Document DGdwnyEEY09Ow9xbD1QXN597N

514 CHAPTER 20 1957 Guide supply mains and radiators practically at all times. In the sub-atmos pheric system, atmospheric pressure or higher exists in the steam supply piping and radiators only during severe weather. Under average winter temperature the steam is under partial vacuum-which in mild weather may reach as high as-25 in. Hg, after which further reduction in heat output is obtained by restricting the quantity of steam. The rate of steam supply is controlled by a valve in the steam main or by thermostatically controlling the rate of steam production in the boiler. The control valve may be of the automatic modulating or floating type governed thermostatically from selected control points in the building, or it may be a special pressure reducing valve which will maintain the desired sub-atmospheric pressures by continuous flow into the heating main. In some systems radiator supply valves include adjustable orifices, or are equipped with regulating orifice plates. The sizes of orifices used are larger than for other types of orifice systems because, for equal radiator sizes, the volume flowing is larger. Orifices are omitted on some systems. Radiator traps and drips are designed to operate at any pressure from 15 psig to 26 in. Hg. Steam for heating domestic hot water should be taken from the boiler header back of the control valve so that pressures sufficiently high for heating the water may be maintained on the heater. The sub-atmos pheric method of heating can be used for the heating coils of ventilating and air conditioning systems. The flexible control of heat output secured by this method materially reduces the required size of by-pass around the heaters. Some applications of sub-atmospheric systems are proprietary. TWO-PIPE ORIFICE SYSTEMS Orifice steam heating systems may have piping arrangements identical with vacuum systems. Some of these omit the radiator thermostatic traps, but use thermostatic or combination float and thermostatic traps on all drip points. A return condensate pump with receiver vented to atmosphere, a return line vacuum pump, or a return trap, is generally used to return the condensate to the boiler or place of similar disposition, such as a feed-water heater or hot well. The heat emission from the radiators is controlled by varying the pressure differential maintained. The principle on which these systems operate is based on the fact that the steam flow through an orifice will vary with square root of pressure drop when the ratio of the absolute pressures on the two sides of the orifice exceeds 58 percent. If the absolute pressure on the outlet side is less than 58 percent of the absolute pressure on the inlet side, no further increase in flow will be obtained as a result of the increased pressure difference. If an orifice is so designed in size as to exactly fill a radiator with 2 psig on one side and J4 psig on the other, the absolute pressure relation is 14.7 + 0.25 147 + 20 = -90 or 90 Percent- Should the steam pressure be dropped to K psig on the supply pipe, the pressure on each side of the orifice would be balanced and no steam fk>w would take place. From this it will be apparent that if an orifice of * given diameter will fill a given radiator with steam when there is a given pressure on the main, reducing this steam main pressure will permit filling various desired portions of the radiator down to the point where the mam pressure equals the back pressure in the radiator, provided the supply P,Pe. pressures may be controlled sufficiently close. If orifices are designed on Steam Heating Systems INVERTED LIFT FITTING OR EL- 515 LIFT FITTING Fig. 17. Method op Constbucting a Lift in a Retubn Line SHORT NIPPLES TO VACUUM PUMP iIFT PIPE DUL OF RETURN LONG THREAD TO IXTENO NIPPLE 2 IN. BELOW BOTTOM OF RETURN VSTEM RETURN LINE REDUCING TEE IFT POCKET 2 SI2ES LARGER THAN RETURN Fig. 19. How to Build a Lift Fitting valve steam MAINy \ EQUALIZER LINE i - r* O VACUUM HEATING PUMP' CONDENSATE PUM? AS A. mechanical LIFT Fig. 20. Use of Condensate Pump as a Mechanical Lift