John Baymore on sun 19 apr 98
------------------
A bunch of my thoughts on this kiln stuff that's going on........... take
it with whatever size grain of salt you desire =3Cg=3E.
=3Cclip=3E
I was taught that the chimney had to be a certain distance above the roof
peak nearest.
=3Cclip=3E
The reason for this is that adjacent structures can cause flow
irregularities in the pervailing winds. Sort of eddy currents, like in a
stream that has rocks and promintories in it. If the wind goes across a
structure near the kiln chimney, it is possible that an eddy could flow
back down the kiln chimney, thereby causing a =22backdraft=22, or at the =
least
varying the pressure differential created by the chimney. It is also
possible that the erratic flow could increase the draft on the kiln chimney
suddenly. Either way... not good for the ware, the kiln room environment,
combustion safety equipment, or control of the firing.
Another factor is that airflow over rooflines and other such things with a
poorly terminated stack can cause the effluent to swirl downward and to
possibly be drawn into things like windows, make-up air louvers, and so on.
That gets the junk you were trying to get rid of back into someone's
breathing zone =3Cwg=3E.
There are two types of chimneys...... contigious (directly connected to the
kiln), and non-contigious. Contigious chimneys are more of a problem in
this =22eddy current=22 or =22backdraft=22 regard. Non-contigious chimneys =
are
more typical of forced draft kilns that utilize the mechanical energy of
the blowers to supply the driving force to move gases through the kiln.
The kiln then becomes isolated from and somewhat independant of the
chimney, and is more uniform in firing characteristics no matter what the
weather outside.
An example of this would be the type of chimney recommended on the Bailey
downdrafts in their catalog. The kiln chimney ends, then a small =22hood=22=
is
located over the outlet of that chimney, but not hooked to it directly, and
it continues up through the roof penetration point. Cold room air is also
drawn into the upper chimney at the hood, diluting the flue gases and
cooling them somewhat.
If you must have an excessively long contigious chimney which can cause
excessive draft in a kiln, there are draft regulators available that can
limit the maximum differential pressure generated by the stack. There is a
simple version of this device on most home oil burners.
=3Csnip=3E
........natural draught burners for a long time. I am still convinced that
one needs a strong draught for control.
=3Csnip=3E
A good kiln offers a lot of control options. That way YOU can fire it how
YOU want to, not just have to settle on =22the way the kiln fires=22. There
are too many kilns out there that potters have to fire by waiting for them
to just do their thing. Generally speaking, the first criteria for this is
to have the air handling capacity of the combustion system handle AT LEAST
100 =25 of the required primary air for a BTU input profile that allows for
the fastest possible desired firing profile.
Strong draft is crucial for any kiln using aspirating burners since they
are incapable of drawing in 100 =25 primary air. So the system is totally
depandant on secondary air to complete the picture. The draft is the key
to this additional (but completly necessary) air.
=3Csnip=3E
I fully expect the overwhelming =22weight of experience=22 to favor many
valiant approaches to compensating for difficult high-altitude conditions
in what seem clever ways=3B yet, these conditions can be dealt with more
effectively and inexpensively by employing =22the approach that science
indicates=22=2A rather than by gas-pedal, turbo-charged reflex. =
................
=2Alower gas flow per burner + add more burners
=3Cclip=3E
A lower gas flow on simple atmospheric mixing burners would result in less
primary air being entrained, due to the lower kinetic energy available to
entrain air. So more burners at lower flow would likely result in an
overall DECREASE in the percentage of primary air induced in the overall
burner system. Not at all what would be desireable.
This would be somewhat compensated for by the increased number of square
inches of burner ports that would be necessary to use more burners. These
openings would allow more secondary air to be drawn into the kiln, assuming
that the flues and chimney are sized to induce this additional flow. And
assuming that the kiln design would be able to provide adeguate mixing of
all of this secondary air. Also not taking into account the increased
radiational losses from many open ports.
Generally speaking however, one wants the control options available from
having burners that operate with the highest possible primary air
available. So going to more burners providing LESS primary air
possibility is a BAD choice from the point of view of control options.
