Heat rate

Heat rate is the amount of fuel energy consumed to produce one unit of electrical output. It is commonly expressed as BTU/kWh in US practice or kJ/kWh elsewhere. Lower heat rate means better efficiency.

How it is used

Power-plant operators use heat rate to compare actual performance with design, dispatch assumptions and post-maintenance guarantees. It can be calculated as gross or net heat rate, depending on whether auxiliary power is subtracted from output. It can also be reported at different loads because turbines, boilers and HRSGs have load-dependent efficiency.

Fouling connection

Fouling raises heat rate by reducing useful heat absorption and increasing auxiliary losses. An economiser with ash bridges sends hotter gas downstream and preheats feedwater less effectively. A fouled air heater lowers combustion-air temperature and raises fan power. Fouled superheaters or reheaters can force attemperation changes or lower turbine efficiency. High pressure drop in convective passes also consumes fan margin.

Measurement and diagnosis

Heat-rate diagnosis compares fuel flow, fuel heating value, generator output, steam temperatures, feedwater temperatures, stack temperature, oxygen, auxiliary loads and ambient conditions. Fouling should be separated from condenser performance, turbine degradation, control settings, fuel quality and instrumentation drift. Short tests can mislead if load or ambient conditions are unstable.

Acoustic cleaning relevance

Sonic horns are often justified by heat-rate recovery rather than by cleaning appearance. If horns keep heat-transfer surfaces near clean condition, they can reduce stack losses, fan power and derates. The strongest projects establish a clean baseline, trend pressure drop and temperatures, then compare fuel use before and after acoustic cleaning under similar load.

Verification notes

A heat-rate claim should be normalised for load, ambient temperature, fuel heating value and operating mode. A simple before-and-after fuel bill can be distorted by dispatch changes. Better evidence links cleaning to measured changes in stack temperature, pressure drop, steam temperatures, fan power and unit output. For acoustic cleaning projects, the commissioning plan should state which tags will prove benefit and what clean or pre-installation baseline will be used.

Measurement and interpretation

Heat rate is a performance metric, but it is easy to misuse if the boundary is unclear. A plant may report gross turbine heat rate, net plant heat rate, boiler heat rate or a process-specific fuel-to-output number. Each boundary treats auxiliary power, start-up fuel, parasitic load, steam export and fuel moisture differently. Meaningful comparison therefore requires the same load, ambient condition, fuel basis and correction method.

Operators use heat rate to detect efficiency loss from fouled heat-transfer surfaces, excess air, air-heater leakage, condenser backpressure, steam leaks, turbine degradation, poor combustion, high auxiliary load or derated equipment. A small deterioration can be commercially significant because it repeats every operating hour. However, a heat-rate change should be reconciled with instruments: fuel flow, calorific value, steam flow, temperatures, pressures, generator output and ambient data.

Fouling and cleaning relevance

Fouling raises heat rate when it forces higher stack temperature, more fan power, reduced steam temperature, extra duct firing or lower boiler efficiency. Acoustic cleaning can contribute to heat-rate improvement where it keeps economisers, air heaters, generating banks, SCR modules or hoppers from accumulating dry ash that restricts heat transfer or flow. The effect should be measured through before-and-after trends at comparable load rather than assumed from cleaning activity alone.

Heat rate also helps decide whether a cleaning project is worth doing. If the main loss is condenser vacuum, turbine seals or fuel quality, an ash-cleaning system will not fix it. If the trend correlates with rising gas-side pressure drop and stack temperature, online cleaning may protect both efficiency and availability.

Maintenance use

Heat-rate trending is most useful when it is tied to maintainable causes. A cleaning outage, air-heater seal repair, condenser cleaning or burner tune should leave a visible signature if it affects the selected boundary. If no signature appears, the assumed loss mechanism may be wrong or hidden by load and ambient corrections. This is why performance engineers often pair heat-rate review with pressure-drop, temperature and inspection data.