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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Adjustable Spot Cooler Easily Cools Aluminum Plates

Cooling Unit- Iso View
This setup needed a method to cool a 400g aluminum plate (click for larger view)

When we work with our customers about cooling applications, we usually approach them in one of two ways – either with a need to cool down to ambient temperatures, or a need to cool below ambient temperatures.  If we only need to cool to ambient conditions, we can use Super Air Amplifiers, Super Air Knives, Air Nozzles, or Safety Air Guns.  But, if we need to cool below ambient temperatures, we have to use a vortex-based technology such as a Vortex Tube, Adjustable Spot Cooler, Mini Cooler, Cold Gun, or Cabinet Cooler.

One of our overseas customers reached out to me with a need to cool a 400g aluminum plate from 80°C (176°F) to 27-30°C (80-86°F) over the course of about 40-60 seconds.  They needed something repeatable, dependable, and compact, so we explored a Vortex Tube-based solution.

heat load calc process
Process flow for calculating required air volume in a cooling application (click for larger view)

We’ve shown airflow calculations for our Air Amplifiers in the past, and I used the same rationale (shown above) to determine the required air volume to cool this application.  In these types of applications, we derive a ΔT value between the desired ending temperature of the material and the temperature of the air which will be blown onto the material.  So, for this application, we have a desired end temperature of 30°C, and we have considered an outlet temperature of 0°C from a Vortex Tube device.

After determining all the required values, I performed the calculations shown below to arrive at a volume flow of 15.8 CFM.

Cooling calculations for ASC in aluminum plate application
Calculations for determining the required air volume in this application (click for larger view)

In uncovering that the ambient conditions may change, and sharing that these changes could create variance in the required air volume, the end user opted for two model 3925 Adjustable Spot Coolers to provide the required cooling.  Just like Vortex Tubes or Cold Guns, the Adjustable Spot Coolers allow for repeatable delivery of very cold air to the application, but they also have the additional benefit of easy “fine tuning” through the adjustment valve on the hot end of the unit.

Cooling Unit - Details
The cooling unit for the aluminum plates (click for larger view)

The cold air created by the Adjustable Spot Coolers are fed to the nozzles shown above in red.  By supplying an adequate volume of cold air at the right temperature, the problem in this application has been easily solved.

If you have an application with a heating concern, give us a call.  We’re happy to help.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

Calculating Air Flow to Cool Manufacturing Processes

IMG_7065
This application needed a way to cool steel plates from 150C to 70C

I’ve written before about using ambient air to cool an application, calculating the required airflow to maintain a temperature.  And, I was recently contacted by an end user in India in need of a way to cool electromagnets in a similar application.

The need was to reduce the temperature of high manganese steel plates (dimensions of 1800mm x 800mm x 500mm) from 150°C to less than 70°C, using air at 40°C.  These steel plates have a specific heat of 0.5107896 J/g°C, weigh 120kg each, and protect the coil and insulation of the electromagnets in this process.  So, just as was the case in previous applications, we started with the process shown below.

heat load calc process
Heat load calculation process

In doing so, we calculated a heat load of 279,245 BTU/hr., which will require an air volume of 1,805 CFM to cool as needed.  (Click the image below for an expanded view of the calculations)

Electromagnet calculations
Heat load calculations

The recommendation to provide this cooling was the use of (6) 120022 Super Air Amplifiers, operated at 80 PSIG and installed along the length of the plates to distribute airflow.  As we can see in the chart below, each 120022 Super Air Amplifier will move an air volume of 341 CFM at the outlet of the unit, making (6) of these units suitable for this application.  And, if we consider entrainment of additional ambient air at distances away from the outlet of the 120022 Super Air Amplifier, we can consider these units may cool the steel faster than the 1 minute cycle time used for calculation purposes.

air amp chart
Super Air Amplifier performance chart

This application is a great example of how an engineered compressed air solution can remove process disturbances effectively, and efficiently solve problems.  If you have a similar application or even one that is entirely different, contact an EXAIR Application Engineer.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE