Flow and Capacity

Following on from our previous Blogs, "How do I know what size of compressor I need" and "Pressure and Capacity", we are now looking at Flow and Capacity. Flow and Capacity are the way to summarise how much air is required for a task when using your Compressed Air System and the output of your compressor.

Flow

Flow is the ability of an Air Compressor to continuously perform a task. In the U.K., flow is typically measured in 'cubic feet per minute' (CFM) at a specific pressure.

Compressor Capacity

Compressor Capacity is the actual volume rate of flow compressed and delivered at the standard discharge point, at stated inlet conditions. This is usually expressed in terms of Free Air Delivered.

What are Litres/second, m³/min and CFM?

These are all different terms for flow. They all mean the volume of air that an Air Compressor can supply per minute. In other words, its flow or output.

How do I know what CFM I need?

The amount of CFM or flow required depends on the ability of the Air Compressor to perform a task within a particular amount of time.

A Compressor is under-specified, if it is not providing enough flow and you have to wait for pressure to build up in the reserve tank. Some applications require a constant air flow, while others require the Compressor to provide air only part of the time.

It can also depend on the type of Compressor you are using. Generally, Rotary Screw Air Compressors will put out more flow per kW than Piston Compressors will.

If you have a simple piece of equipment, the manufacturer should be able to provide data sheets that set out the amount of flow needed to operate it. However, this may not work with a more complex system.

When the compressor you buy does not have the capacity to match your tools, you will find yourself constantly waiting for the tank to fill up.

How do you work out the capacity required?

Find the required CFM of the tool you will be using. This is usually found in manual or if you are buying new it should be found in the specification.
If you are using more than one tool at the same time, you need to add up the required CFM for each tool.
Once you have added up the CFM of the tools you will be using, it is good practice to add in a safety buffer. This is so the compressor will not be working at its maximum capacity all the time. So, multiply the final number by 1.2 and this will give you your airflow requirement.

For example, you are going to be using an Nail Gun (3.5CFM) and an Impact Wrench (4CFM) at the same time.

7.5CFM X 1.2 = A compressor with at least 8.7 CFM (FAD).

For peace of mind your best option is to ask a Compressed Air Engineer to carry out an energy audit of your system, so that the air compressor engineer can provide a bespoke solution. See previous blogs on Data Logging