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100 Energy Conservation Guidebook

FUEL OIL-BURNING SYSTEMS

Fuel oil is a very common form of fossil fuel that can be effectivelyburned to produce heat. Comfort heating systems that employ this typeof fuel are found widely in commercial, industrial, and residential build-ings today. The use of fuel oil is primarily the result of convenience andthe cleanliness of oil when compared with coal. In the northeastern sec-tion of the United States, fuel oil is a major source of energy for manycomfort heating systems.

Fuel oil is a by-product of petroleum refining. It is graded accordingto the amount of distillation performed during its production. Gradesrange from 1 to 6 with the number 3 omitted. Lower-grade numbers

contain fewer impurities and are more expensive. Number 1 and 2 oilis best suited for comfort heating system applications. No. 1 fuel oil pro-duces 133,000 to 137,000 Btu/gal, with No. 2 developing approximately136,000 to 142,000 Btu/gal. Higher-numbered oil grades are thicker,produce higher Btu/gal values, and are used primarily in industrialheating applications.

Heat produced by natural gas or fuel oil is achieved by essentiallythe same basic type of system components. This includes such thingsas a burner, fuel igniter assembly, combustion chamber, heat exchanger,thermostatic control, venting, a blower fan, and a duct network. Figure4-18 shows a forced-air fuel oil-fired comfort heating furnace with itsoutside jacket removed. Central systems of this type range in heatingcapacity from 85,000 to 200,000 Btu per hour. In addition to its use in thecentral heating type of system, fuel oil is used to energize unit heaters inindustrial and commercial buildings. Figure 4-19 shows an assembledoil-fired unit heater.

Fuel Oil BurnersThe burner assembly of a fuel oil furnace represents the primary

difference between gas- and oil-fired heating systems.The basic reason behind this difference is in the physical state of

the fuel being used. Fuel oil in its liquid state simply does not burn veryeffectively. For burning to take place, it must either be heated to produce

a vapor or mixed with air. Vaporizing is rarely used in comfort heatingsystems because it is somewhat difficult and expensive to achieve. Atom-izing, which is a fuel/air mixing procedure, is used in nearly all comfort





Energy Conservation Guidebook

Figure 4-19. Oil-fired unit heater. (Courtesy of Modine ManufacturingCo.)

Pressurized burnersare made up of an elec-tric motor, a blower, apump, and a fuel unit. Theblower is attached to the

motor shaft and developsair for mixing with theoil. A fuel pump is usedto develop pressure ac-cording to the design of the system. Low-pressureunits respond to a rangeof 1 to 15 psi, whereashigh-pressure systemsmay reach levels of 100psi. The fuel unit contains

Figure 4-20. Fuel oil burner assembly.(Courtesy of The Carlin Company)



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a pressure-regulating valve and a filter for straining foreign particlesfrom the fuel.

In an operational sequence, when heat is called for by the ther-mostat, a solenoid-controlled fuel-level valve opens to admit fuel intothe line from the storage tank. It first passes through a filter strainerwhere any existing solid particles are removed from the fuel beforeit is applied to the pump. Outflow of the pump must be at a selectedpressure level according to the design of the system. An in-line pres-sure-regulating valve follows the pump and maintains the pressureat a desired operating level. Fuel then proceeds under pressure to thefuel nozzle.

This nozzle simply changes pressurized fuel into tiny droplets

which are ejected and mixed with air from the blower. A 10,000-voltspark developed by a high-voltage transformer is used to ignite theatomized fuel at this point. The burning process starts and is continuousas long as fuel is being supplied. When the heat demand of the ther-mostat setting is met, fuel flow ceases and the burner shuts down. Theunit then remains in a ready state waiting for the next heat demand.

A second basic type of oil burner assembly employs the rotaryprinciple. Rotary oil burners are quite compact and usually have a ratherlimited number of moving parts. Oil in this case is applied to a hol-low metal shaft that turns during normal motor operation. The same

rotary action of the motor shaft is also used to turn the blower of theair source. The unique feature of this unit is that it does not necessitatepressurized oil to function. The rotating shaft may employ small tubesor a cup structure for oil emission. Figure 4-21 shows a cross-sectionalview of a rotary oil burner with an atomizing cup.

