Fans in Series

Well, it depends upon the characteristics of the ducting, Brother, which you haven't told me about.

No ducting - one after the other and the only resistance would be the filter in the intake of the first one.

How do you propose the filter to work without ducting?

If you want the air stream to go through the filter you need a duct otherwise the air will bypass the filter follwing the path of least resistance.

Indeed so it looked from my qwestion ! (Took me some time to remember why I asked it. Reread all the posts.) In deed - the fans' frames being "ducting".

No really a reasonable reply. This is a straight forward question regarding multiple fan operations, which Joshi provided a good reference with a correct description of the amount of pressure and flow increase that might be expected.

Thank you, Ried. These days, most either don't have time or are too lazy, to read & fish out the answer they need. They prefer to be spoon/breast fed with a direct answer to their question.

Good link. The diagrams of parallel and series fan operations should give a pretty clear effect of multiple fan operation on a system.

Joshi, your link has not led me to anywhere...

Joshi's link was a Google search on the term "effect of two axial fans on the extraction of air volume".

I looked at the PDF file titled "CHAPTER 10 FANS" and found a textbook explanation that I thought covered your issue very well.

If you take your system flow-pressure-curve, then correctly overlay the two fan curves, you will have a close approximation of the change in the flow for the particular system. If the fans are in series, the curves are stacked in the pressure axis. If the fans are in parallel, the curves are stacked in the flow direction.

Increase.

In an ideal case, none. Think of a bicycle with both wheels driven at the same speed. But there is no such thing as the ideal case, the second helps by overcoming some of the losses of the first.

If there were no advantage, no one would be using multi-stage pumps.

I disagree, after Crabtree's insightful question. A large room may have both intake and exhaust fans. These two types of fans will clearly be in series with each other and increase the net flow of air through the room.

I don't see any reason to disagree. I think we are saying the same thing.

Modest increase, say ~10%.

Agreed. A waste of time as described.

If you are looking for more flow, put them in parallel or get a bigger fan.

Hi

Fans are funny things the flow will be limited to the lesser of the two plus a small margin. Think of flow through first fan, now secound fan can only get what is delivered to it by the first, plus it will lower the pressure on the delivery side of the first thus allowing the first fan to deliver slightly more. If the fans are slow turning then the chance of the secound fan to suck through the first is considerably more. But if they are fast turning the first one acts like a wall and only allows the secound fan to get what the first delivers. Not sure if this makes sense but for efficiecy one fan with more pitch or diameter could be better

Thats what I think too. The effect will show more if ducting is in place and no other openings are available.

Resultant pressure will be a little less then the sum of the pressure developed by each fans and discharge will be higher than the discharge of a single fan.

The maximum airflow will not, indeed cannot increase, however what will increase is the maximum pressure differential. It will almost double

Fans in parallel will increase maximum airflow, but not change the maximum pressure differential. across them.

The second fan will help a little but not much if it is fed air that is already moving.

The truth is that Crabtree is correct, it depends on where and what the air is being pushed through. I worked on a number of power station retrofits, adding SO₂ and Particulate Removal systems. The boilers were balanced draft with a Forced Draft fan on the combustion air side and an Induced Draft fan on the flue gas side, and ducting to the chimney. We cut the ducting and lengthened it to the baghouse or electrostatic particulate removal and the SO₂ scrubbers. We had to add big supplementary fans to push the gases through this new ductwork, we had to minimize the internal bracing of the ducts because, and I quote, "pressure drop is expensive".

An 800MW unit had 28'x28' cross section ducting, the rotor of the added fan weighed about 35T.

Pressure drop is friction, and it takes energy to overcome friction. The solution is to understand the pressure flow characteristic of the system (system curve) then select the fan(s) to match the needs of the system. In my opinion, plotting the system curve over the fan curve is the simplest method of developing a solution to get the flow needed. Sometimes the system will require multiple fan sections.

The flow will increase slightly, my guess is about 20% increase.

Francis.

Since there is no ductwork, the fans must be close together or the question is meaningless. I think that with two box fans bolted together, the blade of one could be in the wrong place for the other, say the outlet blade could be in the path of the maximum flow from the inlet fan and the blade of the inlet side blocking the pull zone for the outlet side, unless they were synchronized in the best position.

