I'm planning something

I'm planning something similar, although my runs will be much shorter in the basement where I want to put the spray booth. In my garage, I want to have a full exhaust system for a laser cutter and a soldering/BGA reflow station, as well as dust collection for various woodworking tools.

Which brings me to the point of not using metal ducts or those plastic flexible tubing with ridges. There are PVC pipes that could be used starting from the house Central Vacuum System at 2" going up to 4-6" such as PVC SDR pipes.

Come to think of it, do you really need more than 2" exhaust for the spray booth/laser cutter? Smaller pipe means better suction/pressure and it's not like you are pulling wood dust through it.

I suggest to take a look on YouTube and what people do to build dust collection systems for woodworking. Basically we need the same thing for spraying and laser cutting.

I think in-line fans are going to be fine for hobby applications, as long as you leave access to them. Any machinery with movable parts eventually breaks down so you want to be able to fix it. And since you will have filtration before that air gets sucked out of the spray booth, any leftover particulate matter in the pipes will probably not affect the fans.

Ducts and Fans

Pipe: I have used galvanized and aluminum duct for vents, as it is fairly inexpensive and easy to install. "Real" aluminum duct tape does a great job of sealing joints.

Dampers: The older passive dampers leaked badly and often didn't keep birds and vermin out. Some of the newer dampers are OK, but if your really want to keep the heat in or out go with an active damper.  If the duct work is in a heated and or cooled area I would plan on insulating the first several feet of duct next to the dampers.

Fan: You can get paint buildup on any fans, but It only seems to be a problem if you also get a lot of dust and other crud on the fan. So if you properly seal any leaks and maintain your filters your fan will last for many years. I had add tape around the edge of the filters in one spray booth because of leaks around the filters. Now if the other users would just make sure the filters were sealed, I would be a happier camper.

Exhaust Fans: One of the booths I help maintain uses a squirrel cage fan in line with a squirrel cage exhaust fan. No problems so far.

Some Guidelines For You

First, I would make sure to use a fan/blower that has the motor outside of the air stream . . . even for water based paints.

A centrifugal blower is usually better suited than a blade type fan for this application as they tend to handle higher static pressures.

Most fans do much better blowing air rather than sucking it.  When designing an HVAC system for example, it's always best to keep the return air duct run as short as possible.  Also, the return duct is sized with a lower static pressure drop than the supply ductwork (it's usually larger than the supply side ductwork).

Don't compare a vacuum system for cleaning to a ventilation system.  They operate on totally different parameters.

For a given CFM of air moved, the smaller duct will have greater losses.  Obviously, a larger duct will allow more air movement with a given blower and static pressure drop.  Bigger is better in this instance.

Use metal ductwork.  Metal ductwork fittings are a larger radius than pipe fittings, hence less loss in them.  Also with metal you don't have to worry about any possible fume reaction to solvents over time.  Smooth duct is far and away the best (but the hardest to run).  Corrugated and flex duct has a significantly higher pressure drop, and will capture more of the solids that make it past the filter (and some will).

If you're calling a damper "active" because it is powered open or closed, that's good.  You can generally get a better damper that way.  All dampers add to the static pressure of the system, and a gravity type backdraft damper will generally have higher loss than a powered damper as the air has to hold it open (hence more pressure drop).

Fume hoods, paint booths, and similar are designed with a certain "face" velocity of air moving across the booth entrance.  Simply put, the air across the open face has to move at this velocity to properly capture the fumes and exhaust them.  A booth with a large open front will require more air than a smaller one.  All of the hobby booths that I've seen fall woefully short of what is recommended with regards to their CFM.  But they're better than nothing.

I wouldn't exhaust anywhere except to the exterior, even if it means having to run some ductwork to the exterior.  I've a friend who made up a piece of plywood that fit inside a raised window.  The duct collar connection is on that piece of plywood.  He raises the window, places the plywood (shutting the window down on it to secure it), then connects the duct to the collar in the center.  Works just fine, though it's something that has to be done every time he paints.

A couple of comments

I use pvc for my exhaust for my laser. The fumes are toxic to humans depending on what substrate is being printed.  The laser internal exhaust fan is all I use.  It is eventually vented outside.

.

Second, I use a paint booth over at the train park and I do know they have made it as safe as possible.  They use special outlets and wiring including the fan system and light fixtures to insure fire safety.  I think they called them 'explosion proof outlets' but your local electrical shop can give you the correct parts.  Paint & paint fumes are extremely flammable when being sprayed.

