How do I determine HP and speed(r.p.m) of agitator

Its called G value which is defined as [p/(u*V)]^0.5, where p is power(W), u is viscosity (cP) and V is volume (m3). You need to know G before you can determine power. From that you can work out rpm

Okay, I can give you a reasonable answer on this as I used to design them for a living. If you want to just measure speed, the obvious answer is to look at the motor speed and confirm the EXACT gear ratio with the gearbox manufacturer. As an alternative you could of coarse measure directly.

As for power, you would not be able to measure that directly, as it would include too many unknown loses, so you would need to calculate it. I am going to assume that the flow is turbulent to simplify things (if not you will need to calculate Reynolds number too). You can then calculate impeller power by: -

P=Np x density x Speed^3 x Impeller Diameter^5

Where Np is a constant the manufacturer should be able to tell you.

Having said all that there is no good way to calculate if this will do what you want it to do. The G factor calcs mentioned earlier are used a bit in water treatment, and can be related to the Kolmogroff microscale of turbulence. This only looks at small scale mixing and not larger bulk flows. You should also consider what you mean by 'homogeneous', is that so it looks mixed, are you promoting a reaction that meeds to occur at the molecular level, are you suspending solids etc.

As you can see this is a relatively complex subject. I would recommend that you either contact a good manufacturer who should be able to advise you on this, or if you are feeling up to it contact Chemineer and see if you can get them to send you their ChemScale design procedure. That is probably the best I have seen for an easy to understand approach to agitation based on what you can see in the tank. However I would also take the opportunity to discuss your actual application with them, as they may have specific knowledge on it.

Although calculating the HP and Speed of an agitator via the G-value is the traditional way, there has been much literature published to show it's deficiencies.

The reasons are simply:

1. it requires the agitator power to be proportional to viscosity

2. the implied scale-up is on a power/volume basis

3. no allowance is made for the impellar size

4. no allowance is made for the impeller type

To remedy this, correlations have been developed which define all attributes of agitation in terms of the impeller Reynolds number.

A good article is found in CEP March 2007, "The G-Value for Agitator Design: Time to Retire it?"