Buyers making their first air compressor purchase often select small-sized tanks or low-pressure compressors for at-home or lightweight use. Over time, the air compressor may not keep up with their growing needs for compressed air. Pneumatic tools tend to be more affordable than their electric counterparts and pack an impressive punch if the compressor can provide enough airflow.
How can you increase the airflow, known as cfm, on your compressor? Five easy and effective ways to increase CFM on an air compressor are:
- Reducing pressure
- Changing your outlet size
- Hooking up two air compressors
- Using an auxiliary storage tank
- Improving the efficiency of your system by servicing your compressor.
Read on to get a better understanding of how the airflow on your compressor works and the five ways you can increase your output airflow.
How to Increase CFM on an Air Compressor
If you need to increase airflow output from your air compressor for a specific tool or task, there are a few options that you may consider:
1, Reduce Outlet Pressure on your Air Compressor
Reducing the output pressure of your air compressor will help to increase the output airflow. Once your air compressor’s tank is pressurized, you have a certain volume of air that you can use until the compressor kicks back on again. If your tank pressure is much higher than your output pressure, you will increase the total volume of air that will leave your tank on a single charge.
For the same power rating on the same air compressor, by adjusting the pressure, you should get more airflow. Power is equal to the pressure multiplied by the volume over time – where volume over time is the cfm. If the power stays the same, then:
Old pressure x cfm = New pressure x cfm
This is the same assumption we used earlier for calculating for a new airflow at a different pressure than listed. If you had originally set the pressure to 125 and got 5 cfm, but wanted to see how much more airflow you would get at 90 psi, then:
125 psi x 5 cfm = 90 psi x New Cfm
New Cfm = (125 x 5) ÷ 90 = 6.94 cfm
The air compressor will run for a longer time when the output pressure is lower, as the tool will use a lower volume of air at a slower rate. At higher pressures, the air has a higher density, therefore, the air compressor has to work harder to compress the same volume of air, which will take longer. At this higher pressure, your tools will require a higher volume of air, emptying the tank faster than at a lower pressure.
You will be able to use your tools for a longer total period of time at a lower pressure than at a higher pressure because the air compressor will have an easier time keeping up with the demand for air.
The actual amount of airflow that your air compressor will produce may not actually be equal to the amount you calculated. This is because the outlet airflow is actually related to the difference in pressures between the output pressure and the tank pressure, rather than how much power the compressor needs to compress the air.
As you use the air in the tank, the pressure will decrease, even as the compressor works to fill it. The airflow will be highest when the pressures have the biggest difference, which is when the tank has been fully charged. The airflow will be at the lowest when the tank pressure is approaching the outlet pressure.
To explain it in a more mathematically correct way, we can use Bernoulli’s equation.
v2=c A (2 (p1-p2)g ρ)
V2 = the outlet velocity
A = the area of the outlet
C = the coefficient of discharge, for a free opening of a tube assume 0.6
p1= The initial pressure, psi
p2= The final pressure, psi
ρ= The density of air, lbm/ft^3
G = the gravitational constant, 32.2 lbm*ft/lbf*s^2
You will need to look up the density of air at the temperature you’re using the compressed air in and the pressure you’re using the air at. You can find some charts online, such as on the Engineering Toolbox. It’s important to note, as well, that psi stands for pounds-force per square inch but density is typically noted in pounds-mass per cubic foot, so you’ll need to convert between inches and feet. You’ll also need to convert seconds in the gravitational constant to minutes.
If you have a ⅜” outlet, which is 0.11 in2, you can calculate the outlet velocity and therefore the cfm at your starting conditions. If you start with a tank pressure of 125 psi and are using the air at 90 psi:
cfm = 0.6 0.11 in21 ft2144in260 sec/min ((2 (125 lbfin2-90 lbfin2)144 in21ft2 32.2 ft s2)0.534lbft3)
cfm = 21.44 cfm
Knowing this equation, you can now calculate the starting airflow at your original pressures, or even an intermittent airflow at a lower pressure. Note that this equation is theoretical, meaning that it doesn’t count for losses in your system. Losses of efficiency can occur due to friction in the hose, friction in the fittings, or variations in the loss coefficient, or losses in your tool.
