Many times when we provide the air consumption of an EXAIR product, we get a response like…. “I’ve got plenty of pressure, we run at around 100 PSIG”. While having the correct pressure available is important, it doesn’t make up for the volume requirement or SCFM (Standard Cubic Feet per Minute) needed to maintain that pressure. We commonly reference trying to supply water to a fire hose with a garden hose, it is the same principle, in regards to compressed air.
When looking to maintain an efficient compressed air system, it’s important that you use properly sized supply lines and fittings to support the air demand (SCFM) of the point-of-use device. The smaller the ID and the longer the length of air supply line, it becomes more difficult for the air to travel through the system. Undersized supply lines or piping can sometimes be the biggest culprit in a compressed air system as they can lead to severe pressure drops or the loss of pressure from the compressor to the end use product.
Take for example our 18″ Super Air Knife. An 18″ Super Air Knife will consume 52.2 SCFM at 80 PSIG. We recommend using 1/2″ Schedule 40 pipe up to 10′ or 3/4″ pipe up to 50′. The reason you need to increase the pipe size after 10′ of run is that 1/2″ pipe can flow close to 100 SCFM up to 10′ but for a 50′ length it can only flow 42 SCFM. On the other hand, 3/4″ pipe is able to flow 100 SCFM up to 50′ so this will allow you to carry the volume needed to the inlet of the knife, without losing pressure through the line.
Pipe size chart for the Super Air Knife
Another problem area is using restrictive fittings, like quick disconnects. While this may be useful with common everyday pneumatic tools, like an impact wrench or nail gun, they can severely limit the volumetric flow to a device requiring more air , like a longer length air knife.
1/4″ Quick Connect
For example, looking at the above 1/4″ quick disconnect, the ID of the fitting is much smaller than the NPT connection size. In this case, it is measuring close to .192″. If you were using a device like our Super Air Knife that features 1/4″ FNPT inlets, even though you are providing the correct thread size, the small inside diameter of the quick disconnect causes too much of a restriction for the volume (SCFM) required to properly support the knife, resulting in a pressure drop through the line, reducing the overall performance.
If you have any questions about compressed air applications or supply lines, please contact one of our application engineers for assistance.
Flow rate is the quantity of material that is moved per unit of time. Generally, the quantity of material can be expressed as a mass or a volume. For example, mass flow rates are in units of pounds per minute or kilograms per hour. Volumetric flow rates are stated in cubic feet per minute, CFM, or liters per hour, LPH. The trick begins when volumetric flow rates are used with compressible gases. In this blog, I will go over the various acronyms and the reasons behind them.
What acronyms will be covered?
CFM – Cubic Feet per Minute
SCFM – Standard Cubic Feet per Minute
ACFM – Actual Cubic Feet per Minute
ICFM – Inlet Cubic Feet per Minute
The volumetric component of the flow rate is CFM or Cubic Feet per Minute. This term is commonly used for rating air compressors. From the history of air compressors, they could calculate the volume of air being drawn into the air compressor by the size of the cylinder. With the volume of the compression chamber and the rotations per minute of the motor, RPM, they could calculate the volumetric air flows. As conditions change like altitude, temperature, and relative humidity, the volumetric value of CFM changes. To better clarify these conditions, compressor manufacturers have decided to add terms with a definition. (For your information, air compressors still use CFM as a unit of air flow, but now this is defined at standard temperature and pressure).
The first letter in front of CFM above now defines the conditions in which volumetric air flow is being measured. This is important for comparing pneumatic components or for properly sizing pneumatic systems. Volume is measured within three areas; temperature, pressure, and relative humidity. We can see this in the Ideal Gas Law, reference Equation 1.
Equation 1:
P * V = n * R * T
Where:
P – Absolute Pressure
V – Volume
n – Number of molecules of gas
R – Universal Gas Constant
T – Absolute Temperature
The volume of air can change in reference to pressure, temperature, and the number of molecules. You may ask where the relative humidity is? This would be referenced in the “n” term. The more water vapor, or higher RH values, the less molecules of air are in a given volume.
