 In this module we will go to discuss the other part of the agitation system as I have told you that the agitation system or agitator consist upon a motor, a regulator, seal, shaft and impeller. But in this whole system impeller has a very vital and critical importance, why? That is why because first is that how many number of impellers and what is the height of the impellers and we have studied in our previous module about the height of the impellers during the design of the fermenter. Apart from the number of impellers and that how many blades are in the impellers, so there are different types of the impellers depending upon their shape, depending upon their construction, so there are different type of the impellers. One impeller that is mostly used in common fermentation that is the russian type impellers are russian turbine impellers. I will talk about in detail about what is russian turbine impellers. So first we have to talk about the motor and mostly we know that there is no manually that a person is sitting beside a fermenter and that manually agitate that with the handle as we have a hand pump, but we have mostly the motors, electric motors and we know that there is single phase motor, three phase motors. So in case of a single phase agitated drive motor that can only for a small fermenters or a reactors, however in case of the large reactors, three phase motors are mostly used or should be used because when there is a three phase motor then there is a less heat generation because when you know that there will be a motor driven and then there will be a heat produced as case of resistance. So that is very critical because motor is attached very near to the body vessel. So if the motor will produce the heat that can affect the temperature of the fermentation vessel. So that is very critical so we have to keep in mind the type of the motor and the capacity of the motor that is very critical because when we have a high power motor that can have a more power consumption that can increase our process cost. So the speed control and the speed reduction are improving that should be attached to that is only case when we have automatic control. Sometimes we have only the monitoring that at what speed that motor is running. Sometimes we have to control our motor with the current supply and then sometimes there are different models of controlling the speed of the agitator. So there are the different methods, engineering methods that can be utilized for controlling the speed. In this diagram we can see here that there is a very large scale industrial scale fermenting having you see 1 impeller, 2 impeller, 3 impeller, 4 and 5 and there is a long series of impellers. You can see here what kind of the impeller these are the blades but here these are the wronging coils that is basically used for the cooling and heating system which we will talk about in later modules but here you can see only we focus on the impellers. As observed that first is the motor and then is a speed controller and then here we will talk about the type of impeller. So impeller are mostly two type, one is radial flow impellers and others are axial flow impellers. On other hand when we talk about the impeller but first we have to thought about the placement of the motor. Sometimes the motor is placed on the top and sometimes and in some fermenter the motor is attached to the bottom. So in both case the purpose is same just to agitate. So there is some positive and pro and some are the prawn in case of top motor and sometimes in the bottom motors. So here you can see that if the impeller shaft can enter from the bottom or from the top. So in this case you can see that in this diagram the motor on the top and the shaft enter from the top of the surface. So in this case it is more expensive to install on the top of that because here in the head space there is a chance of the air exit and exit air in. So that is why the seal is very critical because we have to keep in mind the passage and the space in the seal with respect to the air and the size of the particle. So a reactor with bottom entry impellers however will need higher maintenance because as I have told you that either the shaft enter from the bottom or from the top there is some limitation. So when the shaft enter from the top then there is a more expensive because the seal is very critical but when we fix the motor on the bottom and the shaft enter from the bottom then there is also a limitation because when there is a medium then there is a precipitation of the crystals or those crystals having some mechanical disturbance with the shaft and with the seal and then the life of the seal is very short. So there is a limitation when we have the motor attached to the bottom. So in this diagram we can easily observe that how the shaft when that entered from the bottom of the fermenter because the motor placed on the bottom. So that is why the shaft have to enter from the bottom. So in this case we have the problem of when there is a precipitation or the crystal like in some other salt that present in the medium that can affect and that have a disturbance in the seal quality.