A high pressure fan system delivers air to induction units around the perimeter of the building. The induction units have heating <and cooling> coils. The high pressure air goes through nozzles at high speed, creating a venturi effect which 'induces' a flow of recirculated room air through the unit coils.
There are many conditions in which the high pressure air flow is not required to maintain a comfortable space temperature. When the air flow is reduced or turned off, the induction units perform more like traditional radiators. Under lower load conditions, radiant heat from the heating coil is usually sufficient to heat the space. So long as adequate ventilation air is being delivered, there is no reason not to slow down the induction fan. Also, space temperature can be reset (and air flow reduced or stopped) when the building is unoccupied.
We recommend installing VFDs and inverter-duty motors on inductions fans, to slow down and turn off the fans in response to space temperature and occupancy schedules. This will save a significant amount of fan energy and reduce induction 'hissing' noise in the occupied space. It will also reduce the volume of excess ventilation air and the associated heating and cooling. Similarly, during heating season, the fans can be slowed down to match cooling loads rather than adjusting chilled water supply temperature. <Any summer reheat at the induction units should be eliminated.>
These are noisy systems. Control that slows the fans will reduce ambient noise very significantly, improving the working environment most of the time.
Issues and Concerns
Energy savings from VFDs come from the basic laws of fan or pump operation. Fan power input is related to the cube of the flow rate (load), according to the equation:
W2 = W1 x (Q2/Q1)3
Q1 = first flow rate
Q2 = second flow rate
W1 = first fan power input
W2 = second fan power input
For example, if the flow rate is halved (Q1=1, Q2=1/2), the fan power input W2 will be one-eighth of W1.
In practice, the drive itself dissipates some of the power input as heat, so the savings are not quite as great as this.
When calculating the savings, project the number of hours operating annually at the current operating point. Then project capacity reduction from peak through the year using temperature bins and determine the new power consumption in each bin.