Help with HVAC System

No... You need a more powerful fan blower...what is your cfm?

https://www.hamiltonhomeproducts.com/vb/Repository/InstallationManuals_PartsLists/Ductwork/Duct%20Sizing%20Chart.pdf

This was helpful! thanks !!

About 860 cfm I believe

Well a good leaf blower is about 500 cfm... You can figure about 400 cfm per ton cooling....Your duct size for 800 cfm would be about 12"....or more probably 14"...A typical 4 " duct would be used for bathroom exhaust systems, about 50-75 cfm....1 ton of cooling 400cfm would be good for 350-500sqft depending on load and insulation....

If you have a lack of airflow, check your air filter if it's clean then the design might be too restrictive remove the filter temporarily and check the airflow...If that's not it then check the evaporator coil, it might need to be cleaned(professional only job)...If that's not it, you may be low on freon, this sometimes appears to be low airflow, when it's just lack of cooling....

I'm guessing it would lower the pressure a little by restricting the flow. Any kind of pump has a flow vs pressure curve. When you lower the flow rate, you raise the pressure and vice versa. How much depends on the characteristics of your blower.

https://www.rs-online.com/designspark/fan-types-why-choose-a-backward-curved-centrufugal-fan

Air is "pulled" by a vacuum. (pressure differential)

Restricting the intake won't help. It will lower the work that the motor has to do.

Also motors don't pull. They drive fans, blowers, air pumps that pull, or push.

Get a larger impeller, a different fan or a faster motor.

Just to clarify, your system has several ducts through which air enters and is pulled through a duct to a fan and filter.

Is that correct?

When you say you want to increase the pressure at which the air is pulled, what exactly do you mean? Do you mean increase the pressure at the inlet of the fan, or increase the differential pressure between inside the air duct and outside?

Why do you want to do this?

I want to increase the strength of the air intake so that it would be able to pull very small objects of mass, like a string. The strength of pull at the first set of intakes are much stronger than the rest. I need the rest of the intakes to be a strong as the first ones. There are 3 blower motors connected in series that are 1725 rpm each. I was wondering if I were to maybe split the responsibility of intakes between the 3 motors separated, that it would work. Here is a drawing.

<...1725rpm...> is irrelevant. What is relevant is the performance curve of the <...blower...> and the performance curve of the system it is attached to, for where those two curves intersect is where the system will operate.

<...split the responsibility of intakes between the 3...> With the currently available level of data only an experiment will tell whether this approach is viable.

Passing <...string...> through a <...blower...> is not recommended.

You can do a crude test to see if splitting up duty between the three blowers would work:

-Tape over five (try six later if five was close) of the eight suction holes.

-Make arrangements so that just one of the three blowers can be powered and run it alone.

- check to see if suction is adequate at the non covered suction holes.

Some gains may be available in increasing duct diameter and decreasing the number of turns (and the curvature of each turn).

If this for a central vacuum system? If so,you could consider a ready made off-the-shelf solution instead of trying to reinvent the wheel.Save time and money that way.

Putting the pumps in parallel will increase flow volume ,and result in a higher velocity (More Pressure,as you put it) in the suction side of the existing system,all other things being equal. 1 large pump,properly sized would be more efficient. As I said,a COTS unit would be better.

Airflow through your duct causes a pressure drop. The farther from the blower, the greater the drop. There is also a drop through each vent. Since the differential between the internal pressure and the outside ambient is greatest for the nearest vent, there is more airflow there.

It's like this electrical circuit, where R2, R4, and R6 represent the restriction of airflow through vents and R1, R3, and R5 are losses through the duct. R2 gets more current (airflow) because there is more voltage (pressure difference), than R4, and R6 gets even less.

http://nreeder.com/eet1150/mod10.htm

To equalize the airflow for all vents, you need to handicap the closer vents with a baffle to restrict the airflow through these vents to allow more airflow through the farther vents.

This seems to be more of dust and fume extraction system....

