Two Pipe Induction Systems
When air moves through a space with speed, additional air from the space is caught up in the flow, and moves with the flow of the air. When this occurs, the room air that is caught up in the flow is called entrained air, or secondary air.
The two-pipe induction system uses ventilation air at medium pressure to entrain room air across a coil that either heats or cools. The ventilation-air, called primary air, is supplied at medium pressure and discharged through an array of vertical-facing nozzles. The high-velocity air causes an entrained flow of room air over the coil and up through the unit, to discharge into the room. The flow of room air through the unit has little energy, so obstructing the inlet or the outlet with furniture, books etc. can seriously reduce the performance of the unit.
The coil in the induction unit is heated or cooled by water. For cooling, the coil should be designed to run dry, but it may run wet, so a condensate tray is normally necessary. In a hot, humid climate, to minimize the infiltration of moist air and reduce the likelihood of the coil running wet, the building pressure should be maintained positive. A lint filter should be provided to protect the coil. This filter will need to be changed regularly, so good access to the front of the unit is required.
The induction unit produces some noise due to the high nozzle velocity. This makes it less suitable for sleeping areas. The air noise is tone-free, though, and thus not annoying in most occupied spaces if silence is not a prerequisite.
The units are typically installed under a window, and when the air system is turned off the unit will provide some heat by natural convection, if hot water is flowing through the coil.
