Calculating CFM of Air Needed for Cooling

It’s easy to know that EXAIR’s vortex tubes can be used to cool down parts and other items, but did you know that our air knifes can be used to cool down these same things? It’s the same process that we do every day to cool down hot food by blowing on it. Every molecule and atom can carry a set amount of energy which is denoted by physical property called Specific Heat (Cp); this value is the ration of energy usually in Joules divided by the mass multiplied by the temperature (J/g°C). Knowing this value for one can calculate the amount of air required to cool down the object.

Starting out you should note a few standard values for this rough calculation; these values are the specific heat of Air and the specific heat of the material. Using these values and the basic heat equation we can figure out what the amount of energy is required to cool. The specific heat for dry air at sea level is going to be 1.05 J/g*C which is a good starting point for a rough calculation; as for the specific heat of the material will vary depending on the material used and the composition of the material.

Heat Flow Equation
Using the standard heat equation above add in your variables for the item that needs to be cooled down. In the example I will be using a steel bar that is 25 kg in mass rate and cooling it down from 149 °C to 107 °C. We know that the specific heat of steel is 0.466 J/g°C therefore we have everything needed to calculate out the heat load using air temperature of 22 °C.
Calculating Joules/min
Using the heat rate, we can convert the value into watts of energy by multiplying the value by 0.0167 watts/(J/min) which gives us 16,537.18 watts. Furthermore, we can then convert our watts into Btu/hr which is a standard value used for cooling applications. Watts are converted into Btu/hr by multiplying by 3.41 Btu/hr/watt, giving us 56,391.77 Btu/hr.
Converting Joules to Btu/hr
Once you have Btu/hr you can plug the information into a re-arranged Cooling power formula to get the amount of CFM of air required for cooling.
Calculating CFM
As you can see in order to cool down this steel bar you only need to 343 CFM of air at 72°F. This can be done very easily and efficiently by using one of EXAIR’s Air Amplifiers or Air Knife. Sometimes you don’t need to use a vortex tube to cool down an object; sometimes simply blowing on it is good enough and its pretty simple to calculate out which product would fit your application the best.

If you have any questions about compressed air systems or want more information on any EXAIR’s of our products, give us a call, we have a team of Application Engineers ready to answer your questions and recommend a solution for your applications.

Cody Biehle
Application Engineer
EXAIR Corporation
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EXAIR Digital Flowmeters With Wireless Capability

“You can’t manage what you don’t measure” is a well-known axiom in engineering & process improvement circles.  We talk to callers every day who are keen on conserving compressed air use in their facilities by making a few tweaks, considering a complete overhaul, or more often, some point in between.  Bottom line (literally) is, compressed air isn’t cheap, so small gains in efficiency can add up.  And large gains can be complete game-changers…following our Six Steps To Optimizing Your Compressed Air System has resulted in users being able to shut down 50 and 100 HP air compressors, saving thousands of dollar A MONTH in operating costs.

Step #1 is measurement, and that’s where the EXAIR Digital Flowmeter comes in.  They’re easy to install, highly accurate, extremely reliable, and available for just about any size pipe used for compressed air distribution.  They can output a 4-20mA signal straight from their PCB board, or serial comms (RS485) through an optional control board.  USB Data Loggers and Summing Remote Displays have proven to be value-added accessories for data management as well.

Summing Remote Display (left) for remote indication and totalizing data. USB Data Logger takes data from the Digital Flowmeter to your computer and outputs to its own software (shown above) or Microsoft Excel.

If you want to go wireless, we can do that too: using ZigBee mesh network protocol, a radio module is installed in the Digital Flowmeter with wireless gateway to transmit data to an Ethernet connected gateway.  The transmitting range is 100 ft (30 meters,) and the data can be passed from one radio module to another, allowing for multiple Digital Flowmeter installations to extend the distance over which they can communicate with the computer you’re using for central monitoring.  Advantages include:

  • Wireless monitoring of EXAIR Digital Flowmeters throughout your plant.
  • Prevents unwanted joining upon the network.
  • Monitoring software is included at no extra charge.
  • Measures & transmits both current air usage, and cumulative air usage data.
  • 128 bit encryption for wireless transmissions.
  • Comes configured & programmed, out of the box, available for installation on 1/2″ to 4″ SCH40 iron pipe, or 3/4″ to 4″ Type L copper pipe.

Digital Flowmeter w/ Wireless Capability, Gateway, and Drill Guide Kit

If you’d like to find out more about how easy it is to measure, manage, and optimize your compressed air usage, give me a call.

Russ Bowman
Application Engineer
EXAIR Corporation
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Proper Air Supply Line Size Equals Proper Performance

Pipe_460top
Properly sized piping will allow your compressed air operated equipment to operate efficiently!

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.

Chart2

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.

If you would like to discuss Line Vacs or any EXAIR product,  I would enjoy hearing from you…give me a call.

Steve Harrison
Application Engineer
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CFM, ICFM, ACFM, SCFM: Why so many volumetric flow rates?

Air Compressor

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 = 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 (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)

Where:

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:

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 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.

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