Lets try to reason out your question

i'm not exactly the best person to do this

1. 2y(of 4y) Computers' Sysytems (Halted)
2. 2y(of 4y) Theoretical Physics (Halted)
3. Random Production Automation rel. jobs
4. Random www self-study
5. No practicals in ventilation design

lets start from the case of a tube where therse intake and pull-out vent ...

1. the intake vent is limited by the length of the tubing behind
   -- it's because it creates overpressure behind it
   -- the pressure that gradually releses along the tube
2. the pullout vent /!\ likely /!\ generates the reverse occurance
   -- creates pressure drop in the tube in front of it
   -- the pressure that gradually releses along the tube
• the vets have constant RPM & productivity curves
  -- no matter how many there's in series - the sys
  doesnot go better than this
• the tube resistance can be minimized however by stucking the vents at reasonable¹⁾ interleaves
  -- ¹⁾ there'd /!\ likely /!\ occurs turbulence behind each vent that cant be "fed" that way to the next one - << would generate a musical instrument (something in between drumm and bassoon)

Using your definition of one fan giving the maximal airflow, there will be no increase. However your are wrong about that, so there will be a modest increase. How much will depend on the characteristics of the duct or instrument to air resistance.

Just consider the intake fans of a jet engine. The first ring of blades suck in from a fairly "open" mouth, eg usually not so much tube before the blades.

Then the next row of blades, a static row to straighten the flow.

Then the next row of blades, again speeding up the flow /and/or increasing the pressure.

And so it goes. Jet engines wouldn't have these successive rows of blades to no purpose. Each adds to what it got. Faster, faster, more compression to feed the combustion chamber.

Not So ?

series fans can achieve a greater pressure differential at the same flow.

A jet engine is seeking to achieve a good air density in the combustion chamber to keep the fire lit and burning a high efficiency.

It is a slightly different animal, in that it operates over a very wide range of input pressures (sea level to 30,000ft) .... at 30,000ft the air density is very low, however this is compensated to some degree by the ram air effect generated by its forward motion. Over this wide range of input densities it has to maintain a suitable density for efficient combustion in the chamber ......

At a given set of conditions (air density, fan speed, fan size, blade design, number of blades, pressure differential). Adding a second fan of the same design in series will share that pressure differential across the two fans, therefore each will have approximately half the backpressure to deal with, therefore the flow will increase slightly, fan speed may increase slightly if it is not electronically controlled. a figure of 10 to 20% was mentioned by someone previously, and that is not unreasonable.

If the pressure differential is doubled the two fans in series will flow approximately the same as one at half that differential.

The Manufacturers fan performance curves are very useful to sort this out.

For each fan there is a pressure differential at which flow will cease. This is easily determined from the manufacturers curves, or experimentally by blowing the fan into a sealed box and measuring the pressure differential between the ambient and inside the box. Placing two identical fans in series blowing into the box will almost double the pressure differential, however placing two or twenty blowing into the box in parallel will not increase the pressure differential above what one can achieve.........

Do it and see.....

Thanks for your answers !

I asked because I have made an "air purifier" ("dust extractor") with a good fan intended for cooling an electrical box, and was wondering whether an other fan put in front of it would allow me to even "suck" dust from clothes ! - a bit like vacuum cleaner

^{"this is actually unfilled market cluster" - a portable or "hand held" vacuum-cleaner for precision work such as parts of assembly , PCB-s , atomotive -- i tried to find 1 cople of timez but they're eigther unusable toys or overspec/-sized plug into mains - i have neatly an e-motor from an old hot air fan + random Ni-MH-s -- ((must revisit the topic some day))}

. . . Google foud this ^{(?only 2 videos for 2kyCE & 2 centuries of tech.-age - somewhat sad for a common interest indication)}

by: videosOf: [ tube "air flow" visualization "pressure field" elasticity ]

http://www.powershow.com/view/9a80-ZWZmY/Properties_of_Fluids_for_Fluid_Mechanics_powerpoint_ppt_presentation

http://www.comsol.com/products/4.3b/

It depends upon the characteristics of three fans,their separation distance(for pressure) etc. In water supply schemes using centrifugal pumps in several stations connected in series if one station is under maintenance, they operate pumps in the first station in series to boost the pressure to deliver over double the distance.

That qwestion I asked for an air cleaner - just fan and filter. But now am making a small air handler with recuperator and am going indeed to place axial fans in series.