There are many different kinds of compressors, such as scroll compressors, reciprocating compressors, rotary screw compressors, and centrifugal compressors, but they all do the same thing: compress air.
But how exactly do air compressors work? How does air get compressed differently depending on the compressor type? And what's the big deal, anyway?
Let's start with the last inquiry and address it.
The size of the air stream is dependent on the intended usage of the compressed air.Pressure, velocity, and quality of the surrounding air are the most important of these factors. All compressors similarly compress air, but the specific procedures that each type takes to do so are unique. The type of compressor you need is partly determined by the purpose for which the compressed air will be used, as the aforementioned variations can limit the practical values for various dimensions of the air stream they create.
The first question may be revisited now that we have a better understanding of why the compressor type matters. But before we get into that, let's quickly examine where air compressors obtain their power from.
Compressors' Primary Energy Sources
Compressed air can be used to power a wide variety of tools and instruments, but the compressor itself needs to be powered to function. Compressors typically draw power from the following sources:
Smaller outdoor compressors are often powered by gasoline engines. Typically, their output is less than 50 cfm (1.4 m3/min), which is measured in cubic feet per minute. They are carried by hand (often in a huge suitcase-style container on wheels).
Larger compressors used in outdoor settings, such as commercial construction, are often powered by diesel engines. Compressors that are permanently installed and cannot be wired to an electrical outlet are included. Diesel fuel has more energy per volume than gasoline, making diesel engines more fuel-efficient.
Compressors used indoors are powered by electric motors, and their applications range from DIY tasks to large-scale industrial operations like fabrication in factories. Single-phase power is typically used for compressors in the home and small shops, whereas three-phase power is used for larger shops and industrial activities.
Compressing Air: The Fundamentals
Intake, compression, integrated storage, integrated cooling, and discharge are the main stages of air compression, though not all compressors require integrated storage or cooling.
1. Intake
The first step in the process of compressing air is, of course, taking in air. An air inlet valve allows outside air to enter the compressor during the intake process.
A filter is typically installed before the air inlet valve to protect the compressor from debris and dust.
2. Compression
The compressed air then exits the compression chamber.
• When air is compressed, kinetic energy from the power source is transformed into potential energy.
When we get to the compressor kinds, we'll take a closer look at how each kind of compressor accomplishes this, but for now, know that two basic ideas underpin all compressors.
Displacement
Compressors use either positive displacement or dynamic displacement (sometimes called nonpositive displacement) to squeeze air into a smaller volume.
• Compressors that use positive displacement to squeeze more air out of a given space.
• Displacement in motion Compressors (also known as non-positive displacement) are devices that raise atmospheric pressure by increasing the air's velocities (and thus its kinetic energy) and then slowing it down again.
Oil Lubricated vs. Oil Free Compressors
All compressors, regardless of the method employed, are either oil-lubricated or oil-free.
• Compressors that are oil lubricated (also known as oil-flooded compressors) use oil in the compression chamber for a variety of purposes, including lubrication, sealing, and cooling. Compressing air has the consequence of introducing a trace amount of oil into the air itself. Depending on the end goal, this may not be the best use of the compressed air.
• When oil is not used in the compression chamber, as in oilless compressors (also known as oilless compressors), no oil is released into the airflow.
However, that does not rule out the possibility of oil being used in any aspect of the compression mechanism (bearings, for example, require oil to function effectively).
3. Integrated Storage
After compression, some compressors direct the compressed air directly into an inbuilt receiver tank (also known as a storage tank or air tank).
Compressor types will be discussed in detail, as will many of the applications of integrated storage. However, two explanations hold across the board.
• Compressors with a limited duty cycle, as opposed to a continuous duty cycle, include built-in storage to ensure that air is always available, even during maintenance.
• To have access to air while operating at reduced capacity, modulating compressors (compressors with controls that allow for reduced capacity operation) incorporate storage.
Keep in mind that modulating compressors are not the same as those with a limited duty cycle.
• Compressors with a limited duty cycle will shut off if they are left on for too long. (While this isn't how large-scale commercial and industrial compressor’s function, it's typical for smaller compressors used in homes and workshops.)
• Modulating compressors can operate continuously, if necessary, but can also reduce their output to save power during periods of low demand.
4. Integral Air Conditioning
When air is squeezed, it generates heat. Although it is not necessary to cool the air before it is discharged from the compressor, most three-phase electric compressors and certain diesel compressors (of any kind) feature built-in aftercoolers to do so.
