Tag: flow

Volumetric Flow Rates: CFM, ICFM, ACFM, SCFM.  What Does it All Mean?

Published on by John BallLeave a comment

Flow rate is the quantity of material that moves over a period of time.  Generally, the quantity of material can be expressed as a mass or a volume.  For example, mass flow rates are generally in units of pounds per minute (lbs./min) or kilograms per hour (Kg/hr).  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 for a compressible gas.  In this blog, I will cover 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 above is CFM or Cubic Feet per Minute.  This term is commonly used in rating air compressors and pneumatic equipment.  From their history, they would calculate the volume of air being drawn into the air compressor by the size of cylinder.  With the rotations per minute of the motor, RPM, they could calculate the volumetric flow rate.  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 definitions.

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 air compressors and pneumatic systems. Volume is measured in three areas: temperature, pressure, and relative humidity as seen in the Ideal Gas Law.

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 values, the fewer molecules of air are 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 one 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, 68oF, and 0% RH.  Since we have the reference point, we still need to know the actual conditions.  As an example, it is like having a location for a restaurant as a reference, but if you do not know your current location, you cannot find it.   Similarly, we are “moving” the air from one 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 use the ACFM value to calculate velocities and pressure drops in a system.  We can also correlate between SCFM and ACFM:

Equation 2:

ACFM = SCFM * [Pstd / (Pact – Psat Φ)] * (Tact / Tstd)

Where:

ACFM = Actual Cubic Feet per Minute
SCFM = Standard Cubic Feet per Minute
Pstd = standard absolute air pressure (14.5 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) or (oF + 460)
Tstd = Standard temperature (528oR) or (68oF + 460)

ICFM, or Inlet Cubic Feet per Minute, is one of the newer 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 rise at the “Inlet”.  If an intake filter is used, then the pressure drop will decrease the incoming pressure at the “Inlet”.  These devices that affect the volumetric flow rate of an air compressor should be considered.  Equation 3 shows the relationship between ACFM and ICFM:

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)

Tf = Temperature after filter or inlet equipment (°R)

Tact = Actual ambient air temperature (oR)

To expand on my explanation above about SCFM and ACFM, a technical question comes up often 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 needed.  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 do they actually?  To simplify Equation 2 above, we can compare the two nozzles at the same temperature, 68oF, and 0% RH. This equation can be reduced to:

Equation 4:

ACFM = SCFM * 14.5 / (P + 14.5)

@60 PSIG Competitor:

ACFM = 60 SCFM * 14.5 PSIA/ (60 PSIG + 14.5 PSIA)

= 11.7 ACFM

@80 PSIG EXAIR Super Air Nozzle:

ACFM = 60 SCFM * 14.5 PSIA / (80 PSIG + 14.5PSIA)

= 9.2 ACFM

Even though the SCFM rating is the same but at two different pressures, the actual flow shows that you are using 21% more compressed air with the competitive nozzle. 

Another example would be for sizing an air compressor.  Since air compressors are rated at sea level (14.5 PSIA), 68oF and 0% RH, what happens if you are in Denver?  A manufacturing company was looking for a 500 SCFM air compressor to run their plant.  They were located at 1,000 feet above sea level with a site temperature of 85oF and a relative humidity of 60%.  Since they were not at the standard conditions, we can calculate the ACFM to properly size the air compressor.  At 1,000 feet above sea level, the atmospheric pressure is 14.2 PSIA.  The saturation pressure at 85oF is 0.595 PSIA.  From Equation 2, we can calculate the ACFM.

ACFM = SCFM * [Pstd / (Pact – Psat Φ)] * (Tact / Tstd)

ACFM = 500 SCFM * [14.5 / (14.2 – 0.595 * 60%)] * (85oF + 460) / (68oF + 460)

ACFM = 500 SCFM * 1.0474 * 1.0322

ACFM = 540

For this manufacturing plant, they will need to increase the capacity to 540 SCFM to run their 500 SCFM pneumatic system at their location.  If they had to add any inlet equipment to the air compressor, then we could use the ICFM equation, Equation 3, to provide the proper volume of compressed air.

When it comes to rating compressed air products or air compressors, always ask about the conditions for the pressure, temperature and RH.  Your local compressor dealer can help you in selecting the proper unit.  The more you know about volumetric flow rates, the better decision that you can make in selecting the correct product.  If you need any help with saving compressed air on your point-of-use blow-off devices, you can contact an Application Engineer at EXAIR.  We will be happy to help you with our efficient and safe engineered products. 

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

Photo: Air sign by Barney Moss.  Creative Commons 2.0

Tools of The Trade – Glass Rotameters

Published on by Brian FarnoLeave a comment

Today, I wanted to share one of the many tools that we utilize here in the EXAIR Efficiency Lab. The video will show how a Glass Rotameter is used and works under both compressed air and atmospheric air volumetric flows. If you want to see how your products flow, give us a call, and we will set up an EXAIR Efficiency Lab for you.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

Carburetors and Venturi Tubes: Thank You Giovanni Battista Venturi

Published on by Brian FarnoLeave a comment

I know it has been a little while since I blogged about something with a motor so it should be no surprise that this one ties to something with a combustion chamber. This all starts with an Italian physicist, Giovanni Battista Venturi. His career was as a historian of science and a professor at the University of Modena. He gave Leonardo da Vinci’s creations a different perspective by crediting da Vinci to be a scientist with many of his creations rather than just an amazing artist. He then began to study fluid flow through tubes. This study became known as the Venturi Tube. The first patents in 1888 came to fruition long after Giovanni passed away. So what was this Venturi effect and how does it tie in to carburetors let alone compressed air?

