Spray Booths & Air Flow Designs

Spray Booths have evolved quite a bit since the original early Cross drafts and I would venture to say that a lot of that change and improvement came not only from changes in paint technology but also stricter regulations concerning VOC emissions. 

When I say changes in paint technologies for those of us that are old enough to recall, the lacquer days were very forgiving. Dirt and overspray were not a problem, sanding the finish and laying on another coat would fix most problems or just buffing it with rubbing compound and a wool pad would do the trick.

Those paints went by the wayside and in came the sophisticated 2 and 3 stage paints with tricky and delicate application methods and beautiful long lasting finishes that were here to stay. One requirement for these new paints was a clean environment to apply them and with that the paint booth business grew.

Down draft, Semi down draft, Side draft or Cross draft, these different types of Automotive spray booth all get their names from the direction the air flows when it enters and exits the booth. The only exception to this might be an open face booth which has only a plenum that consist of filters and exhaust fan to filter and remove paint overspray from the shop. These are most often used to cut in parts so as to not tie up the painter or the production in the main booth. They are sometimes used in combination with plastic curtains as a makeshift small booth.

 

Prep Station and Prep Station with Side Curtains

Cross draft

The original and still very much used Cross draft booth is your basic entry level booth. The concept here is that air flows from the entry double door which is typically where the air entry filters are placed and crosses the vehicle and then gets filtered again on the way out to remove the overspray paint before being exhausted out of the shop. The downside to this design is the dragging of the overspray across already painted surfaces and producing dry dull finishes. A good painter will compensate for this and paint from the air entry side to the exhaust side, and paints over any overspray to avoid this issue.

You will see all kinds of modifications to the Cross draft design by manufactures to try and improve on the design while still offering an affordable booth. There's also a Reverse Cross draft where the doors are moved to the exhaust side to try and control dirt from entering through the entry doors and this is accomplished by changing the design of the exhaust plenum to a bridge instead of a box so the vehicle can enter from the other end of the booth. If dirt enters through the doors it's caught by the exhaust filters instead of dragged across the paint finish.

Cross draft

Reversed Cross draft

Down draft

The Down draft concept was designed with the idea of eliminating the paint overspray problem that existed with the Cross draft.  These fall into two major groups, the original take on this design which entailed a lot of floor work because the booth rested flush with the floor and the air had to be vented under the booth. The later models were elevated with raised floors and the down flow concept could still be accomplished without the floor work.

Down draft with raised floor

Conventional Down draft

There's of course many variations to the Downdraft design, Semi down draft is one where the air enters through the ceiling but exits like a Cross draft. Side down draft is another where air enters through the ceiling but exits through the bottom of the side walls eliminating the need to elevate the booth. 

There has definitely not been a shortness of ideas and designs from manufacturers here and they're usually all on display at the Autobody convention on a yearly basis for you to get a closer look at.

Side Down draft

Positive and Negative pressure

In plain terms the booth fans either push air into the booth or pull air through it. The importance of this is that a positive pressure booth by design is a cleaner booth. Depending on what part of the country you're located you'll quite often see a heater unit along with what's called the air makeup unit. This not only produces the positive air pressure in the booth but also warms the intake air in cool weather before it enters the booth. On booths with baking capabilities the heating unit is designed to elevate temperatures high enough to bake the applied finishes at the recommended times and temperatures.

Most of your basic Cross drafts are negative pressure booths and on these types of booth you don't want anyone entering or leaving the booth when there's painting in process, you'll notice that when side doors are opened the air will be drawn in not out and with it plenty of dirt.

Some of the issues that I most often see and should address here regardless of the type of booth are as follows. Maintenance is number one, often I see beautiful expensive booth that are not maintained properly and it's almost a shame to spend that kind of money and still get low quality paint finishes due to dirt. Filter replacement and booth cleaning need to be on a schedule that fits the production and volume of vehicles flowing through the booth, it will only help improve your final product.

Proper quality and installation of filters, on your Cross draft intake filters there's usually a front and a back side, one's sticky and the other is not. This helps in trapping dirt as it enters the filter, many times have I seen them installed backwards. As important are door seals on Cross drafts, if they're worn and torn it presents a steady stream of dirt and contaminants. A good way to check for any openings in the booth due to wear and tear is to get inside, close it and turn off the lights, you'll easily see if light is coming in through any holes, if so then dirt is also making it's way in. Fix and seal these holes and you will not only see improvements in your finishes but also a happier polishing and finishing department.

Finally an important thing to remember when purchasing and installing a spray booth is placement. Carefully consider where you will place the booth or you might pay dearly later with many filter changes and dirt contamination. Install booths away from areas where there will be sanding or the use of any chemicals that are airborne and might enter the spray area.