Depending on secondary makes the kiln highly subject to firing variance
based on such things as weather, stacking variations, and wind. Depending
on secondary also makes the kiln sort of have its own firing cycle..... you
can't vary it too much from what is will do. (Less control options.)
You need more air, but not more fuel, as is said in the quote above (if
that input was calculated correctly to start with). High altitude problems
are summed up in one issue....... move more air through the system. That's
it. You need roughly 10 cubic feet of air to supply enough oxygen to burn
1000 btu's of fuel at the =22typical=22 sea level 14.7 lbs / sq. ft. air
pressure. The higher you go, the more cubic feet you need to get the same
total O2 content.
At some point in the altitude climb, the amount of N (relative to O) in the
air being heated by the flame would become an issue........ but that would
be really high. We aren't climbing Everest here =3Cg=3E.
At high-ish altitude, the choice of the type of atmospheric mixing burner
becomes more critical than it is at sea level. The cheapie one-piece cast
=22venturi=22 burners that barely work OK (with little control options) at =
sea
level are a poor choice at 7000 feet. Go for a good Eclipse or North
American true venturi burner mixing unit and add a good retention head.
These are precision castings. Or switch to forced air if you have power
available.
=3Cclip=3E
How do you determine the size of the opening from the kiln chamber into the
chimney? I vaguely recall mention of a precise ratio of some sort.
=3Csnip=3E
The flow of gas through a =22hole=22 is based on the density of the gas, the
square area of the hole, the shape and materials the hole is made of, and
the pressure difference from one side of the hole to the other. This is
the same science that governs the operation of a gas orifice in a burner.
If you know these factors you can calculate pretty accurately the volume of
gas passed through a hole.
The formula is:
Q=3D1658.5 x K x A x (the square root of) Delta P/d
Q is the flow in CFH
K is a factor of the efficiency of the hole....... this is determined via
experiments... but you can assume for a square brick hole is it very
low.... maybe .3-.4
A is the area of the opening in square inches
Delta P is the differential pressure from the high side to the low side.
d is the specific gravity of the gas that is flowing. You can assume a mix
of mostly CO2 and N, with a small percentage of H2O.
Capacity varies directly (double area ...double flow) with the change in
area. Capacity varies as the square root of the pressure drop change....
so changes in pressure need to be large to give much flow change.
You figure out how much gas you have to burn at peak flow to get your
chamber hot in a reasonable amount of time, and how much combustion
byproducts that creates. You then know how many CFH of a mixture of CO2,
CO, H2O, NOx and so on that has to go through your hole.
Chimney height mainly affects the pressure differential across the
hole...... the taller it is, the more pressure differential. The larger
the cross section of the chimney the larger the volume it is capable of
handling, and the slower the velocity of the gas stream. Also the lower
the friction losses from the interior surface of the chimney itself, as
more of the gases flowing in it are in what is called laminar flow, and are
not hindered by the turbulance caused by the boundary layers bumping into
the roughness of the surface of the chimney walls.
The =22friction factor=22 of the refractories around the flue holes is =
pretty
much a constant (square hole of rough material... terrible amount of
friction =3Cg=3E). If you can make these openings more gradual and =
smoother,
they pass more gas (terrible turn of phrase =3Cg=3E).
The density of the flue gases changes with temperature and is the most
troublesome part of the deal. Generally the hotter the =22thinner=22 or =
less
dense. Also there is some compressability of the gases. You can work this
out at STP (standard temperature and pressure), and constant volume, and
add in a =22fudge factor=22 to correct for any inaccuracies.
Most forced air gas kilns have far too big an exit flue =22hole=22. The
mechanical blower determines the pressure differential across the flue
holes. This mechanical energy can set up very high differentials compared
to natural draft kilns, so the hole in the chamber can be quite small
(small hole / high differential pretty much the same as big hole / small
differential.......... except for friction loses based on the ratio of the
perimeter of the holes to their square areas being different).
