airflow balance - static pressure

[platypus83]

I have been asked to balance a supply air system with 8 supply bell mouths. Normally this would be a simple balance. Each branch would be balanced by a duct traverse. However as well as a design airflow I have been given a requirement for a static pressure at the index branch.

I have been requested to do an initial scan,find the true index, and balance proportionally to design while maintaining 50pa at the index branch.
I have found that the balanced system only has 25 pa at the index branch. This can not change as the branch damper is located before the duct traverse location, so closing it will only reduce SP, and the index branch will be fully open anyway.

I have now been requested to instead create a pressure drop of 50 pa across the index damper by closing it and then balancing the system to design airflows. This is based on the assumption that having 50pa DP across the index damper will give at least 50 pa DP across all other dampers on system because of the regulation of them for balancing.

The requirement for the 50pa is to allow for future additions of components on each branch supply duct. I have been told that when they are added the index damper can be reopened fully and the airflow will return to design.

I have some questions on this
How do I begin to balance it this way and what procedure should be used?

If I have 50pa dp across the index branch damper the airflow will only be resultant.
Static pressure and airflow change together so how can this be done. Is this correct?

If this condition is achieved and the components are fitted to each branch in the future with the loss of 50pa, opening the index damper only will not bring the system back into balance. every damper will have to be opened back up, i.e a complete rebalance.

 

[hoodguy]

I will take a shot at this although I am not an expert in system balancing.

You will need to get the static pressure upstream of the index damper, so drill a hole in the duct for your pitot tube.

Open all the branch dampers fully.

Throttle the index damper to 50pa upstream of the damper.

Measure the flows in all the branches.

Balance the branch airflows to match the branch with the least amount of airflow (assuming they are all supposed to have equal airflow).

Now the branches are balanced.

Go back and adjust the index damper so that the branch flows are in accordance with the design flow.

Measure and record the static pressure upstream of the index damper.

If the static exceeds 50pa then great, you've got more to play with when the system is modified.
If not then you may not have the right fan for the job in the future.

How does this sound?
Have I misunderstood the system?
Are there mistakes in my logic?

Let me know.

 

[ChasBean1]

That sounds reasonable to me.

platypus83, can you post of sketch of this? I tripped over some of your initial wording, the term 'index,' - being the main duct feeding the take-offs? I'd assume each bellmouth take-off has a volume damper? If so wouldn't you just keep the main duct volume damper fully open and balance back the take-offs?

I guess I may not be envisioning this correctly...

 

[platypus83]

I will clarify...

Its an AHU serving one bellmouth per floor through 8 floors, each take off has a vcd, then I have a pitot traverse point drilled in the duct, then it goes out the supply bellmouth. Each floor is identical.

When I say "index" I mean the branch recieving the lowest % of design airflow after an initial scan with all branch vcd's fully open.

The fan has a variable speed drive. There is no main line vcd.

In a normal situation the proportional balance method would be used as follows
- Set up main volume airflow to about 110% of design
- Scan all branch airflows with all vcd's fully open
- The lowest % of design is the "index", this vcd is left fully open
- Balance, starting closest to the index, each branch into the index so that all branches are within max +10% of the index.

However in this situation I dont think this method can be used because I have been requested to create a resistance of 50Pa across the index vcd by regulating it, and then balance all the airflows to design.
When I say 50pa across the vcd I mean the difference between the sp upstream of the vcd and the sp downstream of the vcd.
This has not been attempted yet but what I'm wondering is how do I begin to achieve this...

and for example if I regulate the index vcd to a DP of 50pa the airflow is only resultant and will always be the same as long as there is the 50pa DP.

So if I get 50pa DP across vcd and measured airflow is 60% of design obviously the DP will need to be higher to achieve 100% of design airflow.

 

[ChasBean1]

This helps. Thanks for the clarification.

But I don’t get it, which I think means that I agree with your assessment. If you need 50 Pa in your original index branch duct, downstream of the volume damper, then throttling back that volume damper in any way reduces your balanced 25 Pa to something lower.

If that index branch achieves it’s airflow at 25 Pa and you tinker with things to increase it to 50 Pa, flow will be about 1.4 times the value to which you originally balanced.

So what comes into play now are end/terminal devices. Do these branches run straight and unhindered to diffusers or registers?

This is leading toward a recommendation; based on conditions that helped to create the need for flow controlled VAV boxes and pressure independent systems…

If this can’t be done, keep the current balanced conditions. Have future additions or changes tapped from the vertical main, PRIOR to the branches. If there are later additions to the main riser, the fan VFD will compensate and the branches shouldn’t be affected too much.

If you want future tie-ins onto the index branch (or any other branch for that matter), without pressure independent controls, this will require rebalance of the system at all take-offs at that time. In other words, achieving the “minimum 50 Pa” value now would not make sense.

 

[SAK9]

Here is my two cents:

You need to set the AHU for a much higher flow than design flow.Increase the AHU flow by increments until you achieve 50 pa at the index outlet.This could be 20 or 30% higher than design flow.When you hook up the duct work equivalent to 50 pa,it will achieve design flow rate.

Alternatively use an orifice plate at the index that will create 50 pa and balance for design flow.

I am not sure your AHU motor will allow this.

 

[iken]

The idea of supplying excess air, then restricting this to achieve the desired 50Pa seems logical. The only issue would be the waste of energy, and running costs to acheive this. If this is for future allowances, then I would assume the AHU is VSD controlled, and you are only currently supplying a portion of the total design air volume of the AHU. When additional runs are installed, then the only way to truely ensure all previously installed/commissioned systems and new ones recieve their design flows, is to recheck all.