Paasche is a good spray booth

Joe,

I have owned the 22” version for at least 15 years, with no problems. It’s located in my basement and vented through the band joist, terminated on the exterior with  a standard dryer vent.  I spray both acrylic and enamel paints.  It captures effectively and I have been happy with it.
 

As far as general thoughts, I don’t understand why you would prefer a downdraft booth.  I have some experience with small paint booths and fume hoods from my working life (I’m a recently retired engineer) and we never used downdraft booths (hoods) in those applications. Those hoods always vented out the back and then upward, which is a similar arrangement to the Paasche (of course these were lab/industrial hoods and used design features a “hobbiest” booth like the Paasche doesn’t have.  Still, the arrangement is similar.)  As far as modifying the Paasche to make it downdraft, one thing you should consider is what that will do to the fan, and therefore capture, performance.  Your homemade base will probably cause more pressure drop than you realize, reducing air flow and therefore effective particle capture.  If you go downdraft, make sure you still have a  sides and a top. Open style hoods are notorious for poor capture performance, which often isn’t evident. Regardless of which way you go, keep the exhaust duct run as short as possible and keep all directional changes, including in your base, to a minimum.

Parting thoughts: The model number you mention appears to be a 30” booth.  I have found the 22” plenty big for almost all HO structures.  In a few instances where I had something too long for the booth (e.g. a RR ferry that I scratch built) I found I could just hang the model end outside the booth,  paint one end inside the booth, then  flip it around to do the other end….  a little inconvenient but manageable given long structures are rare or can be painted in sub assemblies.  I too thought about building my own booth since I DIY in most instances and have some engineering knowledge to do so.  But the Paasche has held up well and allowed me free time to build models, not a spray booth. Assuming Paasche has maintained their booth quality since 15 years ago, I’d say you will be happy with it if you buy it.

Happy modeling!

PS: I recommend that, even with a booth, you wear a respirator. I do, even when spraying water based acrylics.  No model is worth potentially damaging your health.

Suggest reading alot

Joe,

I suggest doing a Google search for 'spray booth design and fan selection'. If you are going to DIY or Bash someones product, I recommend understanding  what it is you are dong first. My first and fore most recommendation is to choose a fan that the motor WILL NOT BE IN THE AIR FLOW. Second, don't guess at parameters, know what you need.

Thanks for the Replies

It seems like the Paasche would do the trick then, but the cost is still high, and I do think I want something larger in the 30 x 16" range that will do everything I need and last a lifetime. 

I have done the detailed research in the past to build my own, but I'm not sure I feel like experimenting in this area, or if my math and understanding of blowers working in parallel is right.

I looked at a 30 x 16" cross draft design (based on 30" x 16" furnace filter at the back).

- Needs about 333 CFM to get 100 LFM (industry standard apparently for LFM in a spray booth). Calculating the SP in my ducts (about 46' equivalent of 6" ducting) at 333 CFM I get 0.391" SP.

- 2 x Dayton 1TDR3 Dayton blowers (squirrel cage type with motor out of airflow) spaced evenly apart (like what Paasche does for their 30 x 16) working in parallel will produce 546 CFM at 0" SP.

- If I link them together (2 x 4" outlets to a 6" duct), I will get about 366 CFM at 0.391 SP. (each blower produces 183 CFM at 0.40 SP).

- Those blowers should be quieter at about 1600 rpm each and won't blow the circuit with less than 1amp draw each at 115V single phase AC.

- Not recommended for speed control, but could make an adjustable vent in the plenum  if I find the flow rate is too high.

- Also not explosion proof blowers (but those are cost prohibitive), and I could avoid the use of solvents if necessary.

- Probably some additional CFM loss from filter and 4" to 6" duct transition.

This setup would cost me around $550 CAD all in, including exterior lighting at the top through plexiglass I already have, and would be superior I think to the much more expensive Paasche.

What are everyone's thoughts on that?  Can someone check the math and or concept for soundness?

  "What are everyone's

I wouldn't worry too much about industry standards as painting models with an airbrush is a pretty small job and doesn't make much over spray compared to most industrial tasks. Have you tried painting in your space to see how big a problem overspray actually is?  I paint on a work bench in my workshop with just a nearby exhaust fan drawing the spray away and out the wall with no problems. I only use water based paint these days but years ago I sprayed floquil and scalecoat  too.  It might turn out that you only need a small fan and airflow for your tasks so can avoid some cost and excess fan noise.Keep in mind that your primary goal is to get the paint on the model so you don't want to be sucking too much of it away in an overkill air stream....DaveB

Acrylic sheets

cut at plastics dealer fit well and sealed well also.  I used a Dayton fan and got my design from the internet a 10 or 15 years ago.  No complaints.  I use mainly solvent paints so it is important that it works properly.  
 

