Document NGQOkbkE54mra8jZ8aN6p0BEp

510 CHAPTER 34 1960 Guide to 12 in. water. A portion of the air or primary air required for combustion is entrained from the atmosphere by the stream of gas iaaring from an orifice, and the air-gas mixture is delivered through a venturi mixing tube to the burner head and parts. About 40 to 60 percent of the air required for combustion is in the form of primary air and the re mainder, known as secondary air, is supplied from the air surrounding the dame. Each system embodies characteristics essential for certain applications. It is important that proper and qualified engi neering authorities be consulted prior to the purchase, in stallation, or operation of industrial gas equipment. FUEL BURNING RATES The burning rate for automatic fuel-burning devices is de termined by the gross heat output required of the boiler, or furnace, to carry the net heating load, plus allowances for system tosses and pickup. General values for these allowances have been given in preceding text. Detailed information for piping and pickup allowances for steam and hot water sys tems, is given in Chapter 35, and for warm air systems, in Chapter 18. When the gross output, operating efficiency, and heat value of the fuel are known, the required rate of burning can be determined by means of Tigs. 17, 18, and 19 for the several fuels. As the rate of fuel-burning is directly propor tional to the load for a given efficiency, these charts can be extended by moving the decimal points the same number of digits in both vertical and horizontal scales. The correct fuel-burning rate can be determined directly from the several charts for oil or gas burning installations, as these customarily operate on a strictly intermittent basis. These fuel burning devices usually introduce the fuel at a single fixed rate during the on periods, and this rate should be sufficient to carry the gross load. In the case of coal stokers, which are usually capable of variable rates of firing, it is desirable to operate at as low a rate as weather condi tions will permit, but the maximum firing rate of the stoker should be sufficient to carry the gross load. This rate may be determined by the same method as used for oil or gas. Rg. 17 .... Coal Fuel-Burning Rate Chart * This chart isbeaeduponNo. t ail haring s hestcoctectof 143,409 Bta per piIon. II either grades of oS in need multiply the value obtaiaed bom this chert by the foQowiag tutors: No. 1 oil (129,000 Bta per gslioa) 1.00; No. 4 oil (144,500 Btu per gallon} 0-993; No.Soil (14fi,000 Bta per gallon} 0.982; and No. 0 oil (150.000 Bta per gallon) O.SSS. Rg. 18 .... Oil Fuel-Burning Rate Chart* o CROSS CALORIFIC MLUE 8 s2 btu sen cubic foot CONTROLS FOR AUTOMATIC FUEL-BURNING EQUIPMENT Controls for the automatic fuel-burning equipment de scribed in this chapter are outlined here. The basic require ments for oil burners, gas burners and coal burners (stokers) are included and the term burner refers to all three types of fuel-burning equipment. Controls for these burners can be classified as operating controls, limit controls and primary controls. The schematic diagrams shown in Tig. 20 indicate the relationships of the basic components. The power supply may be line voltage for each type of burner. In control sys tems for domestic burners the line voltage may be reduced by means of a transformer for those portions of the circuits indicated by dotted lines. Such transformers are usually built into the stoker and oil burner primary controls. Control systems for domestic gas burners frequently contain a sepa rate transformer as shown. Some domestic gas burner control systems obtain their electrical power supply from the con version of heat to low-voltage electrical energy, in which case no transformer would be required. As the burner sizes in crease, the tendency is to use line voltage controls. Rg. 19.... Gas Fuel-Burning Rate Chart tTCMA OC euMCR CAS OWCR (O =* Opeiufarg Control, P = Primary Control; 1 limit Control) Rg. 20 .... Schematic Diagrams of Some Typical Burner Control Systems Automatic Fuel Burning Equipment 511 Operating Controls Operating controls initiate the starting and stopping of the burner. For residential and small commercial heating plants with fuel inputs up to aproximately 500,000 Btuh, the operating controller is usually some form of room thermostat which automatically starts and stops the burner through the primary control as the limit controls permit. On larger in stallations the operating controller for the burner becomes an iusertion thermostat (for furnaces), immersion thermostat (for hot water boilers), or pressure controller (for steam boilers). On some installations, outdoor compensated con trollers may find application. Most large burners, over 2,000,000 Btuh, are equipped to vary the rate of fuel and air input in response to the varying load demands. Limit Controls limit controls guard against unsafe temperature, pressure or water level to assure that safe conditions prevail for the operation of the burner. Where the operating control is other than a thermostat, separate operating and high-limit con trols should be used. It is not recommended and codes seldom permit the use of one and the same instrument as both an operating controller and a high limit control. The operating controller will be adjusted to control at the desired set point while the high limit control will be adjusted to stop the burner at some higher point in the event that the operating controller fails to stop the burner before the higher shut-off point is reached. For example, to maintain a constant water tempera ture in a hot water boiler, the operating controller may be set to stop the burner when the boiler water temperature reaches 180 F. The high limit control should-then be set to stop the burner at a higher temperature, say 200 F. Primary Controls Primary controls provide for safe start and operation of the burner. They are somewhat different for each of the three types of fuel burners, and may also vary considerably between domestic and commercial-industrial burners. Domestic Burners. Primary controls for domestic burners are discussed in the following paragraphs 1 to 3. 