If you need to operate at a different pressure because you require less or more force or simply operate at a different line pressure, this formula will allow you to determine the volume of air being consumed by any device.
Lets first consider the volume of the 1100 Super Air Nozzle at a higher than published pressure. As shown in the formula and calculations it is simply the ratio of gauge pressure + atmospheric divided by the published pressure + atmospheric and then multiply the dividend by the published volume. So as we do the math we solve for 17.69 SCFM @ 105 PSIG from a device that was shown consume 14 SCFM @ 80 PSIG.
Now lets consider the volume at a lower than published pressure. As shown it is simply the ratio of gauge pressure + atmospheric divided by the published pressure + atmospheric and then multiply the dividend by the published volume. So as we do the math we solve for 11.04 SCFM @ 60 PSIG from a device that was shown to consume 14 SCFM @ 80 PSIG.
When you are looking for expert advice on safe, quiet and efficient point of use compressed air products give us a call. Experience the EXAIR difference first hand and receive the great customer service, products and attention you deserve! We would enjoy hearing from you.
On any given day myself and my Application Engineering Brethren here at EXAIR have discussions with customers on air starvation of any given EXAIR Product. The calls generally start off the same, “The Line Vac is not performing like it should”. We at EXAIR absolutely want to help you get the most out of our products and we certainly want them to perform to your expectation. However they must be supplied with clean/dry compressed air at sufficient pressure and volume.
Just the other day I was discussing a performance issue with a customer on a 1″ Line Vac. The customer thought he needed a larger Line Vac. I asked the questions regarding the diameter of his Supply Line and if he was using Quick Connect or Push Lock connectors. He was attempting to feed this Line Vac with 1/4″ Poly Tubing through a elbow Push to Loc fitting.
This 1″ Line Vac was being severely starved for air and therefore not performing as expected. The 1″ Line Vac require’s 14.7 SCFM @ 80PSI to reach the rated performance of 42″ of water column.
Below is a table for Pipe/Hose sizing from the Line Vac installation manual that you can use as a reference guide. It is recommended that if using hose for the supply air to go up to the next size over the pipe recommendation.
Don’t forget that quick connects and Push Lock fittings are not recommended and could restrict the air flow which will have a negative impact on performance.
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 or liters per hour. The trick begins when volumetric flow rates are used for a compressible gas. 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 history of air compressors, they could calculate the volume of air being drawn into the air compressor by the size of 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 value of CFM changes. To better clarify these conditions, compressor manufacturers decided to add terms with 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 the volumetric air flow is being measured. This is important for comparing pneumatic components or for properly sizing pneumatic systems. Volume is measured with three areas: temperature, pressure, and relative humidity. We can see this in the Ideal Gas Law: P * V = n * R * T or Equation 1:
V = n * R * T / P
V – Volume
n – Number of molecules of gas
R – Universal Gas Constant
T – Absolute Temperature
P – Absolute Pressure
The volume of air can change in reference to pressure, temperature, and the number of molecules. Where is the relative humidity? This would be referenced in the “n” term. The more water vapor, or higher RH value, the less molecules of air is in a given volume.
SCFM is the most commonly used term, and it can be the most confusing. The idea of 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 deg. F, and 0% RH. Since we have a reference point, we still need to know the actual conditions for comparison. It is like having a location of a restaurant as a reference, but if you do not know your current location, you cannot reach it. Similarly, we are “moving” the air from its actual condition to a reference or “Standard” condition. We will need to know where the air began in order to reach 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 system. We can correlate between SCFM and ACFM with Equation 2:
ACFM = Actual Cubic Feet per Minute
SCFM = Standard Cubic Feet per Minute
Pstd = standard absolute air pressure (psia)
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 the 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 the ACFM to ICFM is with Equation 3:
ICFM = ACFM * (Pact / Pf) * (Tf / Tact)
ICFM = Inlet Cubic Feet Per Minute
Pf = Pressure after filter or inlet equipment (PSIA)
Tf = Temperature after filter or inlet equipment (°R)
Examples of these different types of flow rates can be found here in this EXAIR blog by Tyler Daniel.
