controlling two fans in series 1

[Lalbatros]

Hello,

Here is a simple flowsheet:


|input>|pipe1>|fan1>|filter1>|neutral point|filter2>|fan2>|pipe3>|output>


In this flowsheet:

- the fluid is air
- the input and output are atmospheric pressure
- there are two fans
- there is a "neutral point", whose pressure should be close to atmosperic (zero depressure)

How would you control the fans of this system in order to mantain the neutral point close to the atmospheric pressure?

Thanks

 

[MintJulep]

Turn them both off.

 

[trashcanman]

It appears to me you have a condition of 0 flow.
If inlet and outlet pressure = atmospreric, ===> no flow.

It would be the same as an open pipe under water. No flow (at least not in this world).

 

[Lalbatros]

There must be some misunderstanding.

But, trashcanman, if you do as requested by MintJulep, then indeed there will be no flow.
I am sure that you know that most applications are just like that: input and output at atmospheric pressure. Just conider a car engine: it takes air and even fuel at atmospheric pressure, and it releases exhaust gases at the atmospheric pressure. In your kitchen too, you aspirate air in your kitchen at the atmoshperic pressure and you release it at atmospheric pressure, but you need a fan for that.

 

[trashcanman]

But with a kitchen fan or floor fan or window fan, the air can expand immediately after leaving the fan blades. In a closed system like you describe, how can the air expand to drop pressure to atmospheric? The air will compress a bit, and pressure will be slightly above atmospheric pressure. Now, if the duct expands enough, the air would drop to atmospheric pressure, like a supply duct entering a room and then leaving through a return duct. Within the room, the velocity approaches 0 and pressure approaches atmo. There is your solution. Build a large plenum so the air can expand and slow down.

 

[MintJulep]

You may find thread403-209806 entertaining.

 

[Lalbatros]

trashcanman,

What do you call a closed system first of all?
If you mean a thermodynamic cycle, then I cannot understand what you mean.
After all, there are many closed cycles in which the fluid cycles its pressure.
It is true that in the system I proposed, you can recycle the output to the input.
But why could there be no depression?

I know practically the plants I am working on.
The gases are dusty, and depression is always by design.
Working in depression reduces dust losse by leakage, since there can be only in-leakage.
A few times I took time to measure thing by myself, pressure drops, temperatures, velocities, dust load, humidity, ... . I have really checked that I am not telling tales.

Consider a filter as in my example.
You can either push the dusty gas to the filter and get a positive pressure.
Or you can use a fan at the exit of the filter to take gas out of the filter, then you get a depression.

Why does that make any problem?

Maybe my question was too elementary?
After all, the two fans have to be dimensioned correctly according to the expected flow and pressure losses. Then if the system is pertuned in some way, like an increased pressure drop, it is easy to figure out how we can react.

Assume pipe1 has an increased pressure drop. Then clearly increasing the speed of fan2 is not the right feedback. To keep the flow constant, it is better to act on fan1. This is all trivial. However, I would like to know what measurements should be used.

Finally, note that my question was a simplified topic.
The real system contains three fans and about 10 branches and temperature varies.

 

[Compositepro]

Lalbatros, there is no problem with your question. There are several ways to accomplish what you want. The simplest is to have adjustable damper at the suction or dischrge of each fan and adjust them appropriately but you could also have variable speed fan. The process of adjustment is usually called "balancing the system".

You would start with one damper wide open and adjust the other for the flow you want. Then adjust the first damper to control the pressure at the neutral point. This will change the flow so re-adjust the second damper to the correct flow. The idea, though is use one fan, or damper to control flow and the other to control pressure at your "neutral point". You probably would have gotten a better response in the HVAC forum.