Over the years, air compressors and air tools in general have evolved to play more integral and greater roles in the home and industrial applications. It is a source of compressed air for aerospace production devices and provides contaminant-free, clean pressurized air.
Regardless of which type of air compressor, all of them have one common function – converting a source of power into stored energy in form of compressed air.
Simply put, a compressor sucks in enough air and decreases its volume. As a result, both the temperature and pressure rises.
The aspect of decreasing the volume of air is the “compressing” that air compressors do. This principle is what positive displacement compressors use to increase pressure.
But a dynamic compressor (we’ll talk about this below) uses a method different from the one positive displacement compressors use.
Even though there are many different kinds of air compressors, we will focus on three in our discussion. One is the reciprocating compressor, the other is rotary screw compressor and the third one is the rotary centrifugal compressor.
Every type uses a distinct air compressor filter. Other classification may be based on:
• How each works (low, medium, high)
• The number of compression stages (single-stage, 2-stage, and multi-stage)
• Cooling method (air, water or oil)
• Drive method (direct-coupling, gear, turbine, chain, belt, motor, engine, steam)
• Lubrication method (either splash, forced lubrication, oil or oil-free)
Perhaps the above five points are the ones you should consider when buying an air compressor.
Each of these three air compressors has their own benefits, regardless of which one you use.
Table of Contents
- I. Benefits of Air Compressors
- II. Types of Air Compressors based on the operation principle
- III. Comparison based on popularity
- IV. The bottom line
I. Benefits of Air Compressors
Besides inflating tires, cleaning, and painting, drilling, air compressors are capable of doing so much more. With the right air tools, you can accomplish a lot just with a standard compressor. Here are some of the many benefits of having an air compressor:
1. Easy to use air tools
The compressed air – that an external compressor supplies – is the one that powers air tools. Hence, these tools will not need separate internal motors to work.
In addition, air tools are lighter, designed ergonomically and more compact than standard power tools. Therefore, you can use them for long without fatigue.
2. You’re no longer at the mercy of air pumps at the gas station
Some years back, air at most gas stations was free. Nowadays, that’s seldom on the contrary.
Even if you come across a gas station with a compressor, it is most likely coin operated.
But with own air compressor, you can fill tires at the comfort of your house, with no inconvenience. Just fix a tire inflator with your gauge.
3. Air tools are highly economical
When air tools are continuously used in industrial or commercial environments, they consume lesser electricity than their mains-run counterparts.
Moreover, air tools are more powerful especially when you want to loosen or tighten a fitting.
4. Air compressors are useful in other ways
Even more, you can use it as a propellant for applicators and airbrushes to spray different substances. For instance, sandblasters can help you remove coatings or rust quickly when you depend on compressed air.
Closing a pond or swimming pool for certain season needs you to flush water out of the area. Indeed, a compressor will help you do this easily.
With this in mind, let’s discuss the 3 different kinds of air compressors.
II. Types of Air Compressors based on the operation principle
Air compressors can be categorized in different ways, the common one being based on principle of operation. In that case, we get:
- Positive displacement
- Roto-dynamic compressors
Positive displacement air compressors can be further categorized into:
- Rotary compressors
- Reciprocating compressors
Probably, this is the kind that you’re familiar with. As mentioned earlier, this one traps air and decreases its volume.
Under reciprocating compressors we have:
- Diaphragm compressors
- In-line compressors
- “V”-shaped compressors
- Tandem piston compressors
- Single-acting compressors, and
- Double-acting compressors
On the other hand, rotary compressors are divided into:
- Lobe & scroll compressors
- Vane type compressors
- Screw compressors, among others
As for the second category of Roto-Dynamic compressors, the working principle is different. It uses an impeller in its rotating element.
As a result, air velocity increases and converts to pressure by slowing it via a diffuser.
Under this category, we have:
- Centrifugal compressors
- Axial flow compressors
In our discussion, we will focus on three types: reciprocating, rotary screw and rotary centrifugal compressor.
1. Reciprocating (piston) compressor
Just as a rotary screw compressor, the reciprocating compressor is also a positive displacement machine. This simply means the compressor boosts air pressure by decreasing its volume.
These compressors take a high volume of air and confine it in a closed tank. Consequently, the machine elevates this air to a high pressure.
