0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Steam Output

Figure 27.5: Example Boiler Performance Curve from Short Term Monitoring.104

Direct Method

The direct method (i.e., the input-output method) is the simplest method to determine boiler efficiency. In this method, the heat supplied to the boiler and the heat absorbed by the water in the boiler in a given time period are directly measured. Using the direct method, the efficiency of a non-reheat boiler is given by:108

where

Qa moho mihi =

Vfuel

HHV Qc heat absorbed (Btu/hr) = Smoho - Sm^hj mass flow-enthalpy products of working fluid streams leaving boiler envelope, including main steam, blowdown, soot blowing steam, etc.

mass flow-enthalpy products of working fluid streams entering boiler envelope, including feedwater, desuperheating sprays, etc.

heat inputs (Btu/hr) = Vfuel x HHV+Qc volumetric flow of fuel into boiler (scF/ hr)

fuel higher heating value (Btu/SCF), and heat credits (Btu/hr). Heat credits are defined as the heat added to the envelope of the steam generating unit other than the chemical heat in the fuel "as fired." These credits include quantities such as sensible heat in the fuel, the entering air and the atomizing steam. Other credits include heat from power conversion in the pulverizer or crusher, circulating pump, primary air fan and recirculating gas fan.

Direct Heat Loss Method in the direct heat loss method the boiler efficiency equals 100% minus the boiler losses. The direct heat loss method tends to be more accurate than the direct method because the direct heat loss method focuses on determining the heat lost from the boiler, rather than on the heat absorbed by the working fluid. The direct heat loss method determines efficiency using the following:109

Qloss x 100

where

0 0

Post a comment