Document MYpn790kMxmzJzY71oM25Gb7

414 CHAPTER 28 1960 Gvide adaptability of each arrangement for proper hydraulic bal ance 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, desig nated 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 de gree from chemical action inside the system and from the movement of low temperature air-saturated water from the -AUTOMATIC AIR VENT --TO VISIBLE ORACM iATC VALVE PITCH OOWN-- fig. 12___ Typical Automatic Air Vent Valve Installation expansion tank to the system." Loss of air by absorption in the water may be prevented by a flexible diaphragm pre venting contact of water and air in the expansion tank and having adequate movement to permit the expansion of the water in the system. The air is eliminated from the flow circuits by trapping it into the expansion tank as explained in connection with dis cussion of dip tubes in the section Boiler Room Piping and piped to a drain to prevent 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 temperature- pressure 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 plugor disc-type manual-automatic valves is recommended for radiation. Piping Principles Piping should be pitched up in the direction of flow in all cases when pump operation is constant, and wherever pos able when pump operation is intermittent to assure con sistent air elimination during conditions of either flow or no flow. Piping is pitched V\ in. per 10 ft as standard practice. When r.hfljigjntr the size of pipe in horizontal runs, ec centric reducers should be used to keep the top of the pipes in line. A means of manual capacity adjustment should be pro vided 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 convection-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 ex pansion of the pipe without imposing undue stress on con nections 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 prop erly located anchors should be used where required. See Chapter 31, 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 con duit 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 watertight. fig. 13___Typical Air Vent Valve Installation on Heating Element 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 sendee of each valve without draining the system (Fig. 12). The drip from each air vent valve should be Grculatmg Pumps Circulating pumps used in hot water heating systems can vary from small M-hp booster-type pumps with a ca pacity of 5 gpxn 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 analy sis of operational characteristics of the pump and system. Both first cost and operating cost are involved. See dis cussion under Pipe Siring 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 nonoverloading 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 inter section of the pump curve with the system curve. Pump curves may be obtained from manufacturers. System curves can be constructed by plotting head versus gpm for dif Hot Water Hearing Systems 415 ferent flow rates. The head for various flow rates can-be estimated by use of the equation: where S^ = calculated system head. = head at new flow rate. Gj = calculated Sow rate. Gi = assumed new flow rate. complish system pressure control. The excess water volume' in the system resulting from increased temperature is stored in the expansion tank during periods of high operating tem peratures and returned to the system when the system water temperature is lower. The expansion tank must be designed to store the required volume of water during maximum de sign operating temperatures without exceeding the maxi mum operating pressure allowable, and to maintain required minimum pressure when the system is cold. Some designers provide an automatic fill valve to maintain the minimum For economic or for operational reasons two similar pumps are sometimes used in parallel on a single circuit. When so used the combined pump curve showing the relationship of flow to head due to operation of both pumps is established by adding the flow capacity of one pump to the flow ca pacity of the other at identical heads. If two pumps are placed in series, the relationship of flow to head due to operation of both pumps is established by adding the pump heads at identical flows. When using pumps in series, they must be identical or similar since the maximum flow will be limited by the smaller pump. Gate .valves should be pro vided to enable isolation of either pump when only one is in operation. EWT EXPANSION The Heat Source or Boiler The most frequently used source of heat is the 30 pti hot water heating boiler, constructed of cast iron or steel in ac cordance with the ASMS Boiler and Pressure Vessel Code, Section IV for Low Pressure Heating Boilers. See Chapter 35, Heating Boilers, Furnaces, Space Heaters. Boilers, classed as low-pressure heating boilers, may be used for hot water with pressures as high as 160 psig if the temperature of the water does not exceed 250 F and if they are con structed according to the ASME code for the pressure-at which they are operated.1* The ASME code requires installation of a relief valve on each hot water heating boiler to limit the pressure exerted on that boiler^ to the value for which it was designed and tested." Similar requirements exist for converters and elec tric water heaters. This operating pressure limit for the boiler is an important factor in the design of the hot water heating system. Together with the size and characteristics of the expansion tank, it determines the permissible height from the boiler to the top element or pipe of the distribution system, as discussed under Expansion Tanks. In addition to the pressure relief valve required by the ASME code, a hot water heating boiler should be equipped with a ther mometer, located at or near the top of the boiler, and a reliable pressure or altitude gage with a range at least twice that of the maximum operating pressure or altitude allow able for the boiler. I CIRCULATING PIPE INSIDE SEWER 0RAIN fig. 14.... An Open Expansion Tank ORAIN VALVE-*'^ RETURN COLD T water-*--NhO*H FILL /'/' iLVE / VALVE OPEN EXCEPT WHEN T2DRAINING TANK PRESSURE RELIEF VALVE rsJ MiMp-LOCATE IN POSITION RECOMMENDED BY MANUFACTURERS --^ ORAIN fig. 15.... A Closed Expansion Tank EXPANSION TANKS AND SYSTEM PRESSURE CONTROL The objectives of system pressure control are to limit the pressure at all equipment in the system to the allowable working pressure of the equipment; maintain minimum pres sure for all normal operating temperatures in order that air can be vented and cavitation at the pump suction and boiling of system water prevented; and accomplish these objectives with as little addition of new water to the system as possible. An expansion tank is the primary device used to ac pressure in the system by supplying water to make up for y losses due to leakage. Others prefer that water added to the system be supplied by a manually operated valve. The relief valve installed on a hot water boiler in ac cordance with the ASME code performs the function of limiting the maximum pressure at the boiler. Since it is a safety device, it should not be considered an operating control. Systems are designated as open (Fig. 14) or closed (Fig. 15). The open tank system is vented to the atmosphere and is generally limited to installations having operating tempera-