A reciprocating compressor does so with the help of a piston. The compressor achieves rotary motion by the use of an electric motor.
The piston moves up & down inside a cylinder. Vapor coming from the suction line moves via the intake valve whenever the piston moves down.
When the piston moves up, it squeezes the vapor refrigerant. This vapor is then pushed via the exhaust valve into the condenser.
Reciprocating compressors are either water-cooled or air-cooled in non-lubricated and lubricated configurations. Hence, they produce a broad range of capacity and pressure choices.
Both two stage and single-stage reciprocating compressors are available in the market.
A reciprocating compressor is single acting when it compresses air only from one side of its piston. This category of compressors is normally used for pressures that range from 70 psi to 100 psi.
Alternatively, a compressor that uses both sides of the piston is double acting. In like fashion, a two-stage air compressor is used in operations that need high pressure between 100 psi and 250 psi.
1 HP to 50 HP is normally for reciprocating compressors. On the same note, compressors with 100 HP or more normally are rotary centrifugal or screw compressors.
Both the single-acting and diaphragm are the same. The only difference is in how the piston moves.
Here the piston moves the diaphragm that expands and contracts (as opposed to it moving on the inside of a container).
Maybe you have come across a diaphragm pump in a place – pumping water.
Pros of Reciprocating Compressor
- The compressor has many uses both industrial and home
- Low installation cost
- Low maintenance costs, hence popular in gas and oil operations
- Able to produce high pressure (400 bar) and power (over 500hp)
- These compressors do not have problems related to oil carry-over
- Easy to operate/repair
Cons of Reciprocating Compressor
- The most expensive of the three types
- Requires constant inspection for it to last long and withstand internal pressures
- Low reliability since the parts are prone to wear & tear
- The large size, volumes, cost, and number of cylinders makes this compressor unattractive
2. Rotary screw compressor
As mentioned, rotary screw compressors are positive displacement. They are the workhorses in many manufacturing industries.
If you come across a huge manufacturing building, most likely, there’s a rotary screw compressor running their production process. And there’s some justification to this.
For one, an industrial rotary screw compressor has a hundred percent duty cycle. It can operate 24/7 continuously. To be precise, it lasts longer and works better when used that way.
Unlike compressors that use pistons, rotary screw compressors do not like stopping and starting regularly.
How rotary screw compressors work
The spiral lobe oil flooded and single-stage helical are the most popular types of screw compressors.
On this type, 2 spinning helical screw/rotors help to compress air (as opposed to pistons): hence the name.
These air compressors have 2 rotors in a casing. These rotors will compress air internally. It doesn’t have valves.
These compressors are oil cooled (with either water cooled or air cooled oil coolers). The oil seals any internal clearance.
The operating pieces do not experience high temperatures as the cooling occurs on the inside. Therefore, the rotary compressor is a continuous-duty, water cooled or air cooled package.
For effective operation, a rotary screw compressor uses a special filter. That is why operating and maintaining this kind of compressor is easy. The variable displacement and speed are in control of the capacity for this compressor.
One of the variables is a slide valve inside the casing, which controls its displacement. When the capacity for this compressor reduces, the slide valve will open. As a result, some compressed air passes back to the suction.
Rotary screw air compressors have several advantages such as pulse-free, smooth air output in a compressed size and at high output volume.
Rotary screw compressors can either be oil-flooded or oil free. For an oil-free device, it uses specifically designed air-ends to compress air. Thus the air produced will be truly free of oil.
As for the oil-lubricated screw compressor, the engine or motor drives its male rotor. Following, this male rotor drives the female rotor. Simply say, it is driven by the thin film of oil between the two.
Besides driving these rotors, the oil seals the compression chamber whilst acts as a coolant.
Pros of Rotary screw Compressor
- The initial installation and purchase price for a rotary screw compressor is lower than that of a reciprocating compressor
- Taken care of in the right way, this compressor can last around 2 to 5 times longer
- Uses low energy as the compressor operates at the low stage – at least most of the time
- Its cooling cycle can last long
- For the oil-cooled screw compressor, it doesn’t generate any hot spot as the cooling occurs on the inside
Cons of a Rotary Screw Compressor
- Compared to piston compressors, screw compressors are pricey
- The screw compressors with lower than 2000ltrs for every minute requires more maintenance than piston compressors
- If maintenance is ignored or non-standard parts and inappropriate oil is used the compressor will be highly vulnerable
- The wheel for this compressor also needs an experienced person to maintain it
3. Rotary centrifugal compressor
The rotary centrifugal compressor is a dynamic one. It relies on energy transfer from the rotating impeller to the air. This impeller is a disk that has radial blades. It spins forcefully inside the cylinder.
