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of coking-chambers desired. The upper story of the oven was made up of a battery of any number of long, high and narrow coking-chambers extending from side to side of the oven. Intermediate the cokingchambers and parallel with them was a like number of heating-chambers of somewhat similar dimensions. In this alternate arrangement of coking-chambers and heating-chambers, a wall of a heating-chamber was similarly a wall of its adjacent coking-chamber, and so on throughout the battery. Heat was conveyed from a heating-chamber, in which combustion occurred, through the wall dividing it from a cokingchamber to the coking-coals there undergoing distillation.

To facilitate uniformity of heat distribution to its walls, the heatingchamber was divided into many parts by narrow vertical flues extending from near the bottom of the chamber to a channel or flue running horizontally across its top. Nearly midway the heating-chamber, a partition, extending from the bottom upward to this horizontal flue, divided the chamber into two parts. These batteries of coking-cham- ́ bers and heating-chambers, and certain hot air delivery flues presently to be mentioned, together with the supporting and separating masonryall made of highly refractory and heat resisting bricks-comprised the combustion organization of the oven located in the upper story.

The lower story of the oven was largely made up of pillars and walls, likewise of refractory brick, to support the upper story. In the center of the lower story, or at its sides, or, indeed, on its outside, was placed one or more sets of regenerators. A "set" of regenerators comprised two regenerators. Between the regenerators and the sides of the oven, when the regenerators were placed in the center, or between the regenerators and the center of the oven, when the regenerators were placed at the sides, was a system of cooling-flues and open arches used to prevent melting or fluxing of the bottom masonry of the upper story, incident to the sub-bottom combustion system employed.

Á regenerator, described very generally, is an oven-like structure of firebrick which contains at the bottom an air flue or sole-channel and checkerwork of firebrick upward from the sole-channel nearly to the dome. The functions of a regenerator are, primarily, to heat air, and secondarily, to so heat it that it attains an approach to uniformity of temperature before it is delivered to the heating-chambers of the second story, where it is used with gas as combustion material. Regenerators of the prior art-in coke-ovens as distinguished from gas generators were placed longitudinal of the battery of heating-chambers, that is, at right angles to each heating-chamber and to each coking-chamber throughout the length or depth of the battery. This was the organization for supplying materials for combustion.

The connection between the two organizations, situate respectively in the upper and lower stories of the oven, was made by at least two flues leading from the regenerator to each heating-chamber, the first extending vertically to the second, and the second extending horizontally under the vertical flame-flues of each heating-chamber. The horizontal connecting flue was termed "bus-flue," and was in the very bot

tom of the upper story. It had as many air openings as there were vertical flame-flues. There were as many connections of this kind with the one longitudinal regenerator as there were heating-chambers to be served with preheated air.

In the operation of a regenerative coke-oven, unheated air was drawn by smokestack draft through the sole-channel into the body of the regenerator. It was there heated on its passage upward through a checkerwork of bricks, then conveyed from the regenerator to the bus-flues and thence to one side of the divided heating-chambers, where, in its travel, it mingled with inflowing gas and became ignited, thereby producing a gas jet for each flame-flue on that side of the division. Heat was in this way conveyed to the adjacent coking-chambers with some degree of uniformity. The hot, waste gases arising from combustion were then carried horizontally along the flues at the top of the heatingchambers, across the division, down through corresponding though inactive flame-flues in the other side of the heating-chambers, then into the bus-flues and thence on to the companion regenerator, where they served to reheat its checkerwork as they passed off into the outer air.

This operation was reversed at stated intervals, with the result that one regenerator of the set and one side of the connected heating-chambers were actively heating a corresponding part of the coking-chambers, lying intermediate the heating-chambers, while the other side of the heating-chambers and the other regenerator of the set were inactive except in being reheated for use when the operation was again reversed. To understand Koppers' invention, it is necessary to have a better understanding of the prior art than can be had from this inadequate statement. Therefore, we insert at this point diagrams of Hoffman's coke-oven (U. S. No. 492,400-1893), an excellent illustration of the elements of the prior art in combination.

