I Blower/Fan Curve Basics & Questions

[Mormont]

Hello All,

I need some basic understanding between theory and practical with respect to blowers.

Blowers usually come with blower curves which give a relationship between Static Pressure and Flow. Usually, SP is inversely proportional to Flow, as in SP increases as flow decreases. Example of a curve is given in the link.

http://kb.eng-software.com/download/attachments/1442159/Figure 2.jpg?version=1&modificationDate=1409856615537&api=v2

Following are the questions;

1. When I take measurements of Pressure and Flow at different RPMs for a blower, I observe that the Flow increases with increase in Pressure and vice-versa. I concluded that this measured Pressure is not same as Static Pressure mentioned in the manufacturer's curve. Can anyone highlight the difference between these 2 pressures?
2. Is the manufacturer's curve independent of RPM? I am asking since there is no mention of it in the curve and usually they give only maximum speed.

My applications would require a certain flow at certain Pressure, should I use the manufacturer's curve or the curve I got when I plot between measured Pressure and Flow and varying RPM.

Thanks in advance!

 

[JRMichler]

Here's a (randomly chosen) blower catalog with more information: https://www.nyb.com/pdf/products/ce.../straight radial/compactGI/231-Compact-GI.pdf. Their blower performance information includes outlet velocity. That's so that you can calculate velocity pressure, which is in addition to static pressure. If the blower outlet is connected to a diverging duct, some or all of that velocity pressure can be converted to static pressure.

When changing blower RPM, you can convert the manufacturer's curve using the fan laws:
Flow is proportional to RPM
Pressure is proportional to RPM^2
Power is proportional to RPM^3

 

[russ_watters]

Welcome to PF!

1. When I take measurements of Pressure and Flow at different RPMs for a blower, I observe that the Flow increases with increase in Pressure and vice-versa. I concluded that this measured Pressure is not same as Static Pressure mentioned in the manufacturer's curve. Can anyone highlight the difference between these 2 pressures?
2. Is the manufacturer's curve independent of RPM? I am asking since there is no mention of it in the curve and usually they give only maximum speed.

A fan curve is all of the available performance characteristics of a fan at a specified RPM. By changing the RPM, you are changing fan curves and actually plotting the system curve. The system curve is the pressure vs flow characteristics of the ductwork system attached to the fan (there is one system curve per system unless you change something about the system; then you'll create another). These curves are roughly perpendicular to each other and where they cross is the current operating point of the system and fan.

Example:

Most manufacturers will provide you fan curves at multiple RPMs. This is sometimes called a "family of curves."

My applications would require a certain flow at certain Pressure, should I use the manufacturer's curve or the curve I got when I plot between measured Pressure and Flow and varying RPM.

Your measurements are real: use them to get what you want out of the system.

 

[Mormont]

Hello JRMichler and Russ_watters,

Thanks a ton for the quick response. On the blower (that I am interested in ) data sheet I was unable to find any info about RPM for the given blower curve. However, on closer look at the table gives me the following:
N_{free blowing}
N_{typ. working point}
N_static

Can you please explain the difference among the three along with their typical practical application (as in where possibly I could use them). Additionally, is it right to assume N_static is the speed related to the blower curve?

Thanks!

 

[Asymptotic]

Hello JRMichler and Russ_watters,

Thanks a ton for the quick response. On the blower (that I am interested in ) data sheet I was unable to find any info about RPM for the given blower curve. However, on closer look at the table gives me the following:
N_{free blowing}
N_{typ. working point}
N_static

Can you please explain the difference among the three along with their typical practical application (as in where possibly I could use them). Additionally, is it right to assume N_static is the speed related to the blower curve?

Thanks!

The more loaded a blower or fan blade is (that is, the greater the mass it is tasked to move) the slower it will run when powered by a given motor. In that context, it appears these three speeds correspond to:

• minimal back-pressure, maximum mass flow (free blowing speed)
• rated back-pressure and mass flow (typ. working point)
• maximum back-pressure, zero mass flow (static)

Do rated speeds increase in this order?

