Economizer approach temperature drifting wider than design is the boiler telling you that heat transfer in the feedwater preheating section has degraded. The cause may be gas-side fouling, water-side scaling from feedwater chemistry, or flow maldistribution through the tube banks. The approach is a sensitive indicator — small changes in heat transfer show up clearly because the temperature differential is narrow by design. By the time it has widened by tens of degrees, the underlying cause has usually been progressing for months.
Why this matters in the whrs
The economizer is one of the highest-impact heat exchangers in the WHRS, because every degree of additional feedwater preheating reduces the steam-side fuel cost. Approach degradation is a recurring overspend that lasts until the cleaning or chemical treatment campaign that addresses it.
Feedwater chemistry is often the underlying issue. Plants running on marginal feedwater quality see scale formation that progresses across the boiler rather than concentrating in one place, and economizer approach is usually the first place it shows. Treating approach temperature as a feedwater chemistry indicator — not just an economizer issue — catches the cause before it has spread to higher-pressure sections of the boiler.