Run Around Coils
A run-around system is a waste energy recovery system that uses coils in the exhaust airstream to transfer energy back to the supply air via another set of coils.
The heat is transferred between the coils by interconnecting piping and a pump/control package.
Although ½” tubes are occasionally used in run-around systems, usually 5/8” tubes are used because the fluid being pumped typically contains a glycol mixture and the larger diameter results in a lower fluid pressure drop.
When does a run-around system make sense?
When the ratio of exhaust air to supply is close to, or greater than, 1:1.
When exhaust air systems are used many hours a day. For example, hospitals are good applications, but schools are not.
When energy costs are high. Or when energy companies offer incentives for energy recovery, such as when energy saved in summer can be applied as a credit for reducing peak load, making even savings of a few tons lucrative.
In geographic locations that have colder climates, making the difference between exhaust air and supply air larger.
Other factors to be considered with heat recovery coils:
Will the exhaust coil be in a corrosive airstream? If so, a protective coating should be used. Many hospitals, laboratories, and industrial applications may contain corrosive elements in the exhaust airstream that will form acids when condensed.
Will winter weather cause condensation on the exhaust coil? If so, then a cabinet and drain pan need to be provided.
Aluminum is commonly used for the fins because of its good heat transfer characteristics and low cost. There are a variety of fin designs available that may offer enhanced heat transfer or air pressure reduction.
Coil frames (also called casings or flanges) are normally galvanized.
Connections are typically copper, steel, or brass.