Document mBg46QK5VkzDgNexMDoZ8351b

392 . CHAPTER 18 1949 Guide oil fired forced air furnaces. Gravity gas furnaces approved by the Ameri can Gas Association are assigned a rating based on 75 per cent efficiency. All forced air gas-fired furnaces approved by American Gas Association are assigned a rating based on 80 per cent efficiency. DESIGN CONSIDERATIONS " Considerations of prime importance in the design of warm air furnaces and some general suggestions to be observed in connection with each are as follows: 1. Adequate heat transfer surface. a. Heat transfer rates of 2,000 to 4,500 Btu per (hour) (square foot) of heating surface may be obtained without unduly high metal temperatures. ' b. Fins, pins and bosses are frequently used to add surface and to break down superficial gas films, both on gas-to-metal and metal-to-air surfaces. c. Surface and stack (Sue gas) temperatures are good indications of the amount and effectiveness of the heating surfaces. 2. Safe and efficient combustion offuel. a. Proper mixture of fuel and air is necessary for efficient combustion. This necessitates careful attention to the design of grates, nozzles, burners, air inlet areas and location, and combustion chamber baffling. b. Regulation of the quantity and the distribution of the air for combustion - should be provided by use of check dampers, draft regulators, draft hoods, air shutters and air orifices; c. Total draft loss through appliances should not exceed that available from chimneys which would normally be obtainable in the size of building which > the appliance will supply with heat. d. The use of ignition safety devices such as safety pilots, hold-fire controls, and the like is recommended. . Z*. Fuel Capacity of Appliance. a. With solid fuels adequate coal capacity should be provided for at least 5 hr of operation at the maximum rated combustion rate. 4. Adequate circulation of air over heating surface. a. In gravity furnaces, free air space'between casing and heat exchanger should be great enough to permit free flow over all surfaces. b. Forced air furnace design must include fans having; proper capacity and suit able performance characteristics. Internal static pressures must be-mini mized without losing'the advantages of high velocity circulation over the heat exchanger surfaces. c. The air flow over the heating surface must be directed to obtain maximum efficiency and to eliminate hot spots and air noises. d. Air velocities at bonnet should not be much in excess of 1,000 fpm' and air temperature distribution at the furnace outlet should be uniform within approximately =fc 30 deg. N 5. Durability. a. A minimum metal weight for gas-fired heat exchangers is established as No. 20 U.S. Gage for plain carbon steel by the A.GA. Approval Requirements for Central Heating Gas Appliances with some municipal codes specifying 18 gage. Cast-iron sectional thicknesses of in. to | in. are recommended. b. Added strength and reinforced designs may be required to preclude damage in shipment, burning out from overfiring, or corrosion from condensation. c. Maximum heat exchanger surface temperatures which may be used vary with the metal. The American Gas Association Approval Requirements for Cen tral Heating Gas Appliances specify a maximum of 875 F for cast-iron or steel gas furnaces, and the National Bureau of Standards CS109-44 Code for Forced Air Solid Fuel-Burning Furnaces specifies 1000 F as a maximum surface tem perature. These temperatures define the range in which oxidation of non- . alloy ferrous metal begins. The use of proper alloy additions increases the temperature resistance properties of metals. d. Casing temperatures should be controlled so that they do not become hazards to burn those who touch them, or to create fires. Heating Boilers, Furnaces, Space Heaters .393 6. Serviceability. a. Those parts of the furnace which may be subject to soot, fly-ash, or conden sation deposits should be accessible for cleaning. b. Parts which may require adjustments or replacements, such as grates, baffles, liners, controls, should be removable. c. Furnaces should be so designed that they can be installed with a minimum of difficulty. 7. Control. a. Thermostatic controls of various types should be used to correlate space temperatures with unit operation. b. Controls should be provided wherever possible, to prevent the occurrence of excessive temperatures or other conditions in any part of the unit which might cause unsafe operation. 8. General design considerations. a. Furnace casings are normally constructed of formed and painted sheet steel or of galvanized iron. The casing should be protection from excessive radia- tion losses and temperatures by use of insulation or sheet steel air space liners. Liners should extend from the grate level to the top of the furnace and should be spaced from 1 in. to 1$ in. from the outer casing. b. The hood or bonnet of the casing above the furnace should be as high as base ment conditions will allow,'to form a plenum chamber over the top of the furnace. This tends to equalize the pressure and temperature of the air leaving the bonnet through the various openings. It is generally considered advisable to take off the warm air pipes from the side of the bonnet near the top, as this method of take-off allows the use of a higher bonnet and thus provides a larger plenum chamber. c. Warm air outlet and return air connections should be designed so that the ductwork may be easily attached. A | in. flange is normally used for this purpose. d. Suitable provision shall be made in appliances so that the controls and humidifiers may be installed in the proper location. When these auxiliary units are installed in the ductwork, detailed instructions should be provided to insure their proper location. e. The flue connection should be of integral flue pipe size and provision should be made to attach the flue pipe to the flue outlet of the furnace. HUMIDIFICATION EQUIPMENT Water evaporating pans are usually located in air which has been heated by contact with the heating surfaces; To change water into vap6r cap able of being carried in an air stream as part of the mixture, about, 1000 Btu per pound are required. There is a trend in present practice toward heating" the Water in addition'to heating the air. Equipment for doing this may make use of sprays, or it may take the form of water circulating coils placed within the combustion chamber and connected by pipes to the humidifier pans where a constant water level is maintained by some separate float device. All humidifiers require provision for removal of dirt and lime. SPACE HEATERS Space heaters may be classified in several ways, such as: 1. By the type of fuel used as coal, wood, gas, and fuel oil. 2. According to the method of heat distribution as circulators or radiant types. A radiant heater is one in which the heat exchanger surface is exposed directly to the room atmosphere, and the generated heat is dissipated primarily by radiation. A circulating heater is essentially a jacketed radiant heater from which circulation of room air is promoted by the chimney effect caused by the movement of air passing upward between the jacket and heat exchanger surface. 3. According to method of design for particular fuel types, such as: (a)'surface--1 fired and magazine-feed for solid fuels, (b) vaporizing pot type and-blue flame heater