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    Monday, 2 May 2016

    [How To] Select a Motor Capacity for Agitator

    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.

    Equivalent Dia : 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.  

    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.

    Bulk Velocity/Pumping Velocity : 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.

    Pumping Rate :  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.

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

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

    If Scale of Agitation is 10, then the effect of agitation is Violent, 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.

    Power Number : 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.

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

    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:

    About The Author

    Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
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    1. 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 .

      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 .

      1. Here D is the Vessel dia, Teq is the equivalent dia,
        Equivalent 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

    2. Hello Ajay,
      I 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.

      1. Dear ,

        First 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.

        AJAY K

    3. Dear ajay

      Above explanation is quite useful.Now we can also discuss design on NRE>1 LAKH.
      Also calculation of flat blade turbine power number.

      1. Dear Anshu,

        you can take power number as 3.60 for a FBT, and the remaining calculation will be same.
        once refer this site:
        AJAY K.

    4. Dear Ajay Sir,
      If you please tell me when(e,g in the time of crystalization, slurrification, suspension) axial mixing and when radial mixing required?

      1. Dear Pratim,

        During reaction it is preferred to go with radial flow, and while extractions / washing better to go with axial flow,
        During crystalization suggested to go with axial mixing with low RPM, while slurrification move with radial flow.

        AJAY K

    5. Hello Sir,

      I am working at Lupin Ltd..

      In one of our product, the isolation is done in centrifuge..The CF is SS 48".. The issue is the wet cake becomes hard and during unloading we need to use scrapper which causes the CF bag damage..

      Is there any means available by which we can avoid the usage of scrapper..

      The solvent is chloroform..

      1. Dear Rohit,

        As we know that its not a safe practice to unload chloroform rich wet material, and if you cant use a scrapper, try it out with a scoop, use an anti-static nylon double bag or else go ahead with bottom discharge centrifuge.

        AJAY K

    6. hi, I need agitator for rectangle tank with volume of 150 liter , can u suggest ?

      1. Dear,

        go ahead with a gate agitator, enhance the sweep dia to 80% of the tank dia.

        Comment with you name next time,

        AJAY K

    7. what to assume D/T ratio?

      1. Dear Sri,

        For better results don't assume, D / T = Agitator Dia / Tank Dia, which you can get from GA drawings.

        AJAY K



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