Thanks for the replies,

Thanks for the replies, folks

trainzluvr sez:

I intend to route the exhaust in such a way that it will vent my airbrush booth, but also other, unknown things.  A laser cutter crossed my mind, but a soldering station had not. Thanks for mentioning this.  It may be something as simple as a work-surface insert into my booth.  Also, your point about leaving access is a good one that I'll reinforce for other readers. Leaving access to mechanicals was one of the driving factors behind not installing a drop ceiling in my basement.

Logger01 commented that:

I wasn't aware of the active dampers before starting this project. I like the idea of putting one just inside where the exhaust exits the house.  My current booth already has a fan built in - I'd planned to remove it, but I've since gathered that it's appropriate to put fans in multiple places in the pipe network - as boosters; is that correct? (that may allow a smaller-diameter pipe system, but wouldn't solve the noise problem...) 

Metal ductwork and other advice

Jack had a lot of good advice:

Yes, you interpreted my active damper meaning correctly. The one that caught my eye is here:

http://www.solerpalau-usa.com/documents/Submittal/sub-md.pdf

This is a power-open damper, meaning it will close when the fan is off. I'll need to call the vendor to understand where the power is triggered from.

Face velocity is well described in the article I linked to in the original post. For me, this was key to understanding the rest of the recommendations. Without understanding that, fan sizing would be a complete mystery. I encourage anyone interested in this post to read through that article. 

Regarding metal ductwork, I have only two concerns - finding it in 5" diameter, and the lack of ready-to-run branching adapters. I want to have the ability to vent another part of my basement from the same fan - the ducting will be arranged as a "T", with the cross providing a draft to the booth and some future need, and the base heading toward the house's band board. Branching adapters are common in the PVC world.

Russ commented:

I was previously suspicious of built-in fans - having no doubt that they can vent effectively without ducting, but may have trouble pushing a column of air. With your testimonial and another comment from a different forum from someone whose airbrush booth pushes air through about twenty feet of mostly semi-rigid 4" duct, maybe I'm being overly cautious!

A Bit More on Duct Sizing

Matt, describing the how-to of sizing ductwork and accessories is hard to do in just a few paragraphs in a forum.  But it sounds like you're making a serious effort to understand it well enough to have a reasonable chance of success with your project.

Yes, the link you provided shows a typical motorized damper for round duct.  Very common in commercial work, not so common in residential work.  The control system for the HVAC operates the damper, opening it when the blower is energized. You'll have to come up with something that both energizes the blower, and supplies power to the damper motor.  A commercial HVAC contractor should be able to get you on track there.  If you use a damper with a line voltage motor (120v), then it could be as simple as just controlling the damper with another switch (or a single two-pole switch . . . not as common to find).   It's doable, you just need to find the way.

You're right that 5" duct can be hard to find, especially the fittings.  If you move up to a 6" duct, you'll easily find everything you need, as it's the smallest common size for HVAC applications.  And the size will help you greatly in reducing the static pressure (SP) losses in your system.

And speaking of those losses, sizing things can sometimes require a bit of back and forth.  For example, you might have a blower that produces X CFM with a Y static pressure capability.  At that point, you assume a certain size ductwork, and measure the lineal distance of the duct, and count the number and type of fittings.  You can consult a chart that will state the static pressure drop in (typically) 100' of duct.  As you've already discovered, the fittings have an equivalent length as well.  Simply add all those distances to get a total, and then you can figure the SP drop for the ductwork.  If the duct has a pressure drop of .1" per 100' at X CFM, and your calculated total is 50', then your SP drop will be .05" (.1 / 100 x 50).  But you also have to add the static pressure drop for other things: the damper at the end, the filter (a big number usually) and any other things in the air stream, like perhaps a return air grill.  Everything in the air path has a loss associated with it, so don't ignore them.

As you'll see frequently, the SP drop in the system exceeds the capability of the blower.  At this point you can either select a different blower with a higher static capability,  -or- resize the ductwork and accessories to result in a smaller loss.  Or, you could find the blower exceeds what you need, and go the other way.  If you've been looking at the technical data for blowers, you've probably seen that a blower (or any fan) will have a chart with a curve on it (or perhaps several curves).  If you follow that curve you'll see that the blower produces less air volume as the static pressure it's working against goes up, and vice-versa.  The trick is finding the blower that produces your desired airflow at the static pressure you've calculated that it will need.  It's really rather straightforward.

