The difference between pressure and flow

November 25, 2020

6 minutes

Editor's Note: This post was originally published in 2017 and has been completely updated for accuracy, comprehensiveness, and new information.

When it comes to compressed air systems, pressure and flow are two of the most common terms you’re sure to encounter. This is true whether you’ve just started your research into air compressors or have made the decision to request a quote from a compressed air provider! Pressure and flow are key measurements for compressed air systems, given that they help facilities size their compressors to power applications with the proper air volume and flow rate so that no energy is wasted.

What is pressure?

Pressure is the measure of force that’s applied to an area and determines the compressor’s ability to perform a specified amount of work at any given point in time. The compressor must provide the right amount of pressure, or force, needed to complete the process. Too little pressure means the job won’t get done, whereas too much pressure can damage the equipment and cause unexpected malfunctions.

How is pressure measured?

Pressure is measured in pounds per square inch (psi). For example, 50 psi would mean that there is 50 pounds of pressure being applied per square inch. To check the PSI of your existing system, look at the air gauge on the air compressor – this is where the system pressure will be displayed.

What is flow?

Flow is measure of volume that indicates your compressor systems output rate; it’s also considered to be your compressor’s ability to continue performing a certain task. The amount of flow needed depends on the length of time required to complete the task. With insufficient flow, the compressor will require breaks to rebuild pressure in the compressor’s reserve tank.

How is flow measured?

Flow is measured in cubic feet per minute (cfm) at a specific pressure and increases in direct proportion to the horsepower (HP) that is applied. So, when comparing a small compressor sold at home improvement stores to an industrial compressor located at a large manufacturing facility, it’s easy to see why the industrial compressor can achieve a much higher CFM than the smaller, at-home compressor.

Get more information on HP, CFM, and PSI here.

How do pressure and flow work together?

Now that we know what these compressor measurements mean, let’s look at the relationship between them. The goal is to supply an application with the proper air flow (CFM) at the correct pressure (PSI). Say we need to move a 10 lb. block across a long table. While 100 psi of air pressure may not be enough force to move the block, 115 psi will, which is why it’s important to know the minimum pressure needed for your process. Blindly increasing pressure can cause an unnecessary increase in energy consumption. Simply increasing pressure by 2 psi causes a 1 percent increase in the energy needed to maintain the same airflow.

block-psi illustration.jpg

Airflow takes into consideration how often you need to move the 10 lb. block across the table. If the block only needs to move a small distance every hour, then a small compressor with a small air tank can meet those demands. However, if your application requires that you keep the block constantly moving over a span of many hours then you’ll need a larger compressor with more continuous flow.

block-cfm illustration.jpg

How compressed air pressure and flow affect system performance

Understanding how compressed air pressure and flow interact is essential to getting the most out of your compressed air system. While PSI and CFM are often discussed separately, they are deeply interconnected, and a change in one will almost always affect the other.

In any compressed air system, pressure and flow have an inverse relationship. As system pressure increases, the available flow decreases, and as pressure drops, flow increases. This means that simply turning up the pressure on your compressor to solve a performance problem is rarely the right answer. If a tool or process is underperforming, the root cause is often insufficient flow rather than insufficient pressure, and increasing pressure to compensate only drives up energy consumption without addressing the actual problem.

This inverse relationship also means that compressed air pressure and flow must always be specified together. A compressor rated at 100 CFM delivers that flow at a specific pressure, for example 100 PSI. If your system requires a higher pressure, the compressor will deliver less CFM at that higher pressure. Always check that a compressor's CFM rating is stated at the pressure your application actually requires, not at a lower reference pressure, to avoid sizing mistakes.

Getting compressed air pressure and flow right for your specific application is the foundation of an efficient, reliable compressed air system. Undersizing either measurement leads to poor tool performance and production slowdowns. Oversizing leads to wasted energy, higher operating costs, and unnecessary wear on equipment.

Beware of Overpressurization

One thing to keep an eye out for? Overpressurizing your compressed air system! Having a solid grasp on your application’s required pressure and flow can be difficult, which means many times facilities will overpressurize; this means that they will increase the system’s pressure (PSI) in order to make sure processes run consistently. Doing this, however, results in severe energy losses and high energy costs.

