Utility Boilers

A big energy saving and CO2 emission reduction could be reached by improving boiler efciency. Boiler

efciency is measured by industrial manufacturers in laboratories following standardized test procedures, but values reached in operation could differ a lot. The boiler efciency situation in Europe is reported in the product studies of Europe Economics [1]: “There is a

Giuseppe Toniato and Silvio Rudi Stella

34.1 Introduction ................................................................................................................................................................. 705 34.2 Boiler Efciency .......................................................................................................................................................... 706

34.2.1 Energy Losses Through the Chimney qsmoke ............................................................................................ 707 34.2.1.1 Smoke Temperature and Excess of Air ..................................................................................... 707 34.2.1.2 Condensation Index .................................................................................................................... 710 34.2.1.3 Other Parameters Affecting Losses at the Chimney .............................................................. 711

34.2.2 Envelope Energy Losses qenvelope ................................................................................................................. 712 34.2.3 Auxiliary Electrical Consumption qaux ...................................................................................................... 713 34.2.4 Boiler Efciency in Field: Ambient Conditions and Transient Operations ......................................... 713 34.2.5 Boiler Efciency and Emissions ................................................................................................................. 714

34.3 Utility Boilers Main Components ............................................................................................................................. 715 34.3.1 Burner ............................................................................................................................................................. 715 34.3.2 Combustion Chamber.................................................................................................................................. 715 34.3.3 Heat Exchangers ........................................................................................................................................... 717 34.3.4 Boiler Controls .............................................................................................................................................. 717

34.4 Utility Boiler Test Methods........................................................................................................................................ 718 34.4.1 Product Categories ....................................................................................................................................... 718

34.4.1.1 Output Function .......................................................................................................................... 718 34.4.1.2 Capacity and Conguration Storage ......................................................................................... 719 34.4.1.3 Fuel Conguration....................................................................................................................... 720 34.4.1.4 Boiler Flue Gas/Intake Air System ........................................................................................... 720 34.4.1.5 Technology Boiler ........................................................................................................................ 721 34.4.1.6 Power Class ................................................................................................................................... 721 34.4.1.7 Boiler/Burner Conguration ..................................................................................................... 721 34.4.1.8 Boiler Water Temperature Control ............................................................................................ 721 34.4.1.9 Burner Combustion Control ...................................................................................................... 722 34.4.1.10 Installation .................................................................................................................................... 722

34.4.2 EN Product Standard ................................................................................................................................... 722 34.4.2.1 Introduction .................................................................................................................................. 722 34.4.2.2 Classication of Gas Appliances by Type ................................................................................ 722

34.4.3 Eco Design Preparatory Study: Boilers ..................................................................................................... 722 34.4.3.1 Assessment Procedure ................................................................................................................ 723 34.4.3.2 Test Requirement ......................................................................................................................... 723 34.4.3.3 Other Data Input .......................................................................................................................... 724 34.4.3.4 Labeling......................................................................................................................................... 724

34.5 Appendix ..................................................................................................................................................................... 724 References ................................................................................................................................................................................ 728

large gap between the theoretical efciency and the real efciency of boilers. The majority of boilers are oversized (200% → 400%). The part load of condensing boilers is assumed to be 38.8%, but in tests the real average part load is measured at about 9%. The (theoretical) efciency of most existing boilers which are older than, e.g. 15 years, is probably 10-15% lower than the newest condensing boilers available.” To x the boiler efciency concept we have to image a volume around the boiler, measure all energies in and out, and relate energy output to energy input. This seems quite simple, but going deeper the situation is more and more complex. First of all boiler efciency is often confused with combustion efciency. In a heating boiler a fuel is rst burnt, then the produced heat is transferred to a uid (air or water). The rst process controlled by the burner takes place in the boiler combustion chamber; here the produced heat starts to be transferred to the intermediate uid through conduction, convective, and radiative heat process. Downstream the combustion chamber smoke enters a heat exchanger where a prevalent convection mechanism completes the heat transfer to the intermediate uid. Two efciency parameters can be easily identied: 1) the combustion efciency that measures the combustion process or the ability of the burner to burn the fuel and 2) the efciency of the heat transfer from smoke to the water or air. For evaluating heat transfer efciency we have to consider that this process takes place both in the combustion chamber and downstream in the heat exchanger. Combustion efciency depends upon burner and combustion chamber design and is related to unburned fuel and excess air in the exhaust. The combustion chamber design inuences the fraction of heat that is transferred here to the intermediate uid and the smoke temperature at the inlet of the heat exchanger, whose effectiveness is related to the temperature variations of the smoke passing through it. Both combustion efciency and heat exchanger effectiveness contribute to boiler efciency.