The lid keeps the temperature conditioned air from exiting when not in use and the furnace filter should keep the duct and fan somewhat clean.  It vents outside.  I haven’t had to use the fire extinguisher……. yet…….  I have never found direction of flow to be a problem.

My thoughts: Downdraft is

My thoughts:
Downdraft is used in full size automotive paint booths for two reasons, the biggest being the inlet air is clean and filtered, so downdraft keeps any dirt on the car, equipment, operator and floor down and away from the car, rather than being blown up where it can settle in the paint.  The second is it tends to draw the spray away from a standing operator's head, a tiny improvement in safety.
Since an airbrush booth has no filtered air inlet, and the operator is completely outside the booth, neither of those reasons are a benefit.  I think the back vented is just fine, especially since it packages better.

With respect to airflow, I would pay attention to booths made for airbrushing.  The Paasche model says 470 CFM, which I'm sure is at zero pressure drop.  Going super overkill could just result in all your paint going out the exhaust, and not onto the model.

How expensive are things in Canada?  On US websites, that Paasche booth is less than $500, $462 on Amazon.  At that price point compared to your estimate, I would just buy the commercial unit.

Replies

Thanks again for all the replies.

In terms of CFM, based on my calculations, designed for small model spray booths, I do need the 2 Dayton 1TRD3 motors given that the booth will be 30" wide by 16" high, and the equivalent duct length size I will need to use to vent it properly. That is of course unless someone with training or detailed technical knowledge in this area can correct the math I outline above. I'm actively looking for that feedback. Maybe less than 100 LFM is good for airbrush / spray can painting. I don't think one stronger motor centered in a 30" plenum with a 4" duct would pull the air evenly enough through the filter across the full 30" - hence two motors instead of one, and a 6" duct to reduce SP. I think that's why you see the large 30" commercial booths with two fans and two 4" ducts instead of one.

I live in a cold climate and want to be able to airbrush in my basement year round. There are only two very small windows in the basement (one blocked by the railroad). The house is small and I'll be working close to a cold air return for the furnace. I've read a fair amount about how hobbyists generally unknowingly over expose themselves to issues in the air that would not be acceptable in a work place from a health and safety perspective (particularly sawdust in home woodworking shops). I'm concerned that while I wear a mask, I could be exposing my family to particulates that could harm their health, as well as my own once the mask is off, so I'm okay with some added cost and/or time to give me piece of mind.

In terms of the cost of the Paasche, once you factor in the CAD to USD exchange rate, shipping, taxes, and maybe some cross border fees, the Paasche comes in close to $900 to $1000 CAD from retailers here for the 30" model. That price is why I am entertaining maybe doing it myself for about $550 CAD, while maybe building something a bit superior, that will last my entire modeling career, and hopefully fulfill all my future modeling ambitions.

Booth

I built a spray booth made mostly of plywood from plans online.   Plexiglass on the top and front to allow for lighting. I connected a bilge blower cage fan to it and it works great.  I did have to get a 12v power supply for the fan.

a bilge blower is designed to blow out gas vapours from the engine room of a boat so I would hope it should be fine for airbrush applications.

a furnace filter is ahead of the fan to catch particles.

I had a friend run duct work from the fan to outdoors.

The fan was the most expensive part.

Mike

Design thoughts

A few things I'd consider. First is size, I'd make it as tall as possible so you can spray from the top easier if needed. That's why I prefer to just paint on a workbench and let the shop exhaust fan carry away the over spray. I'd also try to shorten the 46 feet of exhaust pipe as it would cost more and reduce airflow.Ideally straight out the wall or nearby window would be best.  As for CFM ,  IIRC  kitchen range hoods blow about 200 cfm on low and maybe 500 cfm on high so that could give you an idea of how much flow you need. Have you checked on youtube for info on making spray booths, I usually find anything I need to know on there.....DaveB

Length and Sources

Hi Dave B,

The actual run of the exhaust duct would be about 12 to 14' long total. When you add 45 and 90 degree bends it increases the equivalent length of straight duct from a static pressure perspective. So my proposed set-up would have about a 10' of straight duct but then a few elbows on top of that make an equivalent extra 36' of straight duct (assuming I did the math right).

