Great day viewers.....!!

Today i wish to have a brief about Motor selection for an agitator, this is quite important topic and before that you need to know how to select an agitator, and right now i'm not interested to demonstrate it as it will divert the topic from this point,

Getting into point, we should be well aware of the basic terminology for this topic, Like Equivalent Diameter, Bulk Velocity, Pumping Rate, Scale of Agitation, Power number, Reynolds Number.

Now, I gonna Elaborate you one by one, upto my knowledge.

**: The defining of Equivalent Dia generally depends on the Pressure loss, Equivalent dia is nothing but just the Diameter of our reactor vessel diameter that gives the same pressure loss as an equivalent diameter of a rectangular vessel.**

*Equivalent Dia*In other simple words, it means that the liquid level inside the vessel is equal to the vessel Dia, ad based on this only the Batch Sizing calculations were done.

*: The Velocity at which the Bulk mass in the vessel mixes/agitates around the shaft after gaining the momentum provided by the Agitator, this will have an high impact on Reaction and Work ups done in day to day production.*

**Bulk Velocity/Pumping Velocity***: The rate at which the Bulk mass gets Pumped inside the Stirred tank, this is partially depends on the HotSpots of the Stirred Tank and the utility in the Jacket of the stirred tank.*

**Pumping Rate***Scale of Agitation*: Scale of Agitation gives the Intensity of Agitation or Mixing, Generally the Scale of Agitation will be in range of 1 to 10.

Scale of Agitation Range Values and their Significance over Agitation :

If Scale of Agitation is 1, then the effect of agitation is

**, and this could be affordable when the operation is a crystallisation, that too if its a supersaturated Crystallisation.**

*Mild*If Scale of Agitation is 3, then the effect of agitation is

*Normal,*this is most probably used for the Dryer's.

If Scale of Agitation is 6, then the effect of agitation is

**, this means the mass that need to be stirred is somewhat denser but the bulk density will be not much.**

*Vigorous*If Scale of Agitation is 10, then the effect of agitation is

**, this is applicable for the mass where there is need for extraction of product in between layers, and also the settling time during this extraction should be higher than regular cases.**

*Violent***: This number signifies the effect of Power consumed over the agitation, in other basic words it is ratio of resistance force to inertial force, it is dimensionless.**

*Power Number***: This number defines the fluid flow, so here it indicates the flow is whether laminar or turbulent.**

*Reynolds Number*So, now you were ready for the Show. I'll start it now,

Things you need to know is the Batch Volume ( V ), Type of Agitator, Agitator to vessel dia ratio ( D/T ), Scale of Agitation, Fluid Density ( rho ) and Fluid viscosity ( myu ).

*Also Read:*

Equate Batch Volume to the basic equation pi/4 x D x D x D., by replacing the Vessel Dia with Equivalent Dia, i.e.,

V = ( pi/4 ) x Teq x Teq x Teq

Then, Teq = CubthRoot ( 4 x V/pi )

Agitator Dia ( D ) = D/T x Teq.

Bulk Velocity ( Vc ) = 6 x Scale of Agitation ft/min.

= 0.03048 x Scale of Agitation m/sec.

Pumping Rate ( Q ) = Vc. ( pi x Teq x Teq )/4 Cu.m/sec

Based on the Agitator, we need to consider the Flow number, find from below table.

From the Flow Number we need to determine the RPM, as shown below,

Nq = Q / ( N x D x D x D),

Agitator Speed , N =Q / ( Nq x D x D x D ),

From this Agitator Speed, calculate the Reynold's Number,

Nre = D x D x N x rho / myu.

rho - Density, myu - Viscosity.

Now after Calculating the Reynolds number, we can get the Power Number from the Agitator Curves, as follows,

From the estimated Power number ( Np ), We can Calculate the Power required.

Power required ( P ) = Np x rho x N^3 x D^5.

You will be getting Power P in KW.

If you can understand all these say cheers, i've done my job, explaining it clearly, if you have any queries please feel free to ask me,

All Comments were Appreciated.

*Also Read:*

**A**bout The Author

Equate Batch Volume to the basic equation pi/4 x D x D x D., by replacing the Vessel Dia with Equivalent Dia, i.e.,

ReplyDeleteV = ( pi/4 ) x Teq x Teq x Teq .

pls. clarify what is D,Teq in above.

Secondly in attached excel sheet if we will take water in batch reactor then vicosity =1 cp ,then reynold's no. will be very high(in lakhs), then what will be the power no. in case of Pitch blade turbine & anchor .

Here D is the Vessel dia, Teq is the equivalent dia,

DeleteEquivalent dia is nothing but just a assumption, for example if you can transform it into a rectangular which will he having same capacity of holding pressure, then the formed circle's dia is the equivalent dia of the vessel,

and what ever the reynolds number you may get don't get shock, it will be correct only

Hello Ajay,

ReplyDeleteI Have to increase the speed of agitator. ( Current detail is Agitator RPM=48, Motor RPM 1400, Agitator type= Anchor with paddle type, Reactor SSR 250 Lit.). We need to increase the RPM up to 75-80 with same agitator. Is it possible to increase the agitator RPM with increasing Motor RPM.

Dear ,

DeleteFirst of all glad to hear from you,

As per the current details, your gear box ration is 29.2,

48 RPM for anchor means the agitation will be some what vigorous,

and you are telling that you need to increase the RPM to 75/80, it is not appropriate to enhance the RPM upto 75/80.

Anyway if you want to increase the RPM, then better change the gearbox, with ration 17.5. you haven't mentioned anything about motor power. as per my knowledge if 75/80 RPM needed, its better to go with a 5 KW motor.

Regards,

AJAY K

Dear ajay

ReplyDeleteAbove explanation is quite useful.Now we can also discuss design on NRE>1 LAKH.

Also calculation of flat blade turbine power number.

Dear Anshu,

Deleteyou can take power number as 3.60 for a FBT, and the remaining calculation will be same.

once refer this site: https://www.chemicalonline.com/doc/flat-blade-turbine-fbt-0001

Regards,

AJAY K.