They're Different

Aircraft paint booths

What are the design differences that make an aircraft paint booth successful? This is a common question that is frequently answered incorrectly or indifferently. I will try to show some distinctions that make aircraft paint booths different from booths that are used to paint trucks, buses, cars, construction machinery, and the like.

These distinctions will suggest to the designer several different design techniques that spell the difference between success and failure.

Large equipment paint booths
Examine carefully the relationship between the product to be painted and the paint booth geometry and you will discover many subtle differences. If you look at the cross-sectional area occupied by a bus or railcar in a typical crossdraft booth, you will find that the ratio of the booth cross section (Area A2 in the figures on the right) and the product cross section (Area A1 in the figures) is about 70 to 80 percent. If a prudent designer uses an average cross-sectional velocity of 100 fpm for the booth, the velocity around the object will be four times that design velocity. This mathematical relationship is what makes the booth work correctly. Overspray is not subject to the 100 fpm velocity but to a 400 to 500 fpm velocity.

Staying, for the present, with crossdraft booths, look at the ratio of the aircraft cross section to the booth cross section. Looking at Figure 1, the aircraft cross-sectional area occupies only 15 percent of the cross-sectional area of the paint booth. If the designer uses a velocity of 100 fpm average across the booth cross section as above, the airflow over the aircraft surface (where the overspray will be found) may only be 125 fpm.
I submit that two things are in play here. The first is that 400 to 500 fpm is too high and probably leads to capture of particles that should be included in the product’s finish. The second is that 125 fpm will not do as good a job as a higher velocity. It is hard to say what the ideal velocity is for capturing overspray. Much depends on the paint and its characteristics (such as high solids vs. low solids, volatiles used, etc.). It should be obvious that there is more to work with when more of the booth is filled with product. If the actual velocity in the booth is running 400 fpm and too much overspray is reporting to the filters, it is easy to reduce the fan speed and get better performance.

What happens in a downdraft paint booth? In the large equipment example of painting a bus, the ratio of product cross section (as seen from above) is very nearly the same 75 to 80 percent as in the crossdraft booth example. Thus, the overspray collection velocity is still about 400 to 500 fpm.

Let’s look at the plan view of an aircraft (A1) in a downdraft paint booth (A2) as shown in Figure 2. You will note that the ratio is still only 20 to 25 percent in this view. However, a 5 percent increase (above the crossdraft example) on such a small number is significant and can materially affect the success of the booth, especially if combined with other design enhancements. The cross-sectional velocity may increase to 150 to 160 fpm. This is a good effect, but if a way could be found to get the airflow faster, it would work even better.

It is obvious that the booth area should be decreased to as low a number as practical to increase the ratio. This may be done by using conformal design. In this practice, the shape of the booth more closely conforms to the shape of the aircraft. For instance, aircraft are low over the fuselage and wings and are high at the tail. Installing conformably shaped surround walls that allow a slot for the tail to fit into with a low roof over the wings will reduce the booth area in a crossdraft booth and improve the area ratio. Any improvement in area ratio represents drastic improvement in overspray control.

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