COMPRESSOR SELECTION GUIDE
The three basic types of air compressors are:
- Rotary screw
- Rotary centrifugal
These types are further specified by:
- The number of compression stages
- Cooling method (air, water, oil)
- Drive method (motor, engine, steam, other)
- Lubrication (oil, oil-free where oil free means no lubricating oil contacts the compressed air)
- Packaged or custom-built
Reciprocating Air Compressors
Reciprocating air compressors are positive displacement machines, meaning that they increase the pressure of the air by reducing its volume. 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 a piston within a cylinder as the compressing and displacing element.
- Single-stage and two-stage reciprocating compressors are commercially available.
- Single-stage compressors are generally used for pressures in the range of 70 psig to 100 psig.
- Two-stage compressors are generally used for higher pressures in the range of 100 psig to 250 psig.
Note that 1 HP ~ 4 CFM at 100 psi and that 1 to 50 HP are typically for reciprocating units. Compressors 100 hp and above are typically Rotary Screw or Centrifugal Compressors.
The reciprocating air compressor is 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.
Load reduction is achieved by unloading individual cylinders. Typically this is accomplished by throttling the suction pressure to the cylinder or bypassing air either within or outside the compressor. Capacity control is achieved by varying speed in engine-driven units through fuel flow control.
Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations and provide a wide range of pressure and capacity selections.
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.
Rotary screw air compressors are easy to maintain and operate. Capacity control for these compressors is accomplished by variable speed and variable compressor displacement. For the latter control technique, a slide valve is positioned in the casing. As the compressor capacity is reduced, the slide valve opens, bypassing a portion of the compressed air back to the suction. Advantages of the rotary screw compressor include smooth, pulse-free air output in a compact size with high output volume over a long life.
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.
The centrifugal air compressor is a dynamic compressor which depends on transfer of energy from a rotating impeller to the air.
Centrifugal compressors produce high-pressure discharge by converting angular momentum imparted by the rotating impeller (dynamic displacement). In order to do this efficiently, centrifugal compressors rotate at higher speeds than the other types of compressors. These types of compressors are also designed for higher capacity because flow through the compressor is continuous.
Adjusting the inlet guide vanes is the most common method to control capacity of a centrifugal compressor. By closing the guide vanes, volumetric flows and capacity are reduced.
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 Right Compressor For You
Reciprocating Vs. Rotary Screw
This is a common question asked by most people looking for a new air compressor. Either compressor depends in your application.
If the application requires air all day or has multiple shifts that use air on a consistent basis, then a rotary screw is probably a better fit for the application due to its ability to run all day every day. Rotary Screw compressors are thermally controlled through an air cooled oil cooler. Some rotary screw compressor packages have built in timers that will turn the compressor off if it runs unloaded for an extended period of time (everyone goes to lunch, or it does not get turned off at the end of the day).
Reciprocating compressors are best suited for applications that require air for shorter durations of time and might have 15-30-45 minutes before they need air again. Because a Reciprocating compressor is air cooled, they need to enter an application with a duty cycle of 70% or less. Either package depends on making sure you purchase a properly sized compressor for your application.
Noise of Reciprocating Vs Rotary Screw
The noise of an air compressor package depends on how it is constructed. Most rotary screw compressors sold in the world today are built in sound dampening enclosures. These do not add to the performance of the compressor package, all they do is lower the DBA of the compressor package. Most reciprocating compressors are built open on a tank with no sound proof enclosure, which is why they sound louder than a rotary screw in a sound dampening enclosure. If you take a reciprocating compressor and build it in the same enclosure as a rotary screw compressor, you will get a similar DBA reading.
Some manufacturers make a reciprocating compressor muffler that can lower the DBA of the compressor. The enclosure for a rotary screw compressor is an added expense up front and to the yearly maintenance cost. It takes longer to service an enclosed rotary screw compressor than an open tank mount.