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ments are such that the boiler must occupy @ minimum of space and weight, the bricks are installed with the 9 x 2Y,-inch face exposed to the gases, giving a wall depth of 4% inches. In other service, where weight and space are not such vital factors, the bricks are generally installed so as to give a depth to the wall of 9 inches. Behind the refractory brick an insulating wall about 5 inches thick is generally installed. Insulating brick has a low thermal conductivity which resists the pas- sage of heat and permits more or less uniform heating throughout the refractory brick. It should never be directly exposed to the heat of the furnace, however, for it has a low melting temperature and a large co- eae of expansion and would soon fail. There are many satisfactory meth- ods of laying up a wall and a far greater number of unsatisfactory ones. An approved method consists of the following steps: Steps in Laying Up Wall (a) The clay or cement used to bond the bricks should have similar refractory and expansive character- istics as the brick in order to avoid unequal expansion and fusion. The mortar should be a good grade of this clay or cement finely ground and thoroughly mixed with water to the consistency of paint. The proper consistency of the mortar can be judged by lifting the upper of two bricks having a thin layer of mor- tar between them. When the lower brick, as it is quickly lifted, just tends to lift with the upper brick, the consistency is approximately cor- rect. A thick mortar will result in thick joints, which are to be avoid- ed; too thin a mortar will have poor bonding qualities. (b) After ensuring the brick to be free of chips and cracks, dip it first in water and drain off. Then dip one end and one side in the mor- tar, avoiding air bubbles, and allow the excess mortar to drain off. Place the brick in position on the wall and tap into place until a thin joint from which no more mortar can be forced out is obtained. Do not use mor- tar on a trowel on bricks in place on the wall. A thick joint must be avoided, and with properly mixed mortar, joints as small as 1 /32-inch can be obtained. As the brick is the actual refractory and the cement pri- marily the bonding element, the effi- ciency of a furnace wall has, there- fore, been said to be only as good as the joints. (c) Care must be exercised in preventing the edges of a brick to protrude beyond the face of the wall; that is, the entire face of the wall should be smooth and flush, with no projecting corners or bricks. (d) Furnace floors, for oil fired Fig. 2—Poor resistance to fusion. Good resistance to shrinkage and cracking. Wall well made up. Bonding property of clay is excellent. Brick of incorrect composition boilers, should be covered with about 4%inches of diatomaceous earth next to the plating. This is then covered with a double layer of split brick with staggered joints. No mortar is necessary to hold the floor bricks in place but dry fire clay should be swept over each course of split brick to a depth of about 1 /8-inch ensur- ing that all joints and cracks are filled. (e) Each marine boiler brick should be anchored. The tempera- ture from the furnace as far back in- to the wall as the bolt head is so high that ordinary iron or steel bolts usually fail due to reduced tensile strength from burned heads. Satis- factory service has been obtained from nickel-manganese alloy bolt ma- terial containing about 4 per cent manganese. (f) Expansion of the brick, par- ticularly in the case of flooring, must be allowed for at the ends of the walls or floor at a rate of 1/8-inch to the foot of length. The rigidity of marine boiler casings demands this in order to prevent bulging: of the floor or wall on an increase in tem- perature. (g) After the wall has been com- pletely layed it should be allowed to thoroughly dry and the mortar to set by a good circulation of air through the furnace. A _ slow fire then lighted will further dry it out and strengthen the bond. (h) After the wall has been thor- oughly dried out a light wash, con- sisting of the mortar used in lay- ing, considerably thinned out, should be applied with a brush. This will fill up any small cracks that devel- Fig. 3—A furnace wall showing poor resistance to shrinkage. MARINE REVIEw—March, 1931 Good resistance to fusion. Wall well made up. Evidence of excessive shrinkage in clay 21