Paint booth

I think you should review that 100 fpm per sqft of intake area. The standard design criteria for such booths is that a minimum flow velocity of 100 fpm shall be obtained over the entire booth cross sectional area...not the cross sectional of any intake opening you may provide. That is to say you would need to provide 100x12x8 = 9,600 cfm of fan flow as a minimum. This is assuming you orient the airflow lengthwise along the booth. The difficulty, of course, comes in obtaining relatively uniform airflow over the booth cross section (the 96 sq ft in the above case). For this reason you may find that most commercial booths have a relatively large intake area relative to the booth cross sectional area. In fact the ACGIH (American Conference of Governmental Industrial Hygienists, an authority on industrial ventilation design of all sorts) recommends that the filter area be at least 75% of the gross booth cross sectional area. There are also usually some baffle requirements to further assure uniform flow across the booth cross section.

To further complicate the matter, the 9,600 cfm fan flow must take into account all frictional resistance and other losses between the inlet to the booth and the free outlet from any fan discharge ductwork. This is in order to obtain the final flow of 9,600 cfm when the fan operates in the system. Just because a fan has a rating of 9,600 cfm at some static pressure (e.g. 1/2 inch water gage) doesn't mean the fan will actually deliver that amount. It can be significantly less than the rating if the total system resistance is greater than the rating point.

You may find the whole thing becomes prohibitively expensive for the occasional user, if done to established criteria. The explosion proof requirements for the electrics are usually a big cost adder. Don't forget a liberal amount of lighting inside the booth, also explosion proof.

Like you, I have routinely in the past reviewed standard design practice and improvised where I thought it reasonable to take risks, knowing full well that I was not (and didn't need to be) in full compliance with the "regulations". My posts on the paint and body work board relative to my "not fully compliant" breathing air system for iso-containing paints is a case in point. I have reduced the personal risks to a level that has been acceptable to me and completely workable for me over a period of many years in that regard. I have also messed a little bit with makeshift spraybooths and after doing a couple of cars in my garage a long time ago decided against it as a reasonable way to proceed. I simply wasn't comfortable with the perceived level of personal safety I was getting at a price I could afford to pay for basically a hobby effort. I therefore do all my painting in the summertime outside, even with the fresh air breathing equipment (another subject completely). This always creates a problem with scheduling around the weather but I only do work for myself and simply work around it.

You will find many who cobble together booths using window fans and such winding up with relatively low flow results. That will generally contain and direct any overspray from getting on equipment outside the booth but really is not adequate from a full safety standpoint or from a standpoint of keeping overspray off whatever in-booth lighting system you may have. The uniform 100 fpm minimum flow velocity over the full booth cs area and the need for explosion proof electrical equipment are the key factors that work against you, cost-wise for a reasonably safe installation. 100 fpm is not a lot; as I recall, it is barely perceptable unless the air is hotter or colder than ambient.

Professional automotive downdraft booths, I believe, are up in the $30K-$50K area. Cross-flow booths would be less expensive but still a great burden for a part-timer who is not making his living at it. Can you put together a home-made system for personal use that is reasonably safe and workable at less cost than a commercial unit with full code compliance? Sure. But, depending on your acceptable level of risk taking, it will not be cheap by any means.

I hate to be so negative but I have been where you are in the past, considered the options, costs and risks, and decided against it. I know from your posts on the other board that you take personal safety issues seriously so I am sure you will gather many opinions, evaluate acceptable standards and risks and make an informed judgment for your own use. If you don't already have reference to it, OSHA 29CFR1910.107 provides some food for thought in this area.

Rod

One other thing I thought of when typing this up is make-up air to the space. A professional booth would take air directly from the outside through a heated/cooled make-up air system - $$$. Make-up directly to the booth is appropriate because of the large air volumes involved. Taking the make-up from the space is OK if the volume rate is small relative to the space volume but may become a problem for small spaces. If you say your enclosed space that the booth is in is 30x30x10 ft high, that's 9000 cubic feet. Exhausting anything on the order of 9000 cfm is a huge amount relative to the space volume. That's one air change every minute! In the winter time your heating system would never keep up with it and you will soon be having cold outside air entering your painting area while you are still painting. You also would have to leave a large door or several windows open (roughly the same CS area as your booth in order not to restrict your airflow. I am not trying to rain on your parade here. Just throwing out some numbers for your consideration before you spend a lot of money. In the end you may be better off doing it the way you are doing it. Cover stuff with tarps or plastic to keep the overspray off. 30x30 is not that big a space, I am surprised that you are able to see in there with all the overspray. Maybe that HVLP is a lot better than I thought in that regard :o)?