SCFM is the most commonly used term, and it can be the most confusing. The idea behind this volumetric air flow is to set a reference point for comparisons. So, no matter the pressure, temperature, or relative humidity; the volumetric air flows can be compared to each other at that reference point. There have been many debates about an appropriate standard temperature and pressure, or STP. But as long as you use the same reference point, then you can still compare the results. In this blog, I will be using the Compressed Air and Gas Institute, CAGI, reference where the “Standard” condition is at 14.5 PSIA, 68 o F, and 0% RH. Since we have a reference point, we still need to know the actual conditions for comparison. It is like having the location of a restaurant as a reference, but if you do not know your current location, you cannot move toward it. Similarly, we are “moving” the air from its actual condition to a reference or “Standard” condition. If we do not know the actual state where the air began, then we cannot “move” toward that reference point. We will talk more about this later in this blog.
ACFM is the volumetric air flow under actual conditions. This is actually the “true” flow rate. Even though this term is hardly used, there are reasons why we will need to know this value. We can size an air compressor that is not at “Standard” conditions, and we can use this value to calculate velocity and pressure drop in a pneumatic system. We can correlate between SCFM and ACFM with Equation 2.
Pact – absolute pressure at the actual level (PSIA)
Psat – saturation pressure at the actual temperature (PSI)
Φ – Actual relative humidity (%)
Tact – Actual ambient air temperature (oR)
Tstd – Standard temperature (oR)
ICFM is one of the newest terms in the history of air compressors. This is where devices are added to the inlet of an air compressor, affecting flow conditions. If you have a blower on the inlet of an air compressor, the volumetric flow rate changes as the pressure and temperature rises at the “Inlet”. If a filter is used, then the pressure drop will decrease the incoming pressure at the “Inlet”. These devices that affect the volumetric flow rate for an air compressor should be considered. The equation to relate ACFM to ICFM is Equation 3.
Equation 3:
ICFM = ACFM * (Pact / Pf) * (Tf / Tact)
Where:
ICFM – Inlet Cubic Feet Per Minute
ACFM – Actual Cubic Feet per Minute
Pact – absolute pressure at the actual level (PSIA)
Pf – Pressure after filter or inlet equipment (PSIA)
Tact – Actual ambient air temperature (oR)
Tf – Temperature after filter or inlet equipment (°R)
To expand on my explanation above about SCFM and ACFM, a technical question is asked often about the pressure when using SCFM. The reference point of 14.5 PSIA is in the definition of the term for SCFM. Remember, this is only a reference point. The starting location is also needed as it gives us the ACFM value where the air values are true and actual. Then we can make a comparison of actual air usage.
As an example, let’s look at two air nozzles that are rated at the same air flow; 60 SCFM. The EXAIR Super Air Nozzle, model 1106, is cataloged at 60 SCFM at 80 PSIG, and a competitor is cataloged at 60 SCFM at 72 PSIG. By comparison, they look like they use the same amount of compressed air, but actually they do not. To simplify Equation 2, we can compare the two nozzles at the same temperature and RH at 68 oF and 0% RH respectively. This equation can be reduced to form Equation 4.
Equation 4:
ACFM = SCFM * 14.5 / (P + 14.5)
@72 PSIG Competitor:
ACFM = 60 SCFM * 14.5 PSIA/ (72 PSIG + 14.5 PSIA)
= 10.1 ACFM
@80 PSIG EXAIR Super Air Nozzle:
ACFM = 60 SCFM * 14.5 PSIA / (80 PSIG + 14.5PSIA)
= 9.2 ACFM
Even though the SCFM is the same amount, you are actually using 10% more air with the competitive nozzle that was reported at 60 PSIG. So, when it comes to rating pneumatic products, improving efficiency, and saving money; always determine the pressure that you are at. The more you know about volumetric flow rates, the better decision that you can make. If you need more information, you can always contact our Application Engineers at EXAIR. We will be happy to assist.
Over the weekend I was working on a car in my driveway and I needed a large volume of air at the far end of the car to try and unplug a clogged sunroof drain line. Rather than trying to move the car while it was mostly taken apart, I just hooked up another air line extension and started to go to the drain. Even knowing what I know as an EXAIR Application Engineer about lengths of tubing, air restriction, and fitting restrictions, I went ahead with the quick and easy “fix”.
An example of pressure drop from a compressed air quick disconnect.
