The Bernoulli effect
The French physicist, Daniel Bernoulli, postulated a set of equations that relate the velocity of a fluid to the ratio of the areas. It simply states that with a fixed volume of air flowing through a system of varying cross-sectional area, the velocity changes in proportion to the area. This is the effect we studied above and it has an interesting corollary for the aircraft paint booth designer.
If air is introduced into the booth in such a way as to give a uniform cross-sectional velocity, the air will degrade in its journey to the exhaust end of the booth, whether crossdraft or downdraft. The causes of this degradation are the introduction of paint spray being released (causing added mass), the mass of the painter and helper, and the slow velocity of the air. At 100 fpm, the air is moving at 1.6 mph, which is slower than most people walk. It doesn’t take much of a disturbance to cause the streamlines to degrade.
The presence of the aircraft, workers, and the spray itself are all factors. Once control of the streamlines is lost, control of the overspray is lost. How do you regain control of the streamlines? Bernoulli tells you! You accelerate the air from the booth intake to the exhaust.
An intake velocity of 100 fpm must accelerate to an exhaust velocity of 150 fpm or better to overcome the challenges of streamline degradation.
Location of overspray
Most of the overspray will be within a few feet of paintable surfaces of the product (regardless of the product). Consider the differences in the product shape of a bus versus an aircraft. In most cases of poorly operating aircraft paint booths, this is a difference that was not appreciated by the designer.
If you are operating a crossdraft paint booth, the air in the booth must travel from the nose to the tail. Looked at from above (Figure 2), it is easy to see that 70 percent of the paintable surfaces are within a few feet of the fuselage. That means that 70 percent of the overspray will be within a few feet of the fuselage.
A prudent designer would look at this “spraying zone” with a critical eye and be sure to concentrate his or her efforts at controlling airflow and streamlines in the “spraying zone.” This can be controlled by careful location of the exhaust chambers. We have seen many paint booths that have exhaust chambers located in the corners of the booth. This may work well with trucks and buses but it is a disaster when the product is an aircraft. In this scenario, the air comes in at the nose of the aircraft and then splits and heads diagonally away from the aircraft to the corners, leaving large clouds of overspray hanging in the air.
An appreciation of spraying zone dynamics is essential to a successfully implemented design.
It is not so obvious with downdraft paint booths. The spraying zone appears to be a much greater area. While that is true, we can use the same principles to control overspray in the spraying zone. In this case, most of the air is directed from the ceiling to the grates in the floor over a short distance (compared with the crossdraft version). If the air is introduced in a reflected shadow of the aircraft and withdrawn in grates that occupy the shadow of the aircraft, then air is forced to move from the ceiling to the floor and hug the surfaces of the aircraft.
Or will it? While it is easy to say this, the reality is that the air wants to expand into the areas where there is no air being introduced. If we apply the Bernoulli effect, we can cause the air to accelerate at just the point where expansion of the air would normally take place, ruining our streamlines. Air emerging from the ceiling will follow its own streamlines for a good distance without expanding into the nearby spaces. When it wants to degrade, the designer must start to accelerate it sharply and continue to carry the increasing airspeed over the wing and under the fuselage to the exhaust grates.
Door styles for aircraft booths
Since getting air to the fuselage is so important, the use of filter doors allows the designer the greatest flexibility in aircraft booths. Used mainly in aircraft painting booths, bottom rolling filter doors are intended to give even distribution of air across the whole width of the paint booth. This affords more even distribution of air and control of airflow streamlines. The filter door also polishes the quality of the air-removing particulates.
Global Finishing Solutions manufactures paint booths, ovens, and finishing systems for parts and aircraft. For technical support call (800) 848-8738. Hours: 7:30 a.m. - 5 p.m. CST.