Guns vs. Bells: The Impact of
Ambient Conditions on
Particle Temperature
Guns vs. Bells: The Impact of Ambient Conditions on Particle Temperature
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.
At 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’s virtually impossible to measure the temperature of individual droplets in the cloud, it’s fairly straightforward to calculate the change in temperature.
Saint Clair Systems has developed tools that perform these calculations quickly and easily to help coaters better evaluate and plan their process control strategies. An example calculation is shown in the figure “Particle Temperature Change Calculations”.
In this figure, we can see some critical scenarios played out together for easy comparison.
Remember our discussion of atomization from the second blog in this series? We noted that guns move particles toward their target much faster than bells.
According to Carlisle Fluid Technologies, bells create particles with speeds ranging from 150 – 300 mm/s, whereas guns create particles with double the speed, ranging from 300 – 600 mm/s.
That means that particles are in the air for an average of about 0.42s – 1.69s.
Despite the large surface area presented to the ambient air, that’s not much 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 fairly common situation for painters on a summer day: the booth temperature is 77°F (25°C) and the paint temperature is at 90°F (32°C) coming from a circulation system that is run through the truss level from the mix room into the booth.
Because a paint spray gun creates high-particle velocities, resulting in shorter time in the air, the paint loses between 0.25°F – 0.75°F — but is still above 89°F when it reaches the part. Even with relatively longer “in air time” (due to lower bell velocities), the paint only changes by 1.1°F – 2.3°F. Worst case, it still reaches the part at nearly 88°F.
If you’re assuming your paint is being applied at 77°F but it’s actually at (or above) 88°F, it might be 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 booth temperature.
In the final post in this series, we’ll look at ways to choose between the two different applicators we’ve been discussing.