Document 0Jo3kKYMXjMGoVByZqw5jLwok

492 CHAPTER 34 1959 cosity and actually interferes with the atomizing process. Underheated oil is too viscous for proper atomization, re sulting in poor combustion, the generation of smoke and soot, and an excessively high oil consumption. Overheated oil, on the other hand, is too fluid for correct atomization and is subject to the same type of poor, operating results as the underheated oil. No. 4 fuel oils, with a maximum viscosity of 125 seconds Saybolt Universal at 100 F, as a rule do not require any preheating before atomization. They are sufficiently fluid at normal ambient temperatures for good burner operation. No. 5 fuel oils .are of two types; a light grade having a viscosity ranging between 150 and 200 seconds Saybolt Uni versal, that does not require preheating and hence is known as a cold No. 5; and a heavier grade having a viscosity ranging between 200 and' 800 seconds Saybolt Universal, that requires preheating and hence is termed a hot No. 5 oil. No. 6 fuel oils (also known as Bunker C) always require preheating, as their viscosities range from 900 to 10,000 seconds Saybolt Universal (50 seconds to 300 seconds Say bolt Furol). A number of factors affect the selection of a fuel oil pre heating method. The three basic factors are; 1. Type of installation (commercial heating, industrial proc ess, power generating; etc.). 2. Type of operation (automatic, semi-automatic, manual, intermittent, continuous). 3. Type of fuel (No. 5, No. 6, Bunker C). The pour point of an oil is another characteristic that could influence the preheating requirements, especially in areas subject to severe winter temperatures. An oil with a sufficiently high pour point may require preheating and con tinuous circulation at temperatures above its pour point at all stages between storage tank and burner to insure its easy pumpability. Oil specifications should be checked for this characteristic and limitations imposed, if feasible. Otherwise the oil piping system must be designed with this high pour specification in mind. Four mediums for preheating fuel oil are in common use: steam, hot water, gas, and electricity. With steam as the heating medium the beater may be of shell-and-tube (either straight or U-tube) type or the heaters may be of a bayonet type inserted in the oil tank. In a bayonet-type heater an open-end tube is enclosed in a larger tube which has a closed outer end. Steam enters through the inner tube while condensate is drained from the outer tube. A heater of bell-shaped type may also be con nected into the oil suction line at the tank for the purpose of raising the temperature of the oil drawn from the tank. The steam preheating method has several limitations. Mainly, it is dependent upon continuity of burner operation to keep steam available to prevent viscosities beyond pumpability. Therefore, its practicability and economy are limited to installations having continuous operation. Another limi tation is that the relatively high temperature of steam may cause changes in the chemical structure of lighter grades of residual fuel oils. To prevent excessively high viscosities in the fuel oil piping, constant oil circulation is maintained or the oil piping is enclosed with steam lines in the same thermal insulation. The economy of the steam preheating method is also dependent on the availability of steam at the pressure and quantity required, and on the distance from boiler to storage tank. The hot water preheating method utilizes equipment simi lar to that used for steam. Because of the possibility that oil may penetrate into the boiler through faulty heat ex changes, some devices have been developed to prevent this condition. Double-transfer heaters and fuel-oil detectors are those most frequently used. Similarly, as with steam, con tinuity of burner operation, or cycling burner operation, is needed to maintain the temperature of the heating medium. Since oil heating is limited by the water temperature in the boiler less transmission and transfer losses, such losses must be taken into account, particularly on installations having a considerable length of oil piping. The indirect gas-fired preheating method is a package ar rangement that uses gas as the primary energy source. It generates its own supply of low-pressure steam in a closed vapor-condensate cycle. The oil is pumped through the steam oil heater as required to compensate for radiation losses in the oil-circuit lines, whenever pumping action stops. During the layover period, the main boilers and oil burners are entirely inactive. If separate oil transfer pumps are not available, a small circulating oil pump is added to the pack age to provide circulation in the oil lines during the gas firing periods. The electrical preheating method involves use of immer sion heaters plus one of two special arrangements for heating the fuel oil piping. One special arrangement is a heating cable (soil cable or. plastic covered wire) wrapped around the supply line or all oil piping together with a vertical-ele ment immersion heater in the suction pipe in the oil tank. The closeness and evenness of the spiral winding of the cable determine the heat input. The on-off periods are thermo statically controlled. For protection of the heating cable, the temperature of the oil piping has to be limited and usually must not exceed 170 F. Where pipe and oil temperatures permit wrapping of all oil piping, constant oil circulation is not required. The efficiency of this preheating method is lower than for element-immersion or electric-impedance heating because of transfer losses between the heating ele ment and the pipe. The other special arrangement for electrical preheating is a packaged system of electric-impedance heating for all oil piping together with immersion element heating sufficient to heat the oil used. Low-voltage high-amperage alternating current of the 60-cycle frequency is passed through the oil piping itself, generating an even amount of heat in the pipe walls. The majority heating effects are due to hysteresis and eddy currents. When removing oil from unheated storage hot excess oil is returned to blend with cold oil from storage in a special tank unit. Since all oil piping is kept at design temperature, under all conditions of flow or standby, con stant oil