Document ppRZZkv4G7ZNpj2zRNox2mKN7

-478 CHAPTER 20 1953 Guide / 3. The cooling load per degree difference in temperature is not constant, but in creases as the outdoor temperature increases. 4. The heat lag of the building complicates the estimation of the cooling load under any specified conditions and makes such estimates, based on the usual methods of computation;.of doubtful value. 5. The seasonal cooling requirements are extremely variable from year to year, and the ratio between the degree-hours of any two seasons occurring within a 10-year period may be as high as 7.5 to 1. Hence, an average value of the degree-hours cool ing per season is comparatively meaningless. 6. The duct system-in a forced-air heating installation can be successfully con verted to a system for conveying cool air for the purpose of cooling the structure. No condensation of moisture was observed when the duct temperatures were not less than 65 F. 7. Cooling by means of water at a temperature of 60 F is not satisfactory unless an indoor temperature of less than 80 F is maintained. 8. In the selection of cooling coils, the additional frictional resistance of the coil to flow of air must be given consideration. 9. Cooling the structure by introducing large quantities of outdoor air at night tended to reduce the amount of cooling required on the following day, and was a practical means of providing more comfortable conditions in those homes where cool ing systems were not available. REFERENCES 1 A Yardstick for the Evaluation of a Forced Warm Air Heating System (National Warm Air Healing 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). * 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, Third Edition, 1950). * 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). I Code and Manual for the Design and Installation of Large Warm Air Winter Air Conditioning Systems (National Warm Air Heating and Air Conditioning Association, Manual 9, Fourth Edition, 1950). 6 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, Third Edition 1950). s Warm-Air Perimeter Heating (National Warm Air Heating and Air Conditioning Association, Manual 4, Second Edition, 1950). 10 Work Sheets for Warm-Air Perimeter Systems (National Warm Air Heating and Air Conditioning Association, Forms 41a, 41b, 42, 43 and 45. II 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). CHAPTER 21 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 STEAM heating systems may be classified according to any one of, or combination of, the'following features; (1) piping arrangement, (2) pressure or vacuum conditions obtained in operation, (3) method of returning condensate to the boiler. 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 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 Snd low pressure conditions with the use of vacuum pump. When automatic controls are employed to vary the pressure conditions in the sys' tern 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 system 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 svstern 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 Bystem, t.e., the friction and^resistance to the flow of 8team 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. 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 i psi, and utilizes a Hartford return connection instead of a check valve on the return,