Document x5Qp5jXE8mgr59MDGXOVZ65pm

512 . CHAPTER 35 1959 Guide d. Air velocities at bonnet should not be much in excess of 1000 fpm, and air temperature distribution at the furnace outlet should be uniform within approximately -- 30 deg. 5. Dvrabiiity. St. A minimum metal weight for gas-fired beat exchangers is established as No. 20 US. Gage for plain carbon steel by the A.GA. Approval Requirements for Central Heat ing Gas Appliances, with some municipal codes specify ing 18 gage. Cast-iron sectional thicknesses of Vi 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. e. Maximum heat exchanger surface temperatures which may be used vary with the metal. The American Gas Association Approval Requirements for Central Heat ing Gas Appliances specify a maximum of 875 F for castiron or steel gas furnaces, and the National Bureau of Standards CS 109-44 Code for Forced Air Solid PueXBummg Furnaces specifies 1000 F as a maximum surface temperature. These temperatures define the range in which oxidation of non-alloy ferrous metal begins. The use of proper alloy additions incresses the temperature resistance properties of metals. d. Casing temperatures should be controlled so that they do not become hazards by burning those who touch them, or by creating fires. 6. SeTtnceabi&v. a. Those parts of the furnace which may be subject to soot, fly-ash, or condensation deposits should be accessi ble for cleaning. b. Parts which may require adjustments or replacements, such as grates, baffles, liners, controls, should be remov able. 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 posable; to pre vent the occurrence of excessive temperatures or other conditions which might cause unsafe operation in any part of the unit. 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 radiation losses and temperatures by use of insulation or sheet steel air space liners. Linera should extend from the grate level to the . top of the furnace and should be spaced from 1 in. to 1V4 in. from the outer casing. b. The hood or bonnet of the cosing above the furnace should be as high as basement 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 tk* 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 y in. flange is normally used for this purpose. d. Suitable provision should 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 pro vided to insure their proper location. e. Hie 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 Evaporating pans are usually located in the outlet air. There is a present trend toward heating the water. Equip ment for doing this may make use of sprays, or it may take the form of water circulating coils placed within the com. bustion 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 ex changer surface is exposed directly to the room atmosphere, and the generated heat is dissipated primarily by radiation. A circu lating beater is essentially a Jacketed radiant heater from which circulation of room air is promoted by the chimney effect caused by the movement of air pasting upward between the jacket and heat exchanger surface. 3. According to method of design for particular fuel types, such as: (a) surface-fired and magazine-feed for solid fuels, (b) vaporizing pot-type and blue-flame heater for oil, and (c) vented and. unvented heaters for gas. (The type of gas burner design, such as injection, yellow flame, power, and pressure, may also be mentioned.) SOLID FUa-FIRED HEATERS Surface-fired heaters normally have a front firing door and are operated with relatively shallow fuel beds. A maga zine-feed heater includes a deep reservoir of fuel to lengthen the attention intervals. In a true magazine-feed beater the rate of fuel ignition would be equal to the rate of burning; and self-feeding should operate to move the unbumed fuel by gravity flow from the magazine section into the hearth area. However, the ideal balance between rate of ignition and rate of burning is virtually impossible to attain for any solid fuel under normal usage, although self-feeding may be obtained with wood and some free burning coals. Thus, a magazinetype space heater is essentially a deep surface-fired heater, its principal difference from other heaters being increased fuel capacity. Materials and Construction There is no accepted code governing the construction of solid-fuel-buming space heaters. In past years cast iron was used predominantly in the construction of coal and wood heaters, with the exception of the so-called airtight heaters designed as low-cost wood-burning units with little considera tion for long life, and stoves were priced on a poundage basis. The present trend is toward fabricated steel parts and welded assemblies, although cast iron is still used for grates, firebox linera, and parts subject to high temperatures. Refractory firebox liners are also used quite extensively. Formed sheet steel is used predominantly for the outer jacket of circulator heaters, although heaters with outer cas ings formed from cast iron are still readily available. Circu lator cabinets normally have surfaces finished with a porce lain enamel, while the casing of a radiant heater is finished with an air-dried japan or & baked enamel. Both welding and stove bolts are used in unit assembly, and stove cement is used on section joints to prevent air leakage. This latter is extremely important to obtain a low rate of combustion when desired. Testing and Rating There is no accepted code governing the method of testing and rating solid fuel space beaters. A tentative procedure, Heating Boilers, Fvmaces, Space Heaters 513 TS-3443, has been issued by the Division of Trade Standards, National Bureau of Standards, but is based on use of an thracite as a rating fuel, although bituminous coals are used predominantly as heating fuels. This procedure, which has been the basis of published ratings, consists of determining the heater