There are two main types of air compressor to choose between – a positive displacement compressor and a dynamic compressor.
If you’re unsure which is preferable for your business or intended applications, here we’ll explain the differences and how dynamic compressors work!
How are Positive Displacement and Dynamic Compressors Different?
The critical variance in these two air compressors is that they use a different process to perform the action of compressing the air.
- Positive displacement compressors physically reduce the size of the air with rotors or pistons.
- Dynamic compressors speed up the air, moving it at high velocity. They then restrict airflow, creating increased pressure.
One of the big positives to a dynamic displacement compressor is that they’re usually oil-free, which can be essential in sensitive production techniques or air processes that must be 100% contaminant free.
Modern dynamic compressors are typically fitted with an ultra-high-speed electric motor.
The motor drives the impellers and creates a compact compressor without requiring a gearbox and oil lubrication system, allowing an oil-free design.
You’ll find two main types of dynamic compressor, and we’ll explore these in a little more detail below.
Axial Air Compressors
An axial compressor isn’t commonly used in industrial applications and uses turbine blades to increase the speed of the air.
Axial compressors work by sending the oil or gas along a compressor shaft through rows of rotating blades.
That action increases the air velocity, with fixed blades also in place to convert the kinetic energy into pressure. The axial thrust is counterbalanced with a balancing drum.
Pressure in an axial compressor is low to moderate, and so this compressor is smaller and lighter than a centrifugal compressor but does tend to run faster.
They are usually used for relatively low-pressure applications but requiring constant or high-volume rates. This compressor type is typically found in ventilation systems, for example.
Axial compressors are also coupled with gas turbines to generate electricity and used in aircraft propulsion systems.
Centrifugal Dynamic Compressors
More often, you’d use a centrifugal compressor in industrial applications. This compressor works by:
- Sucking in air through the centre of a rotating impeller with radial blades.
- Increasing the velocity of that air at the same time as increasing pressure.
- Discharging air from the diffuser, converting kinetic energy into pressure energy.
Dynamic compressors work at a continual pressure level, making them more reliable than other equipment and a relatively straightforward design.
That reliability is owing to the lack of alternative components that might be exposed to repeated loads.
A dynamic compressor works like a turbocharger and can produce enormous horsepower, usually used in larger industrial processes.
In industrial machinery, a centrifugal compressor operates typically to a maximum pressure ratio of three, and any higher-pressure ratios can reduce efficiency.
Many of the applications are low-pressure, single-stage, such as wastewater treatment, or with multiple stages through a single, low-speed shaft in oil or gas industries.
Centrifugal compressors must, though, be sealed to minimise potential leakages around the shaft, where the air passes through the compressor housing.
Seals range in quality but can include ring seals, controlled gap seals, mechanical seals and labyrinth seals.
What Are the Advantages of a Dynamic Compressor?
We’ve touched on reliability, but there are a few other reasons a dynamic centrifugal compressor is a go-to piece of plant for large-scale manufacturers.
- They have a higher flow rate than positive displacement compressors.
- Work at a range of rotational speeds, generating high-pressure ratios compared to axial compressors.
- Don’t require any special foundations, are oil-free, and have few rubbing parts that need greater servicing and maintenance.
- Used for continuous compressed air supply, such as that required to run a cooling unit.
In terms of downsides:
- Centrifugal compressors need a larger frontal area and are sensitive to changes in gas composition.
- They work at high speeds, so they require vibration mounting.
- Not suited to very high compression applications.
What Are Dynamic Compressors Used For?
Sticking with a centrifugal compressor, as the most common type of dynamic compressor, you’ll find these used in a wide range of industries and sectors.
That includes oil-free air compression in food and drink manufacture, where air quality and condition is crucial to the production process.
The continuous air supply makes a dynamic compressor ideal for high demands and is commonly used in refrigeration and air conditioning.
Many manufacturing operations requiring pneumatic tools use a dynamic compressor, and they are also found in natural gas processing, oil refineries, and gas turbines fitted to turbochargers in vehicles.