I've looked online in several places that discuss how to build DIY spray booths for modelling. A couple of sites walk you through how to do the calculations for determining the correct blower you need based on their CFM performance. You calculate the CFM you need for the size of your booth, then determine the SP in the exhausting pipes at that CFM, then you need a blower that will provide enough CFM at the given SP you calculate. Blower CFM performance reduces as SP in the system increases.

The big area I'm unsure about is whether my calculations above make sense for running two blowers in parallel to get a more even draw of air across the entire area of the 30 x 16" filter. I think I did it right, but I have found it harder to find that information. Maybe a single motor with a higher CFM performance is sufficient to do the work of the two lower CFM motors across the entire area of the filter, but I don't know. I was hoping someone online could help me out. I'm also not sure I need 100LFM draw at the filter for air brush / spray cans. It might be fine to get away with less air speed into the filter, and still have nothing escape the booth. With less air speed, the CFM and blower requirements go down (I think).

Joe

I don't know if this will help but here is the info on my airbrush booth that I built approx. 35 years ago and is still in use.

Booth is plywood construction with a roll of white paper in the back/top that is pulled down to provide a clean surface as needed. Dimensions are 18" tall by 18" wide by 16" deep. Originally the exhaust fan was mounted on top but currently is located on the left side......haven't noticed any difference in performance. Lighting is a pair of LED bulbs at same temperature as layout lighting (3200K). The lights are mounted inside the booth in the upper front corners.

 The fan is: Dayton 1TDT2.          https://www.grainger.com/product/DAYTON-OEM-Blower-1TDT2

Venting is 4" dryer vent approx.5' to outside with a 90 turn at wall.

Works very well. 

Your CFM calc is correct

assuming your front opening will be 30 x 16…. CFM= V*A (on a hood,  the 100 FPM “standard” is based on the area (width x ht.) of the open face of the hood, not the filter area. Sounds like the area of the filter and the open face are the same, or nearly the same, in your planned design.)

I can’t advise about your SP calc. It sounds like you have found a reference to calculate fitting and duct SP loss. You need to assume some SP loss through a somewhat dirty filter.  My gut says 0.25” would be reasonable.  Then add this to your duct/ fitting loss. Avoid using a forward curved fan, as that type fan may trip on its internal overload if your SP calc.s were too conservative.  Select a backward curved fan if you can, which doesn’t have this problem… except either type may trip on over-current if arranged in parallel driven by separate motors. Selecting fans for parallel operation isn’t as simple as selecting two fans at half the required CFM, though you might get away with that simple approach.  You might be able to get a local Dayton fan sales engineer (not a distributor) to do a parallel fan selection for you since they should have the software to do it. Finally, I strongly agree with the other poster… in either case keep your duct as short as possible and with as few fittings and direction changes as possible. It can be real easy to underestimate the pressure loss of the duct system, filter, and inlet plenum.

If this all sounds complicated, it can be. You are taking on the risk if you “do it yourself”. But here are some thoughts that might make you feel more comfortable doing it yourself-

100 FPM is a good rule of thumb.  But tiny airbrushes don’t put out much paint, and you should be shooting toward the filter “with” the air flow and away from yourself. So if your CFM/LFM turns out not dramatically lower than your design was supposed to attain, paint/fumes will probably go where they are supposed to, away from you and to the outside. The first time you shoot paint you will be able to easily observe if the paint is being carried away from you. Avoid using the booth for anything but an airbrush. Even spray paint cans should be avoided as they are paint blasters v. an airbrush. Not saying you can’t use a spray can, just saying the hood will be less likely to contain, whether bought or self-built.  And you’ll see your filters load up much faster.

Explosion proof fans: again, assuming an airbrush, the solvent volumes are so low and will be so dilute it seems highly unlikely you’ll reach the flammability limit. 

Do you really need two fans?  Consider if you are ending up with a more complex (= expensive) design because you feel you need a large booth.  We all have a tendency to think more is better.  But unless you are working in the larger scales, I think you’d find the smaller booth adequate… cheaper if you buy it, and less complex and expensive to build if you DIY.

Finally, if you are really going to do routine painting, you are right about protecting you and your family by using a booth.  I have seen numerous computer models of where fumes travel, especially when generated on an open bench top.  Yes they eventually exit the space, but it’s what they contaminate on the way out that isn’t apparent to the naked eye.  You and your family’s health are worth more than any model!
 

Just my two cents, Joe. Good luck. 

