Agitator/Mixer Data Sheet

Hi all,

Today I am going to focus on Agitators/Mixers and how to interpret a data sheet for procuring. This article is largely based on my experience.

Note: 

The vendor/manufacturer decides appropriate mixing equipment for your operation to achieve the desired mixing performance. So it is very vital to give the manufacturer the necessary details of the process. This article will help you to understand what goes into an Agitator and understand its basics, so that you can have effective discussion with your vendors and make the right first choice thus avoiding confusions and misconceptions.

I will strictly focus on Interpretation of Agitators data sheet. This being a very vast topic, it is beyond my scope to discuss its various types and specialties in this article. 

Think & ask yourself:

       1.     What is the purpose of an agitator? How many impellers would I need?

       2.      Components of an agitator?

       3.      What basic details would an agitator manufacturer would require?

Purpose:

Agitator or mixers are of various types and the basic purpose of having them in your industry would be one of the following reasons or sometimes may be to serve multiple purposes.

       1.      Uniform mixing of liquids.

       2.      To keep solid in suspension.

Components:

Motor, gearbox, shaft, impeller all of which forms the mechanical parts of an agitator.
Let’s study them in more detail.

First of all what do you need to run an agitator? A Motor right. It’s as easy as ABC.

Motor:

 Now Motors have fixed rpm (900, 1450 and 2900) based on the number of poles it has. For example, 2 pole motor will have rpm of 2900, 4 pole as 1450 rpm and 6 pole offers 900 rpm. This rpm may vary within a small range around the mentioned rpm depending on the losses inside the motor. The rpm required by your agitator may not necessarily be same as motor’s rpm to serve the process purpose. It may be less or may be more compared to motor’s rpm.  What would you do then? You would then definitely look for something that will reduce/increase its rpm in order to meet the agitator’s rpm requirement. This something is – A Gearbox.

Gearbox:

Gearbox is used in Process industries to match the motor speed with that to the process speed requirement. The combination of gears within the gearbox reduces/increases the rpm to match the requirement. It is thus very vital to get the right gearbox for your process and not get an over sized or undersized one, thus details like make, model, reduction ratio, loading rating and capacity of gearbox are included in the data sheet.
Make:
It is the brand type of the gearbox.
Model:
Model number of the gearbox, typically for cross checking the vendor filled data from the brand catalog/website.
Reduction ratio:
  It is the ratio of motor rpm to the process rpm.
Loading Rating:
The loading rating of the gearbox is the maximum power bearing capacity of the gearbox. It is expressed in the power units - kW or HP.
Service factor: It is the ratio of the Loading rating of the gearbox to the rated power of the motor. It is also called as the safety factor. During the start-up of the system, the torque generated by the gearbox requires a lot of power which is more than the motor power. Thus the gearbox must have this high power bearing capacity in the initial phase and thus service factor is important. It is also known as the safety factor. The minimum service factor usually recommended is 1.5

Coupling:

Coupling ‘couples’ two drive elements to transfer torque/motion between them. The correct gearbox selection ensures that the process runs on required rpm. Thus the output of the gearbox needs to be transferred to the agitator shaft. This transfer is done using Coupling, where the gearbox output is coupled with the agitator shaft.

Shaft:

Gearbox is connected to the shaft. So, the next logical question would be how long my shaft should be? Well, if it is a top entry agitator, then the length of the shaft depends on the vessel height (leaving some clearance at the bottom depending on the type of agitation and the type of fluid handled) including the overhead assembly up to the gearbox.
If it is a side entry agitator, then the minimum length should be half of impeller’s diameter.

Shaft is connected to impeller. So what type of impeller? What would be the diameter of the impeller? It is your call, based on your process application. There are many types of impellers available in the market - pitched blade, hydrofoil type, propeller type, turbine type etc. each having its own characteristics and applicability.

How many impellers would I need? This would depend on the dimension of your vessel and the degree of agitation you aiming for (which would actually depend on the type of system you would be using it for). Discuss it with your vendor before your purchase or place an order, ensuring that the vendor gives you the best fit for your system within your budget and that he guarantees the desired mixing quality as required.

Calculate the pumping flow rate i.e. the rate of mixing (m3/hr) or flow discharged by the rotating impeller.

Critical Speed:

Critical speed is usually considered as 30-35 % over the rpm required in your process. It is at this speed that your agitator would produce a lot of vibrations and noise. Necessary actions should be taken if the process rpm reaches -10% of the critical speed.

For eg:

If the rpm required = 300,

Critical speed should be 1.35*300 = 405 rpm.
Necessary actions to reduce the rpm at -10% i.e. at {405 – (0.1*405)} = 365 rpm. 

Note that, this is just an approximate calculation showing the significance and importance of this parameter. However, in the plant noises and vibration would serve as an indication of increasing speed and immediate action has to be taken at that time to avoid any mishaps.

For top entry agitator:

Pumping rate (m³/hr)

Where,
N_Q = Flow number
D_I = Impeller Diameter (m)
rpm = Speed of an impeller (rpm)

Bulk fluid velocity can be calculated using the following equation:
 

                     

Where,
D = Diameter of the vessel

Bulk velocity table:

Scale of agitation	Bulk Velocity (m/s)	Application
1	0.03	For uniform mixing of  miscible fluids
2	0.06
3	0.09	For uniform mixing as well as keeping solids in suspension.
4	0.12
5	0.15
6	0.18
7	0.21	Vigorous agitation
8	0.24
9	0.27

Where,
N_p = Power number
rpm = Speed of an impeller (rpm)
D_I = Impeller diameter (m)
S.G. = Specific Gravity of the fluid.

Miscellaneous features:

Packing, Material of construction, tank coupling, motor weight, total weight, torque, bending moment all are provided by the vendor.

Packing:
Mechanical seal or gland packing.

Material of construction:
 MOC is to be defined for the wetted parts (parts in contact with the liquid) which include the shaft and the impeller. MOC is selected by studying the nature and the characteristics of the fluid to be agitated.

Weights:
Motor weight and the total weight of the agitator is required not from process but from structural point of view. The structures/columns/beams are designed to bear the full load of the equipