The production of Porcelain for electrical insulation — VI

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 described¹. 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are measured by thermocouples, pyrometric cone fusions or measurement of shrinkage bars made of porcelain and withdrawn from the kilns at intervals.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as well as temperature; however, cones are preferable as they can be observed through peep-holes while bars must be withdrawn and measured.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.