| View previous topic :: View next topic |
| Author |
Message |
km987654
Joined: 25 Jul 2007
Posts: 1547
Location: Australia
TV/Projector: Barco BG808s 9
|
 Link Posted: Sat Mar 17, 2012 10:31 pm Post subject: Fan noise reduction |
 |
|
|
If you run two fans back to back at half the speed do you get the same air flow as one fan at full speed. I have been experimenting and it seems to work but I don't have any way to measure air flow. This does reduce fan noise significantly.
|
|
| Back to top |
|
 |
CasetheCorvetteman
Joined: 09 Nov 2008
Posts: 1949
Location: QLD, Australia
|
| Link Posted: Sat Mar 17, 2012 11:16 pm Post subject: |
|
|
No, you dont. You get the same flow youd get with one fan at half speed. If you want to reduce the noise and increase the air flow, you need to use larger fans.
Im assuming by back to back you really mean front to back, as in one fan pushing air into the next, and not both fans pushing toward each other.
|
|
| Back to top |
|
|
km987654
Joined: 25 Jul 2007
Posts: 1547
Location: Australia
TV/Projector: Barco BG808s 9
|
| Link Posted: Sat Mar 17, 2012 11:30 pm Post subject: |
|
|
| CasetheCorvetteman wrote: | No, you dont. You get the same flow youd get with one fan at half speed. If you want to reduce the noise and increase the air flow, you need to use larger fans.
Im assuming by back to back you really mean front to back, as in one fan pushing air into the next, and not both fans pushing toward each other. |
Yes front to back so both are moving air in the same direction. I don't see why two fans at half speed don't move the same as one at full speed after all if you have two fans at full speed you would move twice as much air so why would this not be true for two at half speed?
|
|
| Back to top |
|
|
CasetheCorvetteman
Joined: 09 Nov 2008
Posts: 1949
Location: QLD, Australia
|
| Link Posted: Sun Mar 18, 2012 12:04 am Post subject: |
|
|
Well they wont, they are moving the same air arent they. If they were side by side they would move more air, but the blades are still pushing the same air through the same space at the same speed. Same air flow as a single half speed fan or very close to it. More resistance and more energy used at the same time.
Its very logical really. If you put one water pump straight into another water pump and turn them on half way do you get double the water flow? No. Youd be lucky to get half the water flow.
|
|
| Back to top |
|
|
km987654
Joined: 25 Jul 2007
Posts: 1547
Location: Australia
TV/Projector: Barco BG808s 9
|
| Link Posted: Sun Mar 18, 2012 12:24 am Post subject: |
|
|
| CasetheCorvetteman wrote: | Well they wont, they are moving the same air arent they. If they were side by side they would move more air, but the blades are still pushing the same air through the same space at the same speed. Same air flow as a single half speed fan or very close to it. More resistance and more energy used at the same time.
Sure the same space is being used but you are assuming that that space is at it max with one fan. I have seen many server cooling systems built this way. If you don't get any improvement then why do that. Perhaps someone has equipment to measure air flow.
Its very logical really. If you put one water pump straight into another water pump and turn them on half way do you get double the water flow? No. Youd be lucky to get half the water flow. |
|
|
| Back to top |
|
|
CasetheCorvetteman
Joined: 09 Nov 2008
Posts: 1949
Location: QLD, Australia
|
| Link Posted: Sun Mar 18, 2012 2:59 am Post subject: |
|
|
That is to ensure continued air flow in the event of failure, not to increase it.
You dont need an air flow meter to use your common logic. Same space, same speed, same number of parallel blades in the same config, same air flow. The 2nd fan is doing NOTHING MORE than moving the EXACT SAME AIR the first fan has allready moved. I dont understand how you could expect it to do anything MORE than that.
In order to move MORE air, you need MORE diameter, MORE rotational speed, MORE fan blades or blades at a greater angle, or MORE fans in parallel. Fans in series is just a waste of fans. ( in terms of their physical configuration, not their electrical connection )
But ill tell ya what, lets not take my word for it without testing it, why dont you join up 5 of them, then see if you get more air flow than 1, if your theory is correct youll get 5 times the air flow, which will be easy to see. Good luck with that 
Test it on the bench and let us know what you find.
|
|
| Back to top |
|
|
km987654
Joined: 25 Jul 2007
Posts: 1547
Location: Australia
TV/Projector: Barco BG808s 9
|
| Link Posted: Sun Mar 18, 2012 3:36 am Post subject: |
|
|
| CasetheCorvetteman wrote: | That is to ensure continued air flow in the event of failure, not to increase it.
