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cold conversion kiln gage for mesuring atmospheric pressure

updated wed 3 nov 99


Paul Taylor on sun 31 oct 99

Dear All

I would like to add to Gavins comprehensive over view of kiln pressures.
Just to say that the adjustments on the kiln can be very fine
depending on the widths of flues etc . I discovered how fine when I bought
an oxygen analyzer. I immediately changed the shape of my secondary air
ports so I could make the fine adjustments. It would be useless of me to
give specific measurements every kiln is different. The secondary air is
pre heated a little under the kiln burners so it makes sense to use lots of
it . As for the flue, one centimeter can make a huge difference in the
pressure of the kiln chamber. I am tempted to rig up an atmospheric pressure
gage if I knew how. because the weather also has an effect I am sure it
would be more convenient than a candle held to a hole in the bottom of the
kiln door . Before I discovered how finally tuned my kiln needed to be I
wasted a lot of gas going from extreme oxidation to extreme reduction.
Also my venturies dont like to go beyond 2.5 ponds per sq in and their
pressure settings also have to be taken into consideration a very slight
change in their setting can make a difference.
If any one has a design or knows a few perimeters for setting up a
water, mercury , or any sort of gage that can give me a more convenient
atmospheric pressure reading I would be grateful. I can build most things
>From: Gavin Stairs
>Subject: Re: cold conversion kiln
>Date: Sat, Oct 30, 1999, 4:50 pm

>----------------------------Original message----------------------------
>At 13:47 1999.10.28 , you wrote:
>>----------------------------Original message----------------------------
>>I have a few comments that may help. As the temperature in a kiln
>>increases the pressure inside the kiln also increases.
>Not so. The pressure inside a firing kiln at temperature must be slightly
>decreased, not increased. What drives the pressure differential in a kiln
>is primarily the stack effect. The pressure above the mouth of the stack
>is atmospheric. The pressure decreases as you move back down the stack,
>against the flow. There are two reasons for this. One is the speed of the
>gas flow: Bernouli famously said that the pressure in a fluid is inversely
>proportional to the speed of the flow. This is the effect of inertia. So,
>as you move from the static atmosphere to the moving stack flow, the
>pressure drops. The other reason is buoyancy: the hot gas is less dense
>than the cold atmosphere. This is owing to Boyle's or Charles' law
>depending on how you phrase it: the density of a gas at standard pressure
>depends inversely on the absolute temperature. The relationship of
>temperature, pressure and density of a gas is summarized in the ideal gas
>law: PV=mRT. The pressure times the volume equals the mass times a
>constant times the absolute temperature. This relationship is one of the
>earliest indicators of absolute zero on the temperature scale. For an
>ideal gas, in which the atoms of gas occupy insignificant volume, the
>volume of the gas goes to zero at T=0 Kelvin.
>Back to the kiln. So the pressure is falling as you descend the
>stack. But another effect mitigates this fall: in a fluid flow through a
>pipe (i.e., the stack), fluid viscosity (friction) causes a pressure drop
>in the direction of the flow. So, the pressure rises as we go against the
>flow. Also, the pressure in a fluid rises as we descend in the
>fluid. This pressure rise due to gravity is called the head.
>All of these effects sum up to give the pressure at any point in the
>flow. The same effects act at the burners. As the air is moving at the
>burner ports, the pressure is below atmospheric. Heat is injected (by
>burning fuel) which increases the volume. The fuel also adds to the mass
>of the flow, also increasing the volume. If the stack effect is not
>working, this leads to a drastic increase in the internal pressure, and the
>burners will blow back through their ports. This is burner stall, and the
>only remedy is to get the stack operating by gently heating it before
>turning up the burners. Pull the damper all the way out, and throw
>something burning down the stack. When the stack is drawing, it sucks the
>burner flames in through their ports. So the pressure at the burner ports
>must be below atmospheric.
>This is not true of forced air, closed burner ports, which can operate at
>positive pressure. I am talking here about open port, natural draft, or
>Venturi burners. Venturi was another Italian natural philosopher who
>studied the pressure of fluids flowing in pipes. The pressure drop in a
>flow at the centre of a smooth constriction is named after him. In a
>Venturi burner, the flow constriction serves to suck the primary air into
>the region of the burner nozzle by reason of the flow of gas from the
>nozzle. So a Venturi burner tends to resist stalling for this reason. A
>burner without the constriction will be somewhat more prone to stalling, or
>back flashing. You get a ball of red and yellow flame around the burner
>instead of a blowpipe flame coming out the end.
>In the middle of the kiln volume, the flow almost comes to rest, so the
>pressure rises again. In the static volume of the kiln, the high
>temperature means that the density (m/V) of the gas will be lower than air,
>so although the pressure will fall with increasing head (as you go up), it
>will do so less than air pressure outside the kiln. So relative to
>atmosphere, the pressure rises as you go higher in the kiln. When a
>downdraft kiln is drawing properly at some elevated temperature, you may
>see that below a certain height on the kiln the pressure is below
>atmospheric (a peep or a crack will suck air) while above it, the kiln will
>be above atmospheric (the peep will blow). The actual pressure gradients
>in the kiln are quite subtle, and are more complex than I want to get into
>here. They involve the flame path, the position of the flame fronts, and
>the thermal gradients in the flows.
>In a downdraft kiln, the stack port is low. It is possible for an
>inversion to form, in which the flame front moves across to the stack port
>without moving high. This is one of the functions of a bag wall, or a
>vertically directed burner: to force a vertical flow.
>Anyway, we now have the pressure profile more or less complete: The
>pressure goes from atmospheric to below in the burners, then rises almost
>to atmospheric inside the kiln, may rise above atmospheric in some parts of
>the kiln, but exits the kiln to the stack in a region of low pressure. The
>flow in the stack, and therefore in the kiln, is defined by the pressure
>gradient in the stack. If it is high, the flow is large. If low,
>small. Stacks are built to have a potentially large pressure gradient, and
>therefore a large flow. This permits a high rate of burning and quick
>temperature rise. How to regulate this? The damper does this by
>introducing a lossy flow restriction. Lossy means that, unlike in a smooth
>Venturi, the pressure on either side of the restriction is not the
>same. The pressure behind the damper gate will be less than it is before
>the gate, so it reduces the pressure gradient in the rest of the stack, and
>therefore reduces the flow. Another way of looking at it is to say that
>the damper controls the pressure in the kiln. The farther in the damper,
>the higher the pressure in the kiln. Push the damper in too far, and you
>may stall the kiln, and get a blowback at the burners. Pull it out too
>far, and the burners will run lean, and not heat the air sufficiently. The
>damper setting at the stack and at the secondary ports depends on the
>burner setting.
>To set up the kiln for firing, you need to set the burner gas rate to get
>the proper heat for the temperature ramp you are trying to achieve. Then
>you need to set the primary air to burn much of the gas in primary
>air. Then, set the secondary dampers to give enough air to burn the rest
>of the fuel. If you do this with the stack damper wide open, you will
>undoubtedly have the kiln at negative pressure. That means it will be
>sucking air through every crack, and will probably be in oxidation. The
>ideal damper position is where the secondary air ports can be almost wide
>open. Then the kiln will be balanced in pressure, just a bit below
>atmospheric. It will be easy to control the atmosphere by small
>adjustments of the stack damper.
>When the kiln is drawing strongly, lots of secondary air will be drawn in
>through the burner ports and also through any open peeps and cracks. So it
>will be harder to develop a reduction atmosphere in this case, even with
>strong fuel flow. When the damper is closed down, the kiln pressure will
>rise slightly, and the secondary flow will be inhibited. The burners will
>burn rich, and it will be easier to establish reduction. However, the
>carbon monoxide will also find it easier to leak out of the kiln, so beware
>of CO during reduction.
>Having said all this, every fireman will have his or her own preferences
>about just how to set up the burners and dampers. All that is really
>happening is that the internal pressure of the kiln is being set sightly
>higher or lower. You can successfully fire with the damper wide open, or
>with it some way in. If the kiln was designed properly, the damper will
>most often be perhaps a quarter to three quarters of the way in for most
>That's the story on gas flow and pressure in a kiln. The design of the
>proper rates of flow is another matter which will have to wait until
>another evening.
>Gavin Stairs
>Stairs Small Systems
>921 College St., # 1-A
>Toronto, Ontario, Canada M6H 1A1
>phone: (416)530-0419

