Document jyZNqZnprE4wyqeGZJnqz82XN

574 CHAPTER 21 1958' Guide When'selecting the type of piping arrangement;.such as, one-pipie, twopipe, series-loop, upfeed, and downfeed, the adaptability of each arrange ment for.'proper hydraulic balance should be considered. Air Elimination If-air-and various other gases which occur in hot water heating systems are not eliminated from the flow circuit, they may collect in the heating units or piping and reduce the flow of'water and the heat output. These gases, designated air in this text, are in the piping before it is filled with water, are introduced with the water used to fill and replenish the system, and also are formed to a limited degree from chemical action inside the system and from the movement of low temperature air-saturated water from the expansion tank to the system.11 . . The air is eliminated from the flow circuits by trapping it into the ex pansion tank as explained in connection with discussion of dip tubes in the ..MANUAL AIR VENT ^AIR CHAMBER Fig. 12. Typical, Automatic Am Vent Valve Installation Fig. 13. Typical Air Vent Valve Installation on Heating Element section Boiler Room Piping and by the installation of manual or automatic air vent valves at all high points in the system. Automatic air vent valves may be used to eliminate air from the mains and major equipment. Like other mechanical devices, they are subject to malfunction. Small pieces of dirt, or scale can lodge on the valve seat, causing it to leak. Corrosion, scale, or sludge in the valve can prevent the valve from opening to eliminate air. All systems should be designed to have a minimum number of automatic air vent valves, but where a valve is required there should be no hesitancy about using it. In large systems where large numbers of automatic air vent valves are necessary, provision should be made for service of each valve without draining the system, Fig. 12. The drip from each air vent valve should be piped to a drain to pre vent building damage and facilitate inspection for leakage. Automatic air vent valves should not be installed in piping where, temperatures in excess of 215 F can occur without careful analysis of possible temperaturepressure effects. Each unit of radiation which can act as an air trap should be provided with an air chamber and air vent valve, Fig. 13. The use of manual air vent valves or porous plug or disc type manual-automatic valves is recom mended for radiation. Piping Principles Piping should be pitched up. in the direction of flow in all cases when pump operation is constant, and wherever possible when pump operation is m- f Hot Water Heating Systems 575 termittent to assure consistent air elimination during conditions of either flow or no-flow. Piping is pitched 34 in. per 10 ft as standard practice. ' When changing the size of pipe in, horizontal runs, eccentric reducers should be used to keep the top of the pipes in line. '. . , A means of manual capacity, adjustment should be provided for each heating unit. Manual valves or adjusting cocks are frequently provided' for this purpose. Dampers at the heating element or at the outlet of con-' vection type units may be used. The heating unit located in the area of a zone thermostat should be equipped with a key-operated adjusting device to discourage tampering by unauthorized personnel. . The piping should be installed to provide for thermal expansion of the pipe without imposing undue stress on connections or equipment. Spring pieces of sufficient length, as required by the diameter of the runout and the amount of expansion to be absorbed, should be installed at connections to radiators and risers. Expansion joints or loops and properly located an chors should be used where required. See Chapter 26, Pipe, Fittings, Welding. All mains and risers located in exterior walls should be covered with sufficient insulation to prevent freezing. Trunk mains running between buildings must be protected by conduit or installed in trenches below the frost line and properly insulated. When heating mains are run in locations where they could possibly be immersed in water, the conduit should be water tight. Circulating Pumps Circulating pumps used in hot water heating systems can vary from small 34 hp booster type pumps with a capacity of 5 gpm at 6 or 7 ft head to pumps handling hundreds, even thousands, of gallons per minute at heads limited only by the pressure characteristics of the system. The selection of a pump for a system is based on an analysis of operational char acteristics of the pump and system. Both first cost and operating cost are mvolved. See discussion under Pipe Sizing Methods, Expansion Tanks and System Pressure Control. Pumps with mechanical seals have found wide acceptance in hot water heating. Pump characteristics as affected by system characteristics, should be given consideration to insure non-over loading conditions in pump motor operation. Any given pump, when operated with a given fixed head system, will operate at only one point on its curve; the intersection of the pump curve with the system curve. Pump curves may be obtained from manufac turers. System curves can be constructed by plotting head versus gpm for different flow rates. The head for various flow rates can be estimated by use of the equation: : where ff. calculated system head, B, head at new flow rate, C, calculated flow rate, Bj assumed new flow rate. p used0*"60011011110 or for operational reasons two similar pumps are sometimes cu 111 Parallel on a single circuit. When so used the combined pump e showing the relationship of flow to head due to operation of both