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402 CHAPTER 27 1960 Guide they are made for either gravity or vacuum installations. Typical meter installations are shown in Fig. 3. In dia gram A a continuous-flow type trap is shown installed ahead of the meter, while in diagram B an intermittent type' trap is used. In the latter case, a receiving tank must be placed between the trap and meter to prevent intermittent over loading of the meter. When measuring the discharge from a vacuum pump, a vented receiver should always be in stalled ahead of the meter. Installations of meters in a vacuum return line are shown in diagrams C and D. In diagram C a master, continuousflow trap is installed immediately ahead of the meter to prevent steam from entering it in case of leaking radiator or fixture traps. Where individual traps are reliable, con nections may be made as shown in diagram D. Flow meters are generally used for measuring sendout from plants, high-pressure requirements in buildings, and in in stallations where all of the condensate cannot be returned to a central point. Further information on flow meters is' available from publications of the Fluid Meter committee of the American Society of Mechanical Engineers and from the District Heating Handbook," and are described in Chap ter 4. HOT WATER DISTRIBUTION District heating in the United States is confined almost entirely to the use of steam as the heating medium. Hot water has been used and is being used today, where eco nomic conditions are favorable and good engineering prac tice dictates. The use of low-pressure hot water distribution below 200 F has been practically discontinued for district heating in the United States because of practical and economic difficulties. The use of high-pressure, high-temperature water distribu tion at 275 to 400 F has found favor in Europe, where fuel is scarce and is finding some application in the United States* High-pressure water distribution has some practical and economic advantages over steam (see Chapter 29 High Tem perature Water Systems). Higher thermal efficiencies, bettor daily and annual system load factors, lower water treat ment costs, smaller pipe rises, fewer piping accessories, and feasibility of installing pipe to follow contour of ground are given as a few. On the other hand hot water requires a two-pipe distribution system which increases the capital in vestment in some cases. System maintenance is complicated by the necessity to drain at least a portion of the system when connections are made or leaks repaired. The advantages of hot water have exceeded the disadvan tages in some industrial applications and in Air Force bases in this country. As the cost of fuel increases, additional ap plications will probably become practicable, especially in new installations serving buildings designed for hot water heat ing.1 REFERENCES 1P. L. Geiringer: Recent developments in high temperature water for area heat distribution in the United States (National District Heating Association Proceedings, 1956). fSteam requirements for processes, maximum demand and load factor (District Heating Handbook, National District Heating Association, 3rd cd., p. 346). * Influence of pressure (District Healing Handbook, National District Heating Association, 3rd ed., p. 147). * Meet of looping (District Heating Handbook, National District Heating Association, 3rd ed., p. 155). 'American Standard Code for Pressure Piping (American Standards Association, B 31J, 1955, p. 66). * Steam pressure boosters (District Heating Handbook, Na tional District Heating Association, 3rd ed., p. 329). ' Graphical Solution of Unwin's Formula (chart published by the National District Heating Association). For explana tion of chart see District Heating Handbook (National Dis trict Heating Association, 3rd ed., p. 194). 'Gordon Carlson: Improved method of supporting steam pipes in tunnels (National District Heating Association Pro ceedings, 1955, p. 125). 'District heating piping systems, and Fabrication details (American Standard Code for Pressure Piping, American Standards Association, B 31J, 1955, Sections 4 and 6). " Piping, valves, fittings and accesories (District Heating Handbook, National District Heating Association, 3rd ed., p. 169). "Steam traps and drain pockets (District Heating Hand book, National District Heating Association, 3rd ed., p. 175). "Control of steam flow (District Heating Handbook, Na tional District Heating Association, 3rd ed., p. 211). "The reinsulation of underground steam mains (National District Heating Association Proceedings, 1945, p. 21). u Underground insulation specifications, Thermal conductiv ity, Steam pipe insulation, Soil temperature surrounding a buried steam line, and Heat loss-from insulated buried steam line (District Heating Handbook, National District Heating Amociation, 3rd ed., p. 162, p. 180, p. 184, p. 188, and p. 189, respectively). "Sabin Crocker: Heat loss (Piping Handbook, McGraw-Hill Book Co, 1955, 4tb ed, p. 1016). " Return of condensate (District Heating Handbook, Na tional District Heating Association, 3rd ed, p. 158). " Reducing and rebel valves on consumers' premises (Ameri can Standard Code for Pressure Piping, American Standards Association, B 31.1, 1955, p. 66). "Temperature control (Principles of Economical Heating, National District Heating Association, 5th ed, p. 25). " Hours of heating (Principles of Economical Heating, Na tional District Heating Association, 5th ed, p. 22). "H. T. Kucera: New developments in programming build ing heating (National .District Heating Association Proceed ings, 1956). " Utilization of heat in condensate (Principles of Economical Heating, National District Heating Association, 5th ed, p. 40). "Metering (District Heating Handbook, National District Heating Association, 3rd ed, p. 221). "Hot water for district heating (District Heating Handbook, National District Heating Association, 3rd ed., p. 423). 