One other thing I'll mention is noise.  For a given CFM of air, the smaller the duct, the higher the velocity of the air.  But you reach a point where the noise starts becoming a factor.  But with the lower pressure blowers that you're likely looking at, I doubt you'll have velocity problems, as the blower won't have the oomph to push that much air quickly in a small pipe. 

Good luck.

may have trouble pushing a column of air

I think it is not so much a matter of moving air as it is to move the toxic smoke from the work area and to get it away from the human running the machine.  When I was playing with the initial setup I had a very short piece of PVC as my 'vent' and even then the 'smoke' was pretty well dissipated by the end of the PVC.  I have no way of measuring the toxic density on a short run but it was noticeably better 'smelling'!    Russ

Metal vs plastic ductwork

While this might be bordering abstraction, at the back of my head sits an idea about molecule frictions happening while they are passing through ducts made of various materials.

I do not have the numbers regarding static friction caused by metal or PVC duct material, yet somehow I feel metal would be worse, causing more (positive) ionization, especially at the corner turns. What kind of affect that positive ionization has on the molecules being transported is another question.

Which really brings me to another idea, whether the metal ductwork could be "electrified" negatively, to repel the particles away from its walls so there's no accumulation of material.

A simple negative ion generator could be attached to the entire metal structure, but of course you need to remember to buy insulated ducts, or insulate them yourself beforehand, or else you will be shocked by the electric charge on them.

As I said, going into abstraction...but might be worth while.

Static

For dust collection for woodworking, if you use a non-conductive duct you need to run a wire down the inside that’s grounded.  And of course you’re supposed to ground metal duct work too.  Dust particles scooting along a non-conductive duct can build up a static charge, which could either give you a bit of a start or in extreme cases cause a spark that ignites the dust.

I’m not sure how much that applies to paint and laser cutter fumes.

I’m am pretty sure regardless that you don’t want to purposefully apply a charge to the duct!

Thanks for this topic!

Matt,

Thanks for starting this topic!

I have a salvaged Jenn-Aire downdraft fan that I have planned to use for a spray booth. This discussion is helping me make decisions and turn the plan into action.

Regards from Akron, OH

Tim Moran

Changes in plan and a new question.

Jack, thanks for the thoughtful input.  Prior to seeing your post, I discovered something important namely that  the fan I was looking at is a 6" fan, not a 5" - I'm not sure why I overlooked this pretty important detail, but I did!  The upside is that I'll have a lot more choice in duct choices, as you noted.

I did run into a problem when working through the calculations 6" - the chart on the page I linked in the original post didn't cover the lower CFM range that I was looking at - it stopped at 500 CFM.  Since the other values seemed somewhat linear, I took a SWAG at it and assumed about .2 SP @ both 225 CFM 250 CFM.

Using the interactive diagram tool on the Fantech site, the fan I linked above is a bit too small, even with the reduced resistance of 6" duct. It's rated at 227 CFM with no load - and I need that after 39 equivalent feet of run. The best it will do is 207 CFM with a calculated SP of under .2 (very rounded from my SWAG).

The next size up is rated at 325 CFM, which will move 278 CFM with a SP .2 after the 39' run, according to the diagram calculator. That's a comfortable margin over my needs, and I may need to install a variable speed switch (or vent the duct) if it's too much. That probably also allows for the filter and damper drop, which I do not know how to calculate.

After typing through this, I think I'm getting comfortable with this sizing, and have all the calculations in my notes if anyone is curious.  Assuming that the 6" duct will fit where I need it to, the exit from the house still works with the larger hole and no one sees any obvious flaws, I'll nail this spec down - and take a few aspirins.

Incidentally, I called two fan vendors yesterday. Fantech recommended the 150, but said not to worry about a motorized damper - he figured it was too much complexity for my needs. 

Thoughts?

Fan Isn't Going to Cut It

Matt, I went out to look at the fan you're looking at.  Unfortunately, I don't think this fan is going to cut the mustard.  Two things:  this is an inline fan, and as such, the motor is in the air stream.  Unless the fan is specifically rated for hazardous service (which is what this is), you are at risk.  Even using water based paints, there is a risk.  I don't see anything in the specs or submittal to indicate this fan would be safe for anything beyond "just air".

But aside from that, even the larger of the two fans isn't close to delivering the CFM you're looking for at the static pressure it will be facing.  The problem is that you're trying to move a lot of air through a very small duct.  Yes, even a 6" duct is small for this.  300 CFM moving through a 6" duct will be traveling at about 1500 feet per minute (FPM).  That's pretty high.  The static pressure loss in smooth metal duct will be about .67" per 100 feet.  That will give you .26" SP loss for your (equiv.) 39' of duct.  The hood/backdraft damper will likely double that figure.