How to determine the right compressed air pressure and flow for your application

Determining the correct compressed air pressure and flow for your facility starts with understanding the demands of every tool, process, and piece of equipment connected to your compressed air system. Here is a practical approach:

Start with pressure. Identify the minimum PSI required by the most pressure-demanding piece of equipment in your system. This becomes your target system pressure. Remember that pressure drops occur across filters, dryers, piping, and fittings between the compressor and the point of use, so your compressor will need to generate slightly more pressure than the minimum required at the point of use to compensate for these losses.

Next, calculate your total flow requirement. Add up the CFM requirements of all the tools and processes that may run simultaneously at peak demand. This gives you your peak flow demand, which your compressor must be able to meet reliably. Adding a buffer of 25 to 30 percent above your calculated peak demand is a common and sensible approach to account for system leaks, future growth, and demand spikes.

Finally, match a compressor to both requirements. The compressor you select must be able to deliver your required CFM at your required PSI simultaneously. A compressor that meets your pressure requirement but falls short on flow, or vice versa, will not adequately serve your application. If you are unsure how to calculate your compressed air pressure and flow requirements, a compressed air audit conducted by an expert is the most accurate and reliable way to establish the right specifications for your system.

The Takeaway

Understanding pressure and flow will help you size your compressor based on process demands while reducing energy requirements, improving productivity, and lowering lifecycle costs. Not quite sure how to determine your system’s specifications? We recommend starting with a compressed air audit or reaching out to your local compressed air expert. We’re always available at www.atlascopco.com/air-usa!

Frequently asked questions

What is the difference between pressure and flow in a compressed air system?

Pressure (PSI) measures the force of the compressed air, or how hard the air is being pushed. Flow (CFM) measures the volume of air being delivered per minute, or how much air is available. Pressure determines whether a tool or process has enough force to operate, while flow determines whether the system can sustain that operation continuously over time. Both measurements are essential to compressor sizing and system performance, and they must always be evaluated together rather than in isolation.

Why do compressed air pressure and flow have an inverse relationship?

Pressure and flow are inversely related in a compressed air system because the compressor has a finite amount of energy available to do work. When pressure is increased, more of that energy goes into pushing air harder, which reduces the volume of air that can be delivered per minute. Conversely, at lower pressures, more air volume can be moved. This is why a compressor's CFM rating is always stated at a specific pressure, and why it is important to verify that rating at the pressure your application actually requires.

How does overpressurization affect a compressed air system?

Overpressurization occurs when a facility runs its compressed air system at a higher pressure than the application actually requires, often as a way to compensate for pressure drops, leaks, or undersized piping. While overpressurization may seem like a quick fix, it significantly increases energy consumption. Every 2 PSI increase in system pressure results in approximately a 1 percent increase in energy cost. Over time, running at unnecessarily high pressure also increases leakage rates, accelerates wear on equipment, and can cause malfunctions in pressure-sensitive tools and processes.

How do I know if my compressed air system has the right pressure and flow?

The most reliable way to verify that your system has the right compressed air pressure and flow is to conduct a compressed air audit. An audit measures actual pressure and flow at various points throughout your system, identifies pressure drops, quantifies leakage losses, and compares the system's actual performance against your application's real requirements. The results give you a clear picture of whether your current compressor is correctly sized, whether your distribution system is efficient, and where the best opportunities for improvement and energy savings lie.

Can I increase CFM without changing my compressor?

In most cases, significantly increasing the CFM output of a compressed air system requires either upgrading to a larger or more powerful compressor or adding an additional compressor to the system. However, there are steps you can take to maximize the effective CFM available from your existing compressor. Fixing air leaks in the distribution system, reducing unnecessary pressure drops across filters, dryers, and piping, and eliminating artificial demand created by overpressurization can all free up additional flow capacity without any change to the compressor itself. A compressed air audit is the best starting point for identifying these opportunities in your specific system.

What is a compressed air audit and how does it help with pressure and flow?

A compressed air audit is a systematic assessment of your compressed air system conducted by a qualified expert. It involves measuring system pressure and flow at multiple points, logging demand profiles over time, identifying leaks and inefficiencies, and evaluating the performance of compressors, dryers, filters, and distribution piping. The audit produces a detailed picture of how your system is actually performing compared to how it should be performing, and provides specific recommendations for optimizing compressed air pressure and flow, reducing energy consumption, and ensuring your compressor is correctly sized for your application's real demands.