CCPP HRSG Efficiency Calculation. Why mas flow rate can be ignored

A typical formula used in combined Cycle Power Plants for HRSG Efficiency calculation is

Where:

• Tinlet = Inlet Gas Temperature (°C)
• Toutlet = Outlet Gas Temperature (°C)
• Tambient = Ambient Temperature (°C)

Explanation

Inlet Gas Temperature (HRSG)

• This is the temperature of the exhaust gas entering the HRSG.
• Higher values provide more thermal energy for recovery.

Outlet Gas Temperature (HRSG)

• This is the temperature of the gas leaving the HRSG.
• Lower outlet temperatures indicate better heat transfer to the water/steam system.

Ambient Temperature

• Acts as a reference for measuring the temperature drop in the HRSG.
• A cooler ambient environment improves heat transfer efficiency.

Why Mass Flow Rate is Not Considered in This Formula

The given formula is based on temperature differences, which inherently capture the heat transfer effectiveness — a core indicator of HRSG efficiency. Here's why mass flow rate is excluded:

1. HRSG Efficiency is a Thermal Efficiency Concept

• The formula represents thermal efficiency, defined as the percentage of heat energy successfully transferred from exhaust gases to the steam system.
• Since this is a relative efficiency calculation, it focuses on temperature gradients rather than absolute heat quantities.
• Mass flow rate affects the total energy transferred (in kJ or BTU), but efficiency remains a ratio of energy recovered versus energy available.

2. Constant Mass Flow Assumption

• In practical scenarios, mass flow rate is often assumed to be relatively stable for a given operating condition.
• The temperature drop between inlet and outlet indirectly reflects the energy transfer — provided the mass flow rate is within expected limits.

3. Efficiency is a Performance Ratio, Not an Absolute Energy Measure

• Efficiency calculations focus on how well the available energy is utilized.
• Including mass flow rate would shift the calculation from efficiency to total heat transfer capacity, which is a different metric.

When is Mass Flow Rate Important?

Mass flow rate is critical in:

? Calculating Total Heat Absorbed:

Q=m˙×Cp×(Tinlet?Toutlet)

Where:

• Q = Total heat absorbed (kW or MW)
• m˙ = Mass flow rate (kg/s)
• Cp = Specific heat capacity of flue gas

? Determining Steam Generation Capacity:

• Higher gas mass flow rates increase the total energy available for heat recovery.
• Any deviation in mass flow rate directly affects HRSG performance.

? Performance Degradation Analysis:

• Reduced mass flow rates may indicate fouling, blockages, or equipment deterioration.

The provided formula is ideal for quick efficiency assessment using temperature data alone.

For a deeper understanding of energy transfer rates, mass flow rate must be included in heat transfer calculations or energy balance analysis.

Recommendation:

If you aim to analyze total heat transfer, steam generation rate, or fuel utilization, incorporating mass flow rate into your calculations will provide a more comprehensive view of HRSG otherwise, simple temperature diff is sufficient.

Would you like a combined formula that integrates mass flow rate for a detailed heat recovery analysis? Tell me in comments.