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88 CHAPTER 6 1962 Guide And Data Boole to use commercial stock sixes of lift fittings, but if these are not available in the sizes required a satisfactory lift connec tion can be made from standard pipe fittings as shown by Fig. 19. When a lift assembly requires a union, it should be installed in a horizontal run. The use of a lift in any part of the system results in slower heat-up as well as interrupted return of the condensate from the section ahead of the lift. Lift fittings should not be used in place of.an auxiliary t*nlr in the return main at the inlet to the vacuum pump receiver. A condensate pump may be used as a mechanical lift where severe outlying lift conditions cannot be avoided. This may be piped into the system as shown by Fig. 20 in such a man' ner as not to interfere with the normal functioning of the vacuum pump and without loss of vacuum on the returns preceding the condensate pump. TWO-PIPE SUBATMOSPHER1C SYSTEMS Subatmospheric systems are ghnilar to vacuum systems but, in contrast, provide control of building temperature by variation of the heat output from the radiators. The radia tor heat emission is controlled by varying the pressure, tem perature, and specific volume of steam in circulation. These systems differ from the ordinary vacuum system in that they maintain a controllable partial vacuum on both the supply and return sides of the system, instead of only on TO VACUUM PUMP PtPC i DtA. or RCTUPM LONG TTCAO TO lEXTEND NIPPLE 1 0* BELOW BOTTOM OP ACTUM* TEC ITT POCKET 3 UXC* LAACCA TNAN ACTUAN fig. 19.... How to Build a Lift fitting the return side. In the vacuum system, ateam pressure above that of the atmosphere exists in the supply mains and radia tors practically at all times. In the subatmospheric 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 vac uum which in mild weather may reach as high as 25-in. Hg, after which further reduction in beat 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 thermo statically from selected control points in the building, or it may be a special pressure reducing valve which will maintain the desired subatmospheric pressures by continuous flow into the heating main. In some systems radiator supply valves include adjustable orifices,'or are equipped with regu lating orifice plates. The axes of orifices used are larger than for other types of orifice systems because, for equal radiator sixes, 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 subatmospheric method of fig. 20.... Use of Condensate Pump as a Mechanical Lift 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 bypass 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 atmos phere, a return-line vacuum pump, or a return trap, is generally used to return the condensate to the boiler oi 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 maintain^ The principle on which these systems operate is 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 ride 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 prig steam on one ride and Ya prig on the other, the absolute pressure relation is 14.7 + 0.25 14.7 + 2.0 050 90 percent. If the steam pressure were dropped to V\ prig on the sup- ` ply pipe, the pressure on each side of the orifice would be balanced and no steam flow would take place. From this it will be apparent that if an orifice of a given diameter will fill a given radiator with steam when there is a given pres sure on the main, reducing this steam main pressure will-' permit filling various desired portions of the radiator down to the point where the main pressure equals the back pres sure in the radiator, provided the supply pipe pressures are closely controlled. If orifices are designed on a similar basis for a given system and proportioned to the haling capacity of the radiators they serve, all radiators will beat proportionately to the steam pressure. The range of pres sure variation is limited by the permissible noise level of the steam flowing under the pressure difference required for Steam Heating Systems 89' rnarimum.beat output. The control of the steam supply is obtained by a valve placed in the steam main to maintain a determined pressure, and by varying the vacuum in the return lin The valves are frequently set manually from a remot8 location, guided by temperature indicating stations in-the building; ot thermostatically controlled from a ther mostat on the roof, which automatically measures the differ ential.of outdoor and indoor temperatures. Since the range through which the pressures may be varied is usually from 0 to 4 prig, the control should be capable of maintaining dose regulation of the desired space temperatures, particu larly ini mild weather. A recommended orifice schedule is 6hown in Table 1. Some gyatems use orifices not only in radiator inlets, but also at different points in the steam supply piping for the purpose of MRr,ring the system to a greater extent. In this manner the difference between the initial and terminal pressure in the steam main may be compensated to a great extent. For example, if the initial pressure is 3 psig and the pressure at the end of the main is 2 psig, an orifice could be used in each branch for the purpose of obtaining a more uniform pressure throughout the system. Such a provision may be particularly useful in this system for branches close to the boiler where the drop in the main has not yet been produced. Some orifice systems are proprietary. SIZING PIPING FOR STEAM HEATING SYSTEMS The functions of the piping system are the distribution of the steam, the return of the condensate, and, in systems where no local air vents are provided, the removal of the air. The distribution of the steam should be rapid, uniform, and without noise, and the release of air should be facili tated as much as possible, because an air bound system will not heat .readily nor properly. In designing the piping arrangement, it is desirable to maintain equivalent resist ances in the supply and return piping to and from a radia tor. Arrangement of the piping so that the total distance from the boiler to the radiator is the same as the return piping distance- from the heating unit back to the boiler, tends to obtain such a result. The condensate which collects in steam piping as well as in radiators must be drained to prevent interference with the ready flow of the steam and air. The effect of back pressure in the returns and excessive revaporization, such as occurs where condensate is released from pressures considerably higher than the vacuum or pres sure in the return, must be avoided. ' . It is important that steam piping systems distribute steam not only at full design load, but during excess and partial loads. Usually the average winter steam demand i3 less than half of the demand at the design outdoor temperature. Moreover, in rapidly warming up a system even in moderate weather, the load on the steam main and returns may exceed the maTimiiTTi operating load for severe weather, due to the necessity of raising the temperature of the metal in the sys tem. to the steam temperature, and the building to the design indoor temperature. Investigations of the return of condensate have revealed that as high as 143 percent of the design condensation rate may exist under conditions of ac tual operation. Expressed in gallons per minute per 1000 &q ft equivalent direct radiation the theoretical condensing fate of the system at the design indoor temperature is 0.5. During the warming-up period this value - will approach 0.72 gpm. The piping design of a heating system is greatly influ enced . by its operating characteristics. Heating systems do Table 1.... Orifice Capacities for Low-Pressure Steam Systems--Pounds per Hour Hut table it bawd on data from actual todx* Orific* Oiomafer 6411it of on loth 6 In. Hg Oifferentfof 5 fa. Hg Uitterenbof 4in.Hg Pifferenffuf 7 4.5-5.8 4.0-5.3 3.8-4.8 8 5.8-7.3 5.3-0-8 4.8-6.3 9 7.3-9.0 6.8-8.3 6.3-7.5 10 9.0-11.0 8.3-10.0 7.5-9.3 n 11.0-13.0 10.0-12.0 9.3-11.0 12 13.0-15.5 12.0-14.3 11,0-12.8 13 15.5-18.0 14.3-16.5 12.8-14.8 14 18.0-20.8 16.5-19.0 14.8-16.8 15 20.8-23.5 19.0-21.5 16.8-19.0 16 23.5-26.5 21.6-24.3 19.0-21.5 17 28.5-29.8 ' 24.3-27.3 21.5-24.3 18 29.8-33.3 27.3-30.5 24.3-27.0 19 33.3-37.0 30.5-33.8 27.0-30.0 20 37.0-40.8 33.8-37.3 30.0-33.3 21 40.8-44.8 37.3-41.0 33.3-36.3 Orifice OMSMta 64lht of an Inch 2 in. Hg Oiffwtaiiri - I in. Hg Diffoto/liial OrificeAsea tq fa. 7 2.5-3.3 0.00940 8 3.3-4.3 2.0-2.8 0.01227 9 4.3-53 2.8-3.5 0.01553 10 5.3-6.5 3.5-4.3 0.01917 11 6.5-7.8 4.3-5.0 0.02320 12 7.S-9.3 5.0-6.0 0.02761 13 9.3-10.8 6.0-7.0 0.03241 14 10.8-12.3 7.0-8.0 0.03758 15 12.3-14.0 8.0-9.3 0.04314 16 14.0-16.0 9.3-10.5 0.04909 17 16.0-18.0 10.5-11.8 0.05542 18 18.0-20.0 11.8-13.0 0.06213 19 20.0-22.0 13.0-14.fi 0.06922 20 22.0-24.5 14.5-16.0 0.07670 21 24.5-26.8 16.0-17.8 0.08456 NtU.--The tadfator orifice plette recommended is thi* table ere Bade ot bnee etasptnce M8Z in. tbkk enp-ebaped to be ineetod is radiator eelre un ion. * S. B. Sanford end C. B- Swcnfcr: Flow of eteein throogb orifice* into redi*ua (ASHVE TnAMuemm, VoL 37, UK. p. Z71). not operate under constant conditions, because conditions change continually, due to variation in load. As the system is bring filled with steam, the pressures existing in various locations may be different from those which exist for ap. preciable periods at other locations, although at equilibrium conditions the pressures are approximately the same. In designing piping it is of particular importance to arrange the system to preclude trouble caused by such pressure dif ferences. The systems which readily release the air, permit uniform pressures to be attained in much shorter time in tervals than those which are sluggish. Results are given in Rg. 21 from investigations1 to determine the rate of con densate and air return from a two-pipe gravity heating sys tem. Variations in the steam pressure during the warming-up - period, when the rate of air elimination and condensation is high, are clearly indicated ra these curves. It is -evident that the condensate flow during the initial warming-up period reaches a peak, which is greater than the constant-condensing rate eventually reached when the pressure becomes uniform. Moreover, the peak condensing