McMillan High Pressure or Standard Pressure

McMillan High Pressure or Standard Pressure

So what is the difference, and when would high pressure be required? What is a ‘Two Stage’ compressor’ ? These are questions our customers commonly ask, so here’s the gist:

Relating to pressure there are 2 types of piston compressor pumps – single stage and multi stage. Standard pressure of around 120psi can be achieved with single stage compression, whereas higher pressures require 2-stage compression to achieve around 160psi efficiently. In our market, large truck tyre inflation is the main application which needs high pressure, 2-stage compressors.

Single Stage for Standard Pressure

Operating in a range of 90-120psi (cut-in/cut-out). Suitable for most air tools, spray painting, actuators, valves etc. provided appropriate flow rate is selected.

Single stage, compressor. Notice that each piston has an intake filter.

Two Stage for High Pressure

Operating in a range of 130-160psi (cut-in/cut-out). Suitable for large truck tyres, and applications that require minimum pressure in excess of 120psi. Also suitable for all standard pressure applications, provided appropriate flow rate is selected.

Two stage, compressor. Notice that only the low pressure cylinders have intake filters.

How to identify a two stage pump.

Notice on this 3-cylinder Truck Plus pump the high pressure cylinder has no inlet filter and has an inter-stage safety valve ‘upstream’ of the high pressure cylinder.

The upright style pumps may also be either single or two stage. See the piped inter-cooler with the inter-stage safety valve ‘upstream’ of the high pressure cylinder on this two-stage ABAC pump.

The Inter-stage Safety Valve

Also known as the inter-cooler safety valve, this is more of a ‘tell-tale’ than a safety device. It is set to blow off at 4.5bar / 65psi as an indication that the the pump is ‘single staging’. This means the low pressure piston is working to a higher pressure than intended – due to a blown head gasket or HP valve failure. So when this valve blows it does not need to be replaced, the valve gasket or valve parts need attention.

Understanding the difference.

Single Stage

This means compression is achieved in a single piston stroke. Multiple pistons does not mean the compressor is two stage. Single-stage compressors may have 1, 2, or more cylinders. As a general rule 1 intake filter per piston indicates single stage on the cast-iron V-twin and W-triple pumps. On the upright pumps, the absence of an inter-cooler pipe is a clue that it is single-stage. In all cases the single stage pump will have pistons that are the same size.


Compression is achieved in 2 stages. The 2nd stage piston is always smaller than the 1st stage. There would be an inter-cooler pipe between stages and an inter-stage safety valve would be visible before the 2nd stage piston.

Tyre pumping on large trucks

High pressure is important as it is the differential between the compressor tank pressure and the required tyre pressure which determines the speed at which the tyre is inflated.
Let’s say large truck tyres require pressure of 110psi (sometimes higher). The McMillan two-stage air compressors have an operating pressure range of 130psi to 160psi. This means there is always a large difference between the compressor tank pressure and the requirement in the tyre. Remember, the larger the differential between tank pressure and required tyre pressure, the faster inflation will occur

Two-stage compressors can be used for regular applications. Air pressure regulators (which reduce the pressure of compressed air from the tank to the lower pressure required for the air tools being used) are fitted to all McMillan 240V compressors. However, trying to adjust your single stage compressor to achieve higher pressure is both dangerous and harmful to the compressor, so don’t do it!

Some information on McMillan 2-stage compressors:

C Series High Pressure – 240V and petrol.
AF Series High Pressure – 415V
AFP Series High Pressure – Petrol with optional key start
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