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394 CHAPTER 26 1960 Guide 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 door, 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 possible 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 heaters 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 not be dripped into heater sections. 2. If it is necessary to keep the heater in service at aii times, a bypass with globe or plug-type valve should be installed 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 should be based on the steam load and not on the heater supply connection. 5. Each heater or hank of beaters installed in series should have a separate trap. 6. Return piping from heater to trap should be of the same size as the heater outlet connection. 7. Return piping should not be run to an overhead main which is above the heater return connection, or into mains under pres sure which contain modulating or on-off steam control valves. A fig. 62 .... Typical Unit Heater Connections for 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 and 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 in all 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 43. REFERENCES 'F. C. Houghten and J. L. Blackshaw: ASHVE Research Report No. 954--Condensate and air return in steam beating systems (ASHVE Transactions, 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 B3I.11955, p. 66). 1 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 Transactions. Vol. 40, 1934, p. 33). CHAPTER 27 DISTRICT HEATING Steam Requirements, Boiler Plants, Steam Distribution, Design Considerations, Distribution Pressures, Pipe Sizes, Conduits for Piping, Manholes and Tunnels, Accessory Equipment, insulation, Return of Condensate, Building Piping and Equipment, Metering, Hot Water Distribution THE term district heating refers to the supplying of heat from a central plant to a group of buildings in a city, system) to the maximum hourly send-out, which is a direct measure of the extent to which investment in facilities ts used. It is often employed to determine savings by equipment institution, housing development, or industrial or commeroffering higher efficiency. The load factor for space heating is cial area. The heat may be used for any applicable purpose such as space heating, air conditioning, or processing. It is usually preferable that groups of commercial, industrial, or usually between 15 and 30 percent. For space heating plus additional steam requirements such as water heating, cooking, or laundry, it is between 15 and 40 percent. Where process requirements predominate, as in factories, load factors are institutional buildings be supplied with heat from a central sometimes 50 percent or more. Load factors for various types plant rather than from individual plants in order to permit of buildings are given in Table 9, Chapter 37. design 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 2. Duration of plant output. The cumulative number of hours for the various plant eend-outs and the predicted peak boiler mi on the coldest day are used in conjunction to determine the number and sizes of boilers, in order to effect decreases the investment required. most loading of individual units and the plant as a Those phases of district heating which frequently fall within the province of the heating engineer are treated here, and information is given for solving incidental prob whole. The cumulative number of hours a typical district heating plant will send out steam at various plant outputs, with outputs expremed in percentage of maximum hourly rate, is given in Table 3. It will be noted that the duration of plant lems. Some data are included with reference to special re output for the higher eend-outs is very low. quirements for steam piping in buildings served with district steam. The data are confined principally to.the use of steam as the heating medium, but the use of hot water has gained some recognition in recent years.1 3. Feed-water treatment. Good feed-water treatment is an absolute necessity in a plant in order to be able to operate the boilers at maximum outputs. Dirty tubes will limit boiler capacity. Proper feed-water treatment will prevent corrosion and scale formation and keep boiler outage to a minimum. In the design of a district heating system to provide public utility service in a city, it is advantageous to make a thorough study of the entire problem with competent-men having experience in both design and operation of such systems. STEAM REQUIREMENTS The first step in the design of a district heating system is to determine the maximum hourly and the annual steam requirements of each of the buildings to be supplied. Methods 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. 5. Plant location. A boiler plant location is determined by an economic study of fuel handling, water supply, system 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 tion ordinances will have a direct bearing on the type of firing equipment selected and the dust collection apparatus required. of determining the maximum, hourly requirements will be found for space heating in Chapter 12, for building service- STEAM DISTRIBUTION water heating in Chapter 56, and for various process ap plications in Table 1* Measured demands of several types of buildings are given in Table 9, Chapter 37. Methods of estimating annual steam requirements for heating various types of buildings are also given in Chapter 37. Table 7 in Chapter 37 lists the average annual steam 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 Distribution piping in district heating systems must be designed in accordance with the same basic principles ap plying to any other steam piping. The piping may be tud through buildings, through the air on poles or other struc tures, or underground in conduits or tunnels. Local condi tions and investment will be governing factors in establishing the route of the pipes. Design Considerations this chapter. Additional data on annual steam requirements, including those for process uses and air conditioning, of vari ous types of buildings in a number of cities may be found in the District Heating Handbook, Third Edition.* BOILER PLANTS In the design of a plant for district heating, consideration should be given to the following factors: 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 passible damaging effect of the operating-line heat on lawns and shrubbery and the pos sible damage to the underground structure due to heavy traf 1. Annual load factor. This factor is the ratio of the average fic, etc. hourly plant send-out (defined as rate of How to the distribution 3. Walking tunnels usually are not provided for steam mains 395