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 future posts in the series, check the Guns vs. Bells blog series page over the following weeks on Thursdays and Sundays. Also, watch for the recording of this presentation to be put up on our YouTube channel shortly…
For today’s Guns vs. Bells Series post, we’re going to discuss the similarities and differences between the two types of spray applicators, as well as go over the fundamentals of atomization.
Similarities and differences between guns and bells
Because they both do the same job, it is easy to understand that there are many similarities between guns and bells. Both atomize the coating into a cloud, creating a fan pattern that can spread out over the surface of the target part.
Both use compressed air to “shape” the fan pattern. Both can be used in electrostatic applications, where the paint particles are charged at a high voltage and the part is grounded to create an “attraction” between the paint droplets and the part. This helps to reduce overspray, get more of the paint on the part, and increase transfer efficiency.
Figure: Gun vs Bell Atomizers
While both create a fan pattern, it’s obvious from the figure “Gun vs. Bell Atomizers” that the patterns created can be very different. This is due, in part, to the differences in the way that the atomized cloud is created. We will explore that in detail in the next section.
Bells are larger and heavier than guns. This makes guns more suitable for manual spray applications, providing an operator greater control with less stress and fatigue. Bells are generally limited to robotic, reciprocator, or otherwise automated applications.
While any paint applicator is susceptible to maintenance and cleaning issues, bells are more complex, with lots of moving parts. In general, bells require more maintenance than guns.
Bells are generally used with lower viscosity fluids supplied at lower pressure, whereby guns may be better suited for higher viscosity, higher pressure applications.
Comparing atomization between guns and bells
In short, atomization is the result of applying shear, which tears the fluid stream into a cloud of tiny particles.
The rotating cup on the bell generates shear in the fluid by adding force perpendicular to the direction of the fluid stream as it reaches the edge of the cup. The size of the particles is primarily determined by the design of the cup itself, the flow rate of the coating, which determines the rate at which fluid is delivered to the edge of the cup, and the speed of rotation, which determines the speed of the cup edge relative to the fluid stream.
As a result, most of the energy imparted to the particle is perpendicular to the bell and parallel to the part. Without some means of directing the cloud, it would simply hover adjacent to the part with very little fluid actually reaching the surface. Shaping air is then used not only to “shape” the fan pattern, but also to direct it toward the part.
Guns, on the other hand, generate shear by reducing the diameter of the fluid stream, which increases its velocity, and then forcing it through a small orifice. This breaks the fluid stream into small particles creating the atomized cloud.
Atomization is controlled by the size and shape of orifice, and the flow rate of fluid through it, which is generally a function of the characteristics (size and shape) of the flow path, the viscosity of fluid, and the pressure behind it.
The fan pattern is also both shaped, and directed, by the shaping air, but because the fluid stream is already moving toward the part at a high velocity when it is atomized, guns create particles with a higher velocity toward the part than do bells.
So how do these differences in atomization affect our day-to-day coating operation? We’ll go over that in our next post discussing the in-depth experiments we performed at Carlisle Finishing Technologies’ lab in Toledo, OH.
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.
Images: Gun photograph provided courtesy of FreeImages.com. Bell Atomizer photograph provided courtesy of Carlisle Fluid Technologies.
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