Compressed air is one of the most popular ways for industry to store energy for later use. In fact, it is often referred to as the fourth utility.
Air pressure is a safe and effective way to power a range of equipment and processes, particularly in environments where it could be difficult or hazardous to supply electrical energy directly to machinery, power tools or spray guns.
It is estimated that up to 75% of industrial processes and applications use stored energy provided by compressed air at some stage of their operations.
Air compressor basics
Compressed air systems create stored energy by compressing ambient air beyond the normal atmospheric pressure. The operating principle is for the air compressor to force air into a container and then compress this to build up pressure.
It’s just like blowing up and releasing the air from a balloon, but the released air is then used as energy to power equipment. The air compressor engine effectively turns electrical energy into kinetic energy (or the energy an object has because of its motion).
So in essence, most compressors consist of three key components. The drive activates the system pump. The pump takes energy from the drive to compress the atmospheric air. This compressed air is then passed from the compression chamber into a storage or holding tank, providing temporary energy storage which can be released as power when required.
What is air displacement?
Air displacement, measured in CFM (Cubic Feet per Minute) refers to the nominal capacity of any given air compressor pump. It reflects the volume of air the pump is capable of drawing into the compressor per minute.
This figure will differ dependent on the size and speed of the pump. It does not equate to ‘delivered’ or ‘useable’ air.
Positive displacement compressors
Positive displacement compressors such as rotary screw compressors work by using a drive to force air into a container, or chamber. The volume of this chamber is then mechanically decreased thereby compressing air held there.
There are many different types of positive displacement air compressors as detailed below. Choosing the right compressor is dependent upon application; some are designed for industrial workloads and others for smaller operations.
Dynamic displacement air compressors
Dynamic displacement air compressors use power from an engine to drive a rotating blade which generates airflow. This air is then restricted creating pressure rises, and the resultant kinetic energy is stored in the compressor. Dynamic displacement compressors a primarily used for heavy-duty industrial applications.
Types of positive displacement air compressors
These types of air compressors include the reciprocating compressor, rotary vane compressors, scroll compressors and rotary screw compressors.
Rotary screw compressors
Rotary screw air compressors are one of the most common forms of air displacement type compressors.
The rotary screw compressor operates using two interlocking helical screws or rotors moving in opposite directions, effectively trapping air in the space between and compressing this as the space reduces, resulting in a rise in air pressure which is then available for discharge to power various applications.
The rotary screw air compressor is ideal for continuous use and heavy-duty applications. Rotary screw benefits include safety, resistance to temperature fluctuations, ease of maintenance, high airflow rates, quiet operation and longevity in use.
Rotary vane compressors
Instead of using a pumping motion to compress air, rotary compressors use a spinning movement, thereby causing centrifugal forces. With this type of air compressor, a cylindrical rotor with a number of different partitions is mounted off-centre (eccentrically) in the housing.
Air from intake valves is trapped between the partitions as the rotor spins and is squeezed into increasingly small spaces, resulting in pressurisation.
The pressurized air exits via a discharge valve or exhaust port ready for use. Rotary vane air compressors are very efficient, with the flexibility for use in diverse applications.
Reciprocating Piston Compressors
Reciprocating piston oil-lubricated compressors operate using the positive displacement principle using pistons and crankshafts.
On reciprocating compressors, the air is drawn into the cylinder via the inlet valve. A crankshaft then powers the piston, compressing the air. This pressurized air is then forced through a discharge valve for use as energy.
Delivering powerful compression, the reciprocating air compressor delivers powerful compression. They are efficient and flexible types of air compressors and provide a steady flow rate ideal for high-pressure, low-flow rate applications.
Single Stage Compressors
The reciprocating piston air compressor may be single-stage or two-stage. Single-stage piston air compressors compress the air on one side of the piston. Single-stage units are typically inexpensive and easy to use and maintain.
The two-stage piston compressor has compression chambers on either side of the piston. They typically deliver more CFM (Cubic Feet per Metre) than single-stage models, as well as generate less heat due to the intercooling stage.
Two-stage models are more costly than single-stage options and suitable for commercial applications.
Types of dynamic displacement air compressors
Typically, dynamic displacement air compressors include centrifugal and axial machines. These are generally specified for large industrial or manufacturing facilities requiring significant amounts of energy to power their processes.
This type of air compressor is typically used for high-power applications such as power generation turbines and jet engines. They consist of fixed and moving sets of blades. The fixed blades attach to the outer stationary casing (stator) and the moving blades to a rotor hub.
The fixed blades or vanes guide the air to the moving blades. The two sets are arranged to form diverging passages to the flow slows as it goes through, causing an increase in air compression. Axial compressors are multi-stage designs.
Positive displacement axial flow compressors are characterised by a high flow rate and pressure ratio, with high levels of efficiency and a high thrust rate to the frontal area.
Sometimes referred to as radial compressors, centrifugal air compressors operate by drawing air into the centre of a rotating impeller.
The centrifugal force draws the air in and passes it through a volute and diffuser before pushing it towards the centre, with the radial movement causing a rise in pressure and the subsequent generation of kinetic energy.
Centrifugal compressors are usually multi-stage, with each stage adding to the pressure rise. These compressors are commonly used in the oil and gas and process industries.
Centrifugal, or radial compressors, work by bringing more air into the centre through a rotating impeller, which is then pushed forward through centrifugal, or outward, force. By slowing the flow of air through a diffuser, more kinetic energy is generated.
Wrapping things up
Air compressors work to convert electric motor, gas or diesel power into pressurised air. Most air compressors are portable, low maintenance and reasonably quiet in use. They play an important role in today’s factories and workshops, providing a safe and reliable continuous stream power for equipment like pneumatic tools.
Alongside safety and reliability, air compressor systems are clean and dry, as well as easy and affordable in terms of installation. They can be adapted to provide multiple points of use. Compressed air can be stored in a tank and can be used even under extreme temperature conditions.
Air is clean, without oil contamination, which can be vital for industries where air quality plays an important role. Here, oil-free compressors are an important consideration.
Air compressors work to suit almost every operational requirement. What’s more, regular maintenance and professional optimisation of air compressor systems can result in significant annual savings on compressed air operating costs.