Document 42zROJJOEDbwxoEQDBK0VVmx

Knowinf something about the gen* era) process of combustion and the ' various fuels met in power-plant prac tice, we're now ready to tackle the next - clement -- firing equipment. Its job is to prepare the fuel, bring fuel and air together in proper proportions and, with the help of the furnace, ignite and bum the fuel-air mixture continuously. A On the face of it, burning |gA% gas is simple, because the fuel Is reedy for com bustion and requires no preparation in the strict sense of the word. Never. theless, the remaining parts of the jobproportioning, mixing end burning--can be handled In a variety of ways and their characteristics need to be known for sound selection of equipment and successful operation. Atmespherli Srn*r. Qos burners dif fer mainly in the way air and fuel mix. Perhapa most familiar is the so-called atmospheric burner. One form appears quite generally in the ordinary house hold gas range. In it the momentum of the incoming low-pressure gas stream is used to draw in, or aspirate, part of the air needed for combustion. A shutter or similar device regulates amount of air so induced. Oas and air together pass through a tube leading to the burner ports, mixing in the proc ess. The mixture bums at the ports or openings in the burner head (with a blue, nonluminous Rome. Secondary air is drown into the Rome from the surrounding atmosphere. Larger coun terparts of this general burner type, having ring or sectional burner heeds with many ports, are used to Are small boilers and industrial equipment A single-port atmospheric burner is .shown in Fig. 1. Needle valve controls gas flow through the spud; air is drawn in around the shutter at the end. The' resulting mixture passes through the tube and bums at its end. Single-port burners may be grouped, several bonks high and wide, to serve larger furnaces. Although physically simple, such a burner must be proportioned with con siderable skill to conserve the relatively mall amount of energy In the lowpressure gas stream. It is this energy which entrains the primary air. How well this is done depends on primaryair percentage, gas-oriflee site, ratio of mixer throat to burner port area and, In boilers especially, furnace draft. Srimii Percentage. With burner-port lie and shape Axed, nature of burning depends largely on amount of primary air, or premix. With premix low, Aorae is long end pale blue. It may have a yellow tip, indicating, aa we have seen, tome cracking and presence of free carbon. Increasing primary air shortens the flame as burning becomes more rapid, and a greenish inner cone ap pears. When speed ol burning, or Same propagation, exceeds thot of gas issu--^ ing from the port, flame flashes back Into mixing tube, . .'ft*. Operation la generally satisfactory^, with 30 to 70% premix; in some specialty designs 100% primary air is used. This premix range gives a turn down, or.'-^i | capacity range, of about 4 to 1. Usti-1)^ ally premix and capacity ranges arefljf somewhat narrower. `'revi Secondary, air is usually drawn loV^ around the burner and the amount thu** depends on the areo of the opening a#4.&^ the draft Control may be effected tyV.'- varying draft or, sometimes, by adjuit*, ing opening area by shutters. *. The so-called atmospheric burner one example of a general doss, in whleb'if' the energy of one fluid is used aspirate the other and In which part all primary air mixes with the fuel the burner body. The "high-preuurt burner uses gas at about 20 to 30 and air at atmospheric pressure. other type uses compressed olr, the gas is at atmospheric pressure. Toe. - burner of Fig. 6 expands Mgh-P****u*V gas through two venturi-section* .'gfr series to obtain thorough mixtal * short distance. Refractory Burners. For boiler Bn*.^ a somewhat different type of V finds wide use. It depends on or fan draft to draw in all air itfluj for combustion; hence draft C0"~L are most important Design of Fl-^ .employs multiple gas jets, which dis charge into the olr-stream In such o' tray that violent agitation results In a short mixing tube or tunnel of refrac tory. In the burner of Fig. 3, turbufence vanes impart a swirling motion to the air entering the tunnel. Each of the small jets of gas issuing from the multiple-jet orifice entrainj with the sir and impinges it outward against the tunnel walls. This . action gives focfculent, thorough mixing. In the burner shown completely as sembled, Fig. 4, vertical manifolds CBvicct horixontal tubes, which contain individual gea'orificei for the 15 tunnel Mocks forming the complete burner O high, 5 wide). Louvers in front of the burner assembly control.air admls,^n. Each orifice discharges into a tdractory mixing tube or tunnel. In burners of this type the refractory a`^* 'n heating the mixture for wtion and protects metal burner Pwts from high temperature. The flame J**1 be fnade relatively luminous, for 'adiant-heat transfer. ^ege steam-generating units often ""Moy a high-pressure (2 to 25 -psi) w* burner of the gos-ring. Fig. 20, ..*"|<r*diffusion tube, or turbulent, Fig. " The gas-ring. Fig. 20, has an dar manifold located between air ,ttr and furnace wall surrounding `cner opening. Orifices drilled in this 'pray gai angularly ocross an In- coming air stream controlled in quan tity, velocity and rotation by the registers. Pan-MU Burner. In the burner of Fig. 7, gas Issues from jeta drilled at an angle In a rotating spider. Resulting reaction spins the spider and with it the connected fan. A'shutter controls air drawn in, to maintain desired fuelair rotio. Thorough mixing of gas and air result from the turbulent interac tion of jets and air stream; combustion U completed close to the burner. Thus far we have talked of designs in which fuel and air mix In or at the individual burner. Higher burner head pressure to overcome variable furnace draft, high overload capacity, uniform air-gas mix at all loads, and singlevalve control may be had In a system in which mixture is made at one point and supplied to several burners. Such a system is shown in Fig. 5. This b the low-pressure type; gas Is at atmos pheric pressure while air b at 1 to 2 psi. Inspirator Oe*ernor. Heart of the sys tem b the inspirator governor, ie/t. Fig- 5. Air passes through the venturi tube at high velocity to create a lowpressure region at the end of the straight run where gas ports, ere lo cated. This pulls atmospheric-pressure gas through the ports into the throat end products mixing. As the mixture expands through the inspirator body its velocity is converted to pressure. Oas enters the governor under pres sure and flows through the governor valve. A passage through the governor valve keeps pressure at governor out let and on the under aide of the diaphragm the same. With atmospheric pressure on the other side of the diaphragm, governor delivers gas at atmospheric pressure. The mixture leaving the inspirator governor contains oil air needed for combustion. An air valve controls the burning rote. The complete gas-air mixture goes from the Inspirator-gov ernor to a number of individual burn ers. usuolty the tunnel type shown at the right in Fig. 5. This principle of supplying a complete mixture to a number of burner units b alsofound in systems operating with high-pres sure gas end atmospheric air. In Edition to proportioning fuel and. air, and mixing | them, oil burners must pre pare the fuel for combustion. There are two ways of doing this, with many variations of each: (I) The oil may be vaporized or gasified by heating within the burner, or (2) it may be atomised by the burner so vaporisa tion can occur in the combustion space. Designs of the first group, usually called vaporising burners, are neces sarily limited in range of fuels they can handle and find little power use. T. M (748) SOWEB Dacsi*"1' December 1941 (749] 83