1 CHAPTER 28 HOT WATER HEATING SYSTEMS Features and Design of low Temperature Hof Wafer Systems,- Two-Pipe, One-Pipe, and Series-loop Systems; Piping Principles; Circulating Pumps,- Expansion Tanks? Boiler Room Piping; Use of Sfeam; Temperature Control; System Adjustment; Cleaning Systems AHOT WATER heating system is one in which water is used to convey heat by flowing through pipes connect 4. The OTstem may generally be designed with a minimum of mechanical specialties thereby favonng economical mainte nance. ing a boiler or water heater with radiators, convectors, or 5. Due to practical elimination of air, corrosion within the other suitable heat distributing means. The systems discussed system is minimized. in this chapter have supply water temperatures less than 6. Where extended glass exposures occur, the heating ele 250 F and are classified as Low Temperature Systems. Sys ments, which are relatively large because they operate at low tems having supply water temperatures above 250 F are classified as High Temperature Systems and are discussed in Chapter 29. Classified according to the means of generating temperature, may be well*distributed under such exposures to counteract downdrafts. 7. Quiet operation may be expected in a properly designed and installed system. flow, hot water heating systems are of two types: the Grav ity System in which circulation of the water is due to the difference in weight between the supply and return water In the design and operation of forced circulation systems, it is necessary to take precautions against any condition that columns of any circuit or system, and the Forced System in which a pump, usually driven by an electric motor, maintains would permit the water temperature to fall below freezing, particularly where coils are used to heat incoming outdoor the necessary flow. air or where the heating plant may be shut down for more Since the available head or force producing circulation in than 24 hours in cold weather. a gravity system is limited, the piping must be of such size that the friction loss at the desired flow is not in excess of the If the static pressure in the system is high, it may be neces sary to use - boilers built for operating pressures above 30 available headGravity circulation heads which can be obtained with vari pdg unless a steam boiler with heat exchanger is used to heat the water. See discussion in this chapter in section Expansion ous supply and return temperatures are shown in Fig. 1. The- Tanks and System Pressure Control. basic principles which determine design in gravity and forced systems are the same but since the former has less total head available to produce flow, greater precautions are necessary DESIGN OF LOW TEMPERATURE HEATING SYSTEMS in design to secure even distribution of heat with gravity sys tems. Gravity systems are no longer of commercial impor tance. Complete details on the design of gravity type systems are contained in editions of Thb Gums issued before 1957. The significant factors affecting the design of low tem perature hot water heating systems are discussed in the sec tions that follow. Fig. 1 is also useful in determining effects due to tempera Heat Loss Calculations ture difference in forced systems.when large elevation or tem perature differentials occur. This chapter is devoted to the design of forced hot water systems using supply water temperatures below 250 F. The selection of type of distribution system for heating a particular building should be based upon an objective analy sis of the characteristics of the building, the comfort and temperature conditions required, the system operating and The adequacy as well as the proper distribution of the heat in a building served by a hot water system depends primarily on the proper calculation of the heat loss for the individual spaces and the accurate sizing of the heating units to offset those heat losses. Heat loss should be determined in accord ance with the principles discussed in Chapter 12, Heating Load. maintenance characteristics, the space available for piping Selection and Location of Hearing Units and equipment, and the economics involved. Information on the types of room heating units available FEATURES OF HOT WATER SYSTEMS for hot water systems will be found in: Chapter 14, Radia Advantageous features of forced low temperature hot wa ter systems are: tors and Convectors; Chapter 30, Panel Heating; and Chap ter 15, Unit Ventilators and Unit Heaters. While the style or type of unit selected for a given space may depend on per 1. The piping need not be run at a definite level or pitch but may change up and down as required by architectural and structural elements oi the building. High points should be vented and low points be provided with drain connections. 2. Heat distribution when warming up is uniform. 3. Hot water systems are readily adaptable to simple ap proximate control of capacity and indoor temperature by in struments sensitive to outdoor temperature changes. sonal preferences of the purchaser, the designer should make certain that the heat output at the water temperature avail able is equivalent to the calculated beat loss from the space. The heating units within each separately controlled cir cuit should be of similar types, for example: either all castiron or all fin-tube convector-type units. Fan units should not be included in a controlled direct radiation zone. The 403 X