To expand on my explanation above about SCFM and ACFM, a technical question comes up about the pressure when using SCFM. The reference point of 14.5 PSIA is in the definition of SCFM. Remember, this is only a reference point. The starting location is actually required. This would be the ACFM value where the air values are true and actual. As an example, two air nozzles are rated for 60 SCFM. An EXAIR Super Air Nozzle, model 1106, is cataloged at 80 PSIG, and a competitor is cataloged at 60 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 Deg. F and 0% RH respectively. This equation can be reduced to Equation 4:
Even though the SCFM is the same amount, you are actually using 21% more air with the competitive nozzle that was reported at 60 PSIG. So, when it comes to rating compressed air products or air compressors, always ask the conditions of pressure, temperature and RH. The more you know about volumetric flow rates, the better decision that you can make. If you need help, you can always contact our application engineers at EXAIR.
EXAIR’s Digital Flow Meter offers an easy way to measure, monitor and record compressed air consumption. The Digital display shows the current amount of compressed air flow, allowing for tracking to identify costly leaks and/or inefficient air users.
How exactly does the Digital Flow Meter work? The unit falls under the category of Thermal Mass or Thermal Dispersion type flow meters. Below shows the backside of a unit.
Thermal mass flow meters have the advantage of using a simple method of measuring flow without causing a significant pressure drop. The EXAIR units have (2) probes that are inserted through the pipe wall and into the air flow. Each of the probes has a resistance temperature detector (RTD.) One of the probes measures the temperature of the air flow. The other probe is heated to maintain a preset temperature difference from the temperature measured by the first probe. The faster the air flow, the more heat that is required to keep the second probe at the prescribed temperature. From Heat Transfer principles, the heat energy input required to maintain the preset temperature is based on the mass velocity of the air. Using basic physical properties for compressed air, the volumetric rate can be determined (SCFM), and displayed.
It is important to note that the compressed air should be filtered to remove oils, and dried to remove water, as these liquids have different physical properties from air, and will cause erroneous readings.
If you have any questions about the Digital Flow Meter or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.
Leaks in a compressed air system can waste thousands of dollars of electricity per year. In fact, in many plants, the leakage can account for up to 30% of the total operational cost of the compressor. Some of the most common areas where you might find a leak would be at connection joints like valves, unions, couplings, fittings, etc. This not only wastes energy but it can also cause the compressed air system to lose pressure which reduces the end use product’s performance, like an air operated actuator being unable to close a valve, for instance.
One way to estimate how much leakage a system has is to turn off all of the point-of-use devices / pneumatic tools, then start the compressor and record the average time it takes for the compressor to cycle on and off. The total percentage of leakage can be calculated as follows:
Percentage = [(T x 100) / (T + t)]
T = on time in minutes
t = off time in minutes
The percentage of compressor capacity that is lost should be under 10% for a system that is properly maintained.
Another method to calculate the amount of leakage in a system is by using a downstream pressure gauge from a receiver tank. You would need to know the total volume in the system at this point though to accurately estimate the leakage. As the compressor starts to cycle on, you want to allow the system to reach the nominal operating pressure for the process and record the length of time it takes for the pressure to drop to a lower level. As stated above, any leakage more than 10% shows that improvements could be made in the system.
(V x (P1 – P2) / T x 14.7) x 1.25
V= Volumetric Flow (CFM)
P1 = Operating Pressure (PSIG)
P2 = Lower Pressure (PSIG)
T = Time (minutes)
14.7 = Atmospheric Pressure
1.25 = correction factor to figure the amount of leakage as the pressure drops in the system
Now that we’ve covered how to estimate the amount of leakage there might be in a system, we can now look at the cost of a leak. For this example, we will consider a leak point to be the equivalent to a 1/16″ diameter hole.
A 1/16″ diameter hole is going to flow close to 3.8 SCFM @ 80 PSIG supply pressure. An industrial sized air compressor uses about 1 horsepower of energy to make roughly 4 SCFM of compressed air. Many plants know their actual energy costs but if not, a reasonable average to use is $0.25/1,000 SCF generated.
3.8 SCFM (consumed) x 60 minutes x $ 0.25 divided by 1,000 SCF
= $ 0.06 per hour
= $ 0.48 per 8 hour work shift
= $ 2.40 per 5-day work week
= $ 124.80 per year (based on 52 weeks)
As you can see, that’s a lot of money and energy being lost to just one small leak. More than likely, this wouldn’t be the only leak in the system so it wouldn’t take long for the cost to quickly add up for several leaks of this size.
If you’d like to discuss how EXAIR products can help identify and locate costly leaks in your compressed air system, please contact one of our application engineers at 800-903-9247.
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 run, 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. A 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.
We also explain how performance can be negatively affected by improper plumbing in the following short video:
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.
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.