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An alternate arrangement of coking-chambers and heating-chambers in parallel alignment appears in Boult (British Patent No. 5,9791883); Hoffman (U. S. Patent No. 492,400-1893); Otto (German Patent No. 88,200-1895); Hilgenstock (U. S. Patent No. 649,4501900); Schniewind (U. S. Patent No. 673,928-1901); Koppers (U. S. Patent No. 738,918-1903). Vertical flame-flues are seen as early as Herberz (German Patent No. 25,526-1883) in which air and gas for combustion are introduced and ignited by means for regulating their supply. Flue combustion is found also in Koppers (British Patent No. 3,026-1899), in Hoffman (U. S. Patent No. 492,400-1893) and in many other patents. Regenerators in sets, each set ordinarily containing two, and positioned longitudinally the heating-chambers battery, each regenerator serving the whole battery and operated under the system of periodical reversal of air flow, are shown in the cited patents of Hoffman, Schniewind, Otto, and Herberz, and in the early Koppers.

Vertical connections from regenerator to horizontal bus-flues, and vertical connections thence to flame-flues of heating-chambers, for delivery of preheated air for combustion, appear in the same pat

ents.

Koppers invented none of these elements of a by-product cokeoven; neither did he invent their method of operation. All were old in the art, yet all are found in one relation or another in the byproduct coke-oven of his patent. As so much of the prior art is found in the organization of Koppers' coke-oven, what is there in his oven that distinguishes it from others in the art? What is the invention for which he was granted a patent?

Koppers has answered these questions by the claims of his patent in suit. They are as follows:

"1. In a coke-oven, a series of heating-chambers, and coking-chambers intermediate the heating-chambers, combined with a series of regenerators

below and parallel to the heating-chambers and communicating directly therewith, substantially as specified."

"5. A coke-oven provided with coking-chambers, two sets of heatingchambers intermediate the coking-chambers, two sets of regenerators communicating with the heating-chambers and arranged below the coking-chambers, and a partition between the regenerators, substantially as specified."

Prior to Koppers' patent, the efforts of all inventors, including Koppers himself, to produce uniformity of heat intensity over the walls of heating-chambers of by-product, coke-ovens, were directed along the line of delivering and distributing preheated air to heatingchambers from a set of two regenerators which served the whole battery of heating-chambers, whatever the number, and which were positioned longitudinally the battery and at right angles to each heating-chamber. With heating-chambers crosswise the oven and regenerators lengthwise the oven and therefore at right angles to one another, the delivery of preheated air from regenerator to heatingchamber was necessarily roundabout or indirect. As the air, in its transit from regenerator to heating-chamber, traveled up the vertical flue and then along the horizontal channel or bus-flue to the flame-flues, it encountered inequality of air pressure in the flue, with consequent inequalities in air deliveries to the heating-chamber and in flame distribution over its walls.

This being the state of the art, how did Koppers meet its problems? He took what was good in the art and discarded what was bad. He retained the upper story combustion organization substantially as it was, but he departed abruptly from the prior art organization of the lower story. He abandoned the one set of regenerators which had served the whole battery of heating-chambers and provided in their stead a separate regenerator to serve each heating-chamber. In doing this, he greatly multiplied the number of regenerators and filled up the arched spaces which theretofore had been left open to keep the masonry below the bottom of the coking-chambers from fluxing, and in doing this he overcame the chief defect of the sub-bottom combustion system. He then changed the position of regenerators from longitudinal of the battery to crosswise the battery, placed each individual regenerator below and parallel to each heating-chamber and made multiple connections between the two which were vertical and direct instead of horizontal and indirect, thereby causing the preheated air to pass from each regenerator evenly and directly to its companion heating-chamber, where, on combustion, heat intensity is distributed over its walls with practical uniformity.

When Koppers made these changes, he wrought a change in the art from a stagnant struggle for uniform distribution of heat over heating-chamber walls to the achievement of that result. The change was immediately reflected in economy of operation and increase of production. Coke production for a given coking time was substantially doubled. The coke product was of the best metallurgical grade, and the saving of distilled by-products was increased, with a reduced waste of fuel gas used in the heating operation. The art promptly recognized these achievements and adopted Koppers' arrangement to

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the extent that over eighty per cent. of the by-product coke-ovens built during the last ten years in the United States have been Koppers ovens, amounting to 4,700 and representing an outlay of between $50,000,000 and $100,000,000. They exceed in number all competing ovens combined.

Koppers' changes from prior art ovens, involving changes in the number, size, and position of regenerators and in their relation to and connection with heating-chambers, may best be understood by comparing Koppers' patent diagrams here inserted with the diagrams of the Hoffman oven previously inserted.

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