When I work on my tractor, it may be disabled for lengthy periods (so much to do, so little time) and I cannot afford to tie up my only auto bay (also work space) for long periods. My fix for that situation has been to put my tractor on a low flatbed trailer and keep it on the trailer in the bay. That way, if I need the space for some more urgent work, I simply drag the trailer with the tractor on it out, do my thing and then put the trailer back in. That will be my plan if I ever get around to painting the tractor main chassis. Disassemble on trailer, pull outside to paint and then push back in to assemble.

If the smaller individual parts are your biggest problem, a downsized plan of action could be the most appropriate for you. Smaller, openfaced spray booths (not necessarily walk-in) could be a perfect solution. They are used routinely in industry and work well. They could be sized for one of the 3000+ cfm fans you mention. That would give you about 20 sqft of open face area to work with. For smaller areas like this, the recommended face velocity increases to 150 fpm from the 100 for larger cross sections. That would equate to something like 5 ft wide by 4 ft high. It should be enough for most individual tractor parts, including hoods and fenders.

I have the 1977 ACGIH Industrial Ventilation Handbook (retained when I retired from my employer :o)). It has some excellent basic ventilation design standards for paint spray booths. It has been quite a while and I did not keep your email address. Send it to me if you want and I can email a few pages in pdf format that cover small, medium and full size cross draft spray booths. The baffling shown therein is important for airflow distribution across the open face. It might give you some more food for thought. In fact, If I only had more room, I would consider putting something like that together myself. I have a blast cabinet on order now and am currently wrestling with what stuff I will have to move out of my work area to make room for it!

2)My booth's inside width dimension is 11 foot. Wish I would have went a couple feet wider. Twelve foot is good for tractors, but a little close for vehicles -- once you have some sort of painting setup, you will find all sorts of stuff to paint that might be larger than a tractor.

a 30 x 30 building for painting a couple tractors???

What your asking for is for someone to design your system and is way beyond the scope of this forum.

Why?
Any exhaust system for hazardous, flamable materials takes several code books to correctly design for a SAFE working enviroment.

About the best way to even come close to what your wanting without several hours of running numbers is too look at commerical built units.

T_Bone

A paint booth/room is nothing more than a large plenum box exhaust hood where the air needs to be diffused to the correct velocity for fume, dust, filtration, sufficient oxygen supply, ect; via the design of the intake and exhaust air (ie; fan size), the heart of the system.

This requirement is one of the more challenging designs for air movement and so much that there's many different engineering books published on the subject.

Think of it as working inside of a huge gas tank as there very simular. Get the correct oxygen to carbon mixture, then go boom can happen very easy.

T_Bone

Your thoughts ???

Example: We have wood stove sitting in the main room with bedrooms down a long hall way. The bedrooms will warm some what by the natural air current created from heat rising and cold air sinking but the bedrooms will still be quite cool.

Lets set a fan at mid hieght of a hall way entrance and blow hot main room air towards the bed rooms. We notice a slight rise in bedroom temps but over a very long time period.

Lets reverse the fan air flow to where it sucks from the hall way blowing towards the main room. We now notice a increase in bedroom temps as well as a noticeable faster temperature rise time in the bedrooms.

Why?

(1)With the fan blowing down the hallway, were trying to force the air into a chamber with no outlet and raising static presssure or much like blowing up a balloon.

(2) The first law of Thermodymanics states; "heat flows from a higher heat source to a colder source".

Natural air convection currents in the hallway is the heat is at the top near the cealing with cold air near the floor.

With our fan located near mid hieght and blowing down the hallway, were fighting the natural air current flow in the hallway which is heat down the hallway at the ceiling level and cooler air at the floor moving towards the larger room.

When we reversed air flow were now pumping air from a small chamber into a larger chamber area thus lowering the static pressure or slight vaccum and just like deflating the balloon.

We now are no longer fighting the natural air currents and pumping air with the natural convection flow towards the larger chamber as were now pulling the air in the direction is wants to go naturally.

Now look at air with air borne contaminates (paint droplets, dust, etc ). Trying to blow them to a collection point is pretty much useless as your fighting positve static pressure (just like the hallway example) as well as natural air currents.

Static presssure = The resistance to air flow.

T_Bone