Energy Recovery
Devices and effectiveness
Energy recovery transfers energy between exhaust and incoming outdoor air. Sensible-only devices include plate exchangers, heat pipes, and runaround coils; total (enthalpy) wheels and desiccant-coated cores also transfer moisture. Effectiveness ε is actual transfer divided by the maximum possible, defined against the airstream with the smaller capacity rate (m·cp), not simply the larger temperature difference. Counterflow plate cores reach roughly 70–80%; enthalpy wheels can exceed 80%; runaround coil loops are lower, roughly 45–60%, because of the pump loop and coil approach.
When it pays
Recovery helps only when a real enthalpy or temperature difference exists between outdoor and exhaust air. In mild weather or while an economizer is active, the device must be bypassed or it becomes a penalty. Add the device's pressure drop to fan energy: a unit that saves 50 MBtu but costs 5 kW of fan power may not be economical. ASHRAE 90.1 sets thresholds for when recovery is required, based on supply airflow and operating hours.
Traps
Applying only a temperature relationship to a total wheel (moisture moves too); ignoring frost and condensate on the exhaust side in cold weather, which requires preheat or wheel speed control; and forgetting cross-contamination limits in hospitals and laboratories.
Worked example
ε = 0.75, outdoor 95°F DB, exhaust 75°F DB, balanced flows. Leaving air = 95 − 0.75(95 − 75) = 80°F. In a total wheel the humidity ratio is treated identically.