2 Fans

Rick,

That Dayton 1TDT2 pulls almost the same CFM (549 CFM at 0" SP, 480 CFM at 0.4" SP) as two 1TDR3s combined (546CFM at 0" SP, 366 CFM at 0.40" SP) over a smaller area in your booth than my proposed design, with a similar blower inlet (5 5/16" vs 4 5/8"). You definitely have good airflow I think.

Karle,

That's good detailed feedback. Thank you for that. Can you speak to how effective a single blower with an approx 5" inlet might perform pulling air evenly across the full width of a filter? What are the plenum design considerations? Do I need a second blower given the size of the booth for even airflow?

For the SP calculations - 4" duct: 45 and 90 degree bends generate an equivalent 3' and 6' increase in duct length. For 6" duct they generate and equivalent 6' and 12' increase in duct length. The area of two 4" ducts is about the same as one 6" duct. At 100' of 6" duct, SP is 0.85". At the equivalent 46' in my design, SP is 0.391".

By running in parallel, I mean operating at the same time beside each other with two separate air streams that combine into one in the exhaust vents. Neither system would be connected to each other electrically. I do not plan to operate them electrically in parallel. I think it is the case above that you speak of where two lower CFM blowers are operated separately at the same time to help evenly distribute the airflow across the full width of the booth (that's my theory anyway, but I can't back it up yet). Would this work?

It feels like I'm starting to get into the detailed work again now of doing it DIY. I might as well keep going at this point. I will draw up a sketch in xtrackCAD of the design and post it here.

two or one fan?

Could you use an old kitchen range hood and mount it vertically as the back of the box, that might work as a "manifold" to spread the intake across the filter if that is desired?. They usually have a single 7 inch outlet that can be reduced to 6 inch pipe. ....DaveB

Hi Dave,

That is a good suggestion. I am handy enough to do that or make a bit of a plywood funnel from the edges of the filter to the blower intake. I'm just not sure that will work across a 30" x 16" space with a single blower and still get 100LFM near the edges. I'm hoping someone can confirm that is the case if I design it right, or if I do in fact need two blowers. It is cheaper to go with one higher CFM blower like Rick Sutton and other's have mentioned, as opposed to two lower CFM blowers, but if that is the case, why don't the large commercial models do that? The Artograph, Micromark, and Passche 30" models all look like they use two blowers and two exhaust ports. There must be a reason (maybe not the one I think it is though).

$1k versus $500, I would DIY

$1k versus $500, I would DIY too.

You may be putting too much emphasis on "even draw across the filter" - does it matter if the overspray goes at an angle towards the center of the filter when you spray at the edge of the booth, as long as the overspray exits through the vent?

Maybe some mock ups and experiments with cardboard would provide answers to your open questions.  Perhaps a single fan with the CFM you want, and if the flow pattern through the booth is unacceptable, split the intake with ducting rather than two fans?

Also, there is a lot of good advice in here - match your booth lighting to where the models will be displayed, stay away from spray cans once you have the airbrush, and I'm sure other tidbits I have already forgotten.

Transition?

Joe,

I have not seen your designs, but I am curios if there is a transition from square to round between the outlet side of the filter and the fan inlet. A transition will greatly improve air flow in that zone, easing the air into the fan and reducing sharp edge entrance losses going into the fan. Admittedly adding a transition into the fan will add distance between the filter and the fan.

Just thought it worth mentioning.

Bob C.

Diagram

Here is a picture from xtrackCAD of the plan I have so far with the rough calculations.

It's mostly to scale except for the blowers when you look at them from the back view (middle) or the blower inlets from the front view (right). Trying to do 3D in xtrackCAD is not worth the effort.

Parallel Blowers

This morning I was able to confirm that when selecting multiple blowers in parallel in this plenum application that each blower must perform at the SP for the system, and the CFM for each blower is the CFM required divided by the number of blowers. So I think I did my calculations right there.

I will need to make sure that I always startup and run both at the same time or I will have to include some other measures in the ducting to protect one blower from the exhaust flow of the other. I could do this easily with the switch on a power bar I think. The combined amp draw of LEDs, and 2 x 1TRD3 blowers plugged into the same bar will be very low (full load amps is 0.77 for each blower). I'll have to check my air compressor and see what that draws in amps. Maybe I be won't be able to plug it into the same circuit.

I still don't know if two blowers is required for even draw of air across the filter in this design though. Any takers?

I'll keep looking.

