Relief air fan for VAV system

[remp]

I am air conditioning a very large floor plate approx 2000m2
I am using a single AHU (VAV mostly internal zones)
I have decided to have approx 4 No. relief air fans on the roof.
What is the best way of tracking the supply air fan and the relief air fan.
I was thinking of having a velocity probe int he fresh air intake duct and at the relief air fans. If the O/A goes up the relief air fans will ramp up to discharge the increased volume. Are these reliable means. Is there another waY?

 

[Wareagl487]

Space pressure sensor controlling the relief fans is how I would approach it.

Andy W

 

[SAK9]

Flow tracking(volume measurement) and pressure sensing are two common methods.Flow tracking will be more expensive if you are using multipoint flow measurement.Also for accuracy you would need atleast 6D straight duct upstream and down stream of FMS.

Another cheap and practical option is to get your return air fan VSD to track the AHU VSD.Most VSDs have this option.

 

[cry22]

Good answers here.
SAK, FMS can be located within the fan scroll. A normal practice these days. Check with Flow measuring technology or some other MFTR.

 

[marcoh]

Have a look at the Trane website, there is an article on pros and cons of different methodologies and they recommend using using a pressure sensor