Heat Exchanger Effectiveness (ε = Q/Qmax)

Effectiveness is the one exchanger number a non-specialist can read without a chart: 0.82 means the unit captured 82% of everything thermodynamics allowed. Unlike efficiency in a boiler sense it has no fuel in it, and unlike U it needs no area basis to be meaningful. Typical values: a plate heat exchanger with a close approach runs 0.85–0.95, a shell-and-tube process cooler 0.6–0.8, an air-to-air plate recovery core in an HRV 0.6–0.75, a rotary wheel 0.75–0.85, and a fouled unit somewhere well below where it started.

Its real power is diagnostic. Because ε for a given geometry depends only on NTU and Cr, a drop in measured effectiveness at unchanged flows can only mean UA has fallen, which on a water side means scale or biofilm, and on an air side usually means a plugged coil face. Worked example: an economiser recovering 420 kW where the inlet-to-inlet ceiling is 600 kW runs at ε = 0.70; six months later the same flows return 480 kW against a 750 kW ceiling, ε = 0.64, and the cleaning is overdue. The trap is comparing effectiveness values measured at different flow rates: dropping the flow raises NTU and flatters ε, so an exchanger can look better simply because the pump is throttled.