocating air compressors over 100 horsepower are built as multi-stage units in which two or more steps of compression are grouped in series. The air is normally cooled between the stages to reduce the temperature and volume entering the following stage.

Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations, may be packaged, and provide a wide range of pressure and capacity selections.

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Rotary Screw Compressors:

Rotary air compressors are positive displacement compressors. The most common rotary air compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These compressors consist of two rotors within a casing where the rotors compress the air internally. There are no valves. These units are basically oil cooled (with air cooled or water cooled oil coolers) where the oil seals the internal clearances.

Since the cooling takes place right inside the compressor, the working parts never experience extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, air cooled or water cooled compressor package.

Because of the simple design and few wearing parts, rotary screw air compressors are easy to maintain, operate and provide great installation flexibility. Rotary air compressors can be installed on any surface that will support the static weight.

The two stage oil flooded rotary screw air compressor uses pairs of rotors in a combined air end assembly. Compression is shared between the first and second stages flowing in series. This increases the overall compression efficiency up to fifteen percent of the total full load kilowatt consumption. The two stage rotary air compressor combines the simplicity and flexibility of a rotary screw compressor with the energy efficiency of a two stage double acting reciprocating air compressor. Two stage rotary screw air compressors are available air cooled and water cooled and fully packages.

The oil free rotary screw air compressor utilizes specially designed air ends to compress air without oil in the compression chamber yielding true oil free air. Oil free rotary screw air compressors are available air cooled and water cooled and provide the same flexibility as oil flooded rotaries when oil free air is required.

Rotary screw air compressors are available air cooled and water cooled, oil flooded and oil free, single stage and two stage. There is a wide range of availability in configuration and in pressure and capacity.

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Centrifugal Compressors:

The centrifugal air compressor is a dynamic compressor which depends on transfer of energy from a rotating impeller to the air. The rotor accomplishes this by changing the momentum and pressure of the air. This momentum is converted to useful pressure by slowing the air down in a stationary diffuser.

The centrifugal air compressor is an oil free compressor by design. The oil lubricated running gear is separated from the air by shaft seals and atmospheric vents.

The centrifugal is a continuous duty compressor, with few moving parts, that is particularly suited to high volume applications, especially where oil free air is required.

Centrifugal air compressors are water cooled and may be packaged; typically the package includes the after-cooler and all controls.

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Air Systems:

In addition to the air compressor package that includes the drive, air end, and cooling system, complete air systems include:
• Receiver Tanks
• Air Dryers
• Filters
• Piping Distribution Systems

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Receiver Tanks:

The air receivers:

• provide storage capacity to prevent rapid compressor cycling

• reduce wear and tear on compression module, inlet control system, and motor

• eliminate pulsing air flow

• damp out the dew point and temperature spikes that follow regeneration

• avoid overloading purification system with surges in air demand

A rule of thumb is to provide a minimum of one gallon of receiver capacity for each cubic foot of compressor flow.

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Air Dryers:

Moisture, either liquid or vapor, is present in compressed air as it exits the compressor system. If this moisture is not properly removed, your compressed air system can lose efficiency and require dramatically increased maintenance, which can result in costly downtime.

The majority of pneumatic instruments and processes can not tolerate hot compressed air, compressors are normally supplied with after-coolers and moisture separators. After-coolers are heat exchangers that utilize either water or ambient air to cool the compressed air. As the water and lubricant vapors within the compressed air cool, a significant amount condenses into liquid. An after-cooler discharging compressed air at 100F passes 67 gallons of water per 1,000 scfm per 24 hours.

To avoid these problems, compressed air systems have purification devices available to remove the water vapor and other contaminants. The proper selection of these devices is critical as pneumatic applications and compressed air systems become increasingly sophisticated.

The pneumatic equipment in use and the lowest expected ambient temperature determine the drying method. The most common dryer is a refrigerated unit that cools the compressed air, condenses water and oil vapors, separates them, and drains them from the system. The "dried" compressed air is then fed to the air system.

Dryer performance is specified as a pressure dew point class that is based on a specific inlet and ambient conditions. The lowest pressure dew point class with a refrigerated dryer is Class H. This class delivers a pressure dew point that of 33EF to 39EF. Refrigerated dryers should not operate below the Class H range because the water vapor will freeze in the dryer. The highest practical pressure dew point for a refrigerated dryer is 60EF because higher pressure dew points give condensation in downstream piping.

Refrigerated air dryers remove moisture from the compressed air through a mechanical refrigeration system to cool the compressed air and condense water and lubricant vapor. Most refrigerated dryers cool the compressed air to a temperature of approximately 35EF, resulting in a pressure dew point range of 33EF - 39EF. Keep in mind that this range is also the lowest achievable with a refrigerated design since the condensate begins to freeze at 32EF.

Desiccant dryers utilize chemicals beads, called desiccant, to adsorb water vapor from compressed air. Silica gel, activated alumina and molecular sieve are the most common desiccants used. (Silica gel or activated alumina are the preferred desiccants for compressed air dryers.) The desiccant provides an average -40EF pressure dew point performance. Molecular sieve is usually only used in combination with silica gel or activated alumina on -100EF pressure dew point applications.

Deliquescent air dryers utilize an absorptive type chemical, called a desiccant, to provide a 20EF to 25EF dew point suppression below the temperature of the compressed air entering the dryer. The moisture in the compressed air reacts with the absorptive material to produce a liquid effluent which is then drained from the dryer. Keep in mind that this effluent is typically corrosive and must be disposed of in accordance with local regulations.

While deliquescent dryers are typically used in applications such as sandblasting and logging operations, they are not recommended for industrial applications since the dried compressed air exiting the dryer may contain small amounts of the effluent which may be corrosive to downstream equipment.

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Coalescing filters are the most common form of compressed air purification. These filters remove liquid water and lubricants from compressed air and are installed downstream in a refrigerated air dryer system or upstream in a desiccant dryer system.

Most manufacturers claim a one psi "clean and dry" pressure drop, with the normal operating (wetted) pressure drop between three and six psi. Manufacturers typically require filter changes when the pressure drop reaches 10 psi, which is approximately six to 12 months of operation. Coalescing filters will also remove particulate contamination; however, this will increase the pressure drop across the filter and shorten the filter element life.

Filters are rated according to liquid particle retention size (micron) and efficiency, such as 0.50 micron and 99.99% D.O.P. efficient, or 0.01 micron and 99.9999% D.O.P efficient.

Coalescing filters can only remove previously condensed liquids; they do not remove water or lubricant vapors from the compressed air. Any condensation produced from subsequent compressed air cooling will have to be eliminated. When seeking to remove water and lubricant vapors from compressed air, specify an air dryer.

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Piping Distribution System:

The piping distribution system not only controls how the air gets from the compressor room to the tools, it is a major factor in the energy consumed by the compressor. Poorly designed or maintained systems increase pressure losses and increase operating costs. A common error is to increase compressor delivery pressure to compensate for distribution problems. This substantially increases energy costs. Higher pressure increases leak rates, another major source of waste, thus the waste and increased cost is compounded.

The piping distribution system is the major focus of most Air Audits.

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Mississippi Valley Equipment     •     2819 Larson St.     •     La Crosse, WI 54603     •     800-262-2193