Need a calculation for air flow and pressure required to perform give 1m3/min 2

[CandiceTootell]

Hi! I am NOT an engineer but I supply side channel blowers and vacuum pumps - normally to companies that have their own engineers and just tell me what they want. But in this case, I have a client that wants us to help him figure out which size unit to take.

This is a waste water system at altitude +/- 1600m.
There is a tank with dimensions: 8.4m(L) x 2.4m(W) x 3.0(H)
The depth of the solution is at 2.5m.
Solution specific gravity we normally use in this type of plant would be around 1.2
Blower installation is situated above the tank

So up to this point I can answer the question for the client. But then! He has a 52m length of pipe coiling in the tank which has 2mm diameter holes - 60 holes per meter. The pipe is 75mm (UPVC class 12). I am assuming it's blocked at the end. Normally the client would have airstones or diffusers which the manufacturer would dictate what airflow / pressure is required to open the solenoids / get the air at best flow.

The client has stated that he needs 1m3/min air flow in order to achieve the aeration he wants in the system.

My question is could someone please help me to have a formula that I can use in this situation where I could calculate the airflow and pressure of the system? I dont understand about pressure loss but I'm sure over that distance of pipe work it would exist? Do I just use the 1m3/min plus about 320mbar of pressure to size the unit? Am I over thinking this?

What other variables would be needed to create this formula? We often get asked about this so I would like to have something to send to clients to figure it out with them.

Thanks so much for your help. P.s. Please use as simple language as possible as I'm really not technical and really want to understand this!

Edited to add:
Client has just told me that he didn't explain clearly. What he has is a 75mm header pipe coming off the blower, then it goes into the chamber with 8 x 8.4m parallel 50mm pipes comes out of the 75mm pipe with T-Pieces and at the end of each pipe they are again connected to form a ring.

 

[Snickster]

The pressure required by the blower is the sum of:

Pressure due to liquid height (= 4.27 psig) + pressure drop across holes + pressure drop in pipe back to the blower inlet

I calculate that the pressure drop across holes and in pipe in the tank is negligible for the flowrates.

With 1 CMM or 35.32 CFM (assume it is really Standard CFM) the flow per hole is only 0.0113 CFM and pressure drop will be less than 0.1 psi which is negligible compared to the liquid static pressure.

The pressure drop in 75 mm pipe is about 0.02 psi for 52 meters which is again negligible. This assumes all 52 meters is flowing at maximum flow but the flow actually decreases as it goes down the pipe and discharges though each hole until the end, so 0.02 psi is conservative. I don't know what size the pipe is between the tank and the blower but it should be sized to minimize pressure drop and pressure drop in that segment included in blower pressure output requirements.

There are caculations and tables on the internet that provide calculations for pressure drop through orifices and piping.

 

[LittleInch]

You might want to look up sparger design.

But otherwise I agree with snickster. Total area of the holes I calculated as being approx twice the area of the header / main pipe. Ideally you want the same area to make the pressure in the main pipe the same from one end to the next. But losses in the sparker are very low compared to hydrodrostic head.

I would add 10% extra pressure at the pipe entry to make sure you don't get liquid in or can blow the liquid out when yout turn the air supply off.

If the client really wants to control flow to within say 5%, add 20% more for a control valve to regulate flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pierreick]

Hi,
Take a look at this link, hope not too technical.

https://www.blowervacuumbestpractices.com/technology/aeration-blowers/aeration-blower-requirements

Pierre

 

[CandiceTootell]

Thank you!

I have looked at the sparger design as suggested and understand it to be similar to airstone/ diffuser and what the client is doing here.
I also looked at the weblink sent for info on the wastewater treatment plant, thank you.
And thanks for the answer on pressure loss. It almost lost me, but I understand the basics of what you're saying is yes, there is pressure loss to be considered. I have looked at a bunch of websites but fear I need to have a few engineering degrees behind me to understand how to use them correctly.

I have added an edit about the pipework.

What I'm looking for is a way to put this on excel and have a template.
I do understand that each installation is different and that needs to be taken into account.

However, if I have a worksheet which would help to calculate blower required with all the variables and formulae inputted, it would make this part of the job that much easier! Example: I already have a worksheet which has the size of the tank, shape - eg cylindrical / rectangular, etc and how that effects the calculation of the area, which then helps us to determine the airflow required. I have a formula to calculate the depth / head / pressure required depending on altitude.
What I need now, is to create a formula on the next phase of the system. If there is subsequent pipe work with holes (sparger?) what am I trying to calculate? Is it the area of each hole? Is it necessary to know that? Is it the pressure loss per hole or length of pipe? What variables are important to selection of unit?

