The eleven equipment categories we design cover most of what a process plant runs. Some duties fit none of them — and those tend to be the ones where getting it wrong is most expensive, because there is no catalogue unit to fall back on and no second supplier to check the figures against.
Custom Engineering is where that work is done. It covers multi-stream units that recover heat between three or more process streams in a single body; exchangers that boil on one side and condense on the other; graphite, copper and exotic-alloy construction for acid and halide service that would destroy stainless; cryogenic and supercritical duties where fluid properties move sharply across the exchanger; non-Newtonian fluids and slurries whose viscosity depends on how hard they are being sheared; and units whose geometry is fixed by an existing foundation, pipe rack or headroom limit rather than by the process.
It also covers the work that comes after commissioning. We rate installed exchangers that have never reached their design duty, establish whether the shortfall is fouling, maldistribution, bypassing, an error in the original sizing or a process that has drifted from its design case, and specify what to change. Often the answer is not a new exchanger.
Every custom project is worked from first-principles thermal-hydraulic calculations rather than a correlation borrowed from a nearby geometry, with the assumptions written down where a reviewer can challenge them, and one engineer accountable for the outcome.