Then there is the filter.  I can't give you even a range for that.  You need a pretty efficient filter if you hope to capture paint pigment, and that means it's "dense", less air will flow through it's filter material because the denseness needs to filter very small particulate matter.  To counter this, you use a filter with a larger surface area in order to maintain your airflow with a reasonable static pressure drop.  So what you have to do is determine what type filter will satisfy your filter requirements, then figure the filter area you need to achieve your airflow within the SP drop you can handle.  All these figures added together will give you the SP that the fan has to handle.  Looking at the fan's curve, you draw a line straight up from the desired CFM (the bottom scale) to the curve, then project a horizontal line from that point over to the vertical scale, which will be the SP capability of that fan at this particular CFM.

After going through this drill, you'll likely realize that a fan that can truly handle the job is bigger than most hobbyist assumed it would be (and more expensive).

One of the most common blowers I've seen used in home-brew spray booths is a Dayton utility centrifugal blower sold by Grainger.  These aren't explosion proof, and technically aren't rated for a paint booth, but at least the motor is out of the airstream, and if the booth is working properly, you should be at minimal risk.

It ain't easy, is it?  :-(

Drat

Thanks again for the advice, Jack - and yes, it isn't easy.  Sigh.    I suppose the upside was that my SWAG for SP wasn't not too far off.  Thanks for the SP value for 100' of 6" duct. That helps.

The inline fan concern has always confused me since the Paasche booth I have has an axial fan mounted inline. Based on re-reading the site's comments on this type of fan along with your comments, I'm guessing the difference is in how the windings are protected?  But I recognize that the concern is real, and if I'm going to go through the rest of the work, I should choose an appropriate fan.  As it turns out, the cost isn't that much higher. Thanks for prying my head out of the sand - I was fixated on the inline fans due to previous experience with them (for bathroom venting).

The model paint website (linked in the original post) also called out the Dayton fans and listed a few model numbers.  I've just wandered back from the Grainger site with this fan as a candidate.  Its capacity seems far too large with my previous guesstimated .2 SP, but with your spit-balled doubling for dampers and hood and the unknown restriction for the filter, it may be in the ballpark.  Its specs indicate it'll be in the range I need at .7-.8 static pressure. Time to do some more math and puzzle out a more complex fan mounting (due to the built-in 90-degree turn). And also to re-read your posts to make sure I understand what I think I understand.

Regarding the filter, the filter area in my booth is 15.75 x 7 inches, and the filter material is what Paasche supplied: a layer of fiberglass about .75" thick, a charcoal filter layer, and finally a plastic chicken-wire type diamond mesh (16 openings per foot, each about 5/8") to support the filters. That is backed by a 3" deep plenum and the existing 5.5" axial fan, which I intend to remove.  Besides measuring static pressure directly, is there another way to get a working pressure drop value for this?

Back to the modeling bench for a while to do something I understand better...

< edit> - is the .67 figure you noted specifically for 300 cfm, or does it apply for any flow (I'm assuming the former)

Getting There

Matt, the Dayton blower you're looking at is the style I referred to.  Keep in mind that it still isn't explosion proof, but at least the motor isn't in the air stream.

In the interest of "full disclosure":  a true explosion proof blower would have a totally enclosed motor. It would use dissimilar material for the blower wheel and housing (for example, a steel housing with an aluminum wheel - the combination of which wouldn't produce sparks if the wheel should somehow rub on the housing).  The electrical would also be sealed such that there isn't a possibility of fumes getting to the connections.  And of course, the unit would be labeled as explosion proof.  Such a blower costs MUCH more than a regular blower, and it's unlikely that many hobbyists could afford or would even worry about it.  OK, safety lecture over.

Mounting the blower can be as simple as cutting a hole in the back (or top) of the booth and bolting the blower with it's center opening over the hole (gasket the junction to prevent air leaks).  This blower (which is pretty typical) would need a square to round adapter to connect your duct.  Grainger might have something ready-made in their catalog.  Or any sheet metal shop can fab one up pretty easily.

The static pressure drop for any size of duct depends on the air flow through it (more air volume, more SP drop).  A couple values taken from my "Ductulator" show for a 6" duct: .4" @ 225 CFM, .48" @ 250, .67" @ 300, .82" @ 325 and .92" @ 350 CFM.  For further reference, a 7" duct would have .23" @ 250, .32" @ 300 and .43" @ 350 CFM.  These are all values per 100' of duct.  See the relationship?

