Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSCOMBUSTION IN BOILERSPRINCIPLES OF COMBUSTIONWhen gas, oil, or other fuels are burned, several factors mustbe considered if the burning process is to be safe, efficient, andnot harmful to the environment. The burning process must:1. Provide enough air so that combustion is complete andundesirable amounts of carbon monoxide or otherpollutants are not generated.2. Avoid excess air in the fuel-air mixture which would resultin low efficiency.3. Provide complete mixing of air with fuel beforeintroducing the mixture into the firebox.4. Provide safety controls so that fuel is not introduced withoutthe presence of an ignition flame or spark and that flame isnot introduced in the presence of unburned fuel.5. Avoid water temperatures below the dewpoint of the fluegas to prevent condensation on the fireside of the boiler.– Steam or air atomizing burners use high pressure air or25 psig steam to break up the oil into fine droplets.For modulating or high/low flame control applications therotary or steam/air atomizing burners are most common.GAS BURNERSTwo typical types of gas burners are the atmospheric injectionburner and the power type burner. The atmospheric injectionburner uses a jet of gas to aspirate combustion air and iscommonly used in home gas furnaces and boilers. The raw-gasring burner (Fig. 42) is an atmospheric injection burner.Power burners (Fig. 43) use a forced-draft fan to thoroughlymix air and gas as they enter the furnace. Common power burnerapplications are commercial and industrial.FLUE GAS ANALYSISCombustion can be monitored by flue gas analysis. For largeboilers, over 1,000,000 Btuh, the analysis is typicallycontinuous. For small boilers, flue gas is analyzed periodicallyusing portable instruments.Flue gas composition analysis routinely measures the percentof CO 2 (carbon dioxide) or O 2 (oxygen), but not both. IdealCO2 is in the 10 to 12 percent range. Percent oxygen is themost reliable indication of complete combustion. The ideal O2concentration is in the three to five percent range. Lowerconcentrations are impractical and often unsafe. Higher O2concentrations mean that an excessive quantity of air is admittedto the combustion chamber and must be heated by the fuel.This excess air passes through the boiler too quickly for theheat to be efficiently transferred to the water or steam. Carbondioxide measuring instruments are simpler or lower cost thanO2 measuring instruments.The CO 2 or O 2 concentration plus stack temperature providea burner efficiency in percent either directly or by means ofcharts. This efficiency indicates only the amount of heatextracted from the fuel. It does not account for excess heatingof combustion air or losses from leaks or the boiler jacket.GASINLETA. BURNER ASSEMBLYA. BURNER DETAILSINSULATEDAIR REGISTERTILEC2902Fig. 42. Raw Gas Ring Burner.OIL BURNERSOil burners are usually of the atomizing variety, that is, theyprovide a fine spray of oil. Several types exist:– Gun type burners spray oil into a swirling air supply.– Horizontal rotary burners use a spinning cup to whirl oiland air into the furnace.DETAIL OF INDIVIDUALPORTFig. 43. Multiport Forced-Draft Gas Burner.C2903ENGINEERING MANUAL OF AUTOMATIC CONTROL306