You need one blower for each outlet, individually piped...

https://www.kemper.eu/en/company/references/stationary-and-central-welding-smoke-filter-units

You said, "I want to increase the strength of the air intake so that it would be able to pull very small objects of mass, like a string." In a HVAC system??????

Then, "The strength of pull at the first set of intakes are much stronger than the rest."

You have already been how how to balance the system.

Pushing air is much more efficient than pulling from only 14.7 PSI supply of atmospheric pressure. That's about the limit.

Vacuum is inefficient in moving small particles, no matter the flow.

Have you considered blowing the threads toward the vacuum inlets with small blowers?.

Positive pressure airflow is much more directional than negative pressure airflow....

Not sure if you are agreeing, or disagreeing.

Well it's the difference between a leaf blower and a vacuum cleaner, which works better at a distance?

Are you referring to Feynman's sprinkler experiment?

https://en.wikipedia.org/wiki/Feynman_sprinkler

There seems to be a much weaker reverse response with the sprinkler sucking in...

http://www.feynman.com/science/the-water-sprinkler-experiment/

No, but that's interesting....air exhibits different characteristics when under positive and negative pressures when it comes to flow...

Would you happen to know where I can find these equations ? I am splitting the system up into three separate circuits. 2 blowers are rated 300cfm and the other 800. I was thinking of having the 800 cfm blower pull from 4 (adjusted piping of course) and then split 2 intakes each for the other two 300cfm blowers.

It is Bernoulli's equations, I use Crane Techical Papers 410, Flow of Fluids Through Valves, Fittings, and Pipe, they use Darcy's formula, as seen on Page 3-22, though pages 3-25. Page 2-14 also has equations for Flow Through Nozzles and Orifices. Equation 2-21 q=Cd x A x Sqrt(2g x hL), set up the flow to be identical through each branch.

Those ratings are probably calculated without air intake resistance, free air calculations...your actual cfm will vary according to air restriction....the size of the blower wheel and rpm are important factors here, the blower wheel should decrease in size and the rpm should increase to maintain the cfm in a smaller duct...as the blower wheel gets smaller and is driven at higher speeds it needs to be more heavy duty...most commercial vacuums are motor/pulley/belt driven to get the rpm's high enough...and are rated by hg" , or how high it can lift a column of water in the stated size line....

https://www.vac-u-max.com/userfiles/brochures/Industrial%20Vacuum%20Cleaners%20Brochure.pdf

You can use a manometer to balance the suction...

https://faraday.physics.utoronto.ca/PVB/Harrison/Manometer/Manometer.html

Since it looks like everything is 'hard wired', you need to balance the system. This requires gates or valves at each of the intake (except one if so desired). With a fixed (single) vacuum pump, it would be fairly straight forward, but with three and multiple intakes....I fear it will be tricky. The balance your looking for is 'appropriate capture velocity at each intake', and if your mixing in the main branch (based on your strings idea), the minimum velocity will have to be the carry velocity for your heaviest material. Density plays in if the particulates are all the same size. Take a look in "Industrial Ventilation...a manual of recommended practice for design".

P.S. Like has been mentioned, you will need to protect your vacuum equipment with out plugging the pipeline...separator and/or filter...more tricks of the trade in the book.

Here is a link that will be helpful,no matter what your application. ______________________________________________________________________________________________________________________________________________________________ https://www.wwgoa.com/video/tips-central-dust-collection-013543/

We use a venturi with a high pressure blower to move our light waste material through to a collection system. It is super efficient and has low noise (blower in cabinet with sound dampening). It dumps into an open box for waste removal so no bagger or cyclonic separator required.

https://images.app.goo.gl/HqM6r4N36XWVo4RFA

Why is the filter AFTER the blower? It sounds like your system is backwards. Normally you would draw air through a filter and blow it out through the ducts and out of the diffusers. What is your system used for?

Keep in mind that every 90 degree turn/bend in the duct line adds approximately 8 feet of travel resistance to the air flow. You need better diffusers, larger duct work and a more powerful blower. Smaller ducts = less airflow.