Minimim outside ar control 1

[remp]

I am designing VAV air-handling unit with a mixing box at the back of the unit.
Return air is ducted to the unit through return air damper. Fresh air is brought in through a minimum fresh air damper (fixed) and a modulating fresh air damper for economy cycle operation.

This is a very large air-handling unit and airflow-measuring dampers are impossible to find on the market so minimum fresh air is maintained by keeping a constant negative pressure in the mixing box (set during commissioning) by closing the return air damper when the low load operation of the VAV boxes cause the air handling unit fan to ramp down. This ensures constant flow through the fixed minimum outside air damper.

My question is during economy the fresh air damper the will open when outside conditions are favourable, but should the controls close the return air damper or should it just follow its normal control by maintaining the set point pressure in the mixing box. In other words how do I ensure minim fresh air during economy cycle and off peak load conditions?

 

[SAK9]

You can ensure constant fresh air in a VAV system by having an independent fresh fan fitted with a mechanical type CAV box.So the fresh intake will not change with AHU speed.

I am not sure I got the second part of your question right.When you operate your system on economy cycle you should be drawing a substantial quantity of fresh air and there should not be any ventilation issues.

 

[tombmech]

Ordinarily, the static pressure on the outside air inlet and dampers will always be much less than on the return air inlet. However, I have seen a scenario such as the one you describe - a separate, smaller OA intake. It will have a higher pressure drop than the the return air inlet. So, once balanced for the desired air flow, modulating the return air damper to maintain the same static pressure should work to ensure minimum outside air. That should work for the AHU fan in normal operation.

However, economizer mode is different. If I've analyzed your scenario correctly - the return air inlet will always have a higher pressure drop than the larger, economizer outside air inlet. So, all things being equal, the static pressure sensor should keep the return air damper closed during economizer modes - because the minimum static will never be achieved.

Personally, though, I would include a more authoritative sequence, and command the return air damper to shut during economizer mode, period.

 

[cme]

size to minimum osa damper at 1500 FPM