The unit is completely contained in an enclosure equipped with adjustable louvers to control both exhaust and intake air rates. The manual louvers are adjusted seasonally while the automatic louvers are adjusted continuously via pneumatic mechanisms directed by a temperature controller that maintains the temperature of the air entering the tube bundle at an appropriate level. A recirculation chamber projects beyond the front and rear headers
providing ducts where cold ambient air mixes with warm recirculated air. The flow rate of
recirculated air is controlled by internal louvers in the ducts that open as the external intake
louvers close. Either variable-pitch or variable-speed fans are used in these units. The pitch
or speed is automatically adjusted by a second temperature controller that maintains the
outlet temperature of the process fluid at the desired temperature
A row of steam tubes is installed below the tube bundle to warm the air stream during start-ups and shutdowns in cold weather. These tubes are typically the same type and size as those in the tube bundle, but with a pitch equal to twice that of the bundle. The steam tubes are commonly referred to as a steam coil.