Gas Kiln design & firing

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gas kiln design & fring

integrating material and energy efciency into gas fred kiln plans www.ceramicartsdaily.org www.ceramicartsdaily .org | Copyright © 2010, 2010, Ceramic Publications Company | Gas Kiln Design and Firing |

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Gas Kiln Design and Firing Integrating Material and Energy Efciency into Gas Fired Kiln Plans I you want to build and/or re a gas kiln, there are several things you need to know beore diving in. Whether you plan on doing oxidation, neutral, or reduction ring, and regardless o the type o gas uel you will be using, knowing how a kiln is designed and put together helps you understand what is happening during the ring. Gas Kiln Design and Firing

Integrating Material Material and Energy Efciency into Gas Fired Kiln Plans provides guidance and inormation critical to the success o your ceramic work.

Principles o Gas Kiln Design: How to Plan and Build a Gas Kiln That Suits Your Needs by Frederick L. Olsen From choosing the size o bricks to stacking them into a nished kiln, there are several critical actors and principles to consider beore you begin to build a gas kiln. Olsen guides you through choosing the shape and size o your kiln, calculating fue and chimney size, and even adjusting or oxygen needs at higher altitudes. All o these will require you to make some decisions; some are based on the kind o work you make, and some are based on your workfow and studio capacity. capacity. All o these decisions will be guided by the decades o experience provided by veteran kiln builder Fred Olsen.

Efcient Gas Kiln Firing by Hal Frenzel Most anyone can gure out how to mix gas and air to produce heat in a kiln. What takes a little more expertise expertise is ring a kiln with eciency, eciency, regardless o what t ype o ring is being done. Understanding uel combustion and the kinds o burners and other atmospheric controls that are available will help you understand the processes at work in a gas red kiln, and will help you determine the best possible approach or your ceramic art.

www.ceramicartsdaily.org www.ceramicartsdaily .org | Copyright © 2010, 2010, Ceramic Publications Company | Gas Kiln Design and Firing |

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Principles o Gas Kiln Design How to Plan and Build a Gas Kiln That Suits Your Needs by Frederick L. Olsen

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ix critical actors must be considered beore you begin to design a kiln. This article reviews those considerations, then discusses the principles o good gas kiln design. These basic principles are then incorporated into the our distinct types o kilns discussed in succeeding chapters on crossdrat, downdrat, updrat congurations and multi-directional drat congurations.

Critical Factors Kind o kiln. Will you build an updrat, downdrat, crossdrat, circular dome, or salt glaze kiln? Will the kiln be 10, 20, 25, 45, or 150 cubic eet or larger? You must careully calculate your requirements beore you begin the design. Clay to be red. The type o clay you plan to re will determine the type o kiln you need, its size, the uel to be used, and so orth. Kilns may be planned and built specically to re terra cotta clay, sewer pipe clay, earthenware, stoneware, porcelain, or any o a number o possibilities. In act, the potter should know the clay and ware so well that he can design the kiln to enhance the pottery and to control the eects o ring. Atmospheric conditions. The chamber shape will depend on whether the kiln is intended or oxidation, reduction, or perhaps middle re. Burners and dampers can greatly aect the ability o the kiln to oxidize or reduce. This, in turn, aects clay bodies and glazes and their outcome. Available uel. It may be oolish to build a woodburning kiln in the city; it’s a romantic idea but impractical. Thereore, the relative availability o natural gas, propane/butane, oil, wood, coal/coke, and electricity must be considered. Since propane/butane and electricity are available almost anywhere, and are clean burning, they can be used anywhere except where natural gas is provided. Natural gas is a perect uel or use in cities or highly populated neighborhoods; however, beore one

proceeds, ascertain the amount o gas available to the site. Wood, coal/coke, and oil should be reserved or use in the country. Location o kiln. Whether city, suburb, backyard, garage, manuacturing area, or countryside, all locales te nd to “sel design” a kiln. By this I mean that each location tends to dictate what kind o kiln is easible, a wood kiln in a garage is not the best idea, nor is an anagama in a suburb. Many areas will have building code restrictions that aect what kind o kiln you can use. Be sure to check local regulations beore spending any money. Shel size. Be sure your kiln is designed to accommodate one o the standard shel sizes.

