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458 CHAPTER 19 1951 Guide 7. Determine size of ducts in duct system.and size of registers, as explained in this chapter. 8. Determine pressure loss in duct system and select fan haying proper capacity. 9. Select cooling unit from manufacturer's data. Specify temperature and pres sure of available cooling water, voltage and characteristics of electrical supply, and method of control of apparatus. 10. Select cooling coils from manufacturer's data to take care of latent heat load and to give required drop in air temperature with the weight of air flowing. (See Chapter 35). ** 11. If system is to be used for both winter heating and summer cooling, duct sizes must be checked to insure that velocities and friction losses are reasonable for both conditions of operation. Adjustable dampers will be necessary to make changes in air distribution for the two seasons. Provision must also be made for changing fan speeds for summer and winter operation. CHAPTER 20 STEAM HEATING SYSTEMS Classification of Steam Heating Systems by Types; One-pipe; Two-pipe, Sub-atmospheric and Orifice Systems; Sizing Piping for Steam Heating Systems; Pressure Reducing Valves; Boiler Connections; Condensate Return Pumps; Vacuum Heating Pumps; Traps; Drips; Con nections to Heating Units; Control Valves REFERENCES 1A Yardstick for the Evaluation of a Forced Warm Air Heating System (National Warm Air Heating and Air Conditioning Association, Manual 8,1941). * Performance of a Forced Warm-Air Heating System as Affected by Changes in Volume and Temperature of Air Recirculated, by A. P. Kratz and S. Konzo (A.S.H.V.E. Transactions, Vol. 48,1942, p. 393). STEAM heating systems may be classified according to any one of, or combination of, the following features: (1) by the piping arrange ment, (2) by the pressure or vacuum conditions obtained in operation, (3) by the method of returning condensate to the boiler. * Code and Manual for the Design and Installation.of Warm Air Winter Air Con ditioning Systems (National Warm Air Heating and Air Conditioning Association, Manual 7, Second Edition, 1947). 4 Proposed Design Procedure for Large Mechanical Warm Air Heating Systems, by S. Konzo, R. J. Martin, D. S. Levinson, and R. W. Roose (A.S.H.V.E. Trans actions, Vol. 53, 1947, p. 177). 6 Code and Manual for the Design and Installation of Large Warm Air Winter Air Conditioning Systems (National Warm Air Heating and Air Conditioning As sociation, Manual 9, Fourth Edition, 1950). * Automatic Controls for Forced-Air Heating Systems, by S. Konzo and A.' F. Hubbard (A.S.H.V.E. Transactions, Vol. 40,1934, p. 37). 7 Service Manual for Continuous Air Circulation Technicians (National Warm Air Heating and Air Conditioning Association, Manual 6, First Edition, 1947). * Code and Manual for the Design and Installation of Warm Air Ceiling Panel Systems (National Warm Air Heating and.Air Conditioning Association, Manual 7-A, Second Edition, 1948). 9 Warm-Air Perimeter Heating {National Warm Air Heating and Air Conditioning Association, Manual 4, First Edition, 1950). 10 Work Sheet for Warm-Air Perimeter Systems {National Warm Air Heating and . Air Conditioning Association, Form 5, Second Edition, 1950). 11 Summer Cooling in the Research Residence, by A. P. Kratz, S. Konzo, M. K. Fahnestock and E. L. Broderick {University of Illinois Engineering Experiment Sta tion Bulletins Nos. 290, 305 and 321). A.S.H.V.E. Research' Report No. 1177-- Summer Cooling in .the Research Residence with a Gas-Fired Dehydration Cooling Unit, by A. P. Kratz, S. Konzo and E. L. Broderick\A.S.H.V.E. Transactions, Vol 47,1941, p. 203). 1. By Piping Arrangement. A steam heating system is known as a one-pipe system when a single main serves the dual purpose of supplying steam to the heating unit and conveying condensate from it. Ordinarily, to each heating unit there is but one connection which must serve as both the supply and the return, although separate supply and return connections may be used. A steam heating system is known as a two-pipe system when each heating unit is provided with two piping connections and when steam and condensate flow in sepa'l rate mains and branches. Heating systems may also be described as up-flow or down-flow depending on the direction of steam flow in the risers; and as a dry-return or a wet-return depending on whether the condensate mains are above or below the water line of the boiler or con densate receiver. 2. By Pressure or Vacuum Conditions. Steam heating systems may also be classi fied as high pressure, low pressure, vapor, and vacuum systems, depending on the pressure conditions under which the system is designed to operate. A system is known as a high pressure system when the operating pressures employed are above 15 psig; as a low pressure system when pressures vary from 0 to 15 psig; as a vapor system when the system operates under both vacuum and low pressure condi tions without the use of a vacuum pump; and as a vacuum system when the system operates under vacuum and low pressure conditions with the use of vacuum pump When automatic controls are employed to vary the pressure conditions in the sys tem in accordance with outside weather conditions, the system may be known as a sub-atmospheric, differential, or synchronized system. These latter classifications are proprietary designations. When orifices are employed on the inlets to the heating units the Bystem may be known as an orifice system. , 3. By Method of Returning Condensate. When condensate is returned to the boiler by gravity, the system is known as a gravity return system. In this system all heat ing units must be elevated sufficiently above the water line of the boiler, so that the condensate can flow freely to the boiler. Elevation of the heating units above the water line must therefore be sufficient to overcome pressure drops due to flow as well as pressure differences due to operation. Referring to Fig. 1 it will be noted that the boiler and wet-return form a U-shaped container, with the boiler steam pressure on the top of the water at one end and the steam main pressure on the top of the water at the other end. The difference between these two pressures is the pressure drop in the system, i.e., the friction and resistance to-the flow of. steam in passing from the boiler to the far end of the main and the \ pressure reduction in consequence of the condensation occurring in the system. i The water in the far end will rise sufficiently to overcome this difference in order to balance the pressures, and it will rise far enough to produce a flow through the return pipe and overcome the resistance of check valves, if installed. If a one-pipe steam system is designed, for example, for a total pressure drop of 5 i psi, and utilizes a Hartford return connection instead of a check valve on the return. 4 PA