A runaround coil system consists of two heat exchanger coils connected by piping and a pump, transferring energy between non-adjacent air streams without direct contact. The primary coil absorbs heat or cooling from exhaust air while a secondary coil releases that energy to supply air. A circulating liquid carries the thermal energy between coils, achieving 40-60% sensible heat recovery.
Technical Details
Runaround systems use water or glycol solutions circulated at 0.5-2.0 gallons per minute through small brazed aluminum or copper coils. Each coil typically contains 4-8 rows of tubing with 0.3-0.5 inch diameter tubes. Circulating pumps operate at 10-30 watts, and the piping loop maintains minimal pressure drop of 15-30 Pa. Temperature differentials of 5-15°C activate energy transfer between coils.
Applications and Practical Significance
Runaround coils are ideal when exhaust and supply air streams are geographically separated, such as in hospital operating rooms or manufacturing facilities. They provide flexibility in ductwork routing and allow building additions without major HVAC modifications. These systems cost 15-25% less to install than enthalpy wheels in retrofit applications and require minimal maintenance beyond annual fluid checks.