The two most prominent are piston and rotary screw compressors.
Piston compressors are the oldest and most common of all industrial compressors. They are fairly simple machines (a piston is used to compress air in a cylinder) that are often used by operations with a limited, intermittent compressed air demand. High-pressure compressors generally also rely on piston technology.
Their lower purchase price makes them an attractive option for do-it-yourselfers and small workshops. However, they typically cannot run continuously for long periods of time and have to cool down periodically. In addition, they feature a lot of moving parts, which means they are more subject to wear and tear and may require quite a bit of maintenance.
That is why many companies with a higher compressed air demand often turn to rotary screw compressors.
As the name indicates, there are two screws at the heart of these machines. These are also referred to as male and female rotors and they are driven either by the male rotor (if they are oil-injected) or by a timing gear (if they are oil-free).
In either case, the male and female rotors are spinning in opposite directions. As a result, air is drawn in-between them. As their cycle progresses, the decreased space between the rotors and their housing leads to air compression. This compressed air is displaced to the outlet.
Compared to piston compressors, screw compressors offer many advantages. While a little more expensive to buy, they can run continuously for many hours, are much more efficient, emit less noise (and can therefore be installed in more places) and do not require as much maintenance.
When looking to invest in a compressor, keeping in mind the principle of the total cost of ownership is extremely important and will likely save you a lot of money in the long run.
Total cost of ownership … what does that mean?
With total cost of ownership, we are referring to all of the costs associated with a product. In some cases, that primarily covers the purchasing price. For example, when you buy a couch, then you usually only have to pay for it and there are no other costs associated with that purchase (possibly apart from having to pay people to move it).
This is very different for compressors.
In addition to these two types of compressors, there are many others that are less frequently used.
Here is a brief overview of them:
Diaphragm compressors
In these compressors, the diaphragm is actuated mechanically or hydraulically. While mechanical diaphragm compressors are used with a small flow and low pressure or as vacuum pumps, hydraulic diaphragm compressors are used for high-pressure applications.
Tooth compressors
Like screw compressors, tooth technology features two rotors. They rotate in opposite directions inside a compression chamber. The rotors, which are always synchronized, are controlled by gear wheels. When they turn toward each other, they draw in air that is then compressed before being forced out through an outlet port.
Tooth compressors are often used when especially pure air is needed, for example in the health care or pharmaceutical sectors, whenever the air comes in touch with food or in the electronics industry.
Scroll compressors
Scroll compressors also rely on positive displacement. The compressor element consists of a stator spiral fixed in a housing and a motor-driven eccentric, orbiting spiral. The spirals are mounted with 180° phase displacements to form air pockets with a gradually varying volume. When the orbiting spiral moves, air is drawn in and is captured in one of the air pockets, where it is compressed gradually while moving towards the center, where the outlet port and a non-return valve are situated.
Scroll compressors are efficient and quiet. They are often used for cooling processes – from residential air conditioning units to industrial refrigeration solutions.
Vane compressors
Vane compressors consist of a rotor with radial, movable blade-shaped vanes (usually made of special cast alloys) that is eccentrically mounted in a stator housing. When it rotates, the vanes are pressed against the stator walls by centrifugal force. Air is drawn in when the distance between the rotor and stator increases. It is then captured in the different compressor pockets, which decrease in volume with rotation. The air is discharged when the vanes pass the outlet port.
Vane compressors have many applications. They are used everywhere from the agricultural and energy sectors to the automotive industry. In addition, vane compressors also have pharmaceutical, medical and dental applications.
Roots blowers
Roots blowers are valve-less displacement compressors without internal compression. Because they are not very efficient, they are only used for low-pressure applications.
A roots blower is comprised of two rotors that spin in opposite directions. Once the compression chamber comes into contact with the outlet port, compressed air flows back into the housing from the pressure side. Then, as the volume of the compression chamber decreases with continued rotation, further compression takes place.
While the above are all displacement compressors, there are also different dynamic compressors.
Centrifugal compressors
These compressors are characterized by their radial discharge flow. First, air is drawn into the center of a rotating impeller with radial blades. Then it is pushed out towards the perimeter of the impeller by centrifugal forces.
The radial movement of the air simultaneously results in a pressure increase and the generation of kinetic energy. Next, before the air is led to the center of the impeller of the next compressor stage, it passes through a diffuser and a volute. This is where the kinetic energy is converted into pressure.
The impeller shaft reaches extremely high speeds. At 15,000-100,000 rpm, these are much higher than those of the aforementioned compressors.
Axial compressors
These compressors have an axial flow, which means the air passes along the compressor shaft through rows of rotating and stationary blades. As a result, the velocity of the air is gradually increased at the same time as the stationary blades convert the kinetic energy to pressure. A balancing drum is usually built into the compressor to counterbalance axial thrust.
Axial compressors are generally smaller and lighter than their equivalent centrifugal compressors and normally operate at higher speeds. They are used for constant and high-volume flow rates at a relatively moderate pressure, for instance, in ventilation systems. Given their high rotational speed, they are ideally coupled to gas turbines for electricity generation and aircraft propulsion.
The search for the optimal compressor begins with determining what size it should be. This is so important because of the amount of energy it takes to compress air. If the compressor is too large, you risk wasting a lot of energy (especially if you choose a fixed-speed model). And that will add a lot of unnecessary expenses to your operation.
If the compressor is too small, on the other hand, you will not be able to meet your demand. That is also why it is important to take into account any future expansion. By factoring in your future compressed air needs, you can avoid having to purchase another compressor again too soon.
But how do you properly size a compressor?