The potters =22rule of thumb=22 for natural draft kilns is that the total
square area of the inlet flues needs to be matched by an equal square area
of exit flues. This works for kilns that are dependant on natural draft
to supply a lot of the air for combustion, but is certainly overkill for
forced draft systems. Forced draft systems can even be built with what is
called closed ports. There are no open holes... the burners are =
=22sealed=22
into the kiln wall. So there goes that =22rule=22. Based on that =22rule =
of
thumb=22 there'd be no exit flues=21
The better the quality of the aspirating burners on a natural draft type
kiln, the less the sizing of the chimney and the ports becomes an issue,
since such burners pull in a higher percentage of primary air. Also the
more kinetic energy available in the gas stream coming out of the orifice
(higher gas pressure), the less the issue.
When the flue cross section is too large and the chimney is too tall, a lot
of heat gets pumped up the chimney. This happens as excess air (not needed
for combustion) is drawn through the kiln, heated by the burning fuel, and
dumped out of the kiln .......hot. A lot of this waste happens early in
the firing. =22Too much chimney=22 makes the kiln very =22touchy=22 as to =
precise
control settings, particularly, the damper.
Optimal efficiency is achieved when you have just enough air flowing in the
system to combust all the fuel, and to provide enough extra oxygen to
accomplish any reactions in the clay and glazes. When in reduction
(inherently inefficient =3Cg=3E), you want to have just enough deficiency in
oxygen to achieve the level of reduction necessary to =22get the job =
done=22.
The ideal flow relationship is that you have just enough flow through the
kiln to get rid of the byproducts of combustion and the gas products
created by the wares. No more. If you are allowing more flow..... the
kiln is trying to suck in additional air as a negative pressure is pulled
on all or part of the chamber. More unecessary air, means more air that is
being heated up....... and leaving the kiln hot.
Flue gas analyzers are what makes the world go round in this department.
And they DO make a difference in fuel efficiency as well as consistent
fired results.
=3Cclip=3E
I am presuming that in a down draft kiln the opening should be flush with
the bottom floor (as opposed to the =22false floor=22).
=3Csnip=3E
A true downdraft kiln has the flues located right IN the floor. Not a
single large hole at the back of the chamber at =22floor level=22. The =
flues
penetrating the floor are collected into a horizontal flue that connects to
the base of the chimney. This design has some distinct advantages in
eveness of firing that make the extra difficulties in construction and
expense worth it, in my opinion. I frequently build kilns this way for
clients.
A huge number of the kilns potters build are pseudo-downdraft kilns. Maybe
sort of down-cross draft is a better description =3Cg=3E. (Not really) =
They
have a single large hole located at the bottom of the back wall that leads
into the chimney. This construction is easy and cheap to build, but not
the best actual design.
Visualize the kiln closed up and firing. Think of the kiln filled with
water coming from hoses for burners and going out (down) the drain at the
top of the chimney. Once the kiln is initially full of water...... add
dye to the water coming from the hose/burners. You can then understand
the tendency for the flow of combustion products to =22favor=22 moving =
toward
the bottom rear single exit flue and tending to not flow through the bottom
front area (near the door). And how you can get =22eddy pools=22 where =
little
flow is happening at all.
This =22hole in the back wall=22 design often results in a cold, =
under-reduced
front bottom area, due to less flame flow through that area. To
compensate for this tendency, the bag wall is often adjusted to direct more
heat and gases there. Or the front bottom shelf is stacked to promote more
draw through there. Or target bricks are added to the fireboxes to
splatter the flame. These are actually =22fixes=22 for kiln design issues.
Often this adjusting results in pots near the edges of the front shelves
along the bag walls getting =22blasted=22 on one side, in order to get the =
pots
in the middle front acceptable.
If you couple the single hole in the back wall design with an
underinsulated floor (like 5=22 of hard brick) and leaky (gas permeable) =
door
construction (and/or floor construction) you are asking for a persistent
problem, and higher firing costs than would have been possible with
different construction.
The flue holes in the floor can be sized to promote very even draw across
the floor of the kiln. They get exponentially bigger as you move away from
the entrance to the chimney, because the pressure drop across the opening
(chamber to flue collection trough) decreases as you move away from the
chimney. So to get the same volume drawn through each hole, the hole gets
bigger farther from the chimney entrance. The collection trough can
actually taper in cross section from large near the chimney to smaller at
the last flue hole, since the volume of gases the trough carries at the
distal end are less than at the end connecting to the chimney.