The heat exchanger, blower, duct network, and draft assemblyare quite similar to those of a comparable heating system. Figure 4-22shows a partial cutaway view of an oil-fired unit heater. Note particu-larly the location of the oil burner, burner nozzle, blower fan, and heatexchanger.

An assembled heat exchanger of a oil-fired unit heater is shownin Figure 4-23. Note the location of the flue collector, service inspectiondoor, and combustion chamber clean-out ports. For safety reasons,inspection doors and clean-out ports of this type are not found oncomparable gas-fired heat exchangers.

Heat transfer of a fuel oil-fired unit heater is from the inside of the



EnergyConservationGuidebook

Figure 4-21. Cross-sectional view of a rotary oil burner. (Courtesy of Ray Burner Co.)



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Figure 4-22. Cutaway view of an oil-fired unit heater. (Courtesy of Modine Manufacturing Co.)

exchanger to the outside. Cool air is pulled into the assembled unit fromthe back side and circulated over theoutside of the exchanger. It exits fromthe unit through louvers mountedon the front of the assembly. Figure4-24 shows the location of the blowerfan and burner assembly on the backside of the unit.

COAL-BURNINGHEATING SYSTEMS

Coal is a solid type of fossil fuelthat has been used for a number of

years as an energy source in heat-ing systems. Systems of this typeproduce heat through the burningof fuel after if has been placed on a

Figure 4-23. Heat exchanger of anoil-fired unit heater. (Courtesy of Modine Manufacturing Co.)



Energy Conservation Guidebook

Figure 4-24. Rear view of an oil-fired unit heater. (Courtesy of ModineManufacturing Co.)

metal grate inside the combustion chamber. The transfer of heat energyin this case is from the inside of the exchanger to the outside. As a gen-eral rule, coal-fired furnaces are much larger than other heating systems,have less control, and are somewhat inconvenient to operate. Utiliza-tion of these units in commercial and residential buildings has droppedquite significantly in recent years, although industrial applications arecontinuing at about the same rate.

The fundamental parts of a coal-fired heating system are verysimilar in many respects to those of other fossil fuel-burning units. Fuelis loaded into the combustion chamber either manually or automati-cally. Air is mixed with the fuel to produce combustion and is expelledthrough the draft diverter into an outside stack or flue. Heat developedthrough the combustion process is then transferred to circulating airthat passes around the outside of the exchanger. The resulting outputof the heat exchanger eventually passes through a duct network, whereit is distributed throughout the facility. Heated air may be moved by amotor-driven blower or distributed through ducts by natural circulation.Figure 4-25 shows the essential parts of a coal-fired heating system.



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Figure 4-25. Hand-fired coat furnace.

Coal-fired furnaces have a number of features that distinguishthem from other heating systems. For example, the fuel is loaded intothe system either by hand or automatically by a stoker. Once the burn-ing process is started, it should be continuous. Coal must be supplied

periodically to prevent the fire from going out. A burnout necessitatesrestarting the fire with paper and wood to reach a flashover point thatwill produce ignition of the new supply of coal. Unburned particles orresidue drop into an ash pit under the grate. This must be removed ona regular basis to make the process continuous.

The burning process and heat regulation of a coal-fired unit are alsounique compared with other heating systems. Burning is controlled by theloading of new coal and the regulation of air admitted to the combustionchamber through a damper assembly. Closing the damper reduces theflow of air and slows down the burning process. More coal is burned in

a given unit of time when the damper is open. Many systems employa thermostatically controlled damper motor that regulates airflow intothe chamber according to the heat demand of the system.

Coal-fired heating systems are not used as much today as theywere in the earlier part of the century. Most of this can be attributed tothe inconvenience of operation, inefficiency, fuel storage problems, airpollution, and fuel costs. The availability of coal, wood, and other solidfuels compared with gas and oil may cause a rather significant change inthe future of solid-fuel-fired heating systems. A number of manufactur-ing concerns are now developing multi-fuel heating systems that will



108 Energy Conservation Guidebook

burn gas, fuel oil, or solid fuel according to its availability. Systems of this type provide a backup or alternative in the case of a primary fuel

shortage. Figure 4-26 shows a sketch of a representative multi-fuel heat-ing system.

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