Water separators are installed in compressors with aftercoolers to collect the moisture that condenses from the cooled air stream.
5. Discharge
After passing through a series of dryers and filters (for example, instrument air in a manufacturing plant), the air is released through the discharge valve and sent on its way.
Compressing Air with Various Compressors
All air compressors work similarly, yet there are significant differences in the finer points. Let's take a quick look at how four widely used methods of compression function and how that impacts essential tasks.
1. Compressors for Scrolls
Positive displacement, oil-free compressors are known as scroll compressors.
Compression
A scroll compressor has two scrolls that are interleaved to compress air. The design may call for both scrolls to rotate in tandem or for one to remain stationary. No oil is required because the scrolls are kept apart at all times.
Since the scrolls move in a continuous, unbroken motion, scroll compressors operate silently, with minimal vibration, and without producing any pulses in the airflow.
The greatest airflow that can be handled by a scroll compressor is a significant drawback. Although a scroll compressor can theoretically be scaled indefinitely, its practical limit (at least in terms of efficiently producing air) is determined by the diameter of the scrolls required. Their maximum flow rate is the lowest of all the compressor types discussed here.
Integrated Memory
Scroll compressors don't need inbuilt storage unless they utilize modulation since they produce pulse-free, oil-free air.
2. Rotary-Vortex Compressors
Oil-lubricated or oil-free, reciprocating compressors (also known as piston compressors) are positive displacement compressors.
Compression
The compression chamber in an automobile engine is a good analogy for a reciprocating compressor. While the piston is moving upwards, a vacuum is formed, allowing air to enter the compression chamber. The air is squeezed and expelled from the chamber on the downstroke. Some designs use two stages of compression, in which the air crushed in the first stage is further compressed in the second stage, to efficiently attain higher pressures and larger volumes.
Piston compressors are the loudest of the compressor types discussed here, similar to the sound of an automobile engine.
The pistons' smooth motion is essential for their continued efficiency. They must be lubricated, allowing a trace quantity of oil to be released into the air, or coated with a material that minimizes friction.
Air leaving the chamber has a "pulse" rather than a steady flow and pressure because compression occurs only during half the duty cycle.
Integrated Memory
Reciprocating compressors always need integrated storage, even when they don't use oil, because their compression mechanism causes a pulse in the air stream. By drawing air from the receiver tank rather than the compression chamber, a steady flow and pressure can be maintained in the air stream.
3. Screw Compressors That Rotate
Oil-lubricated or oil-free, rotary screw compressors are positive displacement compressors.
Compression
Compressors that utilise rotary screws have two helical screws (rotors) with lobes throughout their length. Compressing the air involves forcing it from a bigger to a smaller volume as it travels along the length of the compressor's lobes. As with reciprocal compressors, some designs employ two stages of compression to efficiently reach higher pressures and larger volumes
A little quantity of oil is introduced into the air stream, although this is negligible in oil-lubricated screw compressors because the fluid used to seal the gaps between the rotors also permits one rotor to drive the other.
Oil-free screw compressors eliminate the need for fluid by relying on timings with extremely close tolerances between the rotor lobes.
The continual rotation of the rotors results in a steady flow of air with no noticeable fluctuations. Compressors of this type are quieter than reciprocating ones, although still not as quiet as scroll compressors.
Integrated Memory
Whether a rotary screw compressor is oil lubricated or oil-flooded determines whether or not it requires integral storage.
• To recycle part of the oil that is lost in the compression process, oil-flooded screw compressors require inbuilt storage in the form of a receiver tank.
• Because there is no oil to recapture, oil-free screw compressors do not require inbuilt storage unless they employ modulation.
4. Rotary Screw Compressors
Oil is not required in centrifugal compressors because of their dynamic displacement design.
Compression
For the initial boost in airspeed, centrifugal compressors rely on a rapidly spinning impeller. The air is then forced through a diffuser, where its speed is reduced and its pressure increases. Since oiling the air stream is unnecessary, it is not done in the compression chamber. Air is constantly being generated while the impeller rotates, therefore there is no pulsating in the airflow or pressure.
Integrated Memory
Unless modulation is used, integral storage is not necessary for centrifugal compressors because they produce pulse-free, oil-free air.
Do Your Research Before Making a Purchase
While familiarity with compressor operation helps narrow down potential models, there are other factors to think about, such as energy efficiency, maintenance costs, expected uptime, and so on. Choose the ideal compressor for your needs with the help of your local distributor's expertise.
