Hot water heaters differ from steam boilers principally in the omission of the reservoir or space for steam above the heating surface. The steam boiler might answer as a heater for hot water, but the large capacity left for the steam would tend to make its operation slow and rather unsatisfactory, although the same type of boiler is sometimes used for both steam and hot water. The passages in a hot water heater need not extend so directly from bottom to top as in a steam boiler, since the problem of providing for the free liberation of the steam bubbles does not have to be considered. In general, the heat from the furnace should strike the surfaces in such a manner as to increase the natural circulation; this may be accomplished to a certain extent by arranging the heating surface so that a large proportion of the direct heat will be absorbed near the top of the heater. Practically the boilers for low-pressure steam and for hot water differ from each other very little as to the character of the heating-surface, so that the methods already given for computing the size of grate surface, horsepower, etc., under the head of steam boilers can be used with satisfactory results in the case of hot water heaters. It is sometimes stated that owing to the greater difference in temperature between the furnace gases and the water in a hot water heater, as compared with steam, that the heating surface will be more efficient and that a smaller heater can be used; while this is true to a certain extent different authorities agree that this advantage is so small that no account should be taken of it, and the general proportions of the heater should be calculated in the same manner as for steam. Fig. 25 shows a form of hot-water heater made up of slabs or sections similar to the sectional steam boiler shown in Part I; the size can be increased in the same way by adding more slabs. A different form is shown in Fig. 26. This is made of cast iron but is not a sectional boiler.

Hot Water Heater Types 1000103

Fig. 24.

Hot Water Heater Types 1000104

Fig. 25.

It has no horizontal flues for the ashes and soot to collect in and a greater part of the heating surface is directly exposed to the hottest part of the fire. Fig. 27 shows another form of heater similar in principle to the one just described. The space between the outer and inner shells surrounding the furnace is filled with water and also the cross pipes directly over the fire and the drum at the top. The supply to the radiators is taken off from the top of the heater and the return connects at the lowest point.

The ordinary horizontal and vertical tubular boilers with various modifications are used to quite an extent for hot water heating and are well adapted to this class of work, especially in the case of large buildings.



American Radiator Company.

Automatic regulators are often used for the purpose of maintaining a constant temperature of the water. They are constructed in different ways - some depend upon the expansion of a metal pipe or rod at different temperatures, and others upon the vapor ization and consequent pressure of certain volatile liquids. These means are usually employed to open small valves which admit water pressure under rubber diaphragms, and these in turn are connected by means of chains with the draft doors of the furnace, and so regulate the draft as required to maintain an even temperature of the water in the heater. Fig. 28 shows one of the first kind. "A" is a metal rod placed in the flow pipe from the heater, and is so connected with the valve "B" that when the water reaches a certain temperature the expansion of the rod opens the valve and admits water from the street pressure through the pipes "C" and "D" into the chamber "E." The bottom of "E" consists of a rubber diaphragm which is forced down by the water pressure and carries with it the lever which operates the dampers as shown, and checks the fire. When the temperature of the water drops, the rod contracts and valve "B" closes, shutting off the pressure from the chamber "E." A spring is provided to throw the lever back to its original position and the water above the diaphragm is forced out through the pet cock "G" which is kept slightly open all of the time.

Hot Water Heater Types 1000106

Fig. 26.

Hot Water Heater Types 1000107

Fig. 27.