Tag: air velocity

Pressure Drop vs Differential Pressure

Published on by John BallLeave a comment

I find myself interchanging these terms; pressure drop and differential pressure.  This is very common as both are determined by the change in pressure between two points.  In this blog, I will cover the difference between these two terms in my view.

Pressure drop only occurs when the air is flowing.  The higher the velocity, the higher the pressure drop.  Velocity is created when the pressure changes.  So, the higher pressure will go toward the lower pressure.  But we wish that pressure difference to be as low as possible.  Pressure drops are always a loss, and you cannot regain that energy.  Forms of pressure drop that can be found are small diameter pipes or tubing; restrictive fittings like quick disconnects, and undersized conditioning equipment like after coolers and air dryers.  If a pressure drop is too large, the pneumatic equipment will not have enough power to operate effectively and efficiently.  I have another blog with a video that helps demonstrate this, “Pressure Drop and its Relationship to Compressed Air”. 

Differential pressure can be static or flowing.  It is very similar to pressure drop except that the energy is stored.  The most common device that does this is the pressure regulator.  You can reduce the pressure downstream to the point-of-use.  This type of pressure reduction will save you money, instead of wasting money.  For every 10 PSI reduction in pressure, it will save you 5% in energy.  With blow-off devices, you want to use the least amount of pressure to “do the job”.  Over-driving compressed air pressure is a common and wasteful condition found in facilities.

Here is a graph of a typical compressed air system.  As you can see, the typical pressure drop from the air compressor to the point-of-use.  So, if you can reduce the pressure drop through the system and optimize the differential pressure from the regulator to your point-of-use, you can enhance your compressed air system.

Pressure Drop Chart

In a simple statement, pressure drop loses energy while differential pressure stores energy for later use.  EXAIR offers a variety of efficient, safe, and effective compressed air products to fit within the demand side and which can help to reduce pressure drops within a system.  This will include the EXAIR Super Air Knives, Super Air Nozzles, and Safety Air Guns.  If you wish to go further in optimizing your system, an Application Engineer at EXAIR will be happy to help you.

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

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

EXAIR Adjustable Air Amplifiers Helps Clear the Smoke

Published on by John BallLeave a comment

Smoky Foundry
Smoky Foundry

An overseas customer had a problem with their coal blasting furnace. As the workers would open a 1.2 meter by 1.2 meter door to shovel in coal, the foundry would fill with smoke.  This was a hazard and a nuisance for the crew.  They saw articles about how EXAIR Air Amplifiers were used in smaller ovens for exhausting hot flue gases, and they wondered if the EXAIR Air Amplifiers could be used for something much larger.

He sent me an email with some additional details about their furnace system. They had a fan that was mounted in the stack that had a capacity of 50 m^3/min.  This was fed into a filtration collection system to remove the residue byproducts.  The temperature inside the furnace was approximately 450 deg. C.  From this information, I could calculate the required velocity to keep the smoke inside the furnace.

Smoke starting to migrate out of the opening
Smoke starting to migrate out of the opening

In sizing this application, I determined that I could use an equation from Heskestad and Spaulding. This equation was developed to find the minimum velocity required to keep smoke from egressing into corridors during fires.  In this case, we were keeping the smoke from egressing into the foundry.  The formula looks like this:

V = 0.64 * Sqrt(g * H * (T – To)/T)      Equation 1

V – Velocity (m/s)

g – Gravitational acceleration (9.8 m/s^2)

H – Height of Opening (meters)

T – Avg. Fire Temperature (Kelvin)

To – Avg. Space Temperature (Kelvin)

In this equation, we are mainly fighting the forces of the temperature difference from inside the hot furnace area to the outside cooler area.  The outside area was near 40 Deg. C, and this gave me the temperature difference.  In converting these temperatures to the absolute temperature, Kelvin.  I calculated the fire temperature, T, to be 450 Deg. C + 273 = 723 Kelvins; and the space temperature, To, to be 40 Deg. C + 273 = 313 Kelvin.

In placing the given information into Equation 1, the minimum velocity could be found.

V = 0.64 * Sqrt(9.8 m/s^2 * 1.2m * (723K – 313K)/ 723K)

V = 1.65 m/s

 

If the velocity could be maintained at this mark of 1.65 m/s, then the smoke could not egress into the plant.  They had a stack fan that was flowing 50 m^3/min, or 0.83 m^3/sec.  We can determine the velocity that the stack fan was producing by calculating the flow over an area:

V = Q/A      Equation 2

V – Velocity (m/s)

Q – Flow (m^3/sec)

A – Area (m^2)

 

With a door opening of 1.2m by 1.2m, or 1.44m^2, the velocity can be calculated by placing the known values into Equation 2:

V = (0.83 m^3/s) / (1.44m^2)

V = 0.58 m/s

Now we can see why they were getting smoke pluming from the coal furnace into their facility. They required a minimum of 1.65 m/s, and the stack fan was only drawing 0.58 m/s.  If we take the difference, we can determine how much additional velocity will be required to keep the smoke within the furnace: 1.65 m/s – 0.58 m/s = 1.07 m/s.

 

To determine how much air flow would be needed to create a velocity of 1.07 m/s through the door opening, I just had to rearrange Equation 2 to determine the flow, Q.

Q = V * A = 1.07 m/s * 1.44 m^2 = 1.54 m^3/s

To better correlate the flow data, I converted 1.54 m^3/s to 92.4 m^3/min of air flow.

EXAIR's Adjustable Air Amplifier
EXAIR’s Adjustable Air Amplifier

EXAIR Air Amplifiers are designed to have large amplification ratios (the ratio between the amount of ambient air being moved compared to the amount of compressed air used).  This makes them perfect as an efficient air mover.  Being that this was a furnace application, the High Temperature Stainless Steel Adjustable Air Amplifier was required.  This Air Amplifier has a temperature rating of 374 deg. C, and it can be easily mounted at a safe distance to meet this temperature requirement.  The largest unit that we stock is the model 6034, a 4 inch (10cm) Stainless Steel Adjustable Air Amplifier.  It has a 24:1 amplification ratio that can create an outlet flow of 34 m^3/min.  (It would only need 1.42 m^3/min of compressed air at 5.5 bar to create this outlet flow).  For this customer to reach the 92.4 m^3/min to keep the smoke from escaping, he would need to install three units (3 * 34 m^3/min = 102 m^3/min).  He mounted the Stainless Steel Adjustable Air Amplifiers to some extraction wyes in their stack and added solenoids to them.  So, when the crew opened the door to load the coal, the Air Amplifiers would operate to keep the exhaust smoke from filling the room.  The company and operators were very satisfied as it made the environment clear to see and safe to work.

If you have an application where smoke and fumes are a nuisance, you can contact an Application Engineer at EXAIR to see if an Air Amplifier would work in your application.

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