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induced draft / an alternative to forced air burners

updated fri 4 oct 02


John Baymore on thu 3 oct 02

Instead of using a forced air burner to create draft within a kiln it
is possible to have Venturi openings in the stack and to each
attached a smaller 12-15 cmf fax blowing inward and upward?

Or is is possible to get the length of draught required in some other
way than having a vertical stack? A slanted stack, a horizontal

Hi Steve.

Most "faxes" provide very little airflow ...... I assume that you mean=
"fan" and that was a typo . But..... then again...... some political=

faxes are full of huge amounts of hot air..... so maybe that IS an option=

. Recycling at it's best .

Gas flow through a duct (pipe or chimney) is a somewhat complex subject s=
any simple answer takes a lot of latitude with the subject. That being
said..... here's some simple thoughts.........

The idea of using a fan to induce some draft in a flue is a common one in=

other types of heat generating applications and in industry. YES, it
works. They are called .... ahem ...... "draft inducers". The desig=
of such a system is a little complicated..... so if you are going to
attempt it, unless you are comfortable with a lot of math....... a "try i=
and modify as needed" approach might be the way to go. =

Or conversely, there are commercial units available.... but they are a bi=
"pricey". I have used one very occasionally in a situation where some
design constraint has precluded any other solution at a reasonable
price.... and they then become a viable option.

Typically, at the bottom of a vertical duct there is a "T" shaped fitting=

sort of laying on it's side. The vent from the heating appliance comes i=
on the horizontal leg of the "T". One vertical leg of the "T" that point=
upward is the duct that will carry the effluent somewhere. On the bottom=

of the "T" is the location of the fan. The fan supplies air on axis with=

the vertical duct above it. The action of this flow of air across the
opening causing reduced pressure which causes a flow of air to be induced=

through the leg attached to the appliance.

You can visuallize this operation sort of like the compressor driven (or
mouth operated) atomizer sprayers for glazes. You blow a jet of air acro=
the top of a little tube which is in the glaze and the low pressure creat=
at the mouth of the tube by the flow causes the glaze to be "sucked up"
(actually pushed by atmospheric prssure on the surface of the glaze
reservoir) and caught in the jet of air and disbursed. ("Mouth operated"=

is probably not an option in this kiln application .) This is a close=

enough approximation of what the unit does to "get you in the right
ballpark" for thinking about it.

I'd stay away from the "inward and upward" approach you mention above ...=
likely too much turbulance that will interfere with flow and too much of =
chance to create a positive pressure near the connection with the kiln (s=

The flow of the gases in the duct at a point PAST the fan insertion point=

gets a little more complicated. The duct at this point has to be able to=

handle the volume of kiln effluent generated PLUS the volume of air added=

by the fan and keep the pressure inside that duct low. This is a CRUCIAL=

element..... because if the pressure created in the duct increases past t=
normal level of pressure seen at the entry point of the kiln breech
connection...... the gases flow back INTO the kiln from the flue, not OUT=

OF IT!!!!!! Reverse draft. BAD!

To do this whole concept WELL (ie- real engineering) requires some
significant calculations. To just "DO THIS INTUITIVELY" can be
accomplished through "trial and error" and making the system
EASILYadjustable until acceptable results are obtained. The correct sizi=
of the fan itself is complicated and involves issues like the static
pressure developed in the duct, a lack of laminar flow and induced
turbulence, temperature corrections for the volume of gases generated, t=
exact material the duct is mad of, and so on. In general ....... it is a=

pretty large fan. Without a lot of specifics of the installation.......
guessing if 15-20 CFM blowers will work is impossible. Unless it is a
small kiln....likely not.

As a generalization..... and they are dangerous...... for the "INTUITIVE
METHOD"......... the cross section of the duct AFTER the fan needs to be =
bit larger than the cross section of the flue necessary for the kiln
itself. Also be wary of elbows, "T"'s and other such restrictions and
"flow interrupters" in the ducting after the point of the fan insertion. =

Also the longer the run of duct after the fan the larger the pipe. =

Horizontal runs of duct add losses and static pressure to the system as d=
"negative slope" runs (downhill). All of these issuse also mean the larg=
the free air CFM capacity of the fan needed.

The "good news" for this approach is that the fan itself does not see muc=
in the way of heat.... so no special fan is needed. Another method of
accomplishing this is to place a tubeaxial fan in the exhaust duct
itself....... but this fan needs to be able to withstand high temperature=
If this route is atempted..... the fan should be placed near the exit
point of the system to allow the effluent to cool as much as possible
before the fan "sees" it. The ducting between the appliance and the fan
should be designed to cool the gases as much as is possible. Not the pri=
choice for high temperature ceramic kilns.

The need to try to take advantage of the "venturi effect" is minimal in t=
"trial and error" method. If you look at the equation for venturi
operation you will find that there are some important issues with the
precise sizing, friction, and laminar vs. turbulent flow factors that can=

pretty much negate much increase in flow if they are not done properly. =

That being said, if the venturi is constructed PROPERLY it will have the
impact of increasing the negative pressure induced at the connection poin=
to the kiln...... thereby allowing a smaller fan to get the same impact o=
gas flow. HOWEVER...... this venturi .... to have a real impact......
needs to be constructed with smooth curves and somewhat low friction
surface materials to offset the effects that are caused as a fluid (gases=

act as fluids) has to flow through a constriction in a duct (pipe or
chimney). =

(This is also why good precision high quality venturi burners entrain a
higher percentage of primary air than cheap rough cast venturi burners or=

home-made pipe burners.)

A "slanted stack" causes draft. But the impact is lower (due to friction=

loses and such) than the draft that would have been created by a vertical=

chimney (of the same internal diameter) that has a height equal to the
difference between the inlet of the slanted one and the outlet. At some
point in running legnth it is theoretically possible that the losses from=

the long slanted run would completely offset the draft gained from the
height of the outlet..... and it would have no draft at all.

A "horizontal stack"...... usually called a "breech"...... is a method of=

moving gases from one place to another. It creates NO draft. In
fact...... because of the firction of the gases flowing within it and the=

pipe walls and joints and fittings and such...... a horizontal run
DECREASES the volume of flow in a ductin system. SO that one is not an
option....... except to get stuff to the location you want. It also says=

that you want to minimize horizontal runs in the system any time you can.=

Even a slight slope upward is better than none or FAR better than a
negative slope run.

It likely will be a cheaper and simpler solution for you to just use forc=
air burners..... if the ONLY reason for exploring this idea is to not
change the type of burners you already have. It certainly will take less=

of your time working it all out . If there are other reasons..... the=
you'll have to weigh them for yourself.

Think I got all your questions . Hope this is of help in getting the
creative juices flowing a bit and understanding how this idea can work.



John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA

603-654-2752 (s)
800-900-1110 (s)