Turbulence arises for several reasons:

• Spray booths are improperly designed.
• Modifications have been made without taking into consideration the original design factors.
• Spray booths are not properly maintained.
• Painters, supervisors, and engineering managers do not understand how a spray booth functions and therefore do not know how to recognize malfunctions. This author has found this to be true in more than 90% of the facilities he has visited for more than 20 years.

This article will provide examples of air turbulence that were caused for one or more of the previously stated reasons.

How to Measure for Air Turbulence

The methodology is extremely simple. Stake out nine or more locations inside the spray booth that represent positions where a painter can be expected to stand during the painting operation. For instance, Figure 1 represents an enclosed drive-through, cross draft booth (in this article the term "cross draft" also means "side draft."

Here we refer to a spray booth in which the air travels parallel to the floor) in which incoming air enters through the filtered back doors.

In this booth we arbitrarily selected nine points at which air velocity was to be measured. Three points were located approximately three feet from the left wall, three points down the center line, and the last three approximately three feet from the right wall. Points one thru three were above the side access door and a few feet upstream of the exhaust filters. Points four though six were in the center of the booth, behind the side access door, and points seven to nine were approximately four feet in front of the back doors, through which the makeup air entered from the outside.

From experience, we have found that the air velocity along the side walls is often immeasurably low, and hence we rarely stand closer than three feet from the walls.

Air velocity is measured using an air velometer such as an anemometer. If you measure velocity in a cross draft spray booth, the velometer must be held so that only the air travelling parallel to the floor will be measured. In a down draft booth, the velometer is held so that vertical air movement is measured from ceiling to floor.

Turbulence is measured by measuring the air velocity at a single location for several seconds. Because this is a time-consuming process, we usually measure the air velocity for two minutes, recording velocity every five seconds. If the reading remains reasonably constant, the airflow can be considered laminar, but if it fluctuates widely during the two-minute period, it is turbulent. In extreme situations the airflow might even reverse itself.

The spray booth that forms the basis of this article was housed inside a larger building. The painters keep the roll up doors of the outer building open during the painting operation, otherwise the exhaust fans of the spray booth suck out much of the air in the building. When the ever-increasing vacuum in the building equals the maximum pressure differential that the fans can pull, air velocity through the spray booth ceases.

Scenario #1

The first air velocity profile was performed under ideal conditions, and exactly in accordance with what the spray booth manufacturer had intended; the access door and the filtered back doors were closed (Figure 1). Figure 2 provides the profiles or all nine measuring points. At points one to three the air velocity decreased as we moved from the left wall to the right. Table 1 shows the average velocities for all nine points.

In the center of the spray booth the same trend occurred with the air velocity at point four, along the left wall, considerably higher than at points five and six. Toward the back of the booth and close to the filtered back doors the trend was almost the same, but this time the velocity at point eight in the center was lower than at point nine.

OSHA requires that the velocity in a spray booth should be in excess of 100 ft./min., but in scenario number one some of the velocities were extremely high, greater than 200 ft/min In fact, the average velocity for all nine points was 170 ft/min.

In Table 2 we have assigned a coefficient of variation (COV%) to describe the turbulence at each point, and this is based on 20 measurements recorded at five-second intervals. COV% is calculated by COV% – (standard deviation × 100)/average. Based on the many profiles we have performed over the past several years, we have concluded that a COV% less than 20 is fairly normal in a spray booth. Notice that at point three, close to the exhaust filters and along the right wall, the turbulence was higher than at point one, near the left wall. By the same token the turbulence had switched places near the back of the booth where this time point seven, near the left wall, was considerably more turbulent than at point nine along the right wall.

Scenario #2

Figure 3 shows the configuration of the spray booth when we arrived to perform the test. The access side door was wide open because one of the air compressors was out of commission and the painters needed to draw compressed air for their spray guns from a second unit. To do this they had hooked up an air hose to the second compressor and fed the hose through the opening in the door. Had the first compressor been working, the existing piping in the booth would have been used and the access door would have been closed.

When one compares Table 2 with Table 1 it is immediately evident that when the access door was open, the air velocity between the door and the exhaust filters dropped significantly. This time, however, the velocity along the right wall was more than double that in positions one and two.

An analysis of the results at the center of the booth, points four to six, show that the trend remained the same as in scenario number one; namely the velocity was highest along the left wall and then dropped progressively as we moved toward the right wall.

Even though the velocities at the back of the booth, points seven to nine, were almost half those for the same points in scenario number one, the trend from left wall to right wall was all but retained (Figure 4).

Let us now analyze the turbulence. Here we see a dramatic change from scenario number one. As soon as the access door was opened the air became horribly turbulent, especially at points one to three, between the access door and the exhaust filters. The same is true of point eight.

Scenario #3

In this scenario the original intent of the spray booth was abused. Although the access door remained closed, the two filtered doors were opened completely (Figure 5). This scenario is more hypothetical than real, but it does occur in some facilities when the machine or weldment being painted is too long for the booth. Clearly, the physical size of the object demands that the design of the booth be abused, otherwise the object cannot be painted at all.

