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88                         METALLURGY   OF   CAST   IRON.
The valve at T is arranged with piping, through which water runs in order to protect the exposed parts of the valve from burning out. The valves W and D do not require the presence of water, for the reason that when the gas is on, the brick work of the stove absorbs the greatest heat at its bottom, which prevents the highest temperature being confined to the upper part of the stove. One stove, when a furnace is working well, is all that is generally "in blast;" but if there should be a " slip " to chill a furnace or make it work cold, two or three stoves are often put on at one time for a short duration to assist in raising the temperature in the furnace so as to restore it to its normal condition, after which the additional stoves are taken off and the work continued with but one, as in ordinary practice.
The four stoves are placed together as closely as is convenient to leave room for working around them. They cover an area of ground about 40x50 feet. The four stoves are connected by band pipes and separate valves, so that the cold blast coming from the "blowing tubes '' and the hot blast leading to the four stoves come from and lead into one main pipe. The pipes which convey the hot blast to the furnace are either coated with an asbestos covering or have their interior lined with fire brick, the same as is done with the " down-comer " which carries the dead gas from the top of the furnace down to the combustion chamber of the hot blast stoves to protect them and prevent loss of heat.gathers on the combustion chambers for a distance of about twenty feet in height, and on the bottom of the stoves, which have openings as at K and H for getting at or cleaning out the stove, or, if shut off, for repairs.d checkered brickres, etc. It may also beency                :|,