The illustration below showcases the Venturi effect of a fluid within a pipe that has a constriction. The principle states that a fluid’s velocity must increase as it passes through a constricted pipe. As this occurs, the velocity increases while the static pressure decreases. The pressure drop that accompanies the increase in velocity is fundamental to the laws of physics. This is another principle we like to discuss known as Bernoulli’s principle.

1 – Venturi

Some of the first patents using Venturi’s began to appear in 1888. One of the key inventors for this was Karl Benz who founded Mercedes. This is how the Venturi principle ties into combustion engines for those that do not know the history. This patent is one of many that came out referencing the Venturi principle and carburetors. The carburetors can vary considerably in the complexity of their design. Many of the units all have a pipe that narrows in the center and expands back out, thus causing the pressure to fall and the velocity to increase. Yes, I just described a Venturi, this effect is what causes the fuel to be drawn into the carburetor. The higher velocity on the input (due to this narrowing restriction) results in higher volumes of fuel which results in higher engine rpms. The image below showcases Benz’s first patent using the Venturi.

2 – Venturi Patent

While carburetors slowly disappear and now can mainly be found in small engines such as weed eaters, lawn mowers, and leaf blowers, the Venturi principle continues to be found in industry and other items. Needless to say, I think Giovanni Battista Venturi would be proud of his findings and understanding how monumental they have been for technological advancements. For this, we will recognize the upcoming day of his passing 199 years ago on April 24, 1822.

Brian Farno
Application Engineer
BrianFarno@exair.com
@EXAIR_BF

1 – Thierry Dugnolle, CC0, Venturi.gif, retrieved via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/1/16/Venturi.gif

2 – United States Patent and Trademark Office – Benz, Karl, Carburetor – Retrieved from https://pdfpiw.uspto.gov/.piw?Docid=00382585&homeurl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO1%2526Sect2%3DHITOFF%2526d%3DPALL%2526p%3D1%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsrchnum.htm%2526r%3D1%2526f%3DG%2526l%3D50%2526s1%3D0382,585.PN.%2526OS%3DPN%2F0382,585%2526RS%3DPN%2F0382,585&PageNum=&Rtype=&SectionNum=&idkey=NONE&Input=View+first+page

Don’t Fall Victim To Undersized Piping

Published on by Brian FarnoLeave a comment

Pressure drops, incorrect plumbing, undersized piping, insufficient flow; if you hear these terms from tech support of your point of use compressed air products or from your maintenance staff when explaining why a process isn’t working then you may be a victim of improper compressed air piping selection.
Often time this is due to a continued expansion of an existing system that was designed around a decade old plan. It could also come from a simple misunderstanding of what size of piping is needed and so to save some costs, smaller was used. Nonetheless, if you can understand a small number of variables and what your system is going to be used for, you can ensure the correct piping is used. The variables that you will want to consider when selecting a piping size that will suit your need and give the ability to expand if needed are shown below.

  • Minimum Operating Pressure Allowed (psig) – Lowest pressure permitted by any demand side point of use product.
  • System Pressure (psig) – Safe operating pressure that will account for pressure drops.
  • Flow Rate (SCFM) of demand side (products needing the supplied compressed air)
  • Total Length of Piping System (feet)
  • Piping Cost ($)
  • Installation Cost ($)
  • Operational Hours ( hr.)
  • Electical Costs ($/kwh)
  • Project Life (years) – Is there a planned expansion?

An equation can be used to calculate the diameter of pipe required for a known flow rate and allowable pressure drop. The equation is shown below.

A = (144 x Q x Pa) / (V x 60 x (Pd + Pa)
Where:
A = Cross-Sectional are of the pipe bore. (sq. in.).
Q = Flow rate (cubic ft. / min of free air)
Pa = Prevailing atmospheric absolute pressure (psia)
Pd  = Compressor discharge gauge pressure (psig)
V = Design pipe velocity ( ft/sec)

If all of these variables are not known, there are also reference charts which will eliminate the variables needed to total flow rate required for the system, as well as the total length of the piping. The chart shown below was taken from EXAIR’s Knowledge Base.

Once the piping size is selected to meet the needs of the system the future potential of expansion should be taken into account and anticipated for. If no expansion is planned, simply take your length of pipe and start looking at your cost per foot and installation costs. If expansions are planned and known, consider supplying the equipment now and accounting for it if the additional capital expenditure is acceptable at this point.

The benefits to having properly sized compressed air lines for the entire facility and for the long-term expansion goals makes life easier. When production is increased, or when new machinery is added there is not a need to re-engineer the entire system in order to get enough capacity to that last machine. If the main compressed air system is undersized then optimal performance for the facility will never be achieved. By not taking the above variables into consideration or just using what is cheapest is simply setting the system up for failure and inefficiencies. All of these considerations lead to an optimized compressed air system which leads to a sustainable utility.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF