[Trade Journal]
Publication: Journal of the American Institute of Electrical Engineers
New York, NY, United States
vol. 42, no. 9, p. 988-994, col. 1-2
The Production of Porcelain for Electrical Insulation-VI
BY FRANK H. RIDDLE
Champion Porcelain Company, Jeffery-Dewitt Insulator Company
Review of the Subject.—This article contains several excerpts taken from papers presented at recent meetings of the San Francisco and Los Angeles Sections by Dr. Joseph A. Jeffery on "Modern Methods of Firing High-Voltage Porcelain."
The pyrochemical reactions which take place during firing have been described'. The firing is done to bring about a complete vitrification of the body to form porcelain. Complete vitrification means no porosity.
Firing temperatures are measured by thermocouples, pyrometric cone fusions or measurement of shrinkage bars made of porcelain and withdrawn from the kilns at intervals. Pyrometric cones and shrinkage bars show the effect of time as well as temperature; however, cones are preferable as they can be observed through peepholes while bars must be withdrawn and measured.
The two classes of kilns used in firing porcelain are Periodic and Continuous. Periodic kilns have a fuel efficiency of only about 5 per cent, while the continuous kilns not only have a fuel efficiency several times greater than this but also are more efficient as regards production per cubic foot of space required, convenience of operation, labor cost, quality of ware, etc.
In firing a periodic kiln the entire contents and inside kiln walls are slowly heated up on a definite time temperature schedule. The air supplied to the burners and fire-boxes is not preheated but is cold, and as soon as the hot gases pass through the ware they are exhausted to the atmosphere and wasted.
The temperature of the exhaust gases is only a few degrees lower than that of the ware in the kiln, being anywhere from 1100 deg. cent. to 1400 deg. cent. (2012-2552 deg. fabr).
In firing the kilns the heat goes into the kiln through throats and then passes up through the mass of ware conveniently stored in bungs of earthenware containers (saggers). The ratio of the number of saggers close to the fire-boxes where the flames are hottest and longest as compared to the saggers farther away is great and as a result there is no way of giving the ware the same heat treatment in all parts of the kiln.
In continuous kilns the car tunnel kiln is used more often than the compartment kiln. There are two main types, direct-fire and muffle,. The thermal efficiency of both is obtained in the same manner. These kilns are from three hundred feet to three hundred and fifty feet long and of relatively small cross-section, being about four feet by six feet and so arranged that the temperature at any cross-section is practically the same. The cross-sectional area of a periodic kiln is approximately twelve times as great as the continuous kiln.
In these kilns a continuous train of cars of ware passes through in one direction on a definite time schedule. The cars and ware are cold when entering the kiln and are heated up gradually and evenly as they approach the center of the hot zone where the temperature is held constant while the thermal reactions proceed to completion. Then the cars of ware leaving the hot zone are gradually cooled down until they are withdrawn at the exit end cool enough to handle. The air for combustion passes in a counterwise direction to that in which the cars travel. This air absorbs heat from the ware on the cars and not only cools them but also becomes hotter and hotter itself, so that by the time it has reached the fire-boxes or burners it is practically at the same temperature as the firing ware itself. The superheated air combines with the fuel, thus giving a great deal more heat efficiency than it would be possible to get from cold air.
These products of combustion continue to travel in the same direction, that is, counterwise to the direction in which the cars go and thus give up practically all their heat to the incoming ware, leaving the kiln relatively cool.
The muffle kiln has an added advantage in that it is arranged to keep the products of combustion away from the ware and also to create a vertical flow caused by difference in density of hot and cold air. This practically equalizes temperatures from top to bottom.
CONTENTS
Review of the Subject. (600 w.)
Firing. (250 w.)
Measuring Temperature. (300 w.)
Periodic Kilns. (650 w.)
Continuous Kilns. (650 w.)
General Description of Tunnel Kilns. (275 w.)
Dressler Kiln. (13:51 w.)
Uniformity of Real Treatment. (225 w.)
FIRING
NO matter how carefully the forming and drying of an insulator have been done, it is still essential that the piece be properly fired. In fact the firing is one of the most important operations in the manufacture. The success of the firing is primarily dependent upon the type of kiln used and the skill with which the kiln is controlled.
The pyro-chemical reactions which take place during firing are very important and must be carefully controlled.(1)
It is not generally known that the application of higher temperatures will convert any porcelain into a glass and that the successful manufacturer of porcelain depends on arresting this process at a very critical point. Due consideration of this point makes one realize the necessity for almost perfect control in order to attain any degree of skill approaching the ideal.
At low temperatures, such as are employed in making lower grade porcelain wares, the feldspar simply acts as a glass to fuse and cement the quartz and clay together. In making high-voltage porcelain, higher temperatures are used and the function of feldspar is quite different as it acts as a solvent and is thus enriched with dissolved quartz and some of the clay substance, the bulk of the latter undergoing dissociation into sillimanite which at first assumes the amorphous and crypto-crystalline form and later develops as needlelike crystals.
Any porcelains of the type used for modern insulators have an exceedingly narrow firing range within which all of their possible properties are best developed.
MEASURING TEMPERATURE
The methods of measuring temperature and heat work in ceramic kilns are important and of interest, The three general methods are by use of thermocouples, pyrometric cones and shrinkage.
1. Thermocouples are generally known and need not be described. One point, however, should be brough out in connection with them and that , that they measure temperature alone and do not show the effect of time upon the ceramic product.
2. Pyrometric cones, Fig. 30, are small tetrahedral about two inches long having a one-half inch triangular base. They are made from ceramic materials such as kaolin, flint. feldspar, etc., and so compounded in series that each composition deforms by bending at a definite temperature under definite burning conditions. The temperature interval between the softening or bending point of two numbers is practically 20 deg. cent. (36 deg. fahr.). By experimentation it is easy to determine the cone number that deforms when the body has matured properly. This cone and two or three others which mature at lower temperatures are then placed on clay slabs so they will stand in a vertical position between the ware to be fired on the cars and so exposed that they can be observed through the peep holes in the kiln. These cones being made of the same types of material as the ware show the effect of the time as well as the temperature effect and hence are reliable indicators, particularly when used in connection with pyrometers, which register temperature alone.
1. Shrinkage—bars or other pieces shaped from ceramic bodies are drawn from the kiln from time to time and the shrinkage accurately measured. It is necessary to know the exact size of the pieces before use and also the shrinkage necessary to show the proper maturing of the body being fired. The so-called Veritas firing rings are used to some extent for this purpose; however, any shrinkage-bar is difficult to draw from the kiln and the chances for error in measuring rather small variations in shrinkage are great.
PERIODIC KILNS
During the past few years great progress has been made in the United States in the development of kilns in which porcelain is fired. Gradually the manufacturer has given up the use of the so-called up-draft periodic or beehive kiln. This has been replaced to a certain extent by the down-draft kiln which has been developed to a remarkable degree; and although a very inefficient mechanism (its efficiency being only 5 per cent) it is quite satisfactory for firing many classes of porcelain articles. It is not so suitable for firing high-voltage porcelain which must be neither overfired nor underfired.
In firing a periodic kiln the entire mass is slowly heated up on a definite time temperature schedule. The burners or fire boxes are always supplied with cold air and as soon as the hot gases pass through the ware they are exhausted to the atmosphere and wasted. These instruments are at best very inefficient equipment from a fuel saving standpoint.
Another objection to periodic kilns from a consumer standpoint is that there is no way of giving the ware the same heat treatment in all parts of the kiln and this holds good throughout the entire firing process.
The kilns are heated at uniform rates, but towards the end of the firing it is necessary to hold the temperature and to permit the heat to soak into or saturate the kiln contents as much as is possible, in order to secure reasonably uniform temperature conditions throughout the entire volume. The ware near the throats or bags will have been exposed to a maturing temperature much longer than the ware located in the coolest part of the kiln, and it will also have had a very different gas treatment. As glazes change color with different gas treatments, as well as with different temperatures, visual inspection of the ware or glazed test pieces is not altogether reliable, particularly in view of the fact that it is perfectly possible to have the outside of a piece of porcelain properly fired and mature while the inside is underfired and porous. It seems to the writer that it is better practise to have a transparent glaze through which an inspector can see overfiring defects on the surface, and to resort to other means for determining underfiring in the heart, of the porcelain.
Fig. 31 shows the cross-section of a down-draft periodic kiln with the saggers loaded with ware and in the positions in which they are burned. Saggers are refractory containers in which the ware is placed so as to be piled in bungs or columns for economically filling the kiln and to keep the insulators free from strain. The arrows indicate the paths of the heat, flames and gases as they come up through the throats or bags from the fire boxes and pass around and through the saggers of ware into the flue at the bottom of the kiln and out to the stack.
The construction of the up-draft kiln, more commonly used, is very similar excepting that the products of combustion come into the kiln through the bags and also through an opening in the center of the floor through flues placed under the floor and leading from the fire boxes. The gases rise around the saggers of ware and pass out through properly proportioned openings in the kiln crown into the stack. In this case the kiln makes its own stack or chimney while an outside stack is necessary with the down-draft kiln. A glance at the cross-section will show that the saggers of ware directly above and in front of the bags are exposed to the greatest heat and most severe treatment. The quality and composition of the gases are also different here from what they are in the center of the kiln, or even up near the crown, where they have become mixed and completely burned. Practically all periodic kilns are fired with natural draft.
For several years careful investigation has been made of the composition of the gases within periodic kilns and much study given to methods of controlling their composition within the desired limits, but this work came to naught owing to the great difference in the specific gravity of the gases involved and the unavoidable contamination or oxidation owing to the leakage of air into the kiln through the combustion regulating devices, and the holes or cracks that are bound to inevitably develop in a structure made of brick and fire clay which is subject to repeated heating and cooling.
CONTINUOUS KILNS
There are two classes of regenerative or continuous kilns, namely, the compartment and the car-tunnel kilns. The latter kiln, however, is the one which has been adopted in porcelain manufacture to the greatest extent.
Some years ago attempts were made to introduce the car tunnel kiln, taut it was not until 1916 that a company was formed with a competent engineering force that gave us the first really successful tunnel kiln for the firing of high-voltage porcelain. Since that time the tunnel kiln has made rapid advances and we now have two distinct types: One in which the products of combustion mingle with the ware, the most modern of which is known as the Harrop kiln—Fig. 32; the other known as the Dressler kiln—Fig. 33. In the latter the products of combustion are drawn through combustion chambers permitting the ware which is in a separate tunnel to be surrounded by pure superheated air or gases of any desired quality, without in any way affecting the character of gases in the combustion chambers.
The principal argument presented in favor of the tunnel kiln is its great thermal economy. As a matter of fact, the engineers of the Dressler Company guaranteed a saving of 80 per cent of the fuel ordinarily used in the periodic type of kiln. This is not difficult to understand when we consider that the gases toward the end of the firing in the periodic kiln actually leave for the stack at a temperature of 1100 deg. cent. to 1400 deg. cent. (2012 deg. fahr. to 2552 deg. fahr.), whereas those of the tunnel type of kiln have been cooled by the incoming ware to a temperature as low as 200 deg. cent. to 250 deg. cent. (392 deg. fahr. to 482 deg. fahr.) In addition to this the atmosphere which is to supply the oxygen for combustion is fed to the gas at temperatures from 1100 deg. cent. to 1400 deg. cent. (2012 deg. fahr. to 2552 deg. fahr.), and this superheated air is obtained by drawing air past the cooling ware. The heat gradient through which the ware passes while being fired and cooled can best be shown by referring to Fig. 34. This is the actual temperature curve of a tunnel kiln used in burning suspension insulators. The curve is typical of most types of tunnel kilns. Keep this curve in mind in following the description of the kiln.
Each type of tunnel kiln has its particular field of application not only to the ceramic industry but also in the heat treatment of steel plate and iron and steel castings where atmospheric oxygen must be excluded. However, some very recent developments in the firing of high-voltage refractory porcelain indicates that there will be some very marked improvements and changes in tunnel kilns in the near future. In this type of porcelain, extremely rigid control of the chemical composition of the gases in contact with the ware is essential. There are several proposed types of kilns for accomplishing this better control and experiments on a large scale are now in progress.
Next in importance to the great thermal economy of the tunnel kiln is its continuous operation. One tunnel kiln of modest dimensions will replace a considerable number of the largest periodic kilns.
The loading and unloading of the tunnel kiln can be made a continuous operation with a very few men, whereas the periodic kiln is generally loaded and unloaded by a larger crew of men in order to reduce the idle hours of the kiln to the minimum.
Another great advantage is the loading and unloading of the cars in a comfortable room while the men loading and unloading the periodic kilns are exposed to very high temperatures and excessive drafts. Fig. 35 shows insulators being loaded onto tunnel kiln cars.
The handling of the saggers containing the ware for a tunnel kiln is a simple operation while in the periodic kiln the saggers of ware have to be piled upon one another to such a height that they can only be placed by the use of platforms and ladders, and of course during this operation much ware is damaged or subjected to severe strains.
Matters of economy and convenience in the manufacture of porcelain due to the use of the tunnel kiln do not necessarily interest the electrical engineer, but he is interested in the fact that it enables the porcelain manufacturer to develop a porcelain of superior properties at a reasonable cost.
GENERAL DESCRIPTION OF TUNNEL KILNS
In some respects there is a great similarity between the different types of tunnel kilns. As the name implies, the kiln really is a long tunnel-like structure. In modern kilns the tunnel is built almost entirely of brick; low-grade fire brick being used in the zones of low temperature; high-grade fire brick near the central zone; and silica or carborundum brick in the high temperature zone. Inasmuch as a considerable length of brick in the tunnel is to be maintained at a red heat and part of it at a white heat, it is necessary to estimate the expansion of the structure very closely and provide suitable expansion joints.
The draft which is to support combustion should always be provided by an efficient exhaust fan with water cooled bearings, the same preferably being driven by a constant speed electric motor. The tunnel kilns should be operated with little or no excess air, and obviously a natural draft stack cannot be depended upon to maintain the draft.
The cars range from 32 inches to 48 inches in width and are generally about 72 inches long. Each car is provided with a tongue on one end and a groove on the other end in the refractory structure, to form a baffle to prevent the radiation and convection of heat between the cars.
If the tunnel kiln is the sand sealed type, the car is provided with a metal flange running the entire length on each side and projecting downward into a trough filled with sand which is located on a bench on each side just above the track. (Fig. 32.)
For non-sealed kilns the car is equipped with a tongue of refractory material which projects into a groove formed in the brick work of the tunnel. This tongue however, provides against radiant heat only, and is not suitable if the air under the car is in rapid motion. (Fig. 33.)
DRESSLER KILN
In the Dressler tunnel kiln an entirely new principle of continuous firing is introduced. The flames do not come in contact with the ware. Essentially it consists of a tunnel which can be divided into two portions: The heating zone which occupies rather more than half the length, and the cooling or annealing zone. In the heating zone, the refractory arch is covered with a thick layer of insulating powder, which prevents the escape of heat from the interior. Inside this arch there are two trapezoidal-shaped chambers placed on a bench on each side of the kiln, and running toward the entrance end. The combustion of the gas for heating the kiln takes place in these chambers near the center of the kiln, and the products of combustion are carried through these chambers toward the entrance end of the kiln by means of an exhaust fan, which is connected through suitable ducts to the chambers on each side. These chambers are entirely independent of the structure of the kiln and are supported upon a flat bench covered with a thin layer of crushed quartz, thus providing against disruption by the different movement of the combustion chamber during heating, and similar effects owing to contraction in cooling if it is necessary to shut the kiln down for repairs. The ware tunnel through which the loaded cars pass is directly between the combustion chambers. In effect, the Dressler tunnel kiln is similar to a muffle kiln, as the products of combustion are separated from the ware.
The chambers in the center section of the kiln are made of the highly refractory material-carborundum. The next section is made of refractory material of a composition approximating that of the high-grade fire brick, and the following sections of ordinary refractory composition, and finally the gases are drawn through a series of cast iron pipes, which very rapidly give up the last available heat in the gases.
The fuels in general use are natural and artificial gas and these are introduced with a small amount of air through highly refractory burner tubes. The air supporting combustion which is drawn from the cooling ware until it finally has a temperature varying from 1095 deg. cent. to 1495 deg. cent. (2003 Fahr. to 2597 Fahr.), mingles with the gas and causes combustion. This mixing is brought about by the aid of a fan which maintains pressure of approximately 0.02 in. of water at the burners and gradually increasing to 0.10 in. at the end of the cast iron pipes. This suction is sufficient to draw the air into the kiln to the burners and then draw the products of combustion on out of the kiln and exhaust them to the atmosphere. The draft is of special interest, as it is important to maintain extremely low velocities. This becomes more and more important in very high-temperature Dressler kilns where radiation becomes a great factor.
Refractory sliding dampers are provided to accurately control the flow of superheated secondary air, and it has been found possible by close regulation to obtain exhaust gases containing from 16 per cent to 17 per cent of carbon dioxide.
The transfer of heat from the gases in the chambers to the ware on the cars in the tunnel is accomplished by three means, conduction, convection and radiation.
Heat is transmitted directly from particle to particle by conduction, the particles not altering in their relative positions to one another. The rate at which heat travels from one spot to another depends upon the distance, the difference of temperatures, and the thermal conductivity of the material.
The heat in the case of convection is carried by the actual motion of the air in the tunnel. The motion is due to the fact that on heating a gas it expands and becomes less dense, tending to rise, while on cooling it becomes more dense and falls. This is well illustrated by our system of room heating in which we place the steam coils near the floor and depend on the convection of the air to distribute the heat uniformly through the room.
Radiation is the passage of heat across an intervening space. This takes place almost instantaneously, as in the case of the sun's rays, it being understood that reference made here is only to luminous bodies.
In the Dressler kiln, the purpose is to secure the greatest possible efficiency by adjusting the equipment so that the passage of heat takes place under the most favorable conditions. The heat from the gases must pass through the chamber wall by conduction, and there is every advantage in making the walls as thin as possible. They are usually made of carborundum as it has a heat conductivity five times as great as fire brick or clay.
It may be interesting to consider a little more fully how this system of firing by convected heat leads to a great uniformity of temperature throughout the mass of the ware. In the case of the periodic kiln, when the flame has made an upward path for itself among the saggers, this path or flue becomes hotter and draws more and more. As a consequence, it is exceedingly calls for great skill on the part of the firemen to create an even distribution of heat throughout the kiln. With the Dressler tunnel kiln, the opposite is the case. The firing is conducted by means of a falling stream of hot air. The combustion chambers deliver a great volume of hot air to the top of the kiln at a temperature very slightly above that at which the ware is to be fired. As the ware is not so hot as this the air is cooled and immediately falls, being replaced by fresh hot air from the chambers. The rate of fall anywhere depends upon the difference in density between the hot air at the top of the kiln and the cooler air at the bottom. The greater this difference, the more rapid and concentrated is the stream. Thus, the effect is automatically to heat up any spot which may be cooler, until it has the same temperature as the ware around it. By the above described mechanism, the zone of the ware at the same temperature as the stream of air delivered to the top of the kiln gradually becomes deeper and deeper, until the whole mass is evenly fired, the circulation of air being finally maintained by the difference in temperature between the top of car refractory base and the flues in the combustion chamber.
In the hottest part of the high-temperature zone of the Dressler kiln there is an extremely rapid heat transfer by radiation, it being a well established fact that above 1375 deg. cent. (2507 deg. fahr.) a great temperature differential cannot be maintained if the hot walls of the combustion chambers are actually looking at the surface of the carborundum saggers on the cars. In order to facilitate the transfer of this radiant heat sections of the outside walls of the carborundum chambers are removed so that the heat rays may travel directly from the inner walls of the chambers to the saggers containing the ware Fig. 37. However, convection in this zone is of great importance, and the ware is so arranged on the cars that the superheated atmosphere is permitted to pass freely through the masses, and the introduction of any combustible matter in this zone will quickly indicate a rapid movement of the atmosphere that can only be attributed to convection. The regular firing is thus not dependent upon the skill of the fireman, but is an inherent function of the kiln.
The cooling of the ware is accomplished in much the same manner as the heating and is equally gradual, only the ware is now hotter than the kiln itself, and the air instead of falling rises through the ware to the top of the kiln. It is then cooled by the walls of the kiln and the cooling chambers and pipes, dropping down until it reaches the base of the car, and is then once more sucked through the channels and up through the ware. Since the air is being pulled along into the combustion chamber, there is not the continual circulation of the same air as in the heating zone, but the cold air which enters at the exit end of the kiln pursues a spiral path heating up as it passes the cooling ware until it reaches the entrance to the combustion chamber, where it is drawn in. At no point does the hot ware come suddenly in contact with cold air, and the dunting or cracking which is a great source of loss in other kilns is eliminated. It will be observed that cooling pipes are provided. The necessity for this was not at first realized, but it was found impossible to cool the ware completely with the volume of air required to complete the combustion of the gases needed to fire the ware.
UNIFORMITY OF HEAT TREATMENT
That the heat treatment is substantially uniform for each piece of ware passing through the kiln cannot be doubted when the conditions and operation are summarized. The cross-sectional area is small being about one-eighth to one-tenth that of a periodic kiln and of such a size that it is possible to maintain practically uniform temperatures throughout the cross-section at any point along the kiln. The term "practically uniform" is used here as there is a slight difference in temperature between the top and bottom saggers, the bottom ones being the coolest. This difference is normally not great enough to make any difference in the quality of the ware but sufficient so that if there is any underfired ware it will invariably be on the bottom layer first and at its worst in the center bottom sagger. This is a very valuable quality of the kiln as it makes it possible to select a whole insulator for the porosity test with assurance that if this piece is good every other piece on the car is also good. As the cars of ware pass through each of these various zones at uniform speed and at such a speed that there is sufficient time to complete the pyro-chemical reactions necessary for proper maturity and vitrification, it is evident that the heat treatment for all insulators is substantially the same. It is equally obvious that the same cannot be said of periodic kilns.
1. "The Production of Porcelain for Electrical Insulation" JOURNAL American Institute of Electrical Engineers, Vol. 42, No. 6, p. 631, June 1923.