You dont need an air flow meter to use your common logic. Same space, same speed, same number of parallel blades in the same config, same air flow. The 2nd fan is doing NOTHING MORE than moving the EXACT SAME AIR the first fan has allready moved. I dont understand how you could expect it to do anything MORE than that.
In order to move MORE air, you need MORE diameter, MORE rotational speed, MORE fan blades or blades at a greater angle, or MORE fans in parallel. Fans in series is just a waste of fans. ( in terms of their physical configuration, not their electrical connection )
But ill tell ya what, lets not take my word for it without testing it, why dont you join up 5 of them, then see if you get more air flow than 1, if your theory is correct youll get 5 times the air flow, which will be easy to see. Good luck with that
Test it on the bench and let us know what you find. |
Testing is why I put the question. Not to get an argument. How's this for logic. 1 fan at half speed moves x volume of air if you have two side by side you have 2 x or the same volume as one fan at twice the speed. What you're saying is if you place one behind the other then the air output of one fan disappears. I find that illogical.
|
|
| Back to top |
|
|
CasetheCorvetteman
Joined: 09 Nov 2008
Posts: 1949
Location: QLD, Australia
|
| Link Posted: Sun Mar 18, 2012 3:58 am Post subject: |
|
|
Dont know what you expected when you ask a question, and then get an answer only to argue the point i make. Why ask a question at all if youre allready set to believe what you want to believe? Ive explained it in my terms, and you dont seem to want to hear it, you seem to be after confirmation on your own theory.
You can find it illogical as you like, until you or someone else can prove without a shaddow of a doubt that youll double the air flow with such an unorthodox method, then ill stick behind my theory.
You have one fan pushing air into the 2nd fan, x amount of air goes in, x amount comes out. The 2nd fan does not create new air, it simply moves what has ALREADY been moved by fan one.
So it can be said:The output from fan 2 is equal to the input from fan 1. That is so plain and simple i dont think i can put it any other way.
What i dont understand is why you think it could possibly work any other way?? What goes into fan 2 HAS TO have allready passed through fan 1 doesnt it.
Explain your theory to me.
|
|
| Back to top |
|
|
km987654
Joined: 25 Jul 2007
Posts: 1547
Location: Australia
TV/Projector: Barco BG808s 9
|
| Link Posted: Sun Mar 18, 2012 6:05 am Post subject: |
|
|
| CasetheCorvetteman wrote: | Dont know what you expected when you ask a question, and then get an answer only to argue the point i make. Why ask a question at all if youre allready set to believe what you want to believe? Ive explained it in my terms, and you dont seem to want to hear it, you seem to be after confirmation on your own theory.
You can find it illogical as you like, until you or someone else can prove without a shaddow of a doubt that youll double the air flow with such an unorthodox method, then ill stick behind my theory.
You have one fan pushing air into the 2nd fan, x amount of air goes in, x amount comes out. The 2nd fan does not create new air, it simply moves what has ALREADY been moved by fan one.
So it can be said:The output from fan 2 is equal to the input from fan 1. That is so plain and simple i dont think i can put it any other way.
What i dont understand is why you think it could possibly work any other way?? What goes into fan 2 HAS TO have allready passed through fan 1 doesnt it.
Explain your theory to me. |
I guess you have not convinced me the theory is wrong. You fail to realise that the total blade area of the fan is now double the single fan. Please explain how double blade area produces no additional air flow?
So the second fan adds to air flow because it adds to the surface area of the blades.
You are looking at this as two separate fans I am saying they become one assembly and with double the blade area there is double the air flow through the whole assembly and that means twice as much air enters the assembly again because there is double the blade area.
Anyway as mentioned explain how double the blade area produces no addiional airflow? You said to produce more flow you need a larger fan well that has been created by the additional blade area.
|
|
| Back to top |
|
|
CasetheCorvetteman
Joined: 09 Nov 2008
Posts: 1949
Location: QLD, Australia
|
| Link Posted: Sun Mar 18, 2012 6:32 am Post subject: |
|
|
And you fail to relise that the blades are moving the EXACT SAME AIR.
Same blade area, because the blades are not on the same plane!!
Forget it, you continue with your theory!! Let someone else have a shot 
|
|
| Back to top |
|
|
HK-Steve
Joined: 15 Jul 2006
Posts: 564
Location: Switzerland
TV/Projector: Marquee 9500, Sony D50
|
| Link Posted: Sun Mar 18, 2012 6:59 am Post subject: |
|
|
The only thing you increase is the static pressure by adding another fan in front of the first fan,
More fan blade area to push the air harder through the chassis. Not more air, just the pressure of that air.
