Document 2NREpN2NZdD3Jw3NOoQxxeO86

Heating Ventilating Air Conditioning Guide 1938 of about 32 lb per square inch must be maintained in the expansion tank if the water in the highest radiator is to be heated to 225 F without danger of boiling. The type of expansion tank used in a heating system, whether open or closed, has no influence on the operation of the system. The only function performed by the expansion tank is to provide for the variation in the volume of the water in the system, and at the same time to maintain a sufficient pressure in the system to prevent boiling when the water is at the highest temperature for which the system is designed. The capacity of the cushion or expansion tank should not. be less than the tank sizes indicated in Table 9 and in addition provisions must be made for draining it without emptying the system. The capacity of the expansion tank should be at least twice the in crease in volume produced when the water in the system is heated from its normal to its maximum temperature. When 25 gal of water are heated Table 9. Expansion Tank Sizes for Hot Water Heating Systems Tank Size Gallons Equivalent Direct Radiation Installed in Sq Ft Capacity Direct Radiation Installed in Mbh 18 21 24 30 35 40 2-30 2-30 2-35 2-40 Up to 350 Up to 450 Up to 650 Up to 900 Up to 1100 Up to 1400 Up to 1600 Up to 1800 Up to 2000 Up to 2400 Up to 52.5 Up to 67.5 Up to 97.5 Up to 135.0 Up to 165.0 Up to 210.0 Up to 240.0 Up to 270.0 Up to 300.0 Up to 360.0 from 40 F to 200 F, the volume of water increases to 26 gal. A safe rule, therefore, is to make the water capacity of the expansion tank equal to 10 per cent of the capacity of the heating system. In a forced circulation system, the expansion tank can either be con nected to the flow'or return main. In a gravity circulation system, the expansion tank should be connected to the flow riser so that air liberated from the water in the boiler may escape through the expansion tank. The expansion tank should be protected so that the water in the' tank or in the connecting pipe lines cannot freeze. If the water should freeze and the water in the system is heated causing further expansion, the resulting force will burst the boiler or some other portion of the system. RELIEF VALVES A relief valve should be installed on any hot water system using a closed circuit. The valve should be of ample capacity to provide for relief of expansion of the system without allowing anTexcessive pressure rise above the valve setting. 350 Chapter 17. Hot Water Heating Systems and Piping A relief valve should be of the diaphragm-operated or gravity-weighted type without guide wings below the seat. Provision should be made for manual operation to assure that the valve is in the proper operating condition at all times, and valves should be checked periodically. A relief valve installed in conjunction with a compression tank will not operate often provided the tank is of adequate size. It is essential that the relief valve be kept in good condition to eliminate any possible failure when operation is necessary. INSTALLATION DETAILS The detailed installation of the pipe system should be governed by four fundamental rules: 1. All piping must be pitched either up or down so that all gases which are liberated from the water can move freely to a vented section of the system. Whenever practicable, the pipe line should be pitched so that gases flowing to a vent will flow in the same direc tion as the water. When a pipe system cannot be installed without creating air pockets, that is, sections in the system from which liberated gases cannot escape, such sections must be provided with automatic air relief valves or with air valves which may be operated manually when necessary, or trapped into a pressure tank. 2. All piping must be arranged so that the entire system can be drained, either to permit alterationsor repairs,, or to prevent freezing if the system is not to be operated during a cold period. It is well to install a gate valve and union in every riser near the main to permit the draining of individual risers without draining the entire system. It is also well, in large installations, to divide the system into branches and to provide each branch with unions and valves so that any one branch can be drained without disturbing the remaining ones. The dividing of large heating systems into branches or zones and providing each zone with individual valves has the further advantage of permitting a varying temperature control. For example, if a building is equipped with a forced circulating system and if the south rooms are on one branch of the main and. the north rooms are on a separate branch, the valves may be set so that the water will circulate through the north branch ..with a temperature drop of, say, 10 F, and through the south branch with a tempera ture drop of, say, 20 F, thus delivering less heat to the south rooms than to the north rooms. This arrangement is especially valuable when the regulating valves are controlled thermostatically by the temperatures in the two zones, because no.matter how accurately the heating system may have been designed, the heat demand of any group of rooms varies with sunshine and with wind velocity, and these intermittent variations can be provided for only by the individual control made possible by changing the valve settings controllirtg the heat supplied to particular groups of rooms. 3. All piping must be installed so that it is free to expand and contract with changes of temperature without producing undue stresses in the pipes or connections. For this . purpose it is generally sufficient to allow for a variation in length of 1 in. for 100 ft of pipe. 4. The pipe system must be installed so that each circuit has its correct friction head. To bring this about, it is necessary in some cases to minimize the friction, i.e., to make the pipe line as short as possible and to provide as few fittings as possible; and in other cases it is necessary to increase, the length of the pipe and the number of fittings so that, for every circuit, the friction head will be equal to the available pressure head. The connections from the boiler to the mains should be short and direct, to reduce the friction head. It is frequently possible to avoid an elbow and to reduce the length of the pipe by running the pipe in a diagonal direction, either in a horizontal or in a vertical plane. The mains and branches should pitch up and away from the heater, generally not less than 1 in. in 10 ft. The flow main should always be covered; the return main should be covered except where it is to provide the heating surface for the basement. The connections from mains to branches and to risers should be such that circulation through the risers will start in the right direction. Hence, in a'one-pipe system the flow . 351 X