Induction

4-pipe Induction

The design is peak cooling capacity and peak air.

This system is composed of a central, constant volume fan located on the cold deck that supplies conditioned primary air to each room's induction unit. The primary air is delivered at relatively low airflow (~.25 cfm/ft2) and at a very high static pressure (~5 in. wg) to induce room, i.e., secondary air, into the room induction units. The room induction units can be either in the heating or cooling mode at any hour, depending on room conditions. Return air is drawn from a common return air plenum where loads to return air are picked up from all rooms and brought back to the ROA deck for mixing with outside air. The room induction unit is composed of an auxiliary heating coil and an auxiliary cooling coil; the induction unit has no fan since it relies on the velocity of the primary airflow to induce secondary (room) air through the auxiliary coils for additional heating or cooling.

System Simulation

The temperature of the primary air stream is dictated solely by the outdoor air reset schedule. In the winter season the primary supply air temperature is closer to the design heating supply air temperature, while in the summer the primary supply air temperature is nearest the design cooling supply air temperature. This constant volume of (either warm or cold) primary air is delivered to all rooms regardless of what the room temperature might be.

If, after the addition of this warm or cold primary air, the room temperature is above this hour's cooling thermostat, the secondary coil is activated to cool the induced, or secondary, air such that the mixture of the primary and secondary air will maintain the space at the cooling setpoint. In the dead band region the secondary coil is not activated and the room temperature is allowed to drift between the heating and cooling setpoints. If the room temperature falls below this hour's heating thermostat, the secondary coil is activated to heat the induced airflow such that the mixture of the primary and secondary air will maintain the room at the heating setpoint.

If the primary fan is scheduled off for a particular hour, no mechanical heating or cooling is possible that hour.

System Options

1. The value of minimum outside air nominally follows the outside air schedule. The amount of outside air introduced into the ROA deck may be greater than the ventilation minimum if an outside air economizer or nighttime purge is activated this hour. (Normally, however, an induction system will be designed at 100% outside air since the primary airflow is usually about a fourth the design supply airflow of more conventional systems.)

The value of outside airflow may be less than the nominal ventilation value if the supply fan has been scheduled at less than 100%; the outside air percentage for a particular hour will be multiplied by the main cooling fan utilization percent for that hour. If the supply fan has been scheduled off for a particular hour, no outside air can be delivered to the rooms.

2. The primary fan is controlled in time clock fashion, i.e., if the schedule reads 80% for a particular hour, the fan will operate at full rpm for 80% of the hour and remain off the rest of the hour. The output of the primary coil will be reduced by the percent of the hour the primary fan is not on.

3. A supply air reset control is not allowed for either the primary or secondary coils.

Application Notes

Humidity compensated controls are not allowed for induction systems

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