The steam package unit is situated close to the air duct and is sized to meet the maximum requirements; this is usually in winter in the U.K.
A steam pipe ( sometimes hoses are used) passes from the packaged unit to the air duct and steam at 100oC is injected into the air stream via. a sparge pipe. The un-used steam is drained from the system via a condensate tundish and drain. It is important to layout the steam pipework so that any condensate will drain back to the unit.
The psychrometric process is shown below.