The curve in post #1 is for a centrifugal blower with static pressure in inches of water column versus volume in SCFM( typical of American manufacturer specs), but the speed specifications you've quoted above suggests European equipment. It would be helpful to link out to the data sheet you are looking at so we're all on the same page.

The static speed spec would be for when the fan or blower is operating at an infinite back-pressure (fully throttled; zero flow) condition, and would be one end point on the blower curve. For example, I've annotated a curve from Micronel for their series Y133X-024K1-5 blower (found while searching for "Ntyp working point").

The spec sheet gives speeds of 34880 RPM, 35770 RPM, and 37150 RPM for free, typ. and static conditions, which correlate to flow volumes of 260, and 120 liters/minute (for free and typ; zero flow is assumed for the static condition), and back-pressures ("Stat." pressures) of 5860 Pa (typ) and 7220 Pa (static) with zero back-pressure assumed for free flow.

It's important to understand what Russ points out in post #3 regarding the system curve. What the blower can do - blower specifications - is only half of it. The other half is the "system" - what all it is the blower is blowing into or out of - and the back-pressure it contributes.

 

[Mormont]

Hello Asymptotic,

Thanks a lot for the detailed explanation.
It is in fact a micronel blower I'm looking at and specifically this:

http://www.micronelusa.com/uploads/tx_micronel/U65M2-024KX-5_without_integrated_electronics_01.pdf

Your explanation of the 3 types were very helpful. Now I would like to get the measured Pressure versus Flow plot, and for this I'm confused about the testing methods;
Method 1: I run blower with an occluded end, then vary RPM to set pressure ranging from say 1000 to 13000 Pa (with an increase of 1000Pa) and measure Flow for all respective pressure to plot the curve.

Method 2: I run blower and measure Pressure and Flow simultaneously (I assume this would be like free flow) for plotting between 1000 to 13000 Pa

Are both methods acceptable and only depends on my application? Would I get different plots when I do both? Which of these 2 methods are more suited?

Additionally, I understand that the blower curve is obtained with a constant RPM, but I am unable to understand how the plot is achieved, like how is flow/pressure varied (while testing) to get the other respective value?

Also, between 2 points, how do I physically measure Static Pressure and Dynamic Pressure? Is it even possible?

Basically any and all resources for testing would be helpful.

Thanks in advance!

 

[Asymptotic]

Additionally, I understand that the blower curve is obtained with a constant RPM, but I am unable to understand how the plot is achieved, like how is flow/pressure varied (while testing) to get the other respective value?

Blower speed isn't constant.

In the first method, blower speed is adjusted (13 times, once per test) to yield back-pressure in 1000 Pa increments from 1000 to 13000 Pa at zero flow (output blocked). After each speed/static pressure setting, the blockage is removed, and flow rate measured.

In the second, both flow rate and pressure are measured simultaneously in 1000 Pa intervals from 1000 to 13000 Pa. Each pressure increment will require a progressively higher blower motor speed.

Be aware that the tubing in the above illustrations creates back-pressure, and reduces flow below the 'free flow' specification. My advice is to perform both sets of tests, plot the results, and see what the difference is.

Is the manufacturer's curve independent of RPM? I am asking since there is no mention of it in the curve and usually they give only maximum speed.

Take a closer look at the specs.

Motor voltage range is from 14 to 22 with 18 volts (U_n) shown in bold. Speed, and other specs are valid at the U_n voltage. U65M2 documentation provides three curves from zero to maximum back-pressure for operation at 14, 18, and 22 volts.

Also, between 2 points, how do I physically measure Static Pressure and Dynamic Pressure? Is it even possible?

You may find this explanation of static and dynamic pressure informative.

 

[Mormont]

Hello Asymptotic,

Thanks again for that explanation. That link does a very good job of elucidating the difference between pressures keeping different applications in mind.

Just as a future reference for others, the below link might help as well;

http://www.gasho.org/wp-content/uploads/Blower-Application-Basics.pdf