Minimim outside ar control
Minimim outside ar control
(OP)
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?
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?





RE: Minimim outside ar control
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.
RE: Minimim outside ar control
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.
RE: Minimim outside ar control