With regard to the filter area, how much air is passing through that filter in the Paasche application?  If in doubt, (if you can do this) consider placing two filters, one atop the other, to give you twice the filter area (14" of height vs. 7").  This would do two things:  it would slow the velocity of the air across the filter face, thereby increasing it's ability to capture.  It would also tend to lead to longer filter life.  Keep in mind that the Paasche filter is pathetic compared to filters I've seen that are made for commercial spray booths.  But if you don't overload them (CFM wise), they'll probably do OK.  The thing to know about filters is that they are rated to capture certain things, and they are rated for a maximum airflow.  Unfortunately "hobby" equipment and accessories rarely publish all the specs that one needs, so you're pretty much on your own.

I apologize if all I've written sounds so negative . . . I'm not trying to discourage you (or anyone) from building a spray booth.  I'm just trying to point you in the general direction of building a reasonably safe and effective booth.  The BEST hobby booths are at best, mediocre (I'd still wear a respirator using them).  The "cost effective" ones do little more than somewhat "corral" the overspray.

Again, good luck.

While searching for

While searching for information on square-to-round transition ducting (literally "square to round duct adaptor, dayton blowers"), I found a YouTube link that discussed exactly that. What are the chances - except in a world where hobbyists are trying to use a relatively safe Dayton blower to vent their hobby fumes). 

The video in turn had a link to the company's website (Vent Works) and an article about building a booth. I thought it was well done, and the fact that it specifically addresses the needs of hobbyists (including laser engravers) caught my attention. He touches on many of the points mentioned in the tripod site linked in my orginal post, including some discussion of static pressure and blower selection. He's also done a series of short videos to illustrate the text. If I didn't have a booth already, I would consider using these plans and would thank him by buying his parts.

https://vent-works.com/blogs/the-ventilation-blog/15945741-diy-hobby-spray-booth

Unfortunately, his blower and duct sizing won't work for my needs - but they've given me some additional ideas. 

A sort of related question if

A sort of related question if i may... after reading all the interesting info posted so far.

If one needs lets say a 600cfm setup/result, can this be obtained by using two smaller motors ie 2x 300cfm ones instead of a larger 600cfm motor? The two motors could be parallel or in-series i suppose.

Balanc.

Do keep in mind that you will need to balance the system.  If you are using a 600cfm exhaust fan you are creating a lot of negative pressure in the house,  if you don’t have a source of make up air   Either approximately  equal at the source of exhaust or greater than farther away you could start pulling in from some very unwanted locations.  Such as furnace exhaust fumes.

This is a larger issue the closer the exhaust fan inlet (ie the booth) is to things like the furnace,  

So just be careful .  It does no good to expell paint fumes (especially the less problematic waterbased) but to pull back in furnace exhaust...

-Doug M

Good point Doug!

Luckily i do not have a furnace but i totally see your point (and have seen it in practice). As source of air, what is recommended practice?

Backdrafting

Yannis, Doug does make a good point about make-up air.  I was just starting to rub my chin about that same thing, given I keep up-sizing my fan - even before seeing his post.

So I went internetting to confirm what I thought I knew. The furnace exhaust back-drafting isn't an issue with high-efficient gas furnaces that have a closed-loop combustion system. These draw combustion air from outside the house and exhausts the low temp gases the same way (through a separate pipe, of course). The combustion chamber on these isn't exposed to indoor air.  80% furnaces DO draw their air from inside the home, and typically have a hooded exhaust flue that is open to the room to draw dilution air in. There is a valid risk about back drafting exhaust gases in these cases.

I live in an older home with a chimney that was previously used for furnace exhaust - I now have a high-efficiency furnace, so the entrance to the chimney is capped in the basement end (still open to the sky otherwise). If I have drafting concerns, I may pull that cap to draw make-up air down the chimney. If anyone in MRH land believes that may introduce a maintenance issue, let me know. Using this approach may make my painting area a little chilly in the winter!

Many houses are leaky enough that they may not need a dedicated make-up air inlet. Try your fan without an extra inlet - if it doesn't work well, and if the performance increases markedly when you open a door or window, then you know you need more air. Your workaround might be as simple as a cracked window.

Serial Fans

Yannis, I forgot to comment on your question about using 2 300 cfm fans in place of one 600 cfm. My understanding is that if you use them in series (i.e., both on the same pipe), you will still get a max of 300cfm, though a longer run is possible (especially if there is separation between the two). I believe that "booster" fans installed in ducting is installed for this purpose - that fan puts some energy back into the airstream that may have been lost to friction.

