Document 8mKraN3mvKxn3VwNnBDBNX0d

American Society of Heating and Ventilating Engineers Guide, 1936 . 13 What important factors must be considered in the determination of resistance for an exhaust system? The maintained resistance of an exhaust system is composed of three factors, namely, (1) loss through the hoods, (2) friction drop through the pipe system, and (3) collector drop. 14 What special materials may be used to resist chemical corrosion in a system exhausting gases and fumes? Various protective materials are available for exhaust systems depending largely upon the type of fumes conveyed.. Nickel-chrome alloys, aluminum coated metals and rubber linings are extensively used. Also protective rubberized paints are available which may be applied for conveying chlorine and hydrochloric acid fumes. 1 a ? 386 1 ' Chapter 22 FAN SYSTEMS OF HEATING Types of Systems, Blow-Through, Draw-Through, Heating Units, Design9 Temperatures, Weight of Air to be Circulated, Tempera ture Loss in Ducts, Heat Supplied, Heating Units and Washerf Grate Area, Boiler Selection, Weight of Condensate, Static Pres sure, Fans and Control AFAN system of heating depends upon fans and blowers to distribute air through ducts from one centrally located plant. This chapter considers heating arid humidifying systems of this type whereas similar systems arranged for cooling and dehumidifying are discussed in Chapter 9. A special type of central fan system, the mechanical warm air or fan furnace system, which is especially adapted to residences, churches, halls, and other small buildings, is covered in Chapter 23. TYPES OF SYSTEMS In the indirect type of central fan heating and air conditioning systems, steam is usually the medium by which heat is transferred from the boiler, or other source of heat, to the heating units. If the system is intended solely for heating, the air is passed over one or more stacks or batteries of heating units and then conveyed to the spaces for which it is intended through a system of ducts. In some cases, a predetermined amount of. outside air is introduced for ventilating purposes, whereas in others the moisture content is controlled by passing the air through a washer or humidifier. If the apparatus is designed to control simultaneously the temperature, humidity, air motion, and distribution, it is known as an air conditioning system. In the split system, the heating is accomplished by means of radiators or convectors, and the ventilating or air conditioning by means of the central fan apparatus. In the combined system, the entire operation of heating, ventilating, and air conditioning is handled by the central fan system. A common arrangement of the central fan system of heating is illus trated by Fig. 1 and consists of a fan, a heating unit (heater) enclosed by a shget metal casing connected with the suction side of the fan, a sheet metal casing connected to the heating unit casing run to the outside of the building and provided with an adjustable opening inside the building for recirculation of the air when desired, and a duct system attached to the fan outlet to convey and distribute the air to various parts of the building to be warmed by the apparatus. The fan is ordinarily motor-driven; there are, however, many cases when a direct-connected steam engine may be used to advantage. In this event the exhaust from the engine can be con- 387