In recent times there has been a call to replace old Perimeter Induction Units (PIU) with VAV (Variable Air Volume) type Induction Units. Although no in-depth study is available to validate whether such change will entail substantial savings in energy bills for a building owner.
Does the VAV Perimeter Induction Unit Save Energy?
VAV-designed PIU has been on the table for a while with earlier manufacturers have given allowing VAV system in certain applications may have merit. However, when analyzed in the perspective of replacement Induction units on a floor, partial floor or few floors in an office building do not rack up savings as one would expect putting such a system.
Induction Unit primarily works under Induction Principle where Primary Air is fed onto a unit which then induces air from the room across heat transfer coils through venturi effect of its nozzles etc.
In all cases, there are no moving parts or electrical components that generate any energy within the system. Primary Air as treated is fed to the induction unit through an Air Handling Unit (AHU) or DOAS system.
Under the VAV Induction unit scenario, the primary air is controlled through an inlet damper/orifice as such when PIU is throttled from high to low, excess air passed to a unit is generally diverted back to the channel for supply to other areas given the AHU feeding the Pius run at constant volume.
We have seen many times where the building operator is reluctant to slow down the main AHU fan to sync with a less primary entry (under VAV mode) to Induction units, which essentially goes to starve air to other areas of the floor or floors as serviced by that AHU.
Now turning down the fan as an option will push less air under when in fact load and requirement don’t call for such depletion. In commercial office buildings where reduction of fan speed in AHU was not an option given base building design allowed AHU to service concurrent floors and reduction will cut off air to floors or sections by such decision.
Conclusion
In the end, just allowing the VAV device (inlet damper) to control Primary Air in Induction units may not be a wise decision unless all parameters are looked at in their entirety.
Alternately without changing AHU design or looking at an overall scenario with less duty or adopting VFD functionality, it may be an abysmal situation to invest dollars towards VAV type replacement, which in the end may be a waste or not generate the required ROI towards the capital investment.
Hence, it is vital to consider reviewing scenarios from all angles and validate if spending higher investment to put VAV operations on replacement PIU makes a viable investment.
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