As a result of these spins, the gas gains velocity. At this stage, a diffuser then changes the energy into pressure energy. Then, this pressure energy moves into a condenser.
As speed increases, the same case applies to the pumping efficiency. Therefore, a centrifugal compressor is built to run at high speed.
The best part about centrifugal machines is that they do not have cylinders, pistons or valves. Thus you’ll only pay attention to the main bearings – in case they wear out.
The centrifugal air compressor can either be a single-stage or multi-stage. At times it can be 3-stage, whose efficiency is higher than other positive displacement and screw types.
A single stage consists of one impeller. It moves air up to 3:1 compression ratio for either vacuum or pressure duty. It has an overhung impeller or beam design arrangement, which is located at the no-driving part of the shaft.
A single stage compressor is more beneficial over a multistage one. It gives high efficiency and delivers surge or oil-free gas.
On one hand, multistage compressors have 1 to 10 impellers. They are arranged in different configurations. Unlike single stage, the compression ratio and temperature here ought to remain the same in every stage.
A multi-stage compressor can take different arrangement: double-flow, compound or straight-through.
Centrifugal compressors are oil-free by design. Atmospheric vents and shaft seals separate the oil-lubricated operating gear from the compressed air.
Pros of Rotary centrifugal Compressor
- Low weight, easy to manufacture and design
- Ideal for the continuous supply of compressed air like cooling unit
- It is oil-free in nature
- The rubbing parts are few
- Unlike positive displacement air compressors, centrifugal compressors have a high flow rate
- Relatively efficient in terms of energy
- They require low maintenance and are highly reliable
- In comparison to axial flow air compressors, centrifugal compressors have a high-pressure ratio for each stage
- Hardly needs a special foundation
Cons of Rotary centrifugal Compressor
- Since their pressure is limited, the compressor isn’t ideal for really high compression
- It encounters the problem of choking, stalling and surging
- Since it operated at high speed, it needs stylish vibration mounting
- It doesn’t welcome changes related to the gas composition
III. Comparison based on popularity
Both rotary screw and rotary reciprocating are the most popular types of air compressors. Centrifugal compressor is least popular.
Some of the aspects that make reciprocating air compressors most popular are:
• Price: least expensive and most economical
• Application: small shops, body shops, tire shops, small manufacturing facilities
• Ideal for: only for intermittent use (duty-cycle of 50-60%)
Rotary screw compressor
• Applications: any operation in need of 100% continuous duty-cycle
• Price: quite pricey, but the energy savings are high
• Ideal for: any operation in need of a constant supply of compressed air
These are some of the aspects of a centrifugal compressor:
• Application: at full capacity, it is highly effective where demand is constant
• Unlike rotary screw compressors, centrifugal ones are more efficient
• Price: far costlier as compared to the other 2 types
• Ideal for: since it’s the largest among the 3 types, the centrifugal design is ideal for large machinery and industrial compressors used in the production processes.
As you have seen, air compressors differ in many ways. Each has their own lubrication method, compression stages, and cooling method. In addition, they differ based on how each works and drive method.
The popularity of each type of compressor depends on these differences and their functionalities.
Therefore, you should select the right type based on your need for it. Consider the air supply you require. Do you want pressurized air for specialized uses?
Air is measured in cubic feet per minute. You should check this in the specifications. Each compressor type is meant for different applications. In detail:
Rotary screw compressors are normally used for prolonged applications with low-pressure/high velocity – 7/8 bars.
An oil-free rotary screw compressor is best for applications that need zero contact with oil.
On the same note, an oil-injected rotary screw compressor is more energy efficient than the piston type. In like manner, its noise level is low and provides huge amounts of pressurized air at a relatively low temperature.
Finally, a reciprocating compressor is ideal for high-pressure/low-velocity applications – below 30 bars.
With all this information and specifics of major types of compressor, you are now in a better position to make an informed decision on which air compressor will suit your needs.