Consulting Specifying Engineer April 2015-CSE : Page 72

Selecting a boiler above the de-aerator temperature, reducing the amount of fuel needed to turn feedwa-ter into steam. A feedwater economizer is a flue gas to feedwater heat exchanger that transfers flue gas heat into boiler feedwa-ter before it enters the boiler steam drum. This capture of waste heat from the flue gas increases boiler efficiency by reduc-ing the amount of energy (Btu’s) required to bring the feedwater to steam tempera-reduces the heat removed from the flue gas, raising the temperature to above con-densation levels. If the boiler system will be operated normally at less than full load, consideration should be given to flue gas temperature control as discussed above. Burner design for packaged boilers varies with boiler output. Packaged boil-ers can be fueled by a range of fuels, including natural gas, #2 fuel oil, pro-have the forced draft blower as a separate unit. The forced draft fan motor can range from 10 to 1000 hp, depending on boiler capacity. The larger fan motor voltage normally will be medium-voltage design to reduce wiring size resulting in first-cost savings. The forced draft fan is an inte-gral part of the boiler control strategy, and consideration of variable frequency drive versus damper jackshaft control is needed. Boiler size, operator capability, and sys-tem compatibility will help determine the boiler control strategy. The common control strategy imple-mented on many new central boiler (20,000 to 300,000 lb/h of steam) instal-lations is fully metered cross-limited operation with oxygen trim where boil-er combustion is controlled by metered steam header pressure, fuel flow, and airflow. The fully metered cross-limit-ed control scheme improves control of air-to-fuel mixture through metering of these parameters. The oxygen trim schemes allow for continuous control of excess oxygen in the flue gas, which is accomplished by optimizing combustion air into the boiler. Alternative to fully metered cross-limited boiler control is parallel position control where preset firing curves are developed during boiler startup and commissioning, and used to Modulating the flow of feedwater through the economizer reduces the heat removed from the flue gas, raising the temperature to above condensation levels. ture and pressures. The economizer can be integral to the boiler, a box-or round-style exchanger assembly installed into the boiler stack, or it can be a cartridge-type insert into the exhaust ductwork of the boiler. The economizer can be designed as parallel or counter flow, the same as most heat exchangers. The counter flow design will have feed water flowing in the opposite direction of the flue gas. The economizer manufacturer will provide a design based on the end user’s flue gas input temperature, desired output tempera-ture, and packaged boiler design. Economizers Economizers can be fitted to both water tube and fire tube boilers and can be integral to the boiler design. Econo-mizer design typically reduces the exhaust gas temperature to near 270 F; however, decreasing the temperature too far can result in condensation of the flue gas, which may create acidic conden-sate depending on fuel gas properties. Low flue gas temperatures also can cre-ate localized spots of condensation and potential corrosion if not controlled. One way flue gas temperature can be controlled is by throttling the flow of feedwater through the economizer. One method of controlling feedwater flow through the economizer is through a three-way temperature control valve that is modulated based on economizer flue gas outlet temperature. Modulating the flow of feedwater through the economizer pane, syngas, biogas, and #6 fuel oil. Burners can be designed to be dual-fuel units with a primary fuel and secondary or backup fuel. Fuel trains and control systems, regardless of fuel, need to be designed to meet applicable insurance company requirements (Hartford Steam Boiler Inspection and Insurance Co., FM Global, etc.) as well as NFPA 85: Boiler and Combustion Systems Hazards Code or ASME CSD-1 to ensure safe operation of the fuel train and boiler burner. Smaller burners for boilers up to approximately 65,000 lb/h will have the burner forced draft blower mounted directly to the wind-box of the boiler, while larger boilers will Industrial boiler types Fire tube n Condensing (alloy construction) n Non-condensing n Steam generation n Hot water generation Water tube n Steam generation n Hot water generation n Superheater n O, A, and D styles Sizes n 1,000 to 60,000 lbs/hr Sizes n 5,000 to 250,000 lbs/hr n Field or factory erected Economizer Low NOx burners Fuels n Fuel oil n Natural gas n Biogas Economizer Modular Low NOx burners Fuels n Fuel oil n Natural gas n Biogas Table 1: This outlines the industrial packaged boiler types, sizes, and system compo-nent options. Courtesy: Stanley Consultants 72 Consulting-Specifying Engineer • APRIL 2015

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