Cheers
|
|
| Back to top |
|
|
CasetheCorvetteman
Joined: 09 Nov 2008
Posts: 1949
Location: QLD, Australia
|
| Link Posted: Sun Mar 18, 2012 7:26 am Post subject: |
|
|
|
Thats about what ive been trying to say Steve yes...
|
|
| Back to top |
|
|
Boilermaker
Joined: 21 May 2006
Posts: 527
|
| Link Posted: Sun Mar 18, 2012 11:46 am Post subject: |
|
|
| Quote: | | If you run two fans back to back at half the speed do you get the same air flow as one fan at full speed. |
You will not get the results that you desire. To find out exactly what you will get, I suggest you look at the fan curves for the fans that you have which will be available at their website. The curves will show the exact relationship between volume and pressure and you can then determine the results.
| Quote: | | If you put one water pump straight into another water pump and turn them on half way do you get double the water flow? |
There are also pump curves available, but water behave differently from air as it cannot be compressed like air.
Try this: Turn on your home vacuum cleaner and put your hand over the nozzle so no air can flow. Did the fan speed up or slow down? Why? Did the current draw from the fan motor increase or decrease? Why? Answers - Look at the fan curves for any typical squirrel-cage fan and you will see.
Bob
|
|
| Back to top |
|
|
CasetheCorvetteman
Joined: 09 Nov 2008
Posts: 1949
Location: QLD, Australia
|
| Link Posted: Mon Mar 19, 2012 3:19 am Post subject: |
|
|
| Boilermaker wrote: |
| Quote: | | If you put one water pump straight into another water pump and turn them on half way do you get double the water flow? |
There are also pump curves available, but water behave differently from air as it cannot be compressed like air.
Bob |
Yep, i know that Bob It is a reasonably suitable example on why not to run in series though.
| Boilermaker wrote: |
Try this: Turn on your home vacuum cleaner and put your hand over the nozzle so no air can flow. Did the fan speed up or slow down? Why? Did the current draw from the fan motor increase or decrease? Why? Answers - Look at the fan curves for any typical squirrel-cage fan and you will see.
Bob |
Fan speeds up due to reduction in air flow, current draw is reduced due to reduction of load and increase in back EMF.
|
|
| Back to top |
|
|
kabuby77
Joined: 28 Mar 2011
Posts: 109
Location: Italy
|
| Link Posted: Mon Mar 19, 2012 7:48 am Post subject: |
|
|
| HK-Steve wrote: | The only thing you increase is the static pressure by adding another fan in front of the first fan,
... |
Right, You see this into cpu heatsink where the fins offer a high resistance so they use a fan in the front and another on the back.
|
|
| Back to top |
|
|
Boilermaker
Joined: 21 May 2006
Posts: 527
|
| Link Posted: Mon Mar 19, 2012 1:58 pm Post subject: |
|
|
| Quote: | | Fan speeds up due to reduction in air flow, current draw is reduced due to reduction of load and increase in back EMF. |
You da man! Actually da Corvette Man.
|
|
| Back to top |
|
|
CasetheCorvetteman
Joined: 09 Nov 2008
Posts: 1949
Location: QLD, Australia
|
| Link Posted: Tue Mar 20, 2012 6:36 am Post subject: |
|
|
| Boilermaker wrote: | | Quote: | | Fan speeds up due to reduction in air flow, current draw is reduced due to reduction of load and increase in back EMF. |
You da man! Actually da Corvette Man. |
Cheers mate!! Im actually an electrician, so i know abit about current flow
|
|
| Back to top |
|
|
km987654
Joined: 25 Jul 2007
Posts: 1547
Location: Australia
TV/Projector: Barco BG808s 9
|
|
| Back to top |
|
|
rosenbush
Joined: 14 May 2010
Posts: 77
Location: Brownsville Texas
|
| Link Posted: Wed Mar 21, 2012 6:09 am Post subject: |
|
|
The Fig 2 clearly shows two fan in serie will deliver the same airflow as one.
If you're getting new fans, anyway, for the serial experiment, get fans with same CFM and less decibels and replace the original ones.
|
|
| Back to top |
|
|
CasetheCorvetteman
Joined: 09 Nov 2008
Posts: 1949
Location: QLD, Australia
|
| Link Posted: Wed Mar 21, 2012 6:37 am Post subject: |
|
|
| rosenbush wrote: | | The Fig 2 clearly shows two fan in serie will deliver the same airflow as one. |
Yep, it does... Which is what ive been saying since the start of the thread...
Anyway KM, i think you should just drop this sh** and come over for a cold beer mate 
|
|
| Back to top |
|
|
|
|
Forum FAQ
Search
Memberlist
Photo Albums
Register
Profile
Private Messages
Log in
