Document K6nQXkY4oknmdjBXpggBkYd70

Heating Ventilating Air Conditioning Guide 1938 3 Why is baffling inside the casing necessary on fan systems? Because the movement of air is independent of its temperature, air must be guided by baffles of one form or another to bring it in contact with the hot surfaces so it will not pass through the casing unheated. On the other hand, if the air is held against a hot surface too long it might become overheated, for the average register temperature on a fan system should not exceed 120 F. 4 # What practical points should be observed in designing a fan system in order, to eliminate noise? a. Use a large fan so it can be run at slow speed. b. Set the fan and motor on a solid foundation. c. Insulate the fan and motor from the foundation with rubber, cork, or other springy material according to the principles given in Chapter 30, provided, of course, that such insulation is of value. d. See that the air velocity is not too high in the ducts. Properly designed splitters in the elbows will avoid high velocities at the turns in cases where the velocity through the ducts themselves is not too high. e. Use canvas connections between the ducts.and any running equipment. /. Be sure the ducts have a relatively smooth interior and are rigid. 5 Why do furnaces designed to burn bituminous coal, oil, or gas require larger combustion spaces than those designed for anthracite? Anthracite bums largely as fixed carbon whereas gas and oil burn as gases, and as much as 50 per cent of bituminous coal burns as a gas. Ample space must be provided for the intimate mixture of these gases with the oxygen of the air to secure proper combustion. 6 A furnace has the following dimensions: Grate diameter, 24 in.; casing 'diameter for gravity air flow, 56 in.; combustion chamber diameter, 30 in. What is the unobstructed area required for passage of air across the heating surface when a motor-driven blower, operating at an outlet velocity of 1200 fpm, delivers 1600 cfm into the casing near its bottom? For residence applications using small blowers, an air outlet velocity of about one third ' of the blower outlet velocity is considered good practice. Air-pass velocity -- --12g0--0 = 400 fpm. Air-pass area = -"j-- = 4 sq ft = 576 sq in. 7 In Question 6 what would be the gap between the chamber and the baffle when the chamber is centered in the casing? Area of combustion chamber (30-in. diam) Area of air pass ^ 706.9 sq in. 576.0 sq in. Total area 1282.9 sq in. The diameter of a circle with an area of 1282.9 sq in. is 40.4 in. One half of the difference between the diameters is the amount of gap. Gap = ^ = 5.2 in. = approximately 5 J4 in. % 408 Chapter 21 CENTRAL SYSTEMS FOR HEATING AND HUMIDIFYING Types of Systems, Blow-Through, Draw-Through, Heating Units, Design, Temperatures, Weight of Air to be Circulated, Temperature Loss in Ducts, Heat Supplied Heating Units and Washer, Grate Area, Boiler Selection, Weight of Con densate, Static Pressure, 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 and humidifying systems of this type whereas similar systems arranged for cooling and dehumidifying are discussed in Chapter 22. 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 20. 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 sheet 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- 409