Fan characteristics

It is generally recommended to keep the RPM of fabricated impellers of Blowers / Fans (as distinct from machined impellers) to less than 2000. Higher RPM's will start causing vibrations in the fan even if the impellers are dynamiclly balanced.

I encountered severe vibration problems in a Blower that was running at 2400 rpm to exhaust the Phosphoric Acid vapors from the Plant Building. After repeated un-successful efforts to solve the problem by dynamic balancing of the rotor I got the rpm reduced to 1200 and found the performance smooth and also efficient exhaust of gases.

Therefore while increase in rpm will increase the flow of gases you may face vibration problems.

Ignoring any duct or outside system effects, fan cfm will vary directly with RPM

cfm1/cfm2 = rpm1/rpm2

fan static pressure varies with the square of RPM

SP1/SP2 = (rpm1/rpm2)^2

and fan power varies with the cube of RPM

HP1/HP2 = (rpm1/rpm2)^3

In the simple case, doubling fan speed double air volume, pressure increase 4X and motor power increase 8X.

In real life, it is not quite that simple due to duct system effects. Your ductwork probably is not large enough to efficiently handle the increase in volume and velocity.

I also would be worried about whether the fan wheel can withstand the stesses of the increased RPM. Be careful that you do not throw a fan blade and destroy the fan and other associated equipment (or personnel)!

I believe one should also keep an eye on what happens to the efficiency of the fan! With increased volume, velocity also will increase which means velocity head (V2/2g) also will increase. Although static pressure increase by 4 fold, available sp will drop due to increased velocity head, neverthelss overall available static pressure certainly increase by a good percentage.

Increased velocity head will cause a lot more problems including increased noise, vibration etc apart from the drop in efficiency.

A major problem will be the balancing! The impeller is dynamically balanced for a certain speed which means it may not necessarily be balanced for a higher speed. This is my understanding and I would appreciate if some expert on this area elaborate more on this in the forum.

If you really wish to experiment I would suggest you increase speed in steps say 10% at a time and see the performance, but not to go beyond 25-30% (Of course provided the fan has an indirect (belt-pully) drive. If it is a direct drive I believe you do not have many options than to take the plunge and hope for the best. Good luck!!

Thanks Reid.

Here we have been in search for the empirical formula that is used to relate Air Horse Power to Motor Wattage. Can you help us?

The supplied information is inadequate. Could ask at least 3 main questions:

1.Is it propeller or centrifugal type fan?

2.Are you supplying air to a space or ventilating(since the air speed will differ especially if you are dealing with supplying air via an air handling unit,internal S.P of coils and ...)?

3.Is it ducted or not(static pressure of the system will be different)?

Knowing the answer to the above question will help to get a better result to your question.

Good luck

Hi Hamid and friends

First: Thanks a lot for your really valuable responces.

Let me explain it..

Its a propeller fan used in evaporative cooler applications. 300 mm sweep size, 5 blade propeller fan. There is no ducting. Its like table fan sort of but placed in closed chamber.

The problem is like this.. The evapoarative coolers 10 ltrs tank capacity have generally centrifugal plastic blowers with unidirectional delivery and around 0.037 sq meter area of delivery. The velocity of air through them is around 6.2 m/s and delivery is 800 CMH.

However we have an application wherein we are using a propeller fan with 300 sweep dia. The air delivered from 0.072 sq meter delivery end is 1200 CMH;but the air velocity is 4.2 m/s. Although the resultant air delivered is more it is PERCIEVED as less.

We need more air velocity at leaset 5.5 or so...and hence looking for some solution.. like rpm increase etc.

Hi,

The main reason for using centrifugal fan beside the quiet and more uniform air delivery is their ability to overcome static pressure and deliver air with higher speed they come in many different type base on of the S.P. requirements.So the best solution to your problem in my opinion is :

1.use a larger centrifugal fan using the formula that other Friends have already mentioned to achieve your goal in parentheses(the thank volume also should increase to achieve the required humidity of the air).

2. Add another proper size unit at suitable position to provide the total required air.

Good luck

The first thing to ask, is will the fan stand up to the higher speed or will it tear itself apart from centrifical force.

Another important point to consider. Given the greatly increased stress loads, will the fan mounting or bearings be able to withstand the stress loading, or will they disintegrate under the load?

Guest:

Anther view of the problem:

What you have is a comfort problem because occupants don't feel comfortable with the current air flow. This is a heat balance problem. Evaporative cooler performance depends on water temperature and air temperature (DB & WB). Of course increasing flow will help you, but have to be sure there is enough water able to be evaporated to get the air cooled. Normally the evaporation media works by capillarity and if the water replacement velocity is not adecuate the media will be dry.

Additional suggestions:

If possible check with fan supplier if the model you have is provided at higher velocity. Propeller fan of this size normally do.

Review the heat balance of the room and try to reduce heat gains. (Isolation, Shadow, relocation of internal heat sources, reduce infiltration, etc.)

Review air exhaust to assure flow balance. May provide some pressure losses reduction.

Install helicopter type fans inside the room to increase perceived air velocity.

Regards

T

The RPM increase will also result to an increase in noise level, might even become intolerable as it is a high pitch kind of noise.