Let's talk about money.
When feedwater quality drops, the boiler's concentration cycle needs to be lowered. This means more blowdown, more fuel use, and more money literally going down the drain.
The cycle indicates how much more concentrated the salts inside the boiler are relative to the feedwater concentration. If feedwater quality drops, blowdown must be increased to limit the salt concentration inside the boiler water.
A Numerical Example
Take, as an example, a 60 bar boiler producing 100 t/h of steam, fired with natural gas. It operates without blowdown heat recovery, 340 days/year (8,160 h/year) and 85% efficiency.
If it operates at cycle 30 instead of cycle 50, blowdown rises from 2.04 to 3.45 t/h — a 69% increase.
Each kg of this additional blowdown leaves at 275.6°C. Saturated liquid has an enthalpy of 1,213.7 kJ/kg (IAPWS-IF97 steam tables, 6.0 MPa, T sat = 275.6°C). Considering a makeup water temperature of 25°C, with an enthalpy of 104.9 kJ/kg, the energy difference is 1,108.8 kJ/kg.
Thermal energy lost to blowdown: 1,408 kg/h × 1,108.8 kJ/kg = 1.561 GJ/h (434 kW thermal), which corresponds to 12,735 GJ/year. Accounting for boiler efficiency, the energy to be supplied by fuel would be 14,982 GJ/year.
The additional natural gas consumption would be 421,000 m³/year. The additional cost under these conditions would be R$ 1.4 million/year (Gasmig industrial rate, 350,000 m³/month tier at R$ 3.3284/m³).
Operating at higher cycles requires all water parameters to stay within the limits of the applicable standards. That's the role of quality management. If your plant isn't at that level of efficiency, the diagnosis is usually faster and cheaper than the figures above suggest — pinpointing where the problem lies is often the simplest step.
Sources
- IAPWS-IF97 Steam Tables.
- Gasmig — Arsae-MG rate adjustment, May 2026.
- Diário do Comércio and O Tempo, May 2026.