Waste Heat Recovery Calculator

Calculator Inputs

Enter your stream conditions and recovery assumptions.

All temperatures are in °C. Cp is in kJ/kg·K.

Heat Source

Choose the closest match for reporting.

Working Fluid

Used for labeling; Cp is entered directly.

Mass Flow Rate (kg/s)

For gases, use mass basis (not volumetric).

Specific Heat Cp (kJ/kg·K)

Typical: air ~1.0, water ~4.18.

Inlet Temperature (°C)

Upstream temperature before recovery.

Outlet Temperature (°C)

Downstream temperature after recovery.

Heat Exchanger Effectiveness (0–1)

Accounts for approach limits and fouling margin.

Utilization Factor (0–1)

Fraction of time heat has a useful sink.

Operating Hours per Year

Use production hours, not calendar hours.

Boiler / Heater Efficiency (0–1)

Translates useful heat to displaced fuel input.

Fuel Price (per kWh)

Enter your effective fuel cost on energy basis.

CO₂ Factor (kg per kWh fuel)

Use site-specific or default factor.

Include Parasitic Loads?

Fans/pumps can materially impact net savings.

Parasitic Power (kW)

Estimated auxiliary power for recovery system.

Electricity Price (per kWh)

Used only when parasitic loads are included.

Reset Tip: Run once, then download CSV/PDF from results.

Example Data Table

Sample input set and computed outcomes for quick validation.

Scenario	m (kg/s)	Cp (kJ/kg·K)	T_in (°C)	T_out (°C)	Effectiveness	Hours/yr	Recovered (kW)	Net Energy (kWh/yr)	Annual Savings
Exhaust stream baseline	1.50	1.05	350	180	0.75	6000	200.81	1,072,388	$99,490.59
Higher effectiveness	1.50	1.05	350	200	0.85	6000	200.81	—	—

Rows marked with “—” are intentionally incomplete for practice.

Formula Used

This tool uses a practical energy balance and optional adjustments for utilization and auxiliary loads.

Thermal Power Recovered

Q̇_th (kW) = ṁ (kg/s) × C_p (kJ/kg·K) × (T_in − T_out) × ε

With Cp in kJ/kg·K, the result is kJ/s = kW.

Annual Net Energy

E_net (kWh/yr) = (Q̇_th × u × h) − (P_aux × h)

u = utilization factor, h = hours/year, P_aux = parasitic power.

Fuel Savings, Cost, and Emissions

Fuel saved (kWh/yr) = E_net / η

Annual savings = (Fuel saved × fuel price) − (P_aux × h × electricity price)

CO₂ avoided (kg/yr) = Fuel saved × emission factor

η is boiler/heater efficiency translating useful heat to displaced fuel input.

How to Use This Calculator

Enter mass flow, Cp, and the temperatures before and after recovery.
Set effectiveness based on expected exchanger performance and fouling.
Adjust utilization to reflect whether a constant heat sink exists.
Add parasitic power and electricity price for a net savings view.
Click Calculate to show results above the form.
Use Download CSV or Download PDF for reporting.

Where waste heat is typically found in plants

High-grade losses leave with flue gas, engine exhaust, and furnace stacks, above 200°C. Medium-grade sources include compressor aftercoolers and jacket water, often 60–120°C. Low-grade losses can appear as wastewater or ventilation air, 25–60°C. The temperature level drives technology choice: recuperators and economizers suit high-grade streams, while heat pumps or heat exchangers to process water are better for low-grade recovery.

Why effectiveness matters more than nameplate temperature

Effectiveness captures real exchanger limits: approach temperature, fouling, bypassing, and control range. Two systems with the same inlet temperature can deliver very different useful heat if one is constrained by pinch points. In this calculator, effectiveness scales the ideal power m·Cp·ΔT. For early screening, 0.60–0.80 is common for compact gas exchangers, and 0.80–0.90 is achievable with clean liquid-to-liquid service and stable flow.

Translating recovered heat into annual energy value

Annual energy depends on operating hours and whether a consistent sink exists. Utilization below 1.0 reflects batch processes, seasonal heating demand, or mismatched schedules between source and sink. A 200 kW recovery project at 6,000 hours yields 1.2 million kWh/year gross, but a utilization of 0.65 reduces useful delivery to 780,000 kWh/year. This is why the utilization input often dominates business cases for otherwise attractive streams.

Including parasitic loads for a realistic net benefit

Fans and pumps can materially change net savings, especially for gas-side pressure drop or long piping runs. This tool subtracts auxiliary energy and, if selected, its electricity cost from the fuel savings. For example, a 5 kW fan running 6,000 hours consumes 30,000 kWh/year. At $0.12/kWh, that is $3,600/year, which can offset a noticeable fraction of small recovery projects and alter payback comparisons between design options.

Using the outputs for feasibility and prioritization

Use recoverable power to size heat exchangers, thermal storage, or downstream loads. Net recovered energy supports utility displacement planning and sustainability reporting, while fuel saved converts useful heat into avoided fuel input using boiler efficiency. Compare annual savings across candidate sources, then prioritize projects with high utilization, stable ΔT, and manageable parasitic loads. For later stages, validate Cp, mass flow, and outlet limits with field measurements and vendor heat balances.

FAQs

1) What does the effectiveness value represent?

It is a practical multiplier for exchanger performance, capturing approach temperature limits, fouling allowance, bypassing, and control range. Use conservative values when data is uncertain.

2) Should I enter Cp for the hot stream or the recovery fluid?

Enter Cp for the hot stream being cooled. The calculator estimates available heat removed from that stream, which is what can be transferred to a sink.

3) Why must inlet temperature be higher than outlet temperature?

Waste heat recovery extracts heat, so the source stream should cool across the recovery device. If your process heats up, you are modeling a different system.

4) How do I choose a utilization factor?

Start from the fraction of operating time a useful sink exists. Reduce it for seasonal demand, batch cycles, or load mismatch. Increase it if storage or multiple sinks are feasible.

5) What does boiler efficiency change in the results?

It converts useful recovered heat into avoided fuel input. Lower efficiency means more fuel is needed for the same delivered heat, so recovered heat displaces more fuel energy.

6) Can I use this for steam generation or ORC power?

Yes for screening, by treating the recovered heat as useful energy. For steam or power cycles, add cycle efficiency and additional parasitic loads outside this model.