Plate exchangers: simple and contamination-free
A plate heat exchanger is a static stack of thin corrugated sheets. Supply and exhaust pass in adjacent channels separated by solid walls, so the two streams never mix. With no moving parts it is low maintenance and inexpensive, and counter-flow plates reach 80 to 90 percent temperature efficiency.
Rotary wheels: high efficiency with a caveat
A rotary wheel is a rotating matrix that absorbs heat (and, if desiccant-coated, moisture) from the exhaust and releases it to the incoming air. Wheels can deliver very high enthalpy efficiency, but they need a purge sector to limit carryover and they are not suitable for contaminated or hygiene-sensitive exhaust.
Selection rule of thumb
For pharmaceutical, certain industrial and any must-not-recirculate exhaust, specify a sensible plate core. For clean commercial HVAC where humidity recovery pays off, a rotary or enthalpy wheel is acceptable. Never use a wheel where the exhaust could contaminate the supply air.