Hank Murrow on tue 2 nov 99

>----------------------------Original message----------------------------
If any one has a design or knows a few perimeters for setting up a
>water, mercury , or any sort of gage that can give me a more convenient
>atmospheric pressure reading I would be grateful. I can build most things
> `Paul

Dear Paul; The Dwyer company makes several manometers which read in
various ranges of pressure. For example, I use their portable "Slack-Tube"
manometer when I go out to help other potters with their kiln problems. I
use their "Flex-tube" well type manometer on my own kiln and those I build
for others. And if one's on a budget, the U-tube manometers are hard to
beat for measuring gas pressures. A unique variation which might prove very
useful to you is their "Dual Range Flex-Tube U-inclined Manometer". One may
hook it up to read gas pressures by hanging it in the vertical position as
one normally would. The special feature of this device is that by rotating
it slightly more than 90 degrees with the aid of the built-in level, it
will read flue or chamber pressures . For example, it will read gas
pressure from 0-16"wc, and in the laid down position it will read kiln or
flue pressures from -.20" to 2.6"wc. Last I heard, Dwyer was at POBox 373,
Michigan City, IN 46360. 219-872-9141.


You could build your own inclined u-tube manometer by routing out a piece
of plywood to accept a length of Tygon clear plastic tubing and making a
scale of some kind on it of your own invention(RELATIVE pressure the
important thing here) and connect it to the chamber on one leg of the U
while leaving the other end open to the atmosphere. If you are on a severe
budget, you could study the pictures in the Dwyer catalog for a better idea
of how to build it. I think their version goes for around $50. In either
case, let us know what you learn, Hank in Eugene