With multiple types of air compression equipment available, it’s essential to evaluate the functionality of the compressor you require.
One of the options is a positive displacement compressor. That includes:
- Rotary screw compressors
- Reciprocating piston compressors
- Rotary vane compressors
The principle function of a positive displacement compressor is that it draws air into the chamber, which is then reduced in volume to compress that air.
Positive displacement compressors differ from dynamic compressors since they work with constant airflow.
Here we’ll run through the different types of positive displacement compressor and how they work!
Rotary Screw Compressors
A rotary screw compressor is one of the most popular industrial air compressors and typically ranges from 25 to 300 horsepower.
There are also much smaller and larger rotary screw compressors on the market, going right up to 600 HP.
This positive displacement compressor draws air in along with lubrication with two helical rotors. Those rotors decrease the cavity’s size, pass the air-end, and compress the mixture of oil and air.
Oil in a rotary screw compressor lubricates the components, seals off air pockets, collects contaminants, and helps to absorb heat.
Coolant oils can remove up to 80% of the compression heat, which means lower operating temperatures.
Benefits of Using Rotary Screw Compressors
- Less oil consumption than an oil-flooded air compressor.
- Minimal oil carryovers.
- Good at conserving energy, with less heat produced.
Given this lower heat positive displacement compression technique, a rotary screw compressor can operate at higher volumes without damage.
Of course, the oil in the air needs to be removed before it leaves the compressor. That requires an oil removal filter, which needs replacing regularly.
Reciprocating Piston Compressors
Another type of positive displacement compressor relies on piston movements and can be either single-acting or double-acting.
Single-Acting Reciprocating Piston Compressors
A single-acting compressor draws in air and compresses it on one side of the piston. The other side is open to the compressor crankcase.
As the downward piston movement draws air in, the upward motion compresses it.
Most single-acting piston compressors are air-cooled and up to 25 HP, so used in:
- Light industrial applications.
- Domestic processes.
- Automotive sectors.
Single-acting compressors shouldn’t be used for over half an hour at a time and need that break to ensure they don’t overheat.
Double-Acting Reciprocating Piston Compressors
In contrast, a double-acting positive displacement piston compressor has two compression chambers, one on either side of the piston.
When the piston moves down, it draws in the air on the top of the piston, compressing air beneath it.
As that piston returns upwards, it draws air into the bottom end, compressing air on the top. The piston rod on a double-acting compressor must be sealed, so they have a crosshead to ensure the rod cannot move at an angle.
Double-acting compressors differ in applications:
- They can range from 40 HP to over 1,000 HP.
- Most are water-cooled.
- Double-acting piston compressors cool faster.
- Operation times are substantially longer.
The downside of a double-acting compressor is that while it can work to greater capacity, they are relatively expensive to manufacture and require specialist foundations and maintenance owing to the vibration.
Rotary Vane Compressors
This type of air compressor is an oil-injected machine and operates similarly to a rotary screw compressor.
It compresses air in the chamber with a rotor, powering sliding vanes. Those vanes rotate and close the space between the rotor and casing to compress the air.
These positive displacement compressors also use separators and oil system components.
Some of the typical uses for a rotary vane compressor include:
- Air compression in agricultural equipment.
- Food and drink production.
- Welding, tyre inflation and air tools in automotive applications.
- Dry cleaning equipment.
The advantage of using a rotary vane compressor over a rotary screw compressor is that they are simple, durable, and efficient.
This more conventional positive displacement compressor has been around for a long time and is volumetrically more efficient given the lower internal air leakage.
Screw compressors run faster to compensate for air leakage, usually due to the ‘blow hole’ where the two rotors meet. Pressurised air escapes through this outlet when returning to lower pressure.
Are All Positive Displacement Compressors Oil-Free?
Most positive displacement compressors are oil-free or oil-less, which isn’t quite the same thing.
In essence, there isn’t any lubricant injected into the air.
Oil-less means that there is no oil involved in the process at all.
Oil-free means that the compressor has a lubricated crankcase, but this is separated from the compression chamber to avoid contamination.
While there are multiple benefits to minimising oil in air compression – primarily in preserving air condition – machines without oil tend to be slightly less efficient and need more regular maintenance than positive displacement compressors with oil lubrication.