If the plant referred to operates without a tank this equalizing pipe is found to be practically a necessity. In any case an equalizing pipe above the water line between the heater and its tank, or the heater and the governor, is necessary to maintain the same pressure upon the water in the tank as exists elsewhere in the return-water reservoirs.
The water-line of the heating system sometimes becomes an important consideration, especially when it is desired to place radiators in the basement of a building. If these are set so low that the return water is liable to rise above the connections, the radiators will fill with water when turned on, which will prevent the steam from circulating into the radiator and will be sure to give trouble from water-hammer. Besides this, with anything except the overhead-supply systems, the water from the returns will back through the radiator and run down the supply riser, and it is therefore generally necessary to set radiators several feet above the water-line, according to the maximum pressure which is necessary to create circulation of steam through the system in coldest weather. If the system is designed for very low pressure, 1 or 2 pounds, the radiator may be placed within 4 feet of the water-line, but should never be lower than this, especially in parts of the building far removed from the heater. For this reason basements are usually heated by steam coils suspended from the ceiling or placed on the walls, near the ceiling, although radiators are sometimes put on brackets attached to the walls; frequently, in order to lower the water-line, the pump, governor, and heater also, when the open heater is used, are placed in a pit. There are, however, special arrangements of radiator connections which may be used with safety, even though they are set below the water line. These are discussed in the chapter on radiators.
There are many combined automatic pumps and receivers designed for taking care of the return water which are very satisfactory, but all work on the same principle of a tank with a float governor to operate the pump. There are also automatic traps designed to return the water of condensation from the exhaust-steam heating systems, without using a pump, direct to a high-pressure boiler by means of an ingenious combination of float valves, traps, reservoirs and check valves, and some of these work with considerable satisfaction if carefully watched and kept in good repair.
In many mills and factories which use condensing engines, and
