Fan redesign

[srinivasan]

Our designed fan has following parameters,
static pressure = 110 mmWg;
Volume =3500 CMH;
impeller thickness = 5 mm;
motor capacity =12.5 hp/9.3kw;
rated current =17 amps;
efficiency =88.8%;
power factor = 8.5;
coupled drive motor;
pole= 4;
maximum speed @110 mm WG =1460 rpm;

Due to wrong calculation, our persons made a mistake in design of fan with above parameters instead of getting 360 mmWC.
Please give suggestion to re design the fan to achieve 360 mm WG and also clarify ,

1) If we increase the speed the motor by changing the pole of the motor , what is result for pressure.
2) Since it is coupled drive motor, We can not change the motor alone, Am i right?
3) If we increase the speed, Which are the MOC thickness will change?Please give suggestion with out changing the full fan design.
Thank you.

 

[Jraef]

This is not a Mechanical Engineering site.

But if you change the speed of the motor to 2 poles, I can tell you that the POWER requirement of the fan will change by the CUBE of the speed change. So if you require 9.3kW now, at 200% speed you will require 74.4kW to run that fan. In other words, that is not the solution; you need to get a mechanical engineer involved, preferably not the one that was used in the first place...

 

[srinivasan]

This is not a Mechanical Engineering site.

But if you change the speed of the motor to 2 poles, I can tell you that the POWER requirement of the fan will change by the CUBE of the speed change. So if you require 9.3kW now, at 200% speed you will require 74.4kW to run that fan. In other words, that is not the solution; you need to get a mechanical engineer involved, preferably not the one that was used in the first place...

Yeah, We called the fan manufacturer for inspection but what is theory involved in electrical motor. Lets see....

 

[topgone]

Yeah, We called the fan manufacturer for inspection but what is theory involved in electrical motor. Lets see....

What jraef said was that the fan power requirement gets to be boss there. If the fan requires 75kW at twice your original speed, the electric motor should deliver. Else, the magic smoke rises out from the motor windings.:happysad:

 

[rcwilson]

Look at your fan curves, if you have them, and select a fan rpm speed that will deliver the required flow and pressure. Calculate the required power and buy a motor with a variable speed drive, both correctly sized to deliver the required power at the needed speed after accounting for all the increased losses, reduced cooling and other factors.

Or change the fan to a belt drive and select the correct sheave sizes to get the needed speed. change the motor to the new power requirement.

Total cost of either option may be in the same range as buying the correct fan.

 

[cyoung120]

I can tell you that the POWER requirement of the fan will change by the CUBE of the speed change.

Hi All,
I was wondering how is this derived? Does this only apply to fans? Sorry for the basic question, it just that I have seen the above statement twice in two days and was just wondering about it.....

 

[Jraef]

Hi All,
I was wondering how is this derived? Does this only apply to fans? Sorry for the basic question, it just that I have seen the above statement twice in two days and was just wondering about it.....

Read up on the "Affinity Laws".
It relates only to centrifugal machines, known also as "quadratic " because the power relationship to flow and speed is a quadratic equation (remember High School algebra?). So that boils down to centrifugal pumps and centrifugal fans. The load "couples" with the fan blades or the pump impeller vanes more and more as the speed increases, or less and less as it decreases, within the operating curves of the pump or fan of course.

Think of it kind of like a clutch with a variable amount of spring pressure. The faster your engine goes go, the harder the springs press so the less it slips and the more your transmission couples with the engine, so the more power you need from the engine to keep from stalling it. As you slow the engine down, the pressure backs off, the clutch slips more, so the engine needs a lot less power because it's not doing much work.