Understand that as a supplier, I dont create or install the systems or plants that the units go into, and the machinery is used in a variety of industries from waste water to pneumatic conveying and I can't learn everything about every industry. But I do want to know that if a client has done their own calculations and selects a unit based on that, and then it doesn't work, that we can work together to find a suitable replacement that will manage their duty. In this instance, it's water treatment and use of air outlet - in what ever form ie diffusers etc. But I also want to understand vacuum and selection for that, if anyone else here has a good way to explain that to me! I have looked for simple training courses but there don't seem to be any that aren't attached to degrees!

I really am appreciative of the time you take to respond here, thank you!

 

[katmar]

If your aim is to have roughly equal flow through each of the holes then I believe you have too many. If the flow were to be equally distributed across all the holes then you would have a pressure drop through each hole of about 2 mm of water gauge. This means that if the elevation of the holes varies by more than 2 mm the lower holes will see no air flow at all, and in fact liquid may flow into the distributor pipes.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[pierreick]

Hi,
About gas distributor.
Pierre

 

[LittleInch]

I agree with Katmar. There is something not right with your data. It's not just elevation, if the pressure along the tubes varies by more than 2mm of water column then the same would happen even if the tubes were absolutely horizontal.

Not sure if I've quite got it right but think you want to aim for min 0.5m/ sec coming out of your nozzles/ orifices.

For the huge number of holes you have that is circa 300 m3/ min, not 1.

Your system will simply not work as pressure drop along the tubes will be less than the pressure drop across the nozzles and hence the arms will flood and only about 10 to 15% of the holes will emit any air.

As a general rule for headers and branches, you want to aim for the same or lower square area of the branches to the header square area to get equal pressure in the header and hence equal flow in identical length branches or nozzles.

This system is about double the sq area or more so you won't get equal flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

There is a tank with dimensions: 8.4m(L) x 2.4m(W) x 3.0(H).

He has a 52m length of pipe coiling in the tank which has 2mm diameter holes - 60 holes per meter. The pipe is 75mm (UPVC class 12). I am assuming it's blocked at the end. Normally the client would have airstones or diffusers which the manufacturer would dictate what airflow / pressure is required to open the solenoids / get the air at best flow.

A 52m sparger in a 2.4m(W) tank?
60 - 2mm holes per meter. > 3,000 holes?

Frankly, as a blower sales person, I would not touch this. I'd send them the catalog, and say we can supply the one they choose. We do not provide or guarantee detailed engineering services on equipment not supplied by us (the sparger).





Good Luck,
Latexman

 

[LittleInch]

Latexman,

Later clarified to be 8 pipes 8.4m long so one every 0.3m spacing.

But yes, far too many holes for the air volume quoted.

But sounds like the client hasn't got a clue, but neither really has the OP (SORRY)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

LittleInch,

Thanks! 8 - 8.4m vertical spargers in a 2.4m(W) tank. Obviously, a horizontal tank.

It feels like it is far, far away from any reasonable sparger operating window I know of. Thus my advice to not touch this one.

Good Luck,
Latexman

 

[georgeverghese]

Pls ask your client to have a read of page 14-74 in Perry Chem Engg Handbook 7th edn. on sparger design and application for biological waste water treatment and then get back to you on aeration requirement for this tank. At a superficial velocity of 6.1cm/sec in the tank, I get an aeration rate = 73m3/min. (8.4x2.4x6.1x60/100 = 73m3/min).
There are other guidelines on this page also for perforated pipe setup to minimise maldistribution.
Obviously you have to account for elevation at 1600m when accounting for mass flow and power demand at this blower.

 

[CandiceTootell]

Hi all,

Thank you so much for your advise - and agreed, I don't have a clue! I wanted to be sure that this wasn't some simple answer I was overlooking. I agree that the client should have the information at design stage and not where we are on the 3rd unit going out to them as the previous two wouldn't do the job. Bad designs are great for sales, but ultimately, I want to help him achieve his purpose so that he can sell more systems. I have recommended an engineer to him and hopefully he can rectify his design. Thank you for your help!

 

[LittleInch]

on the 3rd unit going out to them as the previous two wouldn't do the job

I suspect the client has got his units wrong based on the above and it should be 1m3/sec, not / minute....

Or it should be 6 holes per metre, not 60??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Pls note there are some units conversion errors in this subsection in Perry when moving from Imperial units in the 6th edn to metric units in the 7th. So, backcheck the numbers and expressions in the 7th with the more reliable 6th edition. However, the values for superficial velocity for aeration in waste water treatment tanks are quoted in Imperial units in both editions, so no such conversion errors here.