output, expressed in Btu per hour, by the indirect method. The measurable heat losses: (1) loss due to moisture in the fuel, (2) loss due to heat in the dry flue gases, (3) loss due to unburned carbon monoxide, and (4) loss due to un burned combustible in the ash and refuse, are measured by test. The total of these four measurable Loses, plus an as sumed value for unaccounted for losses, are then subtracted from the heat input. The difference multiplied by the burning rate in pounds per hour is the heater output. When using anthracite as the rating fuel, the unaccounted for loss has been assumed to be zero. It has been accepted practice to use a 20 percent allowance for the unaccounted for losses when burning a bituminous coal. The value of this factor has been under study and, although test work is in complete, a value of 12 percent of the heat input has been determined as representing the losses due to smoke and un bumed hydrocarbons for a surface-fired heater, when burn ing a high volatile bituminous coal. No allowance is made for a radiation loss, as this is useful heat. Design Considerations Some important considerations in the design of solid-fuel space beaters are: 1. Suitable protection by baffling or insulation against over heating of floors and walls. Although there are..no industryaccepted standards by which floor and wall temperatures may be determined, some indication of heater performance, with re gard to overheating, may be found by the use of the comer booth test arrangement described in Notional Bureau of Stand ards Commercial Standard CS 103-48 and Underwriters Lab oratories Standard, Subject 896, mentioned in following section on Oil Heaters. 2. Tight beater construction to prevent air leakage, and to enable maintenance of a suitably low minimum burning rate. This includes a ground, paper tight joint between the ashpit door and door frame. 3. Sufficient free air space, between the caring and heat ex changer of circulating heaters, to permit free air flow over all surfaces and maintain a suitably low casing temperature. 4. Protection of all metal parts from deterioration due to high temperature. This may be accomplished by: (a) fabrication of certain parts from cast iron or an alloy iron, (b) protection by a refractory liner, (c) use of high temperature enamel coat ings, (d) directing air against hot spots. 5. Proper admission of secondary air to complete the combus tion process. Care should be taken to prevent this air from also functioning as primary air. 6. Strength in assembly to prevent transportation and use damage. Considerable work has been directed toward improvement of the performance of bituminous coal-fired space heaters, with particular reference to smokeless operation under con ditions of normal operation such as obtained in homes. A smokeless coal heater has been developed by Bituminous Coal Research, Inc., wherein smokeless combustion is ob tained by admitting secondary air through a narrow slot, extending from ride to ride, above the edge of the fuel bed where the gas leaves to enter a vertical gas passage* Com plete mixing of the volatile material, released from the coal in the magazine, with the secondary air supplied is obtained as both streams pass under the bottom of the arch. Com plete combustion results from this intimate mixing in a re gion which maintains itself at high temperatures even during banking periods. OIL HEATERS Vaporizing pot-type oil heaters consist of: (1) a metal pot in the bottom of which the oil is vaporized, the vapors burn ing at or near the top of the pot and (2) a secondary com bustion chamber, or heat exchanger, in which combustion is completed. The flue connection is made to this chamber or through a second heat exchanger which may be of the divingflue type installed to increase efficiency. The burner may be designed for operation both with and without mechanical draft. Blue-flame oil burners differ from the pot-type variety in that removable perforated sleeves are provided above an oil pan instead of a metal pot, and lighting rings or kindlers are used for easy lighting. Both types of oil burners operate by the burning of the oil vapor rather than the oil itself, the oil being first fed to a chamber in which the oil is entirely vaporized, then mixed with air introduced through suitably located ports and burn ing at'the top of the pot or perforated sleeves. Such heaters are designed to burn No. 1 oil' (See Table 4, Chapter 33) or kerosine (coal oil). At no time should oil heavier than that for which the burner is designed be used, 'as heavier oils may cause excessive carbonization in the burner or fuel feed line. Materials and Construction Formed sheet steel and welded assemblies are used pri marily in oil heater construction. Standards governing con struction which have been generally accepted are: 1. Commercial Standard for Flue-Connected Oil-Burning Space Heaters Equipped with -Vaporising Pot-Type Burners, CSlOl-43 (National Bureau of Standards). 2. Standard for Oil-burning Stoves, Subject 896 (Under. writers' Laboratories, Inc.). 3. Standard for Construction and Performance of Oil Burn ers for Installation tn Stoves and Ranges, Subject 865 (Under writers' Laboratories, Inc.). Some states or municipalities have codes which apply lo cally, but these usually apply primarily to installation, and the Underwriters' Laboratories label of approval is sufficient to cover acceptance of the unit. Testing and Rating Commercial Standard CS101-43 is intended to provide a uniform standard method for ascertaining the maximum practical heat output in Btu per hour of flue-connected oilburning space heaters' under approximately normal service conditions. This method is based upon the following, equations: Hr = A - B (8) and E m HrfA . where A - total beat of fuel used. B " heat lost in flue gases. Hr = net beat delivered to the room. E * unit efficiency. (9)