The coefficient of discharge might be much lower depending on the type of tool you use. The density of the air you use will also change with pressures and temperatures in your system.
Let’s run another example assuming your tank pressure is approaching the outlet pressure, 95 psi to 90 psi:
cfm = 0.6 0.11 in21 ft2144in260 sec/min ((2 (95 lbfin2-90 lbfin2)144 in21ft2 32.2 ft s2)0.534lbft3)
cfm = 8.10 cfm
As you can see, the airflow will decrease as the tank pressure decreases.
2. Change your Outlet Size
Some tools need a minimum pressure to operate which would prevent you from decreasing the outlet pressure enough to increase the airflow. Increasing the outlet hose size on your air compressor can increase the outlet airflow, by increasing the cross-sectional area where the air flows. The additional area lowers the amount of friction the air experiences as it flows through the length of the hose. The friction in the hose decreases the energy in the air which will slow it down.
As more airflow leaves your tank, your tank will empty faster. While the initial airflow will be higher, your run time will be shorter, decreasing the airflow and the pressure during use.
The hose size was one of the factors in the Bernoulli equation that we used in the previous method. We can demonstrate how the airflow will increase by plugging in different sizes of hose into the equation. Calculate the cross-sectional area of your hose using the area of a circle (or by looking it up online):
A=πr2
If you have a hose that is ½” in diameter, then the area of your hose is about 0.19 square inches, but if you have an inch ¾” tube, then the area is about .44 square inches. The interior diameter of your hose may not actually equal the nominal dimension of your tube. Check the specifications on the tube to find the actual inside diameter or use calipers to measure.
We can show the difference between ½” and ¾” tube, using the same pressures as before, you can see how the area changes the airflow:
cfm = 0.6 0.19 in21 ft2144in260 sec/min ((2 (125 lbfin2-90 lbfin2)144 in21ft2 32.2 ft s2)0.534lbft3)
cfm = 37.0 cfm
cfm = 0.6 0.44 in21 ft2144in260 sec/min ((2 (125 lbfin2-90 lbfin2)144 in21ft2 32.2 ft s2)0.534lbft3)
cfm = 85.75 cfm
Again, these equations are theoretical, so the actual airflow value will be much lower due to losses in the system. However, these results of calculating the airflow show that increasing the size of your outlet hose can increase the airflow through your tools.
3, Hook up Two Air Compressors in Parallel
Another option to increase the airflow at your tools is to hook up to air compressors in parallel. By connecting two air compressors that produce the same airflow rate at the same pressure, you can double the output airflow. The airflows are directly additive. While this doesn’t exactly increase the airflow on your air compressor, it achieves the goal of increasing the airflow at your tool.
To connect to air compressors together, you will need a few additional fittings and an extra length of hose. Connect the discharge hoses together through the straight ends of the tee, and the outlet hose to the side connection of the tee. If you can find a wye fitting, your system will have lower losses, making it more efficient.
If each air compressor can produce 5 cfm of airflow, then adding them together may provide 10 cfm of airflow, theoretically. However, again, the actual airflow produced through the tool might be a bit lower due to losses in the system.
4. Use an Auxiliary Storage Tank
Another method to help produce more airflow through your air compressor expands upon the idea in our first method, reducing the pressure on your regulator.
As we explained in the previous examples, reducing the outlet pressure allows you to produce more airflow for a longer period of time. The same idea applies to use of an auxiliary storage tank. Since the volume of the storage tank is larger than using your compressor alone, then the pressure will drop more slowly over time while you use your tools.