Design Principles Once the basic requirements are determined, according to these critical actors, the ollowing nine principles become an integral part o every kiln design:

PRINCIPLE 1: A cube is the best all-purpose shape or a kiln. The best design or an updrat kiln has the arch on top o the cube, not contained within (fgure ( fgure 1). 1). This allows or the best stacking space. Also, the volume o the arch

1 A cube is the best all -purpose chamber shape.


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2 Increasing the height o a cube chamber decreases ring eciency.

serves as a collection area or the fue gases. Increasing the height o the cube chamber with a xed width decreases the eciency o even-temperature ring (fgure ( fgure 2). 2). I do not know what the ratio actor is between increasing height and uneven temperature. From experience in ring an updrat kiln (2×3-oot base × 5-eet+ high stacking space) with burners in the foor, I have ound rom ½ to 1 cone dierence between top and bottom, no matter what ring schedule was used. However, on the same kiln 1 oot shorter (2×3×4 eet) it can be dead even top to bottom. In a similar kiln (3×3×5-oot stacking space), I ound a constant ½ to 1 cone dierence in the ring temperature between top and bottom but i the width and length was increased to 4 eet, the temperature evens out perectly. My conclusion is that equal height and width is extremely important to even temperature when using

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foor level burners. From my experience, the same ndings also apply to the downdrat and crossdrat kiln design. Increasing the length o the cube has no eect on the even ring eciency o the kiln, hence the development o tunnel kilns (fgure (fgure 3), 3), and other long tube-type kilns used commercially. In circle or round-dome kilns, ( fgure 4), 4), the diameter and height should be nearly equal, depending upon whether it is an updrat or a downdrat kiln. Most small downdrat kilns tend to include the dome in the height measurement, while updrat beehive types tend to add the dome to the height measurement. For ring tall kilns (fgure (fgure 2), 2), the burner becomes important and should be placed up the sides o the kiln. There are other specialty kiln designs, not based on the cube, like the tube, groundhog and derivative kilns, but ollow other principles listed here.

PRINCIPLE 2: The chamber shape is determined by heat direction and ease o fame movement to allow a natural fow. Two important rules to remember are: (l) the fame and heat direction should ollow the arch (fgure ( fgure 5), 5), and should not be at right angles to the arch ( fgure 6); 6); and (2) the fame movement and heat direction should have only two right angles to negotiate within the chamber, usually located at the rebox inlet fue and bag wall and at the exit fues. Right angles can cause irregular heating or hot spots, which could lead to reractory ailures and ring ineciency. Figure 6 also depicts the basic groundhog kiln design which is an eective kiln design.

4 Diameter and height should be nearly equal in circular or round-dome round-dome kilns.

Increasing length does not aect ring eciency.

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6 Heat direction should ollow the arch.

Conguration orces heat direction to fow at right angles to the arch.


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Cross sections o three chambers showing proper heat direction.

Nevertheless the act remains it produces hot areas along the crown ollowing the directional arrow which leads to cool areas along the bottom back side wall and reractory ailure at the back wall. I the design runs contrary to the arch then the transition should be curved or shaped rom the rebox up into the volume and then into the chimney wall or chimney fue. This is possible with the use o lightweight insulation castables. Kilns that run contrary to this principle are the Nils Lou Minnesota fat-top kiln design and some proessionally made gas-red pseudodowndrat kilns. They show that any shape can be red i there is sucient heat produced and a correct fue-tochimney ratio is used. The box, which is the easiest shape to build, becomes their primary kiln shape. There is merit to this style o design and they do work and should be researched. However, a more traditional approach based on natural drat is used here. Three kiln chamber cross sections with proper heat direction and fame movement are shown in fgure 7 .

PRINCIPLE 3: A specic amount o grate area or combustion area is needed or natural drat. Grate area (re box or uel combustion area) depends upon the uel used, ollowing these approximate guidelines to get started: Wood: 10 times greater than the horizontal section o the chimney, or put another way, the grate combustion area to chimney cross section area at the base ratio is 10 to 1. Coal: 1 square oot o grate to every 6 to 8 square eet o foor space. Oil: 1 square oot o combustion area to every 5square eet o foor area. Gas: 4½-inch minimum channel combustion space between ware and wall, usually the length o the wall. This is the most dicult principle o kiln design to apply, but it is the real heart o the kiln, or it determines the drat rate. When in doubt, be generous. It is also better to have a chimney area on the large side instead o too small. When designing de signing kilns the reboxes or combustion area and chamber are usu-

ally designed rst, then the chimney matched to the size o the grate area or combustion area and chamber. I your calculations come out short o brick sizes, then opt or increasing the area up to match the brick sizes. In natural-drat kilns the inlet fue areas must be equal to exit fue areas or the simple reason, “what comes in must go out.” I exit fues are too restricted, this will slow down the fow and retard combustion eciency, thereby aecting the temperature increase. The combined area o the inlets should be equal to the chimney section. I the latter is 162 square inches, then the inlet fue area should be about 162 square inches. Since the normal fue size is a brick size (9×4½ inches), our fues o this size would equal 162 square inches, adequate or a chimney with a cross section o 162 square inches. At the point where the exit fues enter the chimney, they should be restricted so that the chimney cross-section is larger than this fue area. This can be done by using a fue collection box behind the chamber and in ront o the chimney. See fgure 8. 8. In the astre design the exit fue is restricted because o the direct connection into the chimney. I the chimney cross section is made much larger than the inlet and exit fues in

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Flue-to-chimney relationship.


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combination o wood was to be used also, then an auxiliary air source must be provided. Exit fues should be built brick-sized (can be reduced in size i needed) and ollow the natural drat relationships rom fues to chimney. The dierence with orced drat is that the height o the chimney is reduced by at least 25 percent.

9 An updrat gas kiln must have equal i nlet and exit area.

a natural drat kiln, tapering o the chimney must be done to ensure proper drat. To make matters simple without without jeopardizing the kiln design, make the inlet and exit fue areas and the chimney cross-section area all equal, with the chimney point o entry slightly smaller. It is ar better to make these areas too large than too small, or they can easily be altered by plugging them up. When using pressurized gas as a uel, the inlet fue area should equal the exit fue area. An example: For an updrat gas kiln with 10 bottom burner ports with 2½-inch diameters (inlet foor fues) which will equal approximately 4.9 square inches per hole or a total o 49 square inches, the exit fue in the arch should equal approximately 49 square inches. See fgure 9. 9. This is a sae rule o thumb to use, remembering in the case o a downdrat or pseudo downdrat one can block the exit fue. In the updrat the exit fue or fues (i two holes are used) can be made smaller by about 5 percent, but the option to enlarge i needed is possible. Remember, making a hole smaller is a whole lot easier than enlarging. For a downdrat or crossdrat kiln using pressurized gas (orced drat), the inlet fues can be the size o the burner tips or slightly larger since the oxygen is supplied through the burner with secondary air pulled in around the burner port hole. In most cases, the inlet fue will be brick-size then reduced in size to add adjustability to the fue. I a

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PRINCIPLE 4: The taper o a chimney controls the rate o drat. Tapering reduces atmospheric pressure and increases the speed o drat, thereby controlling the rate o drat, which ideally should be 4 to 5 eet per second or natural drat kilns. The drat rate is measured periodically throughout the ring o the kiln, and, in the beginning, it will be very slow. The 4 to 5 eet per second is the rate o drat at most ecient operation, usually ater 1093°C (2000°F) in cone 10 kilns. The drat rate measurement in eet is determined by the inside circumerence o the chamber up the ront wall, over the arch, down the back wall, through the fues and up the chimney (which is 45 eet in Fig. 10). For this kiln to re at the proper drat rate, gases would take about 10 seconds to travel rom X to Y. One way to determine the drat rate is to throw an oily rag into the rebox and count the number o seconds it takes or the smoke to come out the chimney. I it is too slow, tapering the chimney could increase the rate. A kiln chimney that is between 16 and 20 eet, with a base section o 12×12 inches, would normally taper to a minimum o 9×9 inches. In a natural drat kiln, seldom would a chimney be less than 9×9 inches at its base cross section. PRINCIPLE 5: For natural drat kilns there should be 3 eet o chimney to every oot o downward pull, plus 1 oot o chimney to every 3 eet o horizontal pull. The height o the kiln chamber in fgure 11 is 6 eet. Thereore, there are 6 eet o downward pull (dp); and

11 It requires 10 seconds or gases to travel rom x to y (45 eet) in this kiln.

Three eet o chimney length are added or every oot o downward pull, plus 1 oot o chimney or every 3 eet o horizontal pull: A, chamber; B, fue collection box; C, chimney.


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13 Chimney diameter is increased by 50 percent in high-altitude adjustment.

High-altitude adjustments or inlet and exit fues: A, normal; B, fue height increased by 50 percent.

or every oot, 3 eet o chimney are added: Thus, 3×6=18 eet o chimney. Then add 1 oot o chimney or every 3 eet o horizontal pull (hp), which in fgure 11 equals the chamber width (5 eet), plus 1 oot o collection box, plus a 1-oot-wide chimney, totaling 7 eet. Thus, 7/3=2.3 eet; added to 18 eet, we nd that this kiln requires a 20.3oot chimney. To calculate chimney height or any natural drat kiln chamber: When using pressurized gas, the drat is orced and does not need the same height requirements as natural drat does to pull the drat through the kiln.

that you can add or knock out a brick, make the chimney entrance fue adjustable, and build the chimney so that the height is readily adjustable.

PRINCIPLE 6: Chimney diameter is approximately one-ourth to one-th o the chamber diameter. I a chamber is 5 eet in diameter, then the chimney must be at least 1oot in diameter. This principle, when used with Principle 3, can give a more specic chimney dimension or natural drat kilns. PRINCIPLE 7: A tall chimney increases velocity inside the ring chamber. Too high a chimney can cause irregular heating by pulling the heat out o the kiln, not allowing it to build up within the chamber, thereby prolonging the ring. On the other hand, too short a chimney can protract the ring by decreasing the drat rate, which allows build-up in the rebox and does not pull enough oxygen into the kiln to allow proper combustion or temperature increase. PRINCIPLE 8: Critical areas o a kiln should be planned and built to be altered easily. I in doubt as to fue sizes, grate area, or chimney size, bigger is better. Plugging excess space with bricks is an easy matter. Also, or ease o construction, all dimensions should be based on the standard 9×4½×2½-inch brick, or the large standard, 9×4½× 3-inch. Perhaps 80 percent o the time, normal fue dimensions will be one brick standing on end (9×4½ inches). These should be spaced 9 inches apart. There will be no problem in the kiln you can’t correct i you remember to make fues adjustable, planning so

High-Altitude Adjustments Building a kiln at high elevations necessitates adjustments to compensate or decreased oxygen per cubic oot o air. The dierence is very apparent in hot desert elevations over 3800 eet, and in mountain elevations rom 5000 to 10,000 eet. Outside air temperature, as well as elevation, has a direct eect on the amount o oxygen present. For instance, in Aspen, Colorado, the elevation is about 8600 eet. However, at an outside air temperature o 22°C (72°F), the density o oxygen per cubic oot o air is equivalent to the amount ound in air at 10,000 eet. Thus, kiln ring is more ecient at night, when the air cools and becomes denser, and more oxygen is present. There are ve steps in the procedure or making appropriate alterations to a natural drat kiln to compensate or high altitude and low air density. 1). Design the kiln according to standard principles, guring out the chimney diameter, the inlet and exit fue sizes, and the chimney height. 2). Increase the chimney diameter by roughly 50 percent (so it works into the closest bricklaying combination). Thus, a chimney with a diameter o 9 inches would be increased to a diameter o 13½ inches ( fgure 12). 12). 3). Increase the inlet and exit fues by 50 percent. I you have three inlet and exit fues measuring 9 inches high and 4½ inches wide each, increase the height by 4½ inches to make them 13½ inches high by 4½ inches wide (fgure 13). 13). 4). Increase the chimney height by at least 30 percent to pull the greater volume o air needed. 5). It is not necessary to increase the grate area in relation to the chimney (or wood) or to the foor area (or coal and oil). Remember, it is not more uel that is needed, but more oxygen to burn the uel.


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Efcient Gas Kiln Firing by Hal Frenzel

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hat is perect combustion? By reading the description in the North American Combustion Handbook, it seems rather simple: Perect combustion exists when one carbon atom is combined with two atoms o oxygen to orm one carbon dioxide molecule, plus heat. But when you are ring a kiln to achieve a certain consistent atmosphere, it becomes a little more complicated. To achieve complete combustion, the exact proportions o uel and oxygen are required with nothing remaining. In a gas kiln ring this is oten dicult to attain because o the many variables in uel and oxygen (which is derived rom the air) and the equipment used to mix the two. The most common uels used today are natural gas and propane. These are hydrocarbons and when they are properly mixed and ignited, they produce heat, carbon dioxide and water vapor. Air is a combination o approximately 75% nitrogen and 25% oxygen by weight. Unlike oxygen, the nitrogen does not react (combust) but it still absorbs a portion o the heat and thereore creates a cooler fame. During the ring o a gas kiln there are a trio o atmospheres that have to be controlled to achieve both a rise in temperature and the desired glaze results. The rst, and most important, atmosphere is neutral. It is only in a neutral atmosphere that perect combustion can be attained. A neutral atmosphere is the most uel-ecient ring possible. I the amount o air is increased, or the amount o uel is decreased, rom a neutral ring, the mixture becomes uel-lean and the fame is shorter and clearer. The kiln

has now entered an oxidizing atmosphere and the rate o temperature rise will decrease. I the uel supply is increased or the air supply is decreased the atmosphere becomes uel-rich and reduction begins. The fame becomes long and smoky and incomplete combustion occurs. The result is an excess o carbon, which combines with the remaining oxygen and creates carbon monoxide. To convert back to its natural state o carbon dioxide, the carbon takes oxygen rom the metal oxides in the glaze, thus altering the nished color o the glaze. The rate o temperature rise will also diminish under these conditions. Regardless o the atmosphere necessary or the results you desire or your work, a higher level o eciency and uel savings may be attained by ring to a neutral atmosphere whenever possible (see diagrams on page 38). With the enormous increases we have seen and will continue to see in uel costs, it might become highly desirable to buy an oxygen probe and maintain a neutral atmosphere or at least part o your rings. In the early stages o a ring, excess oxygen helps in the decomposition o the organic and inorganic carbonates and sulates. In researching this article, I was unable to nd a potter/ceramist who could explain exactly how excess oxygen during the glaze maturity period enhances the glaze nish or color. This raises the question as to whether the results would have been the same i red in a neutral atmosphere during this period. I, by chance, the results are the same, then an oxidation potter would save both time and uel i he or she red in perect combustion during this period.


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Frng Aor Oxygen to burn uel in an artist’s kiln comes rom the air. The air, however, is not all oxygen. Rather, it is ar rom it. By weight, air is approximately 77% nitrogen and 23% oxygen. What this means to the artist is that or every ONE pound o oxygen rom air that is heated to kiln temperature to burn uel in a kiln, THREE pounds o nitrogen have to be heated to kiln temperature. This is why using “excess” oxygen is expensive. Using a minimum amount o excess air in an oxidation ring saves both energy and money.

Defining the terms

Oxaon Aor: A mixture o uel and air where there is a signifcant excess o oxygen rom the air relative to the uel; defned (somewhat arbitrarily) as more than 3% excess oxygen.

Nral Aor: A theoretical mixture o uel and air where there is a perect balance between the amount o uel and the amount o oxygen rom air necessary to burn that uel.

C + O2 C + 2O2

CO2 + O2 + heat

Oxidation: When excess oxygen is present in the kiln, it plays no part in combustion. However, it does absorb heat energy that would otherwise help re your ware. In this way, it does contribute to uel consumption.

CO2 + heat

Neutral: When exactly two oxygen atoms are present or each carbon atom, neutral (or perect ) combustion occurs, creating carbon dioxide and heat. Perect combustion assumes that turbulence and circulation in the kiln is so complete that every atom nds a partner. This is dicult in even the most ecient kilns, so some excess oxygen is typically necessary to avoid reduction.

