8 replies to this topic

[dmu82]

Agitator shaft length should be calculated on which basis. I am asking this because on low level alarm (20 %) of tank, agitator is not getting immersed in liquid resulting in frequent damage to bearing due to vibration so we have changed alarm value to 50% instead of 20%. My question is on which basis agitator shaft length to be decided to avoide the damage or how to calculate agitator shaft length
Thanks

[ankur2061]

dmu,

No double postings please. Art Montemayor has time and again requested members not to make double postings. You should be a little diligent while making your postings.

Now coming to your question:

Normally for general purpose turbine type agitator impellers the clearance of the impeller from the vessel bottom (in other words the length of the shaft) is given by the following:

Clearance from the bottom © / Impeller Diameter (D) = 1

For further reference regarding power factor with change in clearance check out the following link:

http://books.google....esult#PPA281,M1

Hope this gives a much better insight.

Regards,
Ankur.

[empower70]

dmu,

Pls. specify application, type of agitator, process description, process parameters, batch/continuous etc. for understanding application. Normally minimum liquid level(which corresponds to minimum batch size) determines the bottom impeller level, which in turn correspond to low liquid level alarm(slightly above bottom impeller).

Regards,

empower70

[ankur2061]

Agitator shaft length should be calculated on which basis. I am asking this because on low level alarm (20 %) of tank, agitator is not getting immersed in liquid resulting in frequent damage to bearing due to vibration so we have changed alarm value to 50% instead of 20%. My question is on which basis agitator shaft length to be decided to avoide the damage or how to calculate agitator shaft length
Thanks

dmu,

I did some study regarding number of impellers, their clearance from vessel bottom and came out with some interesting information. See the attached file which provides a table on how to select number of impellers and the impeller location.

Send your feedback on the usefulness of the attached information.

Regards,
Ankur.

Attached Files

•  Impeller.doc   28KB   477 downloads

[Joyy]

Referring to procedures laid down in Engineering Equipment Users Association (EEAU) handbook, EEUA HAND BOOK NO.9,

Clearance between Impeller & the bottom of the tank:
Ratio of bottom clearance to vessel diameter should be between 0.1 - 0.4 for a filling ratio of 1.0

But for an Anchor Impeller, clearance is determined by mechanical considerations; a clearance of 1in. or 1(1/2) in. is recommended.

Definition of Filling Ratio:
The filling ratio is the ratio of Liquid Depth to Tank Diameter, which is normally between 0.5 - 1.5. A value approx. to 1 is recommended for most purposes. When dispersing gas in a
liquid, however, a filling ratio of 2 is recommended in order to maintain a sufficiently long period of contact between the gas & the liquid. Also note Power Consumption
per unit volume increases as the filling ratio departs from unity. (Ref.: EEUA, page 5).

Another method to determine lower impeller clearance is given in "Handbook of Chemical Engineering Calculations" By - Nicholas P. Chopey, Chapter: Liquid Agitation.
This method takes into consideration liquid level (Z), tank diameter (T), number of impellers and fluid viscosity.

Viscosity, cP (Pa.s):: Max Level, Z/T:: Number of Impellers:: Impeller Clearance (Lower : Upper)
< 25000 (<25), 1.4, 1, Z/3: ---
< 25000 (<25), 2.1, 2, T/3: (2/3) Z
> 25000 (>25), 0.8, 1, Z/3: ---
> 25000 (>25), 1.6, 2, T/3: (2/3) Z

Thus, Impeller Clearance is Z/3 or T/3 based on the above table.

Length of Agitator Shaft = distance between bearing & agitator (lower impeller)

[Jiten_process]

hello all...

let me put forward my views and plz correct me if wrong. The no. of impeller is decided by finding the ratio of (height of liquid level * sp. gravity of liquid)/Internal dia of tank. If this ratio is greater than 1.2 then go for two impellers else take one.

As far as distance is concern, the distance between tank bottom to bottommost impeller centre line has to be 1/3 times of tank inside diameter.

The minimum distance between two impeller has to be same as impeller diameter.

Power no. is the relation of reynold's no. and type of agitator (pitch blade, flat blade etc.). The graphs are easily available for various types of agitator as Nre vs Np.

Sometimes in case of elipticle bottom the bottommost impeller dia is kept smaller compare to upper impeller dia. in order to facilitate mixing in bottom.

I hope this is inline to what is normally practiced. (Otherwise i have to change my design which i have just freezed for my on going project. ). Please share your views.

[latexman]

Agitator shaft length should be calculated on which basis. I am asking this because on low level alarm (20 %) of tank, agitator is not getting immersed in liquid resulting in frequent damage to bearing due to vibration so we have changed alarm value to 50% instead of 20%. My question is on which basis agitator shaft length to be decided to avoide the damage or how to calculate agitator shaft length
Thanks

dmu82,
The agitator shaft length is determined by considering your process requirements (#1) and to maintain good reliability of the agitator (#2); there is no one magic formula. Given the shaft length and impeller blades position and properties, the shaft diameter is calculated to give a comfortable safety factor of running less than the first critical speed. If your application was specified correctly and you purchased an agitator from a reputiable company (Chemineer, Lightnin, etc.), I would recommend getting them involved. However, if the agitator is "homemade", we need a lot more data, as other posters have requested, to help assess the situation.

[ankur2061]

Jiten,

Read latexmans' post carefully. He is giving very sound advice. Agitator design transcends the normal boundaries of engineering design and ventures into regions which are more governed by empirical and experimentally defined design. In other words, it is a highly specialized design area and requires expertise gained over the years by people who have been actually making agitators as commercial equipment.

Involving, reputed manufacturers like Chemineer and Lightnin for your requirement is the right way to get the correct hardware for your application as suggested by Latexman. I can add one more name to the list and that would be "Ekato" a germany based company specializing in agitation/mixing equipment.

Goodluck in your venture to select the right agitator.

Regards,
Ankur.

[v8landy]

Why not just put a tunning fork device in at a level just above the agaitor, which is linked into the agitator stop/start. Therfore if liquid is below agitor it can not be started or it is stoped!