Remember that cfm stands for cubic feet per minute. If you use one cfm of air through your tool, then you will use a cubic foot of air in one minute. Since one cubic foot of air equals about 7.48 gallons, if you maintained one cfm of use and the air compressor never kicked on to refill the tank, you could use all the air in a 20 gallon tank in just a few minutes.
Of course, you won’t be able to sustain your target cfm as the pressure drops, and over time your air compressor will likely start to cycle to increase pressure back into your tank. This will allow you to continue using your tools for much longer.
5. Improve the Efficiency of Your Air Compressor
In the previous ways to increase the airflow on your air compressor, we mentioned several times that losses and inefficiencies in your system could reduce the airflow you get out of your tools. The easiest way to boost airflow in your system is to make sure that all the air that your compressor produces actually ends up coming out of your tool.
Quincy Compressors recommends a few ways to reduce losses in the output airflow of your air compressor:
- Fix leaks in the system. Air leaks can often be heard through hissing sounds, but some smaller leaks may not be audible. Leaks often occur around fittings, such as the hose connections, pressure regulators, joints, and disconnects. Make sure all gaskets are in good condition and replace if the rubber hardens, wears down, or breaks down.
Use plumbers tape on fittings to help reduce air leaks and tighten down all threaded fittings properly, without over-torquing. You can fill a spray bottle with soapy water and spray carefully around fittings to detect leaks that aren’t audible while in use. Shut the compressor down to disconnect any fittings that need to be further tightened.
- Reduce the total length of hose from the tank to your tool. As air moves the hose, it will encounter friction against the walls of the hose. The longer the hose, the more friction and therefore, more energy it will lose. Additionally, if there are any kinks, crimps, or fittings, those will also create friction losses in the system.
- Select low pressure drop hoses. Low pressure drop hoses will have smooth interior walls. There are many pressure drop charts available online that show what the pressure loss in a specific diameter hose might be. Measure your length of hose and note the diameter.
For example, a ½” hose might lose 10 psi of pressure in just 50 feet of hose. Unlike reducing the pressure setting on your regulator, the pressure loss in the hose will actually reduce the speed of the air that comes out of the hose.
- Reduce the number of fittings between your compressor and tools. Fittings add a lot of pressure loss to your system. Fittings might include valves, couplers, plugs, tees, and reducers. Every time the air hits a fitting, it loses pressure, velocity, and therefore, total airflow.
- Don’t forget auxiliary equipment: If you use an auxiliary storage tank, second compressor, or inlet hose, check to make sure that you’ve reduced any losses in those components as well.
Ways to Decrease CFM on your Air Compressor
Some pneumatic tools are very sensitive and may need a lower airflow to operate properly without damaging the tool. With prolonged use at higher pressures or airflows greater than what they’re rated for, you may wear out the tool more quickly. The best way to prevent damage to tools with lower airflow needs is to lower the airflow on your air compressor.
Lowering the airflow on your air compressor works in reverse of many suggestions we highlighted to increase your airflow. Here’s a summary:
- Increase the outlet pressure: While you don’t want to increase the output pressure of your air compressor too much above the recommended pressure of your tool, you can increase the pressure on your outlet. When the outlet pressure and the tank pressure have a smaller difference, the output airflow will be smaller.
- Decrease the hose size: Attaching a smaller hose on your air compressor will add pressure drop to your air compressor. This increased pressure drop will reduce how much airflow is available at the outlet. Additionally, you could increase the total length of the hose, add fittings into the system, or otherwise add additional pressure.
- Decouple secondary air compressors or tanks: If you don’t have a second air compressor or auxilliary tank, you can’t reduce your airflow by removing one. However, if you were desperate to reduce the airflow at your tool but had no other means, you could hook up a second tool or a bleed on the line to reduce airflow. While this method wastes compressed air that your air compressor worked hard to compress, it’s an effective way to decrease airflow.
How To Calculate CFM on an Air Compressor
CFM is the abbreviation for cubic feet per minute, a unit to describe the volumetric flow rate of air for air compressors.