Spray Booth Filtration

by Jim MacDonald, Ontario Spray Booth – Toronto, ON, Canada
Originally published: Fall 2010 issue, Collision Quarterly

Like pretty well everything else in our little industry, nothing seems to get easier! In the good old days we had our old crossdraft paint booth and all we had to do was worry about ensuring the belt was driving the fan and the filters were not so clogged up that there was no air moving through the booth.

Three or four times a year, we would change out the paint arrestors (filters in the exhaust bank – they were usually a kraft paper mesh pad that was held in place with wire grids that fit into the filter holes/cells in the exhaust chamber). The other filter was the door filter, which was usually a sticky 2-ply polyester filter held together by a wire frame. This filter would be installed in the filter cells/holes that were in the entrance doors of the booth. Once these items were looked after it was time to paint!

As we have moved into the 21st century, our approach to our equipment, and paint spray booths in particular, has changed big time! Since the onset of clear coat finishes and now the Low VOC legislation that has caused most of us to move to waterborne, we want the inside our booths pristine and clean. We want to best control our painting environment.

Out are the old crossdraft booths and in are the downdraft systems c/w heated air make-up units and bake type burners. Like with today’s automobiles, more sophisticated paint booths have all kinds of OEM and aftermarket options and price points available for them. One of the most important areas, but often misunderstood, is paint booth filters!

Paint booth manufacturers go to great lengths to make the air flow in the booth even and compatible with the appropriate codes and desired conditions. They are engineered to move air through the booth at the proper rates while maintaining a clean, well-lit painting environment!

Over a period of time, wear and tear will take their toll on these OEM/factory start-up conditions; however, a regular maintenance program is well worth the while and will save you money in the long term.

Believe it or not, the first and foremost (and perhaps the simplest) issue to maintaining your paint booth’s original standards is simply regularly and properly changing your filters.

One of the biggest issues we see when investigating a booth that is over pressurized (you know what we are talking about – doors blowing open, overspray appearing on outside seams of booth, etc.) or under pressurized (have to fight to get door open, dirt and dust appearing on inside seams of booth etc.) is pits, exhaust ducts and fans that are coated with overspray. This obviously compromises the booth and will most definitely take its toll on the working parts (fans, blowers, etc). This can all be avoided by simply regularly and properly changing your filters.

How well do you know your filters?

We are always asked, “How often should I change my filters?” Seems like a simple question, however, the answer is different for every shop. How many cars do you paint a week? What are your daily/weekly production rates? How many shifts do you have working? What filters are we talking about (Ceiling? Burner? Pit?) These are the things we need to know before we answer that first question. There are three filters in most of today’s downdraft systems:

1. PAINT ARRESTOR (usually a pad type filter found in the floor pit)
2. BURNER/AMU (found on intake or inside of the burner/amu)
3. CEILING (overhead final filter).

How often you change your PAINT ARRESTOR (overspray trap) filter has a direct effect on how often you will have to change the other two filters.

As a rule of thumb, we recommend that shops change out their PAINT ARRESTOR (paint trap pit filter) once a month. The burner filter can be changed every three to four months and if this schedule is followed the ceiling filter may only need to be changed once every 1 year +. The key here is REGULAR changes as outlined above. If the paint arrestor is not changed it will pass overspray, which will eventually plug up the other two MORE EXPENSIVE filters. As you all know, the ceiling filter is a VERY EXPENSIVE filter and extending its life is in your best interest. The only way of ensuring that is to change the paint arrestor and burner filter on a regular basis.

We have shops that change their paint arrestor filters every week (typically high production shops), their burner filters once every two months and their ceiling filters twice a year. This has become part of their routine and the stock room keeps the filters in-house and part of the shop’s responsibility is to make sure changes are made (usually a Friday afternoon, Saturday or Monday morning). Depending on your production rates you may only need to change your paint arrestors every month/two months, however, a routine must be kept.

Dirty paint arrestors most definitely cause contamination! In today’s pressurized systems, as the pit filter loads and the AMU continues to push air into the booth, the air flow will not transition smoothly through the pit filter and may slide along the floor and up the walls of the booth in a continuous loop (ever notice colour or overspray flashes along the sides where the walls of the booth meet the floor?). This loop will get larger and larger on both sides of the vehicle until someone unplugs the floor filter. This loop can catch overspray coming out of the gun, air dry it and deposit the dried overspray on your wet surfaces, ruining your paint jobs! This costs time, which costs money!

