Radiator Connections. For direct radiators, the connections are commonly 1-inch for sizes up to 40 square feet; 11-inch, for sizes of 72 square feet; 1i-inch to 2-inch for sizes larger than 72 square feet. On floors above the first, the connections may be made smaller if the hor izontal runs are short, the sizes to conform to table.
Expansion Tank Connections. About the simplest arrangementof expansion-tank connections is shown in Fig.106. The expansion pipe is commonly connected with a return line in the basement, there being less likelihood of the water boiling over in case of a hot fire with this arrangement than when the expansion pipe is merely an extension of a supply riser. There must be no valve on this pipe, as its closure would almost certainly result in a bursting of some part of the system. Great pains must be taken to guard against the freezing of the expansion pipe. If there is any danger ever, a circulating pipe should be added, as shown, this pipe being connected with one of the flow-pipes or supply risers, to insure a continuous circulation.
versus Pressure System. The open-tank system, although having its disad vantages, is generally to be preferred to the pressure or closed-tank system. . With the open-tank system, the water cannot get much above 212° at the heater, without boiling in the expansion tank and blowing part of the water out of the system, causing, mean while, objectionable noises in the system. On the other hand, the open expansion tank into which the water can freely expand when jteated is the best possible safety device to prevent overpressure.
With the closed-tank system, a safety valve is used. If it operates properly, well and good; otherwise an element of danger is introduced, and, in case an excessive pressure is developed, the heater becomes far more dan gerous than 'a steam boiler, owing to the much greater volt.me of water in the system.
With this system, two safety-valves with non-corrosive seats should be used, unless some well-tested device of demonstrated merit designed especially for this purpose is adopted.
The advantage of the closed-tank system is that smaller radiators may be used, since they can be heated as hot with water under pressure as they would be if heated with steam.
When full street pressure is applied to a system, and no ex pansion tank is used, the radiators are subjected to an unnecessary strain; and in case of rupture in any part of the system, much greater damage results than would be the case with an open-tank system.
System of Forced Circulation. In extensive systems the water is kept in circulation by pumps, which are capable of producing a much higher velocity in the pipes than could be secured by gravity. This system is used principally in connection with power plants, the water being heated in tubular heaters, by means of the exhaust steam from the engines. Much smaller supply mains may be used in this system than with steam heating, because of the greater capacity of water for carrying heat. On the other hand steam returns are smaller.