I'm getting ready to set up a spray paint booth in my basement. Because my
layout is around the walls, the booth will end up having to be in the
middle somewhere. This means the dryer hose I set up to vent it could end
up being very long by the time I can get it up to the ceiling and over to
an outside wall to vent.
Is there a practical limit to how long that should be? I haven't done any
measurement yet but I think it could be 30' or so. Is that ridiculous?
You should probably consult with someone in the HVAC field (Heating
Ventilation Air Conditioning). I do know that the plastic hose often
used for dryer vents will cause a lot of friction and become less
effective in longer lengths. Paint spray dust also will collect in
the corrugations in the hose. It would probably be a better idea to
use metal ducting. The fan could be sized to be able to provide
enough air exchange to effectively vent the booth.
I use plastic dryer vent material in my booth, but it only runs about 3
Doesn't matter how long it is, how many CFM can you EFFECTIVELY move
through it? Longer run, higher capacity blower needed. Long enough
and you might want more than one blower.
(Keep it short, it just works better that way.)
On 1/26/05 9:14 PM, in article email@example.com, "Jim
The answer is "A", as long as you change the wording to "fan outlet".
This is the simple answer, but it is all interrelated. The opening in the
front of the booth should be of a size sufficient for you to be able to make
use of the booth for its intended purpose. For example, you don't need the
same size opening for N scale cars as you do for HO. Obviously, there are
other factors involved (your hands have to fit, etc). Let's assume the
opening is 24" wide x 15" high. That's 2.5 square feet. To be safe, you must
have a fan capable of moving air at 200 feet per minute (FPM) into the
opening (regardless of opening size). Model Railroader's article in the Jan
88 issue used the formula "Fan Cubic Feet per Minute (CFM) = W X H X FPM",
where width and height are in feet. They came up with 525 CFM required. Now
you got get a fan capable of 525 CFM.
Once you've got that, you have to match the output opening of that fan to
some kind of hose.
In my case, I used the same Dayton 4C445A fan they did, and matched it up to
6" dia air ducting. The fan outlet was about 5" X 4", so I felt pretty safe
using the longer hose, since it was much larger than the outlet.
I can email you the MR article, plus photos of my setup if you like.
I THINK he meant the size of the exhaust opening in the booth.
However, the statement that the duct need only be larger than this is
*NOT* true. The booth exhaust opening in ONE restriction, yes, but the
hose is also a restriction, and they are CUMULATIVE. Its *NOT* a case
where the 'weakest link' defines the system.
On 1/27/05 9:49 AM, in article firstname.lastname@example.org, "Daniel A.
This is true, and is pointed out in the MR article. It's one reason I went
with a 6" diameter duct when the output opening of the fan is about 5" X 4".
I didn't want the duct to be a restriction.
As I've stated, I can spray Floquil, Scalecoat, etc., without any smell. I
don't even use my respirator mask anymore.
How many bends will you have in this exhaust duct? The more, the greater
resistance to airflow and the higher the fan pressure needed. Also, have you
any filters in the system, or will you just exhaust the paint laden fumes
out over the neighbour's cat, dog or (Horror! Horror!) his car! Filters will
in turn add more resistance to the system. Certainly something that an Air
Con. or heating and vent. engineer should look at.
Remember, too, the higher the fan pressure the greater the fan noise,
more annoyance to the neighbours.
Before determining required air flow, an analysis of spray
booth's purposes should be considered.
1. To retain over spray.
2. to remove noxious fumes.
3. To prevent overspray from settling on painted surfaces
without effecting spray pattern.
A wind tunnel is not required nor desired. Excessive air
velocity will definitely effect the latter. A fan speed control
is helpful. Improving the efficiency of the air flow through
the booth will reduce the air volume requirements. The shape
and exhaust placement are critical. An easily replaceable
filter will keep overspray from the fan and exhaust ducting.
Experimental results are discussed on my site.
The cross sectional area of the exhaust is by necessity
much less than the front opening which creates a funnel A
truncated pyramid with exhaust vent on rear wall is the optimum.
Sharp corners create eddies, which may swirl overspray back to
Yes, corrugations, smaller diameter, longer length and sharp
curves increase back pressure. By using reduction fittings, a
larger diameter duct may be used for the long run with smaller
flexible hose at the ends to ease installation.
For more details with methods and extensive discussion of
problems and solutions, see first site below in methods/spray
Hope this helps.
MODELRAILROAD TECHNICAL INFORMATION
PROTOTYPE TECHNICAL INFO FOR MODELRAILROADERS
(Revised. New address)
MR TECHNICAL HELP GROUP
COUPLER HELP GROUP
Vince Guarna wrote:
That's too long a run for the common flexible dryer vent tubing, and the
typical small blower used in most paint hoods. Most of these hoods are
underpowered anyway. A proper hood will provide 100 feet per minute of
air flow over the entire OPEN FACE of the work area. That translates to
MANY times that velocity in the exhaust duct. The smaller the duct, the
higher is the needed velocity.
I should add that ANY venting is probably better than NO venting, but
shoot for a good air flow. This is especially true if you intend to
spray solvent based (other than water based acryic) paints like
Testor's, Floquil, Humbrol, etc. The concentrated fumes from these can
be a real health hazard.
Normal plastic 'dryer vent' hose has a very rough interior wall (it
usually has a big coil spring inside it) that restricts flow badly.
Flexible metal dryer vent hose is a little better in that regard, but
not a lot (though it is more flame resistant). Any long hose, even
smooth walled, creates a lot of backpressure, and reduces air flow. The
smoother, and larger, the tubing the less the restriction.
Also, avoid a lot of sharp turns in the duct. They also cause
backpressure and restrict flow.
For long runs consider all sheet metal duct pipe, like 'stove' pipe, but
usually made of aluminum or thin sheet steel (not the flexible aluminum
foil kind). This is much smoother, and should cause less flow
restriction. Consider 6" diameter instead of 4" if space permits. A
short section of flex tube can be used at the hood end if necessary.
For a run as long as 30' you may need to consider a more powerful blower
to better deal with the backpressure. A second blower could also be
installed near the exterior vent, as a 'booster'.
A far better solution is to move the hood or reconfigure something else
to get the duct shorter. More than ten feet and you're inviting problems.
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