Tip Speed and Centrifugal Compressors, Blowers and Fans

The iirst consideration is compression ratio. Viz:- the higher the tip speed, the greater the difference between suction pressure and discharge pressure. The type of gas and it's condition will also play a part. If you need a fan to deliver air at say 1" water guage or a fan to deliver air at say 10" water gauge, then obviously you will need to rev up the first fan up a bit to get the required "tip speed" The same rule applies to a centrigugal compressor, such as in a water chiller where generally you cannot "rev" it up to give you sufficient compression ratio to allow for an air cooled condenser and you have to use a water cooled unit to reduce the head pressure to keep within the limits.

Hope tis helps

Phil

You cannot exceed the speed of sound or about 990 ft/sec or shock waves will form that will destroy everything.

Hello Nilesh,

From what I understand, you are asking what is the optimum tip speed or the best compressor? If all vendors are complying with operating parameters, Can I reject Vendor with Higher Impeller tip speed ?? It is not fair without knowing the limitation of Tip speed.

Tip speed is not the only criteria. There could be a reduced interior bore or pipe size matched with a faster smaller impeller, (fast tip speed) that would give the same air power as a slower larger impeller (slow tip speed).

So you cannot say a faster tip speed is 'better' than a slow one, it is just a different spec' that is all.

It may be that the faster tip is made from a stronger steel than a slower one, but the air power output is the criteria to look for, plus of course a machine with a size and price that will fit the space you have and the pocket you have! ;=)

You will often find, as in most machinery, that a slower larger impeller may last longer and need less maintenance?

The limit of the tip speed of an impeller is directly connected to the material used for its manufacture. Usually a faster tip speed impeller has to be of better steel, or thicker steel when compared to a larger tip speed one.

with Fans, the 'swept area rather than strictly speaking, the tip speed is what you need for power without noise. It is useless having a Fan in an office if you cannot hold a conversation in the office after all?

Higher Tip speed - with regard to construction of rotor - Higher tip speed to be accompanied by FEM analysis of the impeller specifically blade profile. The region of higher stress to be known and it should be 30 % lesser to the allowable value for the material preferably.

Manufacturing of the impeller - Impeller blades can be brazed, welded, riveted etc. For a given stress value how it can be managed.

Impeller test speed to be used for the tip speed calculation and not the design or operating speed.( it is based on the steam turbine or motor drive). Refer to API 617 - The impeller alone to be tested in the test stand 10 % above the maximum speed it will be subjected and the bore of the impeller dimension to be checked for any deformation ( ovality).If higher speed are used the impeller can fail during manufacturing.So lower the speed and increase the dia. Again check the FEM design for the stress levels.

Sonics - The speed of the gas in the end of the scorll - outlet mouth before the flange,limits the speed upto which you can increase.

The combination of speed, diameter and the velocity of gas at the mouth to be related.Fix the velocity of gas based on the dia of mouth (for the selected model) and find the tip speed. Vary the dia and the speed suitably according to driver.In some combinations smaller impeller ( smaller bore) higher speed can lead to ovality problems.