1. Stokers. Stokers are equipped with refueling, or holdfire, controls. These primary controls cause the stoker to in termittently feed a predetermined amount of coal during the long off periods of the operating controller to maintain igni tion and to reduce the possibility of having idle fuel beds damage stoker castings. In some cases the primary control system includes an out-fire feature which stops the feeding of coal into the firebox if the fire goes out.' 2. Oil Burners. Primary controls required for domestic oil burners depend on the type of burner. a. Domestic oil burners of the vaporizing type usually re quire a primary control that will (1) meter the correct . amount of oil into the burner to maintain the pilot flame; (2) regulate the required amount of oil into the burner for its high flame operation; and (3) completely shut off the flow of oil in the event of a flame failure. b. Domestic oil burners of the pressure-atomizing and verti cal rotary types require a primary control that will (1) energize the burner motor and electric ignition; (2) test for . the establishment of main burner flame and stop the burner and ignition if flame has not been established within a set time; (3) cut off the ignition after a safe burner flame has been established (does not apply to constant ignition type burners); (4) monitor tbe flame continuously during burner operation and stop the burner in the event of flame failure (manual reset is necessary following a flame failure lock out) ; and (5) stop tbe burner when either the operating controller or limit control requires it. 3. Gas Burners. Domestic gas burners of the atmospheric type require a primary control (to insure safe starting and operating conditions) that will (1) prevent opening of the gas valve unless the pilot is operating properly, and (2) stop tbe flow of gas through the burner if safe ignition conditions do not prevail at all times. For domestic power burners a relay is normally added to operate the blower motor as well as the gas valve which may be energized simultaneously with the blower motor or following a slight delay. Commercial and Industrial Burners. The primary controls for commercial and industrial burners, often referred to as programming flame safeguard controls, may in some cases be similar in function to those used on domestic burners but involve two major differences: (1) the use of a means of flame sensing which shortens the speed of response to a flame fail ure to within four seconds, and (2) the more elaborate se quencing of the primary control. The entire control system also becomes more elaborate, requiring added controls to provide the additional sequences and safety interlocks. Pri mary controls for commercial and industrial burners are discussed in the following paragraphs 1 to 7. 1. Pressure-atomizing type burners for commercial and industrial applications use flame safeguard controls to pro vide similar functions to those outlined for domestic burners of the same type except for the speed of response to a flame failure. However, in addition, they may also involve preignition purge, timed ignition (electric ignition, or, on the larger burners, spark-ignited gas pilots). In addition, the over-ail control system, would also include safety oil shutoff valves, provision for low fire start and, on the larger burners, a means of modulating the firing rate. 2. Horizontal rotary type oil burners for commercial and industrial installations use still more elaborate flame safe guard controls. Tbe sequencing included in the flame safe guard control may provide for (1) a pre-purge period before energizing the spark-ignited gas pilot; (2) proving the pres ence of proper ignition means before permitting the flow of oil to the main burner; (3) proving the presence of a stable main burner flame within a pm-determined timed trial for ignition; (4) shutting off the ignition means after a pre determined length of time; (5) monitoring the main burner flame during its entire period and shutting off the main fuel supply within four seconds in the event of a flame failure; (6) stopping the flow of oil when the operating or limiting control requires it, but continuing the burner motor opera tion for a pre-determined purging period after the rnnin fuel valve has been closed; and (7) locking out on flame failure shutdown (requiring a manual reset prior to a restart). 3. When steam, air, or mechanical atomizing burners are automatically controlled, the sequence of control is similar to that described for horizontal rotary-cup burners. 4. Commercial and industrial gas burners of the atmos pheric type may be equipped with a constantly burning gas pilot or have provision for establishing a spark-ignited gas pilot on each cycle--the pilot continuing to bum during the entire burner operating cycle. This will require a flame safe guard control with the following sequence: (1) on a call for heat, provision is made to prove the presence of an existing pilot or to establish and prove an adequate spark-ignited