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Guns vs. Bells: The Impact of Ambient Conditions on Particle Temperature

Posted by Mike Bonner

Mar 15, 2018 2:47:00 PM

Editor’s Note: Our VP of Engineering & Technology Mike Bonner recently spoke at the 2018 Waterborne Symposium held in New Orleans, LA. His presentation went in-depth on the differences in atomization between spray guns and bell atomizers, shedding light on how paint finishers can further improve their applications. For those not in attendance, he’s also prepared his insights here for our blog as a series of posts. To view the rest of the posts in the series, visit the Guns vs. Bells blog series page.  Also, watch for the video of that presentation, arriving soon on our YouTube Channel.

It is widely believed that it is important to carefully control booth temperature because it directly affects the temperature of the paint as it is being applied.

On first blush, this seems like a logical assumption. After all, the atomized droplets are extremely small, and there’s a huge number of them, which presents a large surface area to the ambient air when compared to the bulk fluid.

The reality, however, is much different. While it is virtually impossible to measure the temperature of individual droplets in the cloud, it is fairly straightforward to calculate the change in temperature.

Saint Clair Systems has developed tools to perform these calculations quickly and easily to assist coaters in better evaluating and planning their process control strategies. An example calculation is shown in the figure “Particle Temperature Change Calculations.”Figure: Particle Temperature Change Calculations

In this figure, we can see some critical scenarios played out together for easy comparison. 

Remembering our discussion of atomization from the second blog in this series, we noted that guns move particles toward their target at much higher speeds than do bells.

According to Carlisle Fluid Technologies, bells create particles with speeds ranging from 150 – 300 mm/s, whereas guns create particles with speeds ranging from 300 – 600 mm/s — double that of the bell.1

This means that the average time that the particles are in the air ranges from 0.42s – 1.69s.

In spite of the large surface area presented to the ambient air, this is not a long time to effect a change of temperature.

This is especially easy to understand when we consider the insulative properties of air, which has a U-value of just 0.2 BTU/ft² hr °F.

Consider a situation where the booth temperature is 77°F (25°C) and the paint temperature is at 90°F (32°C) coming into the booth from a circulation system that is run in the truss level from the mix room on a summer day — a fairly common scenario for many painters.

With the high particle velocities created by the gun and resulting shorter time in the air, the paint loses between 0.25°F – 0.75°F — reaching the part still above 89°F.  Even with the relatively longer time in the air caused by the lower velocities of the bell, the paint only changes by 1.1°F – 2.3°F — again in the worst case, still reaching the part at nearly 88°F.

If you are assuming that your paint is being applied at 77°F and it is actually at (or above) 88°F, you may find it very difficult to make the right decisions to keep your finish quality in spec.

This is why modern progressive coaters consider controlling paint temperature at the point of application to be more important to finish quality than controlling both temperature.

In the final post in this series, we’ll look at ways to choose between the two different applicators we’ve been discussing.

Subscribe to our blog to get the next installment in this series delivered directly to your inbox. In the meantime, if you’re having issues with your spray finishing operation, contact us directly or book a meeting to discuss your application.

1 – Bell and gun particle velocity data provided courtesy of Carlisle Fluid Technologies.

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Topics: Viscosity, paint and coating, Guns vs Bells Series

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