Air velocity from fan

[david2008]

Hi there,
How can we know the air velocity from fan? I use different fan motor and blade. I know the fan motor speed (rpm). How can I know the CFM or air velocity of the fan? Thanks.

[US Iceman]

You need to know the fan CFM and the fan cowling diameter.

CFM divided by diameter (measured in feet) = feet per minute velocity.

It's important to realize also that the velocity is not constant, but degrades over time and space. In other words, the air slows down and spreads out.

[david2008]

You need to know the fan CFM and the fan cowling diameter.

CFM divided by diameter (measured in feet) = feet per minute velocity.

It's important to realize also that the velocity is not constant, but degrades over time and space. In other words, the air slows down and spreads out.

:)Iceman, To my understanding, CFM is decided by both motor and blade. What will happen if motor and blade are separately purchased?

[US Iceman]

CFM (or air volume delivered) is determined by the fan characteristics it was designed for. The pitch of blade, diameter, and speed of rotation are the major factors for a propeller type.

The fan manufacturer usually has what we call fan curves. These curves are the manufacturers expected performance data for the fan at various operating conditions (there's that phrase again;)).

If you change the operating conditions, the fan performance changes. At any operating condition (i.e., static pressure and RPM) the manufacturer will have a specific power input required to maintain the fan speed at that condition. That power input is the minimum motor size required (this is the same for compressors too).

However, these performance values are based on the manufacturers assumptions used to list the data. For fans they usually list the performance at various static pressures, which is fine by itself.

What you have to be aware of this.... The operating conditions may be changed by how the fan is installed. These are called system effects. Do a Google search and have some fun reading about these!

[david2008]

CFM (or air volume delivered) is determined by the fan characteristics it was designed for. The pitch of blade, diameter, and speed of rotation are the major factors for a propeller type.

The fan manufacturer usually has what we call fan curves. These curves are the manufacturers expected performance data for the fan at various operating conditions (there's that phrase again;)).

:cool:

If you change the operating conditions, the fan performance changes. At any operating condition (i.e., static pressure and RPM) the manufacturer will have a specific power input required to maintain the fan speed at that condition. That power input is the minimum motor size required (this is the same for compressors too).

However, these performance values are based on the manufacturers assumptions used to list the data. For fans they usually list the performance at various static pressures, which is fine by itself.

What you have to be aware of this.... The operating conditions may be changed by how the fan is installed. These are called system effects. Do a Google search and have some fun reading about these!

I got it. Most components have their operating conditions in manufacturer side. System effect is like field condition for customer side. I am google-ing now. :o. Thank you.

[Abby Normal]

you need a fan curve for the blade

[Brian_UK]

If you want to make up your own fan from parts then you will need to spend a lot of money on a design and research facility.

It has however already been done by the fan manufacturers so it is best to use their catalogues and match the fan to your system.

[david2008]

I understand. Thank you all.
David

[david2008]

Hello again everyone,
I am still thinking about this topic.

1.I want to select a fan for condenser, We should know CFM, static pressure. Is that all? (assume diameter and pitch of blade are known)

2.After known these parameters, how we will choose from fan curve? (for example, we will choose the highest efficiency in fan curve? or other item?)

3.Then can we design the fan and motor separately? We will find motor that can provide as much rpm as they can (in same power). We will find the blade have the ‘best’ shape and material. Then combine this fan and motor. It is a ‘perfect’ fan. Am I correct?


Thanks in advance.
David

[Brian_UK]

David, have a look here at an explanation of fan performance curves etc, it may help...

http://www.greenheck.com/technical/files/Product_guide/perf_basics.pdf

[yinmorrison]

Very Diplomatic you are Brian.

[Brian_UK]

I try..............

[david2008]

David, have a look here at an explanation of fan performance curves etc, it may help...

http://www.greenheck.com/technical/files/Product_guide/perf_basics.pdf

Thank you Brian.

"There are individual tables for each fan size and wheel type." So for every combination of blade and motor, they have different table.