I grabbed another 30′ – 3/8″ i.d. air line with 1/4″ quick disconnects (see why this is wrong with this blog) on both end, rather than getting out the 50′ long 1/2″ i.d. air line that I have with proper fittings that then reduce down to a 1/4″NPT at the end to tie into most of my air tools. By doing so I ended up hooking up a Safety Air Gun which then gave a very light puff of air into the tube and the clog in the line went nowhere. As a matter of fact, it was almost like it laughed because the tubing vibrated as if the clog said, “Pfft I am going nowhere.”
I then, stepped back and evaluated what I had done in a rush to try and get a job done rather than taking the extra five minutes to get the proper air line to do the job. I then spent 10 minutes putting that hose up and getting out the correct hose. Then, with a whoosh and a thud the clog was launched into my yard from the clogged drain port and I finished the repairs.
If only I had watched Russ Bowman’s spectacular video on Proper Compressed Air Supply Plumbing the day before. Rather than wasting time with the quick “fix” that cost me more time and didn’t fix anything I should have taken a little more time up front to verify I had properly sized my lines for the job at hand.
If you would like to discuss compressed air plumbing, appropriate line sizes, or insufficient flow on your compressed air system, please contact an EXAIR Application Engineer.
“To measure is to know – if you cannot measure it, you cannot improve it.” -Lord Kelvin, mathematical physicist, engineer,and pioneer in the field of thermodynamics.
This is true of most anything. If you want to lose weight, you’re going to need a good scale. If you want to improve your time in the 100 yard dash, you’re going to need a good stopwatch. And if you want to decrease compressed air consumption, you’ll need a good flowmeter. In fact, this is the first of six steps that we can use to help you optimize your compressed air system.
Six Steps To Optimizing Your Compressed Air System
There are various methods of measuring fluid flow, but the most popular for compressed air is thermal mass air flow. This has the distinct advantage of accurate and instantaneous measurement of MASS flow rate…which is important, because measuring VOLUMETRIC flow rate would need to be corrected for pressure in order to determine the true compressed air consumption. My colleague John Ball explains this in detail in a most excellent blog on Actual (volume) Vs. Standard (mass) Flows.
So, now we know how to measure the mass flow rate. Now, what do we do with it? Well, as in the weight loss and sprint time improvements mentioned earlier, you have to know what kind of shape you’re in right now to know how far you are from where you want to be. Stepping on a scale, timing your run, or measuring your plant’s air flow right now is your “before” data, which represents Step One. The next Five Steps are how you get to where you want to be (for compressed air optimization, that is – there may be a different amount of steps towards your fitness/athletic goals.) So, compressed air-wise, EXAIR offers the following solutions for Step One:
Digital Flowmeter with wireless capability. This is our latest offering, and it doesn’t get any simpler than this. Imagine having a flowmeter installed in your compressed air system, and having its readings continually supplied to your computer. You can record, analyze, manipulate, and share the data with ease.
Monitor your compressed air flow wirelessly over a ZigBee mesh network.
Digital Flowmeter with USB Data Logger. We’ve been offering these, with great success, for almost seven years now. The Data Logger plugs into the Digital Flowmeter and, depending on how you set it up, records the flow rate from once a second (for about nine hours of data) up to once every 12 hours (for over two years worth.) Pull it from your Digital Flowmeter whenever you want to download the data to your computer, where you can view & save it in the software we supply, or export it directly into Microsoft Excel.
From the Digital Flowmeter, to your computer, to your screen, the USB Data Logger shows how much air you’re using…and when you’re using it!
Summing Remote Display. This connects directly to the Digital Flowmeter and can be installed up to 50 feet away. At the push of a button, you can change the reading from actual current air consumption to usage for the last 24 hours, or total cumulative usage. It’s powered directly from the Digital Flowmeter, so you don’t even need an electrical outlet nearby.
Monitor compressed air consumption from a convenient location, as well as last 24 hours usage and cumulative usage.
Digital Flowmeter. As a stand-alone product, it’ll show you actual current air consumption, and the display can also be manipulated to show daily or cumulative usage. It has milliamp & pulse outputs, as well as a Serial Communication option, if you can work with any of those to get your data where you want it.
With any of the above options, or stand-alone, EXAIR’s Digital Flowmeter is your best option for Step One to optimize your compressed air system.
Stay tuned for more information on the other five steps. If you just can’t wait, though, you can always give me a call. I can talk about compressed air efficiency all day long, and sometimes, I do!
