Mixed air plenum pressure method to control min outdoor air volume at the AHU - Any one use this?

[immsk]

I came across this article here

https://www.hpac.com/iaq-ventilation/article/20928092/controlling-outside-airflow-in-vav-systems

that describes a way of controlling outdoor air flow accurately.

TLDR: This strategy relies on maintaining a fixed differential pressure across the outdoor air damper, to ensure the minimum outdoor air volume is maintained. Outdoor air damper is set at a fixed position. The return air damper OR the return fan can modulate to maintain the pressure across the outdoor air damper. The outdoor air damper is like a fixed orifice, therefore once the correct differential pressure across the damper is determined, maintaining it will always ensure the correct air flow.

Question is, does anyone use this? I worked for two different consulting firms here in Toronto, Canada and never came across this strategy.

 

[EnergyProfessional]

The problem with all air flow measurement with pressure is, there is a square relationship between flow and pressure. so any error in pressure measured, will be a squared error in flow. Especially for low flows this is critical. Second problem is, you need a lot of pressure drop to measure, which is wasteful.

I didn't read the whole article, but browsing revealed the typical blah. The only way to know for sure what you OA is, is to measure with a thermal dispersion or vortex shedder flowstation. That OA needs to be equal or a tiny bit larger than exhaust inc relief. Note that flowstations need a good location (straight duct requirement etc.).

the "once-set-and-done" Oa adjustments in constant flow systems won't work. first, the balancers often have no clue what they are doing, don't have good measuring instruments and methods. Second, as conditions change (wind pressure, seasons, filters...) all what was once set up is not correct anymore.

 

[immsk]

Typical flow stations require straight sections, which isn't available in many cases.

Also monitoring stations have errors which when compounded can result in a huge discrepancy in the actual flow. As described here:

https://www.aivc.org/sites/default/files/airbase_11447.pdf

Wondering if anyone else has any thoughts on the differential pressure across the outdoor air damper method.

 

[EnergyProfessional]

I'm sure there was some progress since 22 years ago.
Where you have a fan you also can measure the flow at the fan inlet. Obviously that doesn't help in paths without a fan. Part of the design has to be airflow station location to provide the required conditions. Unfortunately most designs i see don't detail that out and the flow station gets installed "somewhere". the you have an AHU in the BAS whwere the flow balance is 50% off and you know no airflow reading is useful.

One problem with all the publications (even more recent ones) is they never include ERV. They basically still assume VAV systems from 30 years ago.

A manufacturer rep tried to sell me some sort of flow station that worked on a louver, or was some sort of louver. That would be similar to what you try to do with the damper. IIRC one of the problem there also was relatively high pressure drop required to get good readings. I didn't research that too much back then, though.

You could look at the Vortek systems. They have more favorable duct requirements or in some cases create their own conditions. We used their air valves and they are great for many reasons.

 

[GT-EGR]

What strategy do you end up using? Without an airflow station on a VAV system this is a good method. It’s definitely not the most accurate, but if you have a variable air flow system this is a common way to do it - you don’t have too many other measurements to actually control to, especially ones that at least someone directly track with what you are trying to do.

 

[J.J.Newcomb]

I have used this control scheme and not had any problems with it. Control dampers have pressure drop, even wide open (see damper authority), so the pressure drop is already there. If the differential pressure across the outside air damper is controlled, wind pressure, seasons and filter loading make no difference in the flow through the damper. A fixed pressure drop across a fixed opening will give a fixed airflow. Directly connected damper actuators are very repeatable with very little hysteresis.