DIRECT HOT-WATER HEATING A hot-water system is similar in construction and operation to one designed for steam, except that hot water flows through the pipes and radiators instead.
The circulation through the pipes is produced solely by the dif ference in weight of the water in the supply and return, due to the differ ence in temperature. When water is heated it expands, and thus a given volume becomes lighter and tends to rise, and the cooler water flows in to take its place; if the application of heat is kept up, the circulation thus produced is continuous. The velocity of flow de pends upon the difference in temperature between the supply and return, and the height of the radiator above the boiler. The horizontal distance of the radiator from the boiler is also an important factor affecting the velocity of flow.
This action is best shown by means of a diagram, as in Fig. 83. If a glass tube of the form shown in the figure is filled with water and held in a vertical position, no movement of the water will be noticed, because the two columns A and B are of the same weight, and there fore in equilibrium. Now, if a lamp flame be held near the tube A, the small bubbles of steam which are formed will show the water to be in motion, with a current flowing in the direction indicated by the arrows. The reason for this is, that, as the water in A is heated.
The heat given off by the radiator always insures a difference in temperature between the columns of water in the supply and return pipes, so that as long as heat is supplied by the furnace the flow of water will continue. The greater the
difference in temperature of the water in the two pipes, the greater the difference in weight, and con sequently the faster the flow. The greater the height of the radiator above the heater, the more rapid will be the circulation, because the total difference in weight between the water in the supply and return risers will vary directly with their height. From the above it is evident that the rapidity of flow depends chiefly upon the temperature differ ence between the supply and return, and upon the height of the radiator above the heater. Another factor which must be considered in long runs of horizontal pipe is the fric tional resistance.
For gravity circulation some form of sectional cast-iron boiler is commonly used, although wrought-iron tubular boilers may be employed if desired. In the case of forced circulation, a heater de signed to warm the water by means of live or exhaust steam is often used. A centrifugal or rotary pump is best adapted to this pur pose, and may be driven by an electric motor or a steam engine, as most convenient. • Types of Radiating Surface. Cast-iron radiators and circulation coils are used for hot water as well as for steam. Hot-water radiators differ from steam radiators principally in having a horizontal passage at the top as well as at the bottom.
This construction is necessary in order to draw off the air which gathers at the top of each loop or section. Other wise they are the same as steam radiators, and are well adapted for the circulation of steam, and in some respects are superior to the ordinary pattern of steam radiator.