Document OEr3pLE01Z1D0bq729w1vQ93M

110 CHAPTER 6 1962 Guide And Data Boole CHAPTER 7 DISTRICT HEATING Steam Requireinerrfs, Boiler Phots, Steam Distribution, Design Canriderutionj, Disfr^M/tion Pressures, Pipe Sizes, Conduit* for Piping, Manholes and Tunnels, Accessory Equipment, Insulation, Return of Condensate, Building Piping and Equipment, Metering, Hot Wafer Distribution Fig. 61.... Typical Return Connections to Finned-Tube Blast Heaters with High-Pressure Steam shown in fig. 53. Where the vertical distance is limited and the runouts must'run above the floor, the radiator may be set on pedestals or raised by means of high legs. A method of connecting a unit heater to a one-pipe steam heating system is illustrated in Fig. 2. Typical two-pipe radiator connections are shown in Figs. 54 and 55. While these show top inlet supply connections which are preferred, it is also posable to connect the supply to the bottom of the radiator. Short radiators may be con nected with top supply and bottom return on the same end. A typical method of connecting convectors is shown in Fig. 56. Sometimes the supply valve is omitted on con vector connections, and a damper is supplied in the outlet grille for heat control. A typical connection for finned pipe convectors is shown in fig. 57. Typical connections to blast beaters are shown in Figs. 58, 59 and 60. Some precautions which should be noted with regard to connections for blast heaters are: 1. Steam mains should Dot be dripped into heater sections. 2. If it is necessary to keep the heater in service at all-times, a bypass with globe or plug-type valve should be ingfcwlW* around the automatic regulating valve. 3. A strainer should be provided on the steam supply side of a regulating valve. 4. The sizing of regulating valves diould be based on the steam load ana not on the heater supply connection. 5. Bach heater or hank of heaters installed in series should have a separate trap. 6. Return piping from heater to trap should be of the size as the heater outlet connection. 7. Return piping should not be ran to an overhead main which is above the neater return connection, or into mains under pres sure which contain modulating or on-off steam control valves. A Fig. 62.... Typical Unit Heater Connections fair Two-Pipe System pump and receiver, or boiler return trap, should be installed be tween the heater condensate trap and the overhead main or re. turn main under pressure. 8. Steam piping and heater sections should be supported inde pendently. fig. 61 shows a typical return end connection for blast heaters connected to high-pressure systems. A typical two-pipe connection to a unit heater is indi cated in Fig. 62. MANUAL VALVES Gate valves are recommended m ail cases where service demands that the valve be either entirely open or entirely closed, but they should never be used for throttling. Angle globe valves and straight globe valves should be used for throttling in such cases as bypasses around pressure-reducing valves or on bypasses around traps. For information on automatic controls see Chapter 13 of the 1961 Guide And Data Book. ' REFERENCES 1F. C. Houghteo and. J. L. Blaekshaw: ASHVB Hjcrkabct Repost No. 954--Condensate and air return in steam beating systems (ASHVB Tbansactions, Vol. 39, 1933, p. 199). * Reducing and relief valves on consumers' premises {Ameri can Standard Code for Pressure Piping, ASA B31.1-1955, p. 66). 'American Standard Code for Pressure Piping (ASA B31.11955, p. 66). `N. H. Davidson: Economies effected by combination pres sure and temperature control valve (National District Heat ing Association Proceedings, 1956). * ASHVE Standard Code for Testing and Rating Return Line Low Vacuum Heating Pumps (ASHVE Tbansactioxs, Vol. 40, 1934, p. 33). HE term district heating refers to the supplying of heat usually between 15 and 30 percent. For space tearing phis Tfrom a central plant to a group of buildings in a city, additional steam requirements such as water hearing, cooking, or laundry, it is between 15 and 40 percent. Where process institution, housing development, or industrial or commer- requirements predominate, as in factories, load factors are eial area. The heat may be used for any applicable purpose sometimes 50 percent or more. Load factors for various types such as space heating, air conditioning, or preceding. It is usually preferable that groups of commercial, industrial, or institutional buildings be supplied with heat from a central of buildings are given in Table 9, Chapter 12. 