It is in your best interest to change your filters REGULARLY (ask your booth provider or paint supplier to suggest a schedule and ensure the proper filters are readily available – PS – keep extra paint arrestors IN STOCK so you can avoid the air dried overspray issue outlined above).

Now that you have a schedule you must ensure you are using the correct filters!

Proper Filter Changes

Now that we know when we are going to change our filters, how do we know we are putting in the right ones? The easiest answer to this question is to get your filters from the people who supplied your booth and ensure they are supplying you with the OEM suggested filter.
To follow is a quick primer on the three types of filters we outlined above and what to watch out for:

1. Paint arrestors – Paint arrestor medias vary in quality and in cost! This is the least expensive filter of the three, so when you think you are saving money perhaps you are not! Remember, a filter is supposed to FILTER OUT. A line we often hear in shops is, “I love my paint arrestor filters because I never have to change them!” What does that tell you? A primary PAINT filter that you never have to change? Perhaps it’s NOT FILTERING ANYTHING! In recirculating systems this unfiltered overspray can damage the inside of burners and totally plug up fans, duct and blowers!

Most new booths supply premium paint arrestor filters that feature a 3-dimensional structure to their surfaces, made from a textured polyester media that traps paint very effectively. We have found that some shops do not like them because they plug too fast (at least we know the filters are doing their job!).

Another arrestor we like is a two-ply (kraft paper and polyester) paper/poly product. Years ago, this was used in new booths – primarily by the old DeVilbiss Company (a very popular pit media, however, it comes in differing densities and the very cheap roll is likely NOT your best choice). The old DeVilbiss got away from spun fiberglass in their recirculating booths because they believed the fiberglass threads would get brittle in the continuous ‘bake’ cycles and could break off and become airborne.

2. Burner/AMU filters – This filter’s main job is to catch any potential contamination from outside (filters outdoor air that is coming into your AMU/burner package). This filter also plays a major role in catching anything that may be recirculated during the bake cycle (fiberglass threads mentioned above, dirt and dust in the booth, air dry overspray, etc.) This filter is usually not that expensive and the more often the paint arrestor is changed the longer the life of this filter.

Outdoor, rooftop Air Make-up Units often just have a pleated filter that looks a lot like the furnace filter you might have at home. This filter is usually located on the intake hood of the unit and is exposed to the elements. Due to this fact it must be occasionally checked because if the frame is made of cardboard (we would recommend a re-usable metal frame) it will get waterlogged (due to rain or snow) and fall out of the cell it is meant to protect.

Indoor burner filters are usually bag type filters that slide into the burner right where the duct bringing in outdoor air is located. Depending on the location of the filter inside the burner, a heat- treated filter may be required! The best way to determine the proper filter for your burner is to ask the booth supplier. Improper or inexpensive filters in the burner could create some major operation issues with your burner! Promise us you will check with your booth supplier!

3. Ceiling filters – This is the final filter before the air pours down over you and your freshly painted vehicle! If we have stuck to the schedule on our arrestors and burner/amu filters recommended by our booth supplier, we should have nothing to worry about, right?

WRONG! We often see this filter compromised. This filter (no matter who the booth supplier is) is expensive! Due to the cost, we see regular old 2-ply polys (old x-draft door filters) put in the ceiling! This is a huge mistake because the old polys had no treatment other than the tacky coat and were designed to have air dragged through them (by the fan) NOT to have air PUSHED through them by powerful air make-up/burner units! These filters will come apart and rain polyester pieces all over your fresh paint job!

Proper ceiling filters are specially designed for pressurized (air make-up burner unit) environments. They are engineered to allow the air to be pushed through them; however, the weave of the filter gets progressively tighter until all contamination (usually 10 micron or larger) is trapped before entering your booth space.

Once again, we recommend you go to your booth supplier to ensure you are using the proper ceiling filter.

We hope you find this information helpful. This is an often overlooked area and sometimes the importance is forgotten. There are some very good aftermarket filters out there but please make sure you are using the right ones for the situation! If you follow these tips you will continue to enjoy – GOOD FINISHING!


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