[EnergyPlus_Support] Variable Volume Flow and Energy Usage of Fan in AHU

["Jean marais" <jeannieboef@xxxxxxxxx>]

Dear Group,
In an AHU componants like filters, coils and dampers restrict flow with an insueing pressure loss which is volume flow dependant. I.e. the pressure drop accross the cooling coil is 130 Pa at Volume Flow of 20000 m³/h, but only 60 Pa at 18000 Pa.

So although we have a power vs flow curve for the fan, the fan assumes holding the pressure requirement constant (which is true for the downstream side, except if the damper on the "worst case" branch is changed). As explained in the first paragraph, the fan upstream internal AHU and ducting pressure drops may change with volume flow.

So let's say a AHU runs at 20000m³/h and 900 Pa pressure rise. Then a VAV damper closes to decrease the volume requirement to that zone. The pressure in the system increases, therefore the fan slows to maintain the pressure at 900 Pa resulting in the decreased volume flow. But the volume flow drops in the main ducts, the AHU and the ducting to Outdoors. This means that the upstream pressure drops decrease and the fan has an easier time of it to maintain the pressure and the flow. This basically means that the Pressure for the fan to overcome. This means that the fan slows even more to maintain the pressure at 900 Pa.

The VAV damper actually "saw" this pressure all the way on the other side and closed in such a way as to achieve the reduced Volume Flow Set Point for that zone. So actually the VAV damper closes more than one thinks to achieve the volume flow reduction from the AHU fan. The respective static pressure in the duct network junctures remain constant and the AHU fan maintains the 900 Pa constant, meaning that the distribution of volume to the other Zone VAV dampers remains as they were.

As for fan energy, it remains calculated as is. Flow at Constant Pressure vs. Energy with the normal equation: E = Q*dP/(nfanmech*nmotor).

I guess I just want to run this by the group to see if my understanding is lacking / missing something.

regards,

Jean.



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