2. Duration of plant output. The cumulative number of hours for the various plant aend-outs and the predicted peak boiler demand on the coldest day are used in conjunction to plant rather than from individual plants in order to permit determine the number and sizes of boilers, in order to effect jpcign for- better combustion efficiency with less expensive fuel, and reduction of labor per unit of output. The central plant justifies use of more competent personnel, and often most efficient loading of individual units and the plant as a whole. The cumulative number of hours a typical district hearing plant will send out steam at various plant outputs, with outputs expressed in percentage of maximum hourly rate, decreases the investment required. is given in Table 3. It will be noted that the duration of plant Those phases of district heating which frequently fall output for the higher aend-outs is very low. within the province of the heating engineer are treated here, and information is given for solving incidental prob 3. Peed-water treatment. Good feed-water treatment is an absolute necessity in a plant in order to be able to operate the boilers at marimum outputs. Dirty tubes will limit boiler lems. Some data are included with reference to special re capacity. Proper feed-water treatment will prevent corrosion quirements for steam piping in buildings served with district and scale formation and keep boiler outage to a minimum. steam. The data are confined principally to the use of steam as the heating medium, but the use of bot water Hm gained some recognition in recent years.1 4. Smoke abatement. This is a modern requirement. The installation of suitable fuel-burning and dust-collecting equip-, ment to limit the emission of smoke and fly ash is usually necessary. In the design of a district heating system to provide public 5. Plant location. A boiler plant location is determined by utility service in a city, it is advantageous to make a thorough an economic study of fuel handling, water supply, system study of the entire problem with competent men having experience in design and operation of such systems. piping, land costs, and other associated factors. Such a study usually reveals that the economical location of the plant will be near the distribution center, in order to simplify and shorten the distribution piping. Plant location and the local air pollu STEAM REQUIREMENTS tion ordinances will have a direct bearing on the type of firing The first step in the design of a district heating system equipment selected and the dust collection apparatus required. b to determine the maximum hourly and the annual steam requirements of each of the buildings to be supplied. Methods of determining the maximum hourly requirements will be STEAM DISTRIBUTION Distribution piping in district beating systems must be found for space heating in Chapter 25 of the 1961 Guide And Data Book, for building service-water heating in Chapter 76, and for various process applications in Table 1.* Measured demands of several types of buildings are 'given in designed in accordance with the same basic principles .ap plying to any other steam piping. The piping may be run through buildings, through the air on poles or other struc tures, or underground in conduits or tunnels. Local condi Table 9, Chapter 12. tions and investment will be governing factors in establishing Methods of estimating annual steam requirements 'for the route of the pipes. heating various types of buildings are also given in Chapter 12. Table 7 in Chapter 12 lists the average annual steam Design Considerations .consumption per degree-day for buildings located in all sections of the United States. Annual steam requirements for building service-water heating are given in Table 2 of this chapter. Additional data on annual steam requirements, including those for process uses and air conditioning, of vari es types of buildings in a number of cities may be found in the District Heating Handbook, Third Edition.1 BOILER PLANTS In the design of a plant for district heating, consideration 8hould be given to the following factors: 1. Annual load factor. This factor is the ratio of the avenge toudy plant send-out (defined as rate of flow to the distribution system) to the maximum hourly send-out, which is a direct jneaaire of the extent to which investment in facilities is used. H u often employed to determine savings by equipment ottering higher efficiency. The load factor for space tearing is Important points in laying out distribution piping are: 1. Piping within buildings or basements is normally least expensive to install and to maintain. 2. Underground installations involve excavation which rep resents a substantial portion'of the total cost. The depth of such excavations should be kept to a minimum after due con sideration has been given to the possible damaging effect of the operating-line heat on lawns and shrubbery and the pos sible damage to the underground structure due to heavy traf fic, etc. 3. Walking tunnels usually are not provided for steam mains in the district heating industry because of their relatively high first cost as compared with smaller conduits unless they are re quired to accommodate miscellaneous other services or to pro vide underground passage between buildings. 4. Overhead piping on poles or structures is generally less expensive than underground lines, especially in some institu tional and industrial areas and may be a means of eliminating buried conduit in areas where underground installations would in