I have not fully analysed the formulae and details of pressure change.
However, that is why VAV AHU had problems when a zone has no reheat or when the damper is fully open.
My suggestion was to model an air bypass at AHU so that the return air collected at the AHU can be cooled independent of the zone total air flow rate.
This was done with the CBVAV using a constant volume fan.
In a real VAV damper installation, there is a PSD (pressure sensing damper) to keep the duct pressure to the VAV diffuser constant. The VSD (variable speed drive) fan can increase the fan speed, but cannot increase the cooling rate at the coil of the AHU. The quick solution is to add a preassurized leak at the end of the duct in the plenum. When the VSD fan inscreases speed, the leak opens and extra cool air is fed to the plenum. The VSD kept the diffuser pressure the same at around 50 pa.
The control cycle is very similar to what you are describing.
If all the zones have similar loading changes, the problem is minimal, and have problem only when all the zone dampers are fully open. This was considered as a sizing problem.
The set point not met warnings can be reduced by increase the maximum flow rate of the fan.
Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: jeannieboef@xxxxxxxxx
Date: Mon, 27 Feb 2012 09:42:41 +0000
Subject: [EnergyPlus_Support] Variable Volume Flow and Energy Usage of Fan in AHU
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However, that is why VAV AHU had problems when a zone has no reheat or when the damper is fully open.
My suggestion was to model an air bypass at AHU so that the return air collected at the AHU can be cooled independent of the zone total air flow rate.
This was done with the CBVAV using a constant volume fan.
In a real VAV damper installation, there is a PSD (pressure sensing damper) to keep the duct pressure to the VAV diffuser constant. The VSD (variable speed drive) fan can increase the fan speed, but cannot increase the cooling rate at the coil of the AHU. The quick solution is to add a preassurized leak at the end of the duct in the plenum. When the VSD fan inscreases speed, the leak opens and extra cool air is fed to the plenum. The VSD kept the diffuser pressure the same at around 50 pa.
The control cycle is very similar to what you are describing.
If all the zones have similar loading changes, the problem is minimal, and have problem only when all the zone dampers are fully open. This was considered as a sizing problem.
The set point not met warnings can be reduced by increase the maximum flow rate of the fan.
Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: jeannieboef@xxxxxxxxx
Date: Mon, 27 Feb 2012 09:42:41 +0000
Subject: [EnergyPlus_Support] Variable Volume Flow and Energy Usage of Fan in AHU
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.
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.
__._,_.___
Primary EnergyPlus support is found at:
http://energyplus.helpserve.com or send a message to energyplus-support@xxxxxxxx
The primary EnergyPlus web site is found at:
http://www.energyplus.gov
The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/
Attachments are currently allowed but be mindful that not everyone has a high speed connection. Limit attachments to small files.
EnergyPlus Documentation is searchable. Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.
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