CH4 + 2O2 + 8N2 = CO2 + 2H2O + 8N2 + Heat
Using the concept of the lb-mole (pound molecular weight), it can be shown that:
16 lb CH4 + 64 lb O2 + 224 lb N2 will produce 44 lbs. CO2 + 36 lb H2O + 224 lb N2 + Heat
The result from dividing by 16 is:
1 lb CH4 + 4 lb O2 + 14 lb N2 will produce 2.75 lb CO2 + 2.25 lb H2O + 14 lb N2 + Heat
The equations demonstrate that 1 lb of methane (CH4) theoretically requires 18 lbs. of air to completely burn it, assuming perfect mixing and combustion. Because gas flow rates are usually measured in cubic feet and fans are volumetric devices used to deliver the combustion air, conversions are made from pounds to cubic feet. As a "rule of thumb," 1 ft3 of natural gas theoretically requires 10 ft3 of air for complete combustion at 60°F and 14.7 psi under perfect conditions. Of course, this never happens, therefore burner combustion air fans may be sized for 25% excess air, in which case the fan would deliver 12.5 ft3 of air per 1 ft3 of natural gas. Additionally, many burner manufacturers will size the combustion air fans to provide sufficient air to operate properly up to 2,000 feet in altitude, in which case the sea level capacity will be increased by an additional 6 1/4%. Larger fans are typically required in areas 2,000 feet above sea level.
Obviously, fan selection is a function of the burner design process, and provided the burner manufacturer knows the altitude at which the unit is to be installed, there should not be a fan capacity problem.
The amount of air required for ventilation and cooling in the boiler room is determined by the heat loss from the boiler shell or jacket, the boiler piping, breechings, stacks, and any other heat-generating equipment present in the boiler room. Heat loss from the boiler jacket could range from 1/2% to 4% of the boiler output, depending upon operating pressure or temperature, boiler size, type of construction, and insulation thickness. Typically, the larger the boiler the lower the loss as a percentage of boiler output. Boiler room temperatures at the burner fan inlet should be controlled between 50°F and 100°F in order to limit variability in the amount of combustion air delivered to the burner.
The venting system can affect both the amount of air required in the boiler room and the performance of the combustion air fan regarding its ability to deliver the required amount of air flow. For example, draft hoods and barometric dampers, although necessary in certain installations, remove air from the boiler room that would otherwise be available for combustion. Therefore, these must be accounted for when determining the total air flow required into the boiler room. The installation of flue gas economizers, particularly on a retrofit basis, will add to the pressure loss through the venting system. If the added pressure loss is large enough, it may detract from overall combustion air fan performance. The impact of adding an economizer or fan performance must be reviewed prior to such a change being made.
Automatic draft control systems may also affect combustion air supply if not adjusted properly or installed correctly. If the automatic breeching or stack damper is not set to open sufficiently, then additional back pressure is imposed on the combustion air fan, resulting in a reduction of air flow. The position of stack dampers should be checked prior to ignition by means of a limit switch on the driven part of the mechanism.
Heat transfer-enhancing turbulators may have the same effect as adding an economizer because they can add to the pressure loss through the boiler and thereby detract from combustion air fan performance. Whenever turbulators are added to the flue gas side of a boiler, a stack measurement of excess oxygen and carbon monoxide should be made before and after installation to determine if there is any impact on fan output. Any necessary burner adjustments should be made to bring burner performance back to acceptable levels.
Exhaust fans in a boiler room can pose serious problems to the combustion air supply unless the proper steps are taken to provide similar amounts of make-up air. Manufacturing facilities where combustion air is drawn from the plant itself rather than from outside can quite easily become unbalanced as various processes are added over a period of time.
In one situation, I was asked to correct a combustion problem at a certain facility where modifications had been made throughout the years to plating baths, spray painting booths, and environmental control systems. The exhaust flow was found to be 350,000 scfm compared to what was presumably the original make-up air flow of 125,000 scfm. Management at this particular plant had expressed concern that personnel might experience bodily injury by doors slamming shut because of the pressure differentials that existed in certain areas. This was a good indicator that combustion air problems existed.
In other cases, exhaust fans have been added by boiler operating personnel to provide a more livable temperature environment in the boiler room without considering the other implications. The graph above shows the relationship between excess air and carbon monoxide levels emitted from an atmospheric gas burner equipped with a power venter. The normal setup as left by the installer had a carbon monoxide level of 9 ppm. The effect of the exhaust fan was to reduce excess air and increase carbon monoxide to approximately 70 ppm. The situation was aggravated when the combustion air intakes were blocked because of fumes outside the boiler room. At this point, carbon monoxide production started to increase rapidly at about 10% excess air. The result? The heat exchanger became plugged with soot, causing a decrease in excess air levels and an increase in the release of carbon monoxide into the boiler room. Fortunately there were no fatalities, however, carbon monoxide poisoning was evident.
Air Opening Sizes
In their various forms, building codes and standards define the manner in which combustion air openings should be sized for fuel burning equipment. In general, they follow the requirements of the National Fuel Gas Code, NFPA 54, and the Standard for the Installation of Oil Burning Equipment, NFPA 31.
Typically, two permanent combustion air openings are required in an enclosure in which fuel-burning equipment is installed. One opening should be located 12 inches from the ceiling and the other within 12 inches of the floor. This will provide air circulation in to and out of the equipment room under normal conditions.
The actual sizing of the combustion air openings depends upon the manner in which the air flows from the outside of the building to the boiler room. For example, if the air is drawn in through horizontal ductwork from outdoors, then each opening shall be sized to provide a minimum free area equal to one square inch for every 2,000 Btu/hour of input of all fuel-burning equipment within the enclosure. Alternatively, if the combustion air openings vent outdoors (through an outside wall) or are supplied through vertical ducts, then each opening shall be sized to provide a minimum free area equal to one square inch for every 4,000 Btu/hour of input of all fuel-burning equipment in the enclosure. When ducts are used, they must have a minimum cross-sectional area equal to the free area of the opening.
It has been noted in some installations that a single vertical duct has been installed with combustion air openings located 12 inches from the floor and ceiling (in the same duct). In one known instance of this type, the duct was not sized properly for one opening, let alone two. This obviously did not meet code requirements, and a carbon monoxide poisoning resulted.
Codes allow for specially engineered combustion air supply installations in lieu of those described above if they supply adequate air for combustion, ventilation, and dilution of flue gases, and they are approved by the authority having jurisdiction. These may take the form of automatically operated louvers that shut during burner off-cycles. In cases where louvers of this type are used, they must be proven open prior to the start of the ignition cycle by means of a limit switch on the driven member. Of course louvers may become quite large and in cold weather the boiler room can be cooled quite substantially by the significant quantity of combustion air flowing from outdoors into the boiler room. The proper resolution for this situation is an air heater rather than such tactics as nailing a 4 ft x 8 ft sheet of plywood over the louver assembly, as seen on several occasions. Fouling (sooting) of the flue-gas side of the boiler is the least one can expect from this type of action.
Maintenance and Housekeeping
Maintenance and housekeeping procedures play an important part in preserving the adequacy of the combustion air supply. A clean and tidy boiler room, particularly in the area of the combustion air intakes and the burner air inlet damper, is paramount in maintaining proper combustion. Such things as newspapers or animal hairs on fan inlet screens, dirt-encrusted fan blades, and birds' nests in unprotected stacks have all been seen to contribute to sooting and/or the generation of carbon monoxide at various times.
A periodic boiler flue-gas analysis is the best indicator that an adequate supply of combustion air exists. This, and any necessary burner adjustments, should be performed by a trained technician with the proper equipment to measure the amount of excess oxygen and/or carbon dioxide and ppm of carbon monoxide. In the case of an oil burner, the smoke spot number should be properly measured. People who do not have the equipment to make these measurements should not assume that "eyeballing" the flame is all that is necessary. In the absence of specific manufacturer's instructions regarding periodic testing and maintenance of the combustion system, guidelines can be found in ASME CSD-l, Controls and Safety Devices for Automatically Fired Boilers, Part CM - Testing and Maintenance.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.