Exhaust In line Booster Fan ... Fan Interaction

[RandomUserName]

I am having trouble conceptualizing the following:

I have a piece of equipment that needs to be exhausted. The piece of equipment has a pressure drop through it of 0.25" w.c. at the required flow of 25 CFM.

I would like to tie-in the exhaust to the general building exhaust. Problem is, I am almost certain I will not have the required negative pressure in the system where I can tie in; possibly 0.15" w.c. The general exhaust system is very extensive and re-balancing the entire system just for this new "critical path" is cost prohibitive.

I am looking into installing an inline booster fan in our branch sized for the flow rate required (25 cfm) at the pressure drop through all the branch fittings, branch ductwork, and equipment.

My question is: what exactly is happening at the tie-in point to the general exhaust? That location would be downstream of our booster fan, but upstream of the general exhaust fan. Conceptually (I am thinking) if we install the new duct work branch with inline fan - but we do not turn the fan on - we have created a new path for airflow; the general building exhaust fan negative pressure requirement drops, and the fan flows more air. When we turn the inline fan on; what happens at that tie-in node? How do we prevent the in-line fan from increasing the pressure at the tie-in node; therefore reducing exhaust flows of all inlets upstream of the tie-in point?

Ideally the tie-in point maintains its negative pressure that it had before the inline booster fan was turned on and somehow the booster fan only crates additional negative pressure on the inlet side to overcome pressure losses. I cannot work that through my head on how that would be achieved.

 

[willard3]

Your biggest problem will be controlling the fans in series.

 

[RandomUserName]

That does not help answer by question at all. What do you believe happens at that connection point once the booster fan is turned on?

 

[BronYrAur]

I would be interested in this answer too. Occasionally I run across similar situations.

just thinking out loud here, but what about a damper on the discharge side of the booster fan before connecting to the rest of the system. This could be used to prevent the booster from increasing pressure at other exhaust locations downstream of the tie-in location.

Everything I find on fans in series always shown 2 identical fans and rarely discusses the pressures in between the fans. Most references just show the combined fan curves.

 

[dbill74]

How big is the existing building exhaust fan? CFM/ESP?
You mention a "piece of equipment" is being added, when/how often does it need air flow? What is control sequence of building exhaust? Any scenarios where one will operate while the other does not?
If there is any chance the new equipment will need air when the main exhaust is off (unoccupied hours), then you should exhaust the equipment to outside separately and this becomes mute.

However, assuming that is not the case and the 2 fans will run simultaneously, you are on the right track. As long as the static pressure from the new branch is at least close (+/-10%) to the static pressure in the main duct I don't foresee any problems.

Truthfully, unless the exhaust you are tying into is less than 500 CFM, it will be hard to go wrong adding only 25 CFM.

Regardless, I would recommend that the tie-in is not close (at least 2 duct diameters) to any inlet branches and use a "reverse scoop" to direct air in the direction of flow. Many years ago I did a bunch of work in high rise office buildings that had a central exhaust shaft with main exhaust fan at the roof and or mechanical room floors. When a toilet was added to a floor, the exhaust fan had just enough ESP to get the exhaust to the main shaft. I can't tell you how the main building exhaust was controlled or if adding 100 CFM or there created any problems.

 

[RandomUserName]

BronYrAur,

That was my gut reaction as well and I showing that the contractor is to provide a balance damper on the discharge side of the booster fan but still upstream of the common node.

Let's run with that thought and make some numbers up with it!

Let's tie-in the branch in question to the system including the booster fan but do not turn it on. Let's say the node before connection to the system was -0.15" w.c. pulling 150 CFM of air through one exhaust intake grille. Since adding the new branch the node we have created a new path for air. The system curve just dropped due to a less restrictive system and more air will flow through our node, which has now given up some of its static pressure to velocity pressure. So lets say for easy numbers our node SP bumped up to -0.10" w.c. and our upstream intake which was at 150 CFM drops to 125 CFM while our new branch duct starts sucking in about 15 CFM.

We know we need to develop at least -0.25" w.c. to get 25 CFM in our branch ... possibly -0.30" after duct and fittings. But we also need to get the node SP back down to -0.15" w.c. to get the 150 CFM on the upstream side.

So if we shut start closing said balance damper you and I are thinking about ...

We restrict airflow through our new branch ... back to the point that if we close it all airflow would go back to original on the upstream side (150 CFM @ -0.15" w.c.).

I am struggling working this through my head!

 

[RandomUserName]

dbill74,

System we will tie-in to is on the first floor and goes up to the 7th floor where it is discharge via upblast centrifugal fan. 3025 CFM @ 1.25" w.c. We are literally the second to last branch on the system as the furthest away. Turn this into a fun homework problem and for now assume we don't have the neg. SP required.

The piece of equipment needs airflow 24/7.

Building exhaust is operated continuously.


Yes I know we are talking about 25 CFM on a 3025CFM system but it is the concept that is killing me! I should be able to explain what is going on!

 

[lilliput1]

The CFM addition is so small, only 1.653% addition such that it is within the normal allowable +/- 2.5% error tolerance of instruments. Run new ductwork from the equipment to a point on the existing exhaust ductwork that would have enough negative pressure to draw the 0.25"w.c. + the drop through the new ductwork & branch tap.

On projects where inline booster fan would be required you would need to determine the static pressure capability of the existing main fan at the new CFM. Noting that with the new CFM the pressure drop through existing ductwork P2 = P1 x (CFM2/CFM1)^2 determine where the booster fan need to be located to handle new & existing exhaust points behind it. Use VFD to control the booster fan speed to maintain the static pressure at the point of connection to the existing ductwork which would be equal to the negative pressure exerted by the main exhaust fan at that point at the new CFM duty.

 

[willard3]

I will say again that you will also have to match fan type/pressure characteristics or you may encounter some very unusual results.