circulation or cycle precirculation is- cot needed. The thermostatic control of pipe and oil temperature results in fully automatic operation. Oil burners designed for automatic operation on No. 5 or No. 6 oil are generally equipped with thermostatically con trolled electric immersion heaters. The function of a burnerimmersion heater is to heat the oil at the rate being burned from the transportation temperature (usually at 120 F level for No. 6 oil) to the temperature required for efficient com bustion. This temperature ranges from 130 F to 220 F, de pending on the grade of fuel oil and design of burner. In oh emergency, the burner-immersion heater may be used alone to heat a modulated, very low quantity of oil for combustion. However, in a normal operation, it functions in conjunction with the primary preheating system. Maintaining Piping and Tank Temperature In using No. 5 and No. 6 oils it is necessary to make spe cial arrangements to keep the oil fluid in the piping at all Automatic Fuel Burning Equipment 493 times and to keep at least a portion of the oil in tiie storage tank fluid as well. The piping may be heated by one or more of the following methods: 1. Wrapping the supply lin* with electrically heated tape. 2. Passing a current through the supply line itself. 3. Enclfwing the supply line with a steam or hot water line in the same envelope. - 4. Insulating the supply line so as to decrease the thermal losses. 5. Setting up a dual system in which the primary preheaters have a capacity greater than the burner requirement and re turning the excess heated oil continually to the fuel oil tank. The oil in the tank or a portion of it may be heated by one of the following methods: 1. By steam coils in the bottom of the tank. 2. By hot water coils in the bottom of the tank. 3. By a suction-bell, which is provided with either steam or hot water coils, in the bottom of the tank. 4. By providing a hot-well that extends from the top to the bottom of the tank, and provides enough heated oil to supply the burner under a full load. 5. By submerging an electric preheater in the wall or bottom of the tank. Pipe Sizes for Heavy Oil Table 2 gives recommended good practice pipe sizes for handling No. 5 and No. 6 oils. Oil Storage Tanks and Piping Storage tanks, also the piping and pumping facilities for delivering the oil from the tank to the bumer^are very im portant considerations in the design of an industrial oilburning system. The construction and location of the tank and oil piping are usually subject to local regulations which should always be consulted, and to the standards of the Na tional Board of Fire Underwriters, such as NFBU Pamphlet No. 31. The size of the storage tank is usually based on the rate at which oil is to be consumed, the space available, method of oil delivery (truck or tank car) and the ease with which deliveries can be made under all weather, conditions. A mini mum capacity of at least the maximum weekly usage is de sirable. If deliveries are to be made by tank car, the capacity should not be less than 15,000 gal. The 275-gal oil storage tank'is commonly used for domes tic installations. In most localities it may be installed in base ments without being enclosed. Where a greater storage ca pacity is necessary, local regulations usually permit inside installation of two tanks of 275 gal each in accordance with NBFU Standards Pamphlet No. 31. Controls The control of oil-fired equipment is discussed in the sec tion Automatic Control of Fuel-Burning Equipment. GAS-FIRED HEATING EQUIPMENT A gas burner is defined by the American Gas Association as "a device for the final conveyance of the gas, or a mixture of gas and air, to the combustion zone." Burners used for domestic beating are of the atmospheric injection, luminous flame, or power burner types. Because of the ease with which gas fuel may be controlled, automatic gas-fired heating equipment has become very widely used, and is available in a number of types of domes tic gas beating appliances and systems. These may- be clas sified in types designed for central heating plants and those for unit application. Gas-designed units and conversion burners are available for the several kinds of central systems in which gravity and forced warm air furnaces or steam and hot water boilers are used, and for other applications where warm air floor furnaces and room heaters are installed in the space being heated. Central Heating Systems Boilers and furnaces specially designed for gas-firing in corporate design features for obtaining maximum efficiency and performance. Small flue passes to secure good beat trans fer, the use of materials resistant to the corrosive effects of products of. combustion, and draft hoods are notable fea tures. Control equipment includes gas-pressure regulators, automatic pilots, and limit controls designed to protect the appliance and to insure safety of operation. A boiler designed for gas-burning is illustrated in Fig. 14. Gas-designed boilers for hot water or steam heating are available in cast-iron, steel, and non-ferrous metals. The burners are located beneath the sections, and the flue gases pass upwards between the sections to the flue collector. Some boilers are designed to provide domestic hot water through the use of tankless or instantaneous heaters. Warm air furnaces are of two types, gravity and forced Table 2 .... Recommended Minimum Suction and Return Pipe Sizes for Fuel Oil tines* (for No*. 5 and 6 fuel Oi0b of Suction line SO 56 too *6 Pump Cepedfy, GoDon* per Hour 150 200 300 Grade of Oil (Number) 4*4 Diameter of Pipe, inch** * 4 600 *4 900 4 10-50 60-100 110-150 IH W ix IX 2 2 2 2 2 2 2 2 2 2 2 2 2X 3 3 4 2X 2K 2X 3 334 1H 2 2 2 2 2X 2X 2X 2X 3 3 4 4 4 * Pipe siseaalwwnara based on vertical auction lift not to exceed 10 ft Iron bottom of storage tank to oil pomp. bA liae length in of tOO linear feet is not recommended for extern* using No. S or No. oil. Where the length is greeter e separate pump should be installed near the storage tank. . * Equivalent length of pipe includes ell bomontal end vertical measured [aping plus proper allowance for valves sad fittings so follows: Multiply pipe diameter in inches by factor.below to obtain equivalent pipe length in feet. Elbow -- Factor 8; Tee -- Factor 6; Gale Valve -- Factor 1)4; Globe Valve -- Factor 9. (