This is the zone between the top of the hearth jacket and the bottom of the sloping bosh. Various styles of construction have been adopted for this, as for all the other regions of the furnace. One of these which had much vogue at one time was the use of a heavy cast-iron or even cast-steel tuyere jacket with cooling pipes cast in it, and with openings cast through it of the size required by the coolers, no internal cooling by cooling plates being used.

This construction and all others which depend on external cooling in this zone are in my opinion faulty for the reason that the tuyeres must project a considerable distance into the furnace. If they did not, what I have called above the mushrooming effect would rapidly destroy all the parts adjacent to them; therefore, we have first the hollow-walled cooler as a sort of second line of defense which projects into the face of the brickwork. Then fitted into this and projecting about a foot beyond it we have the tuyeere itself, which is literally on the firing line, its nose being some 3 1/2 or 4 ft. from the exterior of the brickwork.

The mushrooming effect of the blast, if this zone were without any but exterior cooling, would soon eat away two or three feet of the face of the brickwork, which would leave the heavy coolers carrying the tuyeres without adequate support. This in turn would allow them to drop down so that instead of directing the blast in horizontally, as they should, it would be blown downwardly into the bath.

To overcome this and to protect the zone around and between the coolers from this intense mushrooming effect, cooling plates should be used as shown in Fig. 164 (page 240) and described below.

Cooling Plates

These are hollow plates of copper, or copper with one or two per cent. of tin to make it cast more readily, with a taper in the vertical plane, as shown, and also in the horizontal plane. These are supplied with water from the cooling-water system and a constant circulation is kept up through them the same as in the case of the tuyeres.

They are built into the brickwork, but on account of the taper above mentioned can be withdrawn by means of lugs provided in their rear or outer surfaces for that purpose. These plates have great cooling surface and corresponding protective power for the brickwork around them. They increase the resistance of the brick to the action of the fire, and the brick to a great extent protects them from the direct attacks of the latter, although frequently they are directly exposed to it, as are the tuyeres, but able to resist because of the activity of the water-cooling, in exactly the same way that the firebox of a locomotive is able to withstand the intense combustion of the coal within it because the thin plates which constitute its inner surface are so completely cooled by the water around them.

It will be seen that these cooling plates with their ability to withstand the direct action of the fire, protect the firebrick work underneath and around the coolers from the attacks of the iron, the slag, and the mushrooming effect of the blast, and so enable these to maintain their correct position throughout the campaign.

In order to supply the element of strength against the internal pressure of the furnace two constructions are possible. Either bands can be passed around the furnace just above and just below the cooler openings, leaving the space between these bands unprotected, or the bands may be connected by vertical buckstays which protect the intermediate space. The construction which I like best of all, however, is to make a solid jacket of steel plate, as shown in Fig. 164, with the flange at the base resting on the top of the hearth jacket, with holes cut through it at the points where the cooler openings and the cooling plates come. This supplies the means of holding all the brickwork to its place, which is necessary, for, in the absence of such protection, it is surprising how small a mass of unbound brickwork will detach itself from the rest and work out of the furnace under the action of expansion, pressure, etc.