This time, almost across the board, the air velocities were even higher than in scenario number one. The left wall-right wall trend remained for points one to three, and for points seven to nine. In the center of the booth, points four to six, the trend almost remained the same, but the velocity along the right wall was higher than in the middle of the booth (Figure 6).

The degree of turbulence for scenario number three was not very different from that in scenario number two. In terms of low turbulence, airflow in scenario number one was considerably more laminar than in the other two scenarios.

Although it might seem obvious to the reader, the spray booth performed best when it was used as intended; namely with all the doors closed. Unfortunately, in the field, painters are usually not aware of this and often operate when one or more doors are open or partially open. Worse, in many facilities, painters are allowed to enter and leave at will, and this analysis has shown how damaging the practice can be to both air velocity and turbulence. One can imagine how dramatic the change is during the first few seconds after the door is opened when the airflow suddenly changes from the first scenario to the second.

It is remarkable, although understandable, that when the back doors were opened so much more air could enter. Even though the air intake filters were of the standard open-mesh, low-pressure-differential type, they were instrumental in dramatically reducing airflow into the booth.

Another lesson one can learn is the air intake filters serve to distribute the air more evenly across the entire air intake section. When designing a high-quality booth for Class A work, such as for automobile finishing, it handsomely pays to purchase higher-resistance air intake filters so they can perform an even better job at uniformly distributing the air.

Another fascinating discovery was when the side access door was opened, the air velocity in front of it (between the access door and the exhaust filters) dropped nearly by 35%, and the average velocity at all nine points in the spray booth dropped 44%.

Although the spray booth appeared to be well designed and in good order, we were curious why the velocity was so high along the left wall and was almost double that along the right wall. The exhaust filters appeared to be the same on both sides of the spray booth. In addition, the filters in the double doors appeared to be uniform and properly installed across the face of the doors. So why did the trend change near the back of the booth, close to the air intake doors?

The puzzle was solved at the end of the afternoon when we were leaving the facility. I said earlier that the booth was an insert in a larger building and the doors of the outer building were opened before the exhaust filters were turned on. On that afternoon a strong wind was blowing diagonally across the building from right to left. Hence, the air entering through the filtered doors was not evenly distributed. This explains why, when the back doors were open in scenario number three the air velocity along the left side was 456 ft/min and only 258 ft/min along the right. The same trend was evident in all three scenarios. Presumably, had we performed the tests during the early morning or on a quiet day, the results would have been different.

How can one operate a spray booth reproducibly and efficiently when the airflow is determined by external factors that cannot be controlled by the painters? At least two solutions are possible for this facility: the two outside doors should be closed and a separate air makeup (supply) system should be installed; or a screen is required outside the building that shields the spray booth from wind, yet does not impede the amount of air that can enter the booth. By far the better alternative is the former, but it is also more costly.

Conclusions

The reader is urged not to use the conclusions that are presented here and transfer them to another booth, even a booth of similar design. The purpose of the analysis was to demonstrate how valuable air velocity and turbulence profiles can help you understand why a spray booth might not be functioning properly.

Turbulence always leads to paint appearance defects and even worse, poor transfer efficiency. It has frequently been demonstrated that strong turbulent air currents in a spray booth redirect the paint particles as they emerge from a spray gun. In an automotive assembly line where thousands of gallons of coatings are used annually, even a small improvement in transfer efficiency accounts for significant, measurable savings in paint usage, hazardous waste reduction, replacement of spray booth filters, maintenance costs, and more.

If less paint is used to perform a job, emissions of VOC, HAP, and paint particulates decrease by the same proportions. If turbulence reduces paint appearance defects, then by virtue of their relationship to paint usage, VOCs and HAPs are lowered by the same ratios. Hence, from an environmental perspective, it pays to reduce or eliminate air turbulence, because of the numerous associated benefits. It is imperative that the doors leading into a spray booth be kept closed throughout the painting operation. Operators must be prohibited from entering or leaving at will.

When something looks wrong, it probably is. In this case it didn’t make sense that the airflow along one side was so much higher than along another. Further investigation showed there was a reason for this, and that solutions were available to correct the situation.

Open-backed spray booths are vulnerable to strong cross drafts that painters might not be able to control. Yet, these can be the major causes for paint defects.

For spray booths that are inserted into larger buildings, it pays to install an air supply system rather than to keep the outside building doors open. This writer has visited many facilities of this type and during a typical day the outside doors are opened and closed by fork lift drivers and others who transport equipment into and out of the building. This article has demonstrated how detrimental this can be to the painting operation. To avoid such problems one should consider spending a little more time and money to install a properly designed air supply system that provides uniform distribution of air throughout the cross section of the booth.
 

Editor's Note

This article is based on a presentation given by the author at the Air & Waste Management Association's 97th Annual Conference and Exhibition, Indianapolis, Ind.

Bio

Ron Joseph is a paint and coating consultant with Exponent, Inc., of Menlo Park, Calif. He is also the Organic Coatings Editor for Metal Finishing.