The total area of all of the small openings in the floor needs to be a
little larger than the cross section of the chimney, because the friction
factor of the gases interacting with the refractory perimiter surface of
all the small holes is greater than the friction from the outer surface of
a single large hole.
If you go this route and have not built a lot of kilns, make all areas
easily changeable.
=3Cclip=3E
I am in the midst of buidling a down draft kiln with burner openings as
Pitelka recommended. The chamber is 3 feet 6 in high plus a 9 inch rise for
the arch and 3'6=22 wide.
=3Cclip=3E
You are getting to the high end of the height to width ratio that will
produce even firng in a downdraft. You might drop the wall construction a
little so that the arch does not take the kiln too far out of the cube
relationship.
=3Csnip=3E
Bag wall planned for 1ft. 10 in high and 3 inches from the walls.
=3Csnip=3E
The 3 inch clearance from the bag walls to the structural walls IS pretty
tight. Are you firing the burners in from the sides at 90 degrees to the
bag walls, parallel to the bag walls from front or back, or up through the
floor parallel to the bag walls? In the latter two it is possible (but
questionable) that the distance would work OK. In the former, it probably
is at the absolute minimum distance (no margin for error) for that size
kiln.
If you are firing in parallel to the bag walls up through the floor, the
somewhat narrow path might induce a lot of directionality to the
flame....... sending it at high velocity toward the top possibly. Even by
enlarging the openings in the bag wall near the bottom to try to
compensate, it might not suffice to get flame to pull through those holes.
So the kiln might tend to fire a little hotter at the top than it should.
If you are coming in from the sides at 90 degrees and splattering the flame
against the bag wall to disburse it, you can expect more quenching of the
flame in the early stages of the firing (causing some undesired reducing
conditions early on) and probably a bit excessive firebox temperatures.
If you are coming in from the front or back and firing the burner axis
parallel to the bag wall, again you may find that the velocity of the flame
may make it difficult to get flame to pull through the holes in the bag
wall near the bottom. The placement of the openings and the =22target=22 =
brick
in the firebox channel will be critical to tune the distribution of the
gases.
If you run the bagwalls this narrow (in other words restricting the
fireboxes in cubic area), make sure that you are using really good
refractories in the fireboxes. You will have a tendency to have stronger
reducing conditions there, higher flame abrasion, and possibly higher
temperatures than =22normal=22 (whatever that is =3Cg=3E).
=3Cclip=3E
(a) A kiln can be designed to require little or no chimney.
=3Csnip=3E
Absolutely. Usually these are forced air type units. If high pressure gas
is available it is possible to utilize HP aspriating burners (good venturi
casts) and approximate the same thing with slightly less control options,
due to the limited primary air available.
Not to mention your typical updraft, of course =3Cg=3E.
=3Cclip=3E
(b) Bright (blue to blue-white) flame indicates hot, efficient combustion
(even little gas flames, like on a gas kitchen stove, should be blue to
blue-white).
=3Csnip=3E
Actually it indicates a high level of primary areation. Also may indicate
high flame temperature. However, the flame character of a flame running
more than 100=25 primary looks much the same. If the flame is areated over
100=25, the flame temp is decreasing as it is diluted with the excess air.
So it could also be =22inefficient=22 combustion. Hard to tell with the =
naked
eye.
One problem with this whole concept is looking at burners as somehow
separate from the kiln. You can't. What you are working with is a
combustion system which includes the burners, the kiln, and the kiln
venting. All work together to provide useful, efficient use of heat
energy.
You can get a useful kiln system that depends on significant secondary air.
You can also build one that requires no secondary air. On one the flame
will never get short and blue but will look fluffy and yellow at the ports.
On the other, it can be anything you desire. Both can be made to work.
One probably offers more control options.... but if you don't want or need
those options, then ..................
If you've got yellow fluffy flames at the burner ports, as long as the heat
energy potential of the fuel is realized IN THE CHAMBER, then you are
getting the maximum heat value out of the fuel. If the fired results are
good, and no appreciable flame front is burning somewhere up the chimney
(should be adjusted to just burn off at the entrance to the chimney or
close to that) and you are reading minimal CO in the chimney, you are
getting the same =22efficiency=22 as you would if you got good results with =
a
hard blue short flame at the burner tips and the same chimney CO readings.