If you mounted the two 300 cfm fans in parallel, separate ducting, then I believe you would get closer to your 600 cfm goal.

Perhaps @JC can correct/add to those questions/comments.

Some Follow-up Comments

Matt, I'll throw in a couple other comments at this point.  Please be aware that the things I've written are based on industrial/commercial ventilation for spray booths, and I've yet to see a hobby booth that meets all these requirements.

For example, a spray booth would be designed with the first concern being the velocity of the air entering the opening in the booth.  A small booth with an open area of under 4 square feet would ideally have an air volume of 200 CFM per square foot flowing across it's open face in order to achieve the required velocity.  A larger face area could get by at perhaps 150 CFM/SF.  So, suppose you had an opening that measured 24" wide x 18" high.  That would be 3 square feet.  3 SF x 200 CFM = 600 CFM of airflow.  How many hobby booths come even remotely close to this?  Obviously we hobbyists are going to use something considerably less, and we won't be achieving "industrial grade" ventilation.  OSHA wouldn't be happy with us.  So we start right off the bat with a huge compromise.

The next thing is to decide how big the filter has to be.  And here comes the next big compromise.  Filters for "legitimate" spray booths are specifically designed for this application.  As hobbyists, we're either using some "combination" filter offered by the hobbyist spray booth manufacturer, or for DIY builders, a filter designed for household use.  Those filters vary all over the place in how effective they will be in this application, and you won't find any specs that state how they rate in removing paint particles from the air stream.  And that's because they aren't tested and rated for that.  So you're strictly shooting from the hip there.  In general, the larger the face area of the filter, the more effective it will be at a given air flow.  Slow the air down moving through the filter, the more efficient it becomes in removing what ever it's rated to remove.  Generally, you want to move air through the filter at a velocity rate of 100-500 feet per minute.  You've probably seen household air filters with general ratings for removing dust, pollen, smoke, etc.   And those are some pretty good guidelines within a given brand.  But there is a trade-off there.  The better the filter is at removing contaminants, the higher it's static pressure drop.  Sooo, a more powerful blower is required to maintain the air flow you require.  The thinner, less expensive filters are mostly a waste of effort, as they aren't very efficient, and don't remove much beyond coarse dust.  And one other thing about filters to remember:  as the filter becomes dirty, it's resistance to air flow increases (the static pressure drop goes up).  The dirtier the filter, the less air flow through it.  A dirty air filter can more than double it's SP drop.

There are other considerations in the dimensions and configurations of hoods, but I can't even remotely cover them in this forum.  And it's unlikely any hobbyist would follow all those guidelines anyway.

A few words about the ducting:  the square-to-round adapter shown on the website you referenced is a rather poor design, and will result in a higher SP drop than a tapered style of transition.  In general, you'd be better served with something that is along these lines:

With regard to the double blower scenario, as has already been pointed out, it can be done, but there will be balancing problems to consider, particularly on the discharge side.  I doubt it would be reasonable to design or cost effective (unless you already happen to have all that's required).  I personally wouldn't opt for it in a hobby application.

Make-up air.  Yes, this is a concern, especially with a larger (as in CFM) booth.  But again as pointed out, the solution is as simple as opening a nearby window some to allow air to enter the room.  And of course, it isn't generally recommended to have anything with an open flame in the area of a paint booth.

In my view, the largest "mistake" folks make is grossly underestimating the total static pressure that the blower will have to overcome to move the desired quantity of air.  If you see a fan/blower rated at say, 300 CFM at 0" SP, that is a useless spec.  It only states what a blower will do sitting on a bench with nothing connected to either end . . . in other words, it's a "free air" spec.  Look at the specs for the Dayton blower you linked to.  See how drastically the CFM delivered drops as the SP increases?  Remember that EVERYTHING in the air stream adds to the resistance.  EVERYTHING.  Folks build a booth and sometimes they're lucky as it appears the booth is working great (and maybe it is).  Others aren't so happy with the result.  The trouble is, not many hobbyists have the instruments they would need to see if the blower is indeed delivering the air flow that they had hoped for.  So we're more or less stuck with trying to design something as best we can, and the more we adhere to established guidelines, the more likely we are to have a satisfactory outcome.

Is it hard to do?  Absolutely unless you are trained in ventilation design, or simply don't worry about the details.  Eventually I plan to build my "ultimate" spray booth.  Will it conform to industrial standards?  Nope.  But hopefully it will work to a point.