I

I

Joe, The deeper the plenum

Joe,

The deeper the plenum the more even the air flow will be across the filter face, whether 1 fan or 2.  For a given plenum depth and CFM, two fans will inherently have more even distribution though air flow will still be faster in the vicinity of each fan inlet.  I don’t know of a simple way to determine how deepening plenum depth effects air velocity across the face and at what point 2 fans are better than 1 for a given plenum depth and face opening. One thing to consider is how conservative do you need to be because how even does air flow really need to be across the filter face? As the filter loads up it will self balance since the air will always take the path of least resistance. 

I agree with Bob C that you need to include some SP loss for the sharp edge entrance loss because of the plenum design used on the Paasche booth design (which you are copying). Also some pressure drop for the Y fitting (if you use 2 fans) and for the loss through the dryer vent outlet/back draft damper, plus the the drop across the filters. 

My comment about 2 fans possibly  having performance issues is not about the electrical side. It is about the air side. Yes each fan needs to be selected for the same SP at half the required total CFM.  When 2 fans operate in parallel they can hunt and/or be noisy. Fan mfr.s have software to select fans with performance curves (flow v. SP curves… different fans have different curves) to avoid this issue.  In my past line of work we consulted the fan mfr.s  sales engineer to make such a selection, to avoid such problems.
 

Design Adjustment

That's more great feedback Karle, thanks.

Below I have refined the design a little more based on everyone's feedback and improved the calculations for SP.

The plenum is a fustrum shape design (rectangular pyramid) in this version, to reduce SP loss (how much, I'm not sure). The plenum volume is much smaller than the original design which may reduce the even draw across the filter face.

I've included more refined calculations for the duct work, including a MERV 8 filter. I could not find duct equivalent lengths for a 6-6-6" Y-duct with 45D junction, so I added 8' equivalent length. I included the short 4" runs that will have half the required total flow rate (166.5 CFM vice 333 CFM). I have shortened overall equivalent 6" duct length because I realize now I will have to park this thing right below a window in the basement living room when I use it, instead of my workbench. The railroad blocks the other window too much to put a temporary exhaust vent there. I could not figure out the SP loss due to the louvers of an exhaust vent, but as I will only put the vent in the window when in use, I can probably go without louvers.

With these refined calculations I come up slightly to 0.412" SP loss. The 2 x Dayton 1TDR3 blowers still seem to have sufficient CFM. 

Built the Beast

So I finally decided to plunge in and build a spray booth. I only made slight modifications to the design above. All in I did it for about $700 CAD, a little more than I originally estimated, but a lot less than buying a commercial model of similar size and quality. In fact, I think my version is probably better than the commercial models.

Here's the parts list and costs (I think its exhaustive but might have missed the odd part):

 I did away with some of the extra supports around the back and made the blower mount out of 3/4" plywood. Once bolted to the blower mount, the blowers were solidly attached. I secured the ducting to the blower mount as well with a screw through some 4" worm gear clamps.

I also added a turntable with a carriage bolt and T-nut. A wood insert nut would have been better but I have the T-nuts already and some Gorilla glue helps hold it in place. When I'm doing larger structures the turntable just spins up and out of the way.

Now here's some photos (excuse the terrible lighting):

I was able to negotiate a location in the house by a window that dramatically reduced the length of ducting, thereby saving me some SP loss and increasing CFM (I hope). I don't know how to rotate this picture properly here. It was rotated properly on my computer. (Moderator: downloaded, rotated, uploaded fixed version... this is a persistent problem with this site)

The construction of the fustrum shaped plenum was a breeze with my relatively new compound sliding mitre saw.

Apparently you can spray paint the solid ducting and foil tape to the color you like by first cleaning it with alcohol and then etching it with vinegar before applying the spray paint. My wife doesn't care about the shiny ducting so I left it as is (for now anyway).

Another benefit of increasing the thickness of the blower mount to 3/4" was I could route slots for the blower exhaust flanges so the blower inlets could be mounted flush to the blower mount.

The switch on the power bar below the blower on the right is used to ensure both blowers start-up at the same time. So far no apparent issues of the two blowers competing to exhaust air with the asymmetrical design of the exhaust ducts, although I'm sure one blower is able to suck out more air than the other this way.

Here's the turntable design:

Here's what the exhaust vent hood looks like mounted in the basement window when the blower's are on:

And finally here's my unscientific way of checking to see how much air the blowers are exhausting. It seems to pull a loose strip of toilet paper dangling outside the hood quite well into the filter.

I have not tested this with actual paint from a spray paint can or airbrush yet but I intend to over the next month or so as I test out the construction of automated switch stands for the layout.

Final note, a high K LED lamp matching the layout lighting will be mounted outside the booth in the near future.

Joe

A masterpiece!