Is it better to have a lot of small fans in a system or fewer bigger fans? 1

[nandwana]

I have not been able to get a good answer for this question. Can you please explain your answer with affinity laws ?

 

[LittleInch]

First define "better" - cost, Opex, efficiency, flow?
Then define "system".

Depending on the answers you can find opposite results. ...

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[nandwana]

By "better", I mean efficiency of the system.
By "system", I mean a fan wall array.

 

[LittleInch]

Doesn't look like it.

http://www.dac-hvac.com/ask-rick-fan-array-does-it-make-sense/

Affinity laws don't make sense here.

If you've got a large turndown then might be different as one small fan working flat out will be more efficient than a large fan going slowly



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

 

[chicopee]

Do you have a reason for your OP?

 

[Artisi]

Seems to me that lots of small fans is the same as fewer big fans, or should that read more small fans rather than a number of larger fans.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[itsmoked]

The same except for the deafening roar of a bunch of small wind-whippers.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[AJKue]

Another advantage to an array is that if a 1 of several fans goes down, it's not an emergency to replace it; you can get the parts in and still run the system, or maybe even have a handful of smaller fans on-deck.

 

[LittleInch]

Just make sure the little fans have a decent set of flaps / non return valves on them....

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

 

[willard3]

Many small fans will decrease efficiency as the friction losses in the fans may increase

 

[nandwana]

This question was asked in my interview, hence the post.

@Little Inch: Why would affinity laws not make sense here ?

@all it seems like everyone agrees that efficiency wise its the same, why is that ?

 

[RossABQ]

An advantage I have heard is that smaller motors are easier to get when one fails, larger motors can be hard to find on short notice.

I've mainly seen these on installations where zero-downtime is a hard requirement. If one fan unit fails, the others in the array are ramped up to compensate. Efficiency and first cost are secondary.

 

[SAK9]

A single large fan is better than a number of small fans if the the duty point is constant.Operating efficiency of all rotating machines(rotodynamic) be it fans,pumps ,compressors follows the shape of a bell curve when you have flow rate plotted on the X axis.If you were to obtain fan selections from 1 m3/s to 15 m3/s for the same pressure head you will see how the rpm required varies from small fans to large fans and how it impacts the power input per m3/s of air flow.You would notice fans operating at low rates need to operate at much higher speed than one operating at higher flow rates to generate the same pressure.Fan head pressure is created by the momentum of air leaving the fan.This momentum is a product of mass of air and velocity of air leaving the fan.Velocity of air is a function of fan RPM.If a small fan were to generate the same momentum as a large fan it then follows that the smaller fan needs to operate at much higher rpm as it is handling much smaller quantity of air. As you would notice from affinity laws input power required varies as the cube of fan rpm and therein lies the answer to your question.

 

[LittleInch]

Affinity laws only apply for different rpm for the SAME fan.

That's not the same thing as small fans to large fans.

Bigger motors and fans are more efficient if the flowrate is constant.

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

 

[SAK9]

Affinity laws only apply for different rpm for the SAME fan.

Not necessarily.They can be applied across dynamically similar equipment.They are also often used in predicting performance prior to trimming /changing impellers.