=3Cclip=3E
(c) A good, well tuned natural draft (venturi-type) burner will get the
majority of its heat generating feed-air from =22primary=22 airflow (this =
can
be verified, if anyone wants to write directly).
=3Csnip=3E
A good venturi WILL get the majority of its air from primary ............
anything over 50=25 is =22the majoriity=22. But that still is pretty low =
when
you are depending on it as one of your main control options. And it
leaves you dependant on secondary air entrainment (and adequate
post-ignition mixing) for complete combustion.
Actually, I'd like to know the source for venturi burners that pull in
really high levels of P. air if you have one. The best casts that I know
of only entrain a max. of about 70-80=25 primary in the medium to high
pressure (gas) range. And these are pricey=21
=3Cclip=3E
(d) Secondary air is called =22secondary air=22 because its role really is
=22secondary=22=21 A given quantity of gas at a given pressure needs a =
given
quantity of oxygen-rich air to combust -- and beyond that, =22bonus=22 feed =
air
serves as a convection medium at any temperature.
=3Cclip=3E
Secondary air is the air that combines with the already ignited partially
or fully aerated mixture. That definition is from basic combustion theory
texts. It can supply enough air to bring the total mixture to
stoichiometric (=22on-ratio=22), can be in excess of that ratio (oxidising),=
or
leave the total mixture running =22rich=22 (reducing). Depends on how the =
kiln
is adjusted, and how much the kiln design will draw in.
Air flowing through the kiln in excess of that needed for complete
combustion can be utilized by the chemical reactions taking place in the
clay and glazes. At some points in the firing this is desireable .....
even necessary. Beyond this, it absorbs some heat energy and lowers the
temperature of the effluent passing throught the chamber by spreading a
specific amount of heat through a larger amount of material.
This extra air (over 100=25 for combustion plus that needed for clay/glaze
reactions) must be exhausted (along with the CO2 and H2O from combustion,
and the N stuff) to make room for the next instants's burning mixture going
into the kiln. This extra air is exhausted at high temperature..... taking
heat energy with it. If you didn't need or want the extra air..... you are
wasting heat.
This hot flue gas acts as a convective heat transfer medium as long as it
is hotter than the wares it is passing over/through. The greater the
differential between the hot gases and the wares, the more rapid the heat
transfer. For that reason you don't want to dilute the gases too much with
unneeded air.
Too much secondary entering the port around a poorly areated =22yellow=22 =
flame
early in the firing (below color) can cause =22quenching=22 around the outer
edges of the flame front. This simply means that the reactions taking
place in the flame stop short of making H2O and CO2. This leaves unburned
complex hydrocarbons floating through the kiln...... wasting fuel and
possibly causing ware defects.
Secondary air is not inconsequential. If you need it to supply O for
combustion it is critical. If you have too much, it wastes fuel. If the
kiln doesn't mix it intimately with the other stuff BEFORE it comes in
contact with the wares, it causes oxidised spots, cold spots, and possible
cracking.
=3Cclip=3E
(g) Although radiation becomes the principal mechanism for heat transfer
in the kiln chamber at high temperatures (=22glowing and above=22), =
throughout
the entire firing burners deliver their heat in the form of hot gas
(primary and secondary, both). We couldn't see any point whatsoever in
bringing radiation into the discussion of gas/heat delivery.
=3Csnip=3E
In the kiln, the primary and secondary air are combined so fully (one would
hope) with the fuel gases that they lose their separate identities. The
burners just deliver burning gases into the kiln. The open ports allow
secondary to enter. The kiln design mixes and distributes them. If the
kiln doesn't mix them well, you get areas of higher and lower
oxidation/reduction, and hot and cold spots.
Convection does take a major role in heat transfer. However one of its
major roles is to get the radiant gas/effluent molecules into closer
proximity to all the wares. So convection in this case greatly aids
radiant transfer.
Luminous flame burners take advantage of this factor.
