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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Calculating Air Volume for Cooling

Motores_en_el_robot
Robot motor in need of cooling

 

dims
Accompanying information about motor in need of cooling

Usually, when discussing application solutions we can make recommendations for proper product based on experience, empirical test data, and application parameters.  Sometimes, though, we need to take things just a little further and aim to dial in the recommended solution before any testing ever occurs.

I recently had an exercise in this, involving the need to cool the robot motor shown in the photo above.  This motor, existing in two forms (one weighing 23kg and the other weighing 25kg) is currently operating, creating heat, and registering a temperature of 90°C.  The desired operating temperature is 60°C, and we can safely assume an ambient temp. no higher than 35-40°C.

The questions posed to me were:  “Which product should be used to cool this motor?  And, how do you know?”  So, I took a certain degree of liberty (though not much) in considering the motor in question is comprised of copper windings, and these windings comprise the total weight of the motor.

Considering this, our knowns for this application were:

Weight:                              23kg and 25kg

Material:                            Copper

Starting temp:                   90°C

Ending temp:                    60°C

 

What we didn’t know was:

Specific heat of copper:                  (determined to be 0.385 Joules/g°C)

Amount of airflow to cool this motor by 30°C:                     XXX cubic feet per minute

 

This airflow was determined using the process shown below, and the resulting calculations shown below.

heat load calc process
Process to calculate the required airflow in a cooling application

 

heatcalcs
The calculations used to determine the required airflow in this application

 

Super Air Amplifier Performance Specs
Performance specifications of our Super Air Amplifiers

The end result was confirmation that EXAIR model 120022, our 2” Super Air Amplifier, can use just 15.5 SCFM of compressed air at 80 PSIG to produce an airflow to cool this motor.  And, thanks to the skills of the team here at EXAIR we have the numbers to back up that claim.

If you have an application with a similar need and think we may be able to help, contact an EXAIR Application Engineer.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE