Waste Heat Recovery: Match Available Heat With Useful Demand
A waste heat recovery boiler transfers energy from a hot process or exhaust stream into water or steam. Its output depends on the upstream heat source as well as the boiler and steam system.
Assess the source and the useful thermal demand together. Recovered heat has practical value when its temperature and availability match a real process requirement. A change in steam production may reflect a change in the upstream machine rather than a boiler fault.
Reference: DOE — Combined Heat and Power Systems Technology Assessment.
Worked Example: Available Gas-Side Sensible Heat
Illustrative example: these assumed values demonstrate arithmetic, not actual plant results or recommended operating targets.
For a simplified estimate:
Heat transfer rate = gas mass flow × average specific heat × gas-temperature reduction.
Assume 10 kg/s, average specific heat 1.1 kJ/kg·K, and a temperature reduction of 200 K:
10 × 1.1 × 200 = 2,200 kW.
If that entire rate were transferred into an illustrative water-to-steam enthalpy rise of 2,500 kJ/kg, the corresponding steam rate would be 0.88 kg/s, or 3,168 kg/h.
This is an idealized balance, not a guaranteed production rate. Heat losses, blowdown, changing gas properties, heat-transfer limitations, and actual operating conditions alter the result.
Interpret a Change in Output
Record upstream load, gas conditions, steam and feedwater conditions, relevant pressure drops, and maintenance events. Compare similar operating periods before attributing reduced output to fouling.
Do not adopt a universal minimum exhaust temperature from a generic article. Fuel composition, corrosion risk, materials, and equipment design determine acceptable limits. Similarly, upstream equipment constraints can restrict allowable exhaust backpressure.
Next step: Reconcile the source-side heat estimate with measured useful output and the equipment's operating envelope.
Record observations only where it is safe. Inspection inside equipment, electrical testing, chemical work and changes to protective controls require qualified personnel, approved isolation and the installed equipment manual. Never bypass a trip or repeatedly restart unsafe equipment. Engineering disclaimer
Measurements To Record
- Source exhaust flow and inlet/outlet temperatures
- Steam generation, pressure and temperature
- Gas-path pressure drop and source backpressure
- Feedwater and drum-level trends where applicable
Maintenance Checklist
- Record heat-source and steam-side conditions.
- Trend gas-side pressure drop and heat recovery.
- Maintain feedwater quality and level instrumentation.
- Plan access, isolation and pressure-system inspections.
Evidence-Based Diagnosis
Low steam generation
- Possible Causes
- Lower source heat; fouling; bypass leakage; changed feedwater conditions.
- Evidence To Collect
- Compare exhaust heat-source conditions and steam output at similar operating loads.
- Qualified Follow-Up
- Investigate the source, bypass system and heat-transfer surfaces using the approved inspection plan.
Excessive gas-side pressure drop
- Possible Causes
- Fouling; obstruction; damper issue.
- Evidence To Collect
- Check differential-pressure instruments and trend readings against the source equipment limits.
- Qualified Follow-Up
- Escalate source backpressure constraints and arrange inspection; do not exceed the upstream equipment envelope.
Unstable drum level
- Possible Causes
- Rapid load changes; instrument issue; feedwater-control problem.
- Evidence To Collect
- Review level, steam and feedwater trends and the alarm sequence.
- Qualified Follow-Up
- Use the boiler/controls specialist and OEM procedure. Do not disable low-water protection.
Record The Confirmed Result
Keep the asset model, time, load, ambient conditions, alarm code and relevant readings together. Compare with the last healthy baseline. Record the confirmed cause, approved corrective action and readings after restoration; check that the improvement persists before closing the investigation.