Waste Heat Recovery Savings Calculator

Industrial Waste Heat Recovery Estimation

This online tool allows plant engineers, energy managers, and process designers to calculate the potential energy, financial, and carbon savings from recovering waste heat from industrial exhaust stacks. Upstream thermal processes—such as steam boilers, direct-fired kilns, industrial ovens, spray dryers, and thermal oxidisers—frequently discharge flue gases at elevated temperatures. By installing a gas-to-liquid or gas-to-gas heat exchanger, this thermal energy can be redirected back into the process, used for space heating, or preheating boiler feed water.

Governing Thermodynamic Equations

The recoverable thermal power is calculated using the mass flow rate of the exhaust gas, its specific heat capacity, and the temperature drop across the recovery heat exchanger. First, the normal volumetric flow rate is converted to a mass flow rate:

Where ${\dot{V}}_N$ is the exhaust flow in ${\text{Nm}}^3/\text{h}$ and ${\rho }_N$ is the normal density of the flue gas (typically adjusted based on fuel type and excess oxygen levels). The thermal power recovered is then determined by:

Where $\dot{Q}$ is the recovered heat in $\text{kW}$, $c_p$ is the specific heat capacity in $\text{kJ/kg}\cdot \text{K}$, $T_{in}$ is the exhaust inlet temperature in ${}^{\circ }\text{C}$, and $T_{out}$ is the target outlet temperature in ${}^{\circ }\text{C}$.

To calculate the annual financial savings, the tool accounts for the efficiency of the primary heating system that the recovered heat replaces:

Where $\text{Run Hours}$ is the annual operating hours, ${\eta }_{boiler}$ is the decimal efficiency of the displaced boiler or burner, and $\text{Fuel Price}$ is expressed in $\text{£/kWh}$.

Inputs, Outputs, and Key Assumptions

The calculator requires the following inputs:

• Exhaust Stream Parameters: Fuel type (e.g., Natural Gas, LPG, Kerosene), dry exhaust ${\text{O}}_2$ percentage, exhaust inlet temperature (${}^{\circ }\text{C}$), and volumetric flow rate (${\text{Nm}}^3/\text{h}$).
• Recovery Targets: Target outlet temperature (${}^{\circ }\text{C}$) or exchanger effectiveness (%), and cold-side fluid inlet temperature (${}^{\circ }\text{C}$).
• Economic Parameters: Annual run hours ($\text{h/yr}$), displaced boiler efficiency (%), fuel price ($\text{p/kWh}$), and optional capital cost ($\text{£}$).

The tool outputs the recoverable thermal power ($\text{kW}$), annual fuel cost savings ($\text{£/yr}$), annual ${\text{CO}}_2$ emissions reduction ($\text{t/yr}$), and the simple payback period in years.

Worked Example

Consider an industrial baking oven burning natural gas with an exhaust flow rate of $5000{\text{Nm}}^3/\text{h}$ at $250{}^{\circ }\text{C}$. The target stack-out temperature is set to $120{}^{\circ }\text{C}$ to avoid acid dew point condensation. The flue gas density is assumed to be $1.293{\text{kg/Nm}}^3$ and $c_p$ is $1.05\text{kJ/kg}\cdot \text{K}$.

If the plant operates for $8000\text{h/yr}$, displacing a boiler with $80\%$ efficiency using fuel priced at $4.5\text{p/kWh}$ ($\text{£}0.045/\text{kWh}$):