Calculation method for waste heat recovery from exhaust gas

There are two main approaches to calculate the potential for waste heat recovery from exhaust gas:

1. Thermodynamic Approach:

This method uses the principles of thermodynamics to determine the theoretical maximum amount of heat that can be recovered. Here's what you need to consider:

• Mass flow rate (ṁ) of the exhaust gas (kg/s) - This can be obtained from engine specifications or measured with a flow meter.
• Specific heat capacity (Cp) of the exhaust gas (kJ/kg⋅K) - This value varies with temperature and needs to be obtained from tables or thermodynamic software for the specific gas composition of your exhaust.
• Inlet temperature (T_in) of the exhaust gas (°C) - Measured with a temperature sensor.
• Outlet temperature (T_out) of the exhaust gas after heat recovery (°C) - This is the desired temperature after heat is removed for your chosen application (e.g., preheating combustion air, generating hot water).

Heat recovery potential (Q) can be calculated using the following formula:

Q = ṁ * Cp * (T_in - T_out)

2. Simplified Approach:

This method provides a rough estimate and is easier to use for initial assessments. It assumes a specific percentage of the exhaust gas energy can be recovered. This percentage can vary depending on the engine type, operating conditions, and the chosen heat exchanger efficiency.

Estimated heat recovery (Q) can be calculated with:

Q = Exhaust gas energy content * Recovery factor

Exhaust gas energy content can be estimated by:

Exhaust gas energy content = Mass flow rate * Lower heating value (LHV) of the fuel

Lower heating value (LHV) is the amount of heat released during combustion when the water vapor formed condenses (available from fuel specifications).

Recovery factor is a percentage typically ranging from 20% to 50% depending on the engine type, operating conditions, and the chosen heat exchanger efficiency.

Important Notes:

• These calculations provide theoretical or estimated values. The actual heat recovery may be lower due to factors like heat exchanger inefficiencies and piping losses.
• The chosen outlet temperature (T_out) in the thermodynamic approach needs to be realistic based on the application and limitations of the heat exchanger.
• Safety considerations are crucial when dealing with hot exhaust gases. Always consult with a qualified engineer for designing and implementing a waste heat recovery system.

Additional factors to consider:

• Condensation: If the exhaust gas temperature drops below the dew point, water vapor will condense. This can release additional latent heat but requires proper condensate management.
• Fouling: Exhaust gas can contain contaminants that can foul heat exchanger surfaces, reducing efficiency. Regular cleaning or choosing appropriate materials may be necessary.

By understanding these methods and factors, you can calculate the potential for waste heat recovery from exhaust gas and assess its feasibility for your specific application.