Ask the Expert Question-and-Answer Archive

by Ron Joseph

December, 2000

Part Size and Booth Face Area

A. Is there a rule of thumb for determining the the ratio of part size to booth face area?

A. To the best of my knowledge there is no such rule of thumb. If I were to design a spraybooth I would consider the maximum part size, conveyor line speed, hook spacing, and the time it will take the painter to coat the part.

If the parts are to be coated in a batch process, I would consider the size of the racks on which the parts will be suspended. If possible, I would design the rack to hold as many parts as possible, within reason, and keep them closely spaced so that the transfer efficiency of the coating application can be maximized. If two racks will stand in the booth side-by-side, this must be taken into account.

While your vendor will help you specify the dimensions of the spraybooth, you might want to insure that the height is at least 2 ft. higher than the top of the part. If you can afford to make it even higher, then so much the better. On the other hand, the larger the face opening, the more air you will need to supply to insure an adequate air velocity toward the filters. (Many companies still go by the old rule of 100 ft/min.) In addition, the filters will need to be replaced when they are clogged with overspray, and to keep the cost of hazardous waste to a minimum, you will not want to be too generous with the dimensions of the face opening.

If you have a range of part sizes, then consider the lowest and highest points from the floor that the painter will need to bend or reach to get the tops and bottoms painted. Again, I would allow at least 2 ft below and above these two limits to establish the vertical face opening of the booth. If you can afford to be more generous, then allow even more space.

If the part has a complex design and the painter will need to point the spray gun upward when painting the top of the part, or point the gun downward when painting the bottom, you will need to allow additional height to the filter opening to take these issues into account.

If the part is painted in a batch process and sits on the floor, then the filter bank should go down to the floor.

When considering the width of the face opening, you should allow the painters sufficient space along the side walls to walk around the part - a minimum of 3 ft along each wall is preferred.

Let us consider a typical conveyor system. If the part is 3 ft wide, moves through the booth at 10 ft/min, and it takes the painter two minutes to coat the entire part, then the conveyor hook from which the part is suspended will move through a distance of 20 ft. In most cases the painter cannot start painting immediately the hook appears through the conveyor opening in the side wall of the booth, and so we must allow some additional space at each end. Considering a generic part geometry, we might allow 3 ft. at the conveyor entrance and another 2-3 ft at the far end. Based on these assumptions, the booth will be at least 25-26 ft wide.

Despite your accurate calculations, please note that in most cases the filters come in 20" x 20" squares, or come on wide rolls. Before you settle on the final design specification, I suggest that you first establish the widths of the filters you intend to use.

Having given you these estimates, please bear in mind that the capture efficiency of the booth will also depend on the airflow. I have visited numerous facilities in which the filter openings were adequately sized, but because the airflow was wildly turbulent, overspray settled on the ceilings, walls and floors. I'm sure you have seen such booths.