Ron Aor: A mixture o uel and air where there is more uel present than there is oxygen rom the air to burn the uel. For complete combustion to occur in a reducing atmosphere, the uel must react with all the oxygen rom the incoming air and with oxygen rom other sources. For a ceramics artist, the important “other” sources o oxygen are oxides o iron and/or copper in the ware being fred, as those oxides are reduced (relieved o their oxygen molecules) by oxygen-hungry oxygen-hungry uel. This typically results in a color change.

2C + O2

2CO + heat, and later, 2CO + O2

2CO2 + heat

Reduction: When an excess o carbon (uel) or a shortage o oxygen (air) is introduced, incomplete combustion takes place. Carbon monoxide (as opposed to carbon dioxide) is produced along with heat, though not as much as would be produced during complete combustion. The carbon monoxide then looks or more oxygen, which it takes rom oxides in the clay and glaze in the kiln. This is also the reason yellow fames shoot out through spy holes when a kiln is in reduction—the carbon-rich uel is ollowing the oxygen supply.

recipe MODIFIED OHATA KHAKI (Cone 10) G200 Fe Feldspar                   50 % Silica                          21 EPK                           6 Talc                           6 Whiting                        7 Bone As Ash                       10 100 % Add: Red Ir Iron Ox Oxide               13 %

Both o these suraces were glazed with Modied Ohata Khaki, an iron saturated glaze. The piece on the let was red in oxidation, and the piece on the right was red in reduction.


8

Aor conrol The two most common types o burners used today are orced-air and atmospheric venturi burners. How these burners mix the uel and air is o vital importance in accomplishing complete combustion.

FORced-AiR BuRNeRs There are many types o orced-air burners, most o which are used in industrial applications with sophisticated proportional uel-air control. The typical orced-air burner used on a kiln is not as complex. Typically there are two burners that enter the rear o the kiln, which have either individual blowers or one central blower with some orm o rheostatic speed control. When adjusting the gas during the fring process you must also adjust the air ow. Initially, this might require some guess- work or prior experience in determining the proper uel to air ratio. But i there is an oxygen probe available you’ll be able to measure the ratio more precisely and achieve the particular atmosphere necessary or your glazes. (See CM September 2002, or more details on the oxygen probe.)

gas air

A orced-air burner propels air through the burner tube. The turbulence created by that orce mixes the gas with the air. Gas is controlled with a pressure valve, and air is controlled through either a rheostatic speed control on the motor or a manual inlet cover fap that blocks the air rom entering the blower.

AtmOspheRic BuRNeRs Atmospheric venturi burners are oten mounted under the kiln in a vertical position. There is an air shuttle on the inlet side o each venturi burner that allows adjustment o the primary air ow into the burner. The venturi burner is called an inspirator and utilizes the energy in the gas jet coming out o the burner orifce to draw in air or combustion. The jet o gas rom the nozzle produces a high velocity in the throat o the venturi, and the resulting low pressure pulls air in and around the gas jet. I the rate o gas is increased, more air will be induced. Thus the air and gas are proportioned or combustion.

gas air

An atmospheric burner operates according to the orce o gas through a narrow throat in the burner body. This creates low pressure behind the opening and draws air into the burner with the gas. As the gas is increased, more air is drawn into the burner, and the appropriate ratio o gas and air or combustion is maintained. Gas is controlled with a pressure gauge, and air is controlled by a plate at the back o the burner.

dAmpeRs There is one other piece o equipment on every kiln that is ab- solutely necessary in controlling the kiln atmosphere and that is the damper blade in the chimney stack. Even the smallest adjustment in either direction could change the atmosphere rom neutral to either reduction or oxidation. By moving the damper in, you create back-pressure in the ue gases, which reduces the ow o air into the kiln and thus causes a reducing atmosphere. By moving the damper out, you create more drat, which pulls more air into the kiln and thus causes an oxidizing atmosphere.

oxidation damper blade reduction

fue gases

A damper is usually a piece o reractory material, oten a kiln shel, that is placed in the path o the fue gases as they travel up through the chimney stack o the kiln. By moving it in and out, the pressure inside the kiln is controlled. Increased pressure decreases airfow, and decreasing pressure increases airfow.


9

Gas Kiln design & firing

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