Document dDRJ6ZO2bpknMxBppNpoqVqOb

676 CHAPTER 29 1955 Guide of design, ease in testing, accuracy at all loads, low cost, and adaptability to low pressure distribution have made it standard equipment with many heating companies. Condensate meters should not be operated -under pressure; they are made for either gravity or vacuum installations. Typical meter installa tions are shown in Fig. 6. In diagram 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 be tween the trap and meter to prevent intermittent overloading of the meter. When measuring the discharge from a vacuum pump, a vented receiver should always be installed ahead of the meter. Installations of meters in a vacuum return line are shown in diagrams C and D. In diagram C a master, float-type, continuous-flow 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, connections may be made as shown in diagram D. Flow Meters used for district heating work are of three types: Area Meters, Head Meters and Velocity Meters. Meters are described in Chap ter 4, Fluid Flow. Flow meters are generally used for measuring send-, out from plants, high-pressure requirements in buildings, and in installa tions where all of the condensate cannot be returned to a central point. Further information on flow meters is available in the District Heating Handbook and from publications of the Fluid Meter committee of the American Society of Mechanical Engineers. REFERENCES `American Standard Code for Pressure Piping, 1942, with Supplement No. 2, 1947, ASA B31.1-1942 (American Standards Association). - 2 Dislrict Heating Handbook, Third_Edition, 1951, p. 329 {National District Heat ing Association). 3 Graphical Solution of Unwin's Formula (a chart published by National District Heating Association). CHAPTER 30 CENTRAL SYSTEMS FOR AIR CONDITIONING Features of Systems, Zoning, Humidity Control, Cooling Load, Heating Load, Air Quantity and Effectual Temperature Difference, Low and High ' Pressure Induction Convectors, Evaporative Cooling, Precooling, Sensible Cooling with Unwetted Coils, Run-Around System, Selection of Type of. System, Location of Apparatus, Design Procedure THE term, central, applied to an air conditioning system implies that the equipment such as fans, coils, filters and their encasement are designed for assembly in the field rather than in a factory as a unit. As a central system usually serves several different rooms, individual con trols are required for each room. FEATURES OF CENTRAL SYSTEMS One advantage of a central air supply system is that one apparatus serving many rooms may involve a lower investment cost than that for a number of self-contained plants, each serving a single room. A central system may occupy basement or attic space that is relatively unimportant, whereas individual factory-assembled apparatus placed in each room may occupy otherwise valuable space. Another advantage of a central system is accessibility for servicing, since it is possible to provide doors in the encasement for cleaning and inspecting all of the component parts in a manner -usually -superior to thatpracticable with compact factoryassembled equipment. Central air conditioning systems usually are connected by ducts with the various rooms served, and preferably have exhaust fans that may effect complete removal and disposal of any desired proportion of the air. The exhaust fan may return air to the supply system for recirculation, as a measure of economy of fuel or refrigeration. Central air conditioning systems are served by heating and refrigerat ing equipment which may be located at some distance from the air supply apparatus, and which may serve one or more central air supply systems. Year-Round Air Supply System Fig. l is a plan of a year-round air supply system. Outside air may enter from the left at A, desirably from an intake on the side of the build ing least exposed to solar heat, and not close to the ground or to a sunheated or dust-gathering roof. The damper B for proportioning the volume of outside air, is interlocked with the return air damper C in such manner that as the outside air volume increases the return air volume decreases, the return air duct D, shown diagrammaticafly, comes from the exhaust an. All the air, it will be observed, must pass through the filters E, and there is ample room on both the inlet and outlet sides of the filters for servicing theih. The filters may be of mechanically cleaned type, of replaceable cell type, or may be electronic, as described in Chapter 34. The cleaned air passes to the equipment that changes its temperature and humidity. Except in very warm climates, a heating or tempering coil F is required to warm the air to a temperature above freezing. Usu- 677