Pressure can be measured in pounds per square inch (psi), or bar (metric measure of pressure). To think of this in simpler terms, pressure refers to the amount of force needed to perform certain amount of work at any given point in time. A simple example of pressure and how it works, is to imagine moving a wooden block across a table. In the illustration below, it shows that using 75 psi of compressed air is not enough force to move the block, but 100 psi of pressure has the ability to move the wooden block the desired distance.
The air compressor has to provide enough pressure to perform a given task (in this case it was to move a wooden block). Per illustration below, if 100 psi is required to move the block, anything less than that will not accomplish the task. It is important to understand what pressure is needed in order to size the compressor properly, otherwise you will be faced with problems, similar to illustration below where lower pressure was not able to move the wooden block, or perform the job.
Flow, also referred to as free air delivery (FAD) can be measured in cubic feet per minute (cfm), liters per second (l/s) or cubic meters per hour (m3/h) depending on your geographical location. In simpler terms, flow is the ability of the compressor to continue performing a task within an acceptable time frame. The amount of flow required is determined by the length of time needed to complete a given task. Let us consider the wooden block example to explain this further. To move a wooden block a certain distance every hour will require less flow and can be achieved with a smaller compressor and a storage tank. The compressor will cycle on and off and refill the storage tank for the next time the wooden block is required to move. However, if the wooden block has to be moved constantly within a given time frame, a larger flow of air (or CFM) with continuous flow will be needed, thus requiring a larger compressor. If the flow is insufficient, the process will require frequent breaks while the compressor builds up pressure in the reserve tank, therefore indicating that the compressor might be undersized.
Ultimately, when you are looking to supply enough compressed air for a given application, it is important to consider the amount of compressed air flow (CFM) needed at a specific pressure (PSI) required for the process to work properly.
One of the ways to find out the total flow needed is to contact the manufacturer of the equipment that uses compressed air and request data sheets with desired information. However, the best way to measure accurate flow, is to perform a “compressed air audit”, which can be done by a compressed air sales professional. It is important to keep in mind that rotary screw air compressors tend to put out more flow per kW or HP than piston compressors.
To summarize, pressure (PSI) is determined by the job you are performing, while flow (CFM) will require the understanding of how frequently the job has to be done, or how many jobs you are performing at the same time. It is important to understand that under sizing a compressor will result in pressure drops and inability to complete a task, while oversizing the unit can lead to future mechanical problems and potential failure of the compressor. If you are unsure on how to size your new compressor for existing or new application, always contact a compressed air sales professional for an audit.