=3Cclip=3E
(h) =22Forced air=22 burners are not ideal for every natural draft kiln by =
a
long shot. Industrial =22forced air=22 burners are essentially always used =
in
conjunction with high pressure gas delivery, where =22blowtorch=22 type heat=
is
the order of the day (like in steel furnaces, calciners, glass furnaces,
foundries, boilers, etc.). Even there, the amount of air =22forced in=22 is
delivered to balance with the fuel inflow.
=3Csnip=3E
A natural draft kiln is usually called that because it does not have draft
induced by mechanical means.... ie.- a blower on the burner(s).
Forced air burners are useful in that (if they are well designed) they
provide the potter with a wide range of reducing to oxidising flame
conditions, and a high level of =22turndown ratio=22 (the range of lowest to
highest stable BTU input settings). They remove the weather effects from
the naturally induced draft on a tall chimney. They remove dependance on
secondary air for combustion or the oxidation of compounds in wares.
Forced air burners simply provide more control options. The decision to
use them comes from such things as a cost/benefit analysis. And looking at
what control you are willing or able to give up. And often an aesthetic
decision..... no =22life support=22 on the kiln =3Cg=3E.
BTW....... Industrial burners often are using low pressure gas and high
pressure air. Many industrial burner systems utilize 4 inches WC pressure
routinely. 11 inches WC is very common too. These are considered low
pressures. In fact there are burners that utilize the flow of the HP air
to entrain the gas. In fact, these are called =22zero pressure burners=22.
The gas is not forced out the orifice by significant gas pressure at all.
The regulator is set to maintain atmospheric pressure and allow flow only
under the varying negative pressure created by the varying air flow, and
then it releases exactly the right amount of gas to mix with the air for
constant set-ratio combustion.
And industrial =22luminous flame burners=22 are anything but short hot blast
furnace type flames. They take great advantage of radiant heat transfer
with little particles of glowing radiant carbon being produced. Flat flame
burners are another interesting industrial toy.
In industry, most often burners are controlled to be =22on-ratio=22 most of =
the
time. Maximum fuel economy. The only time when this is set as something
else is when the work being fired needs less or more air. And this is
carefully monitored and controlled. Most industrial burners are sealed
port types. No un-considered air flow is allowed. Also no radiant heat
loss out the ports to waste heat or deteriorate the burner and plumbing.
=3Csnip=3E
(i) The willful and wanton misuse of the term =22vacuum=22 should disallow =
its
user from ever again scolding anyone else for the =22misuse of terms.=22
=3Cclip=3E
Nature abhors a vacuum. Forget whose quote that is =3Cg=3E. That is why =
draft
works on a kiln. The diferentail pressure across the ports into a kiln
chamber causes a flow of flame/air to enter to replace the materials being
exhausted. Otherwise a vacuum would be created.
Certain parts of fuel fired kiln chambers operate at a lower pressure that
the surrounding atmospheric (certain parts are often higher). That lower
pressure qualifies as a partial vacuum in my book. It is a miniscule
differential...... but a differential nonetheless. True, it is not =22hard
vacuum=22. And the situation is in a dynamic state.... and trying to =
acheive
equilibrium all the time.
But I think for the general usage =22vacuum=22 works OK to help visualize =
the
concept. People can sort of visualize stuff rushing in to =22fill up=22 the
vacuum.
=3Cclip=3E
Many sound sourcebooks about combustion and heat transfer are available --
the overwhelming majority of which should be found in the =22science=22
departments of bookstores and libraries.
=3Csnip=3E
Yup. This firing stuff is equal parts science and art........ that's the
definition of engineering =3Cg=3E. Do a little research and you'll find a
lot of the stuff many potters believe about kilns and firing just ain't
true. Cause and effect relationships can sometimes be misinterpreted from
the apparent data observed. Limited test data can often cause you to come
to false conclusions. That's why when using only =22seat of the pants=22,
total experiential learning it takes a pretty long time to gain true
insightful control and understanding.
May I recommend the North American Combustion Handbook as a good easily
understood reference (if you can find a copy).
So..... a bunch of my thoughts on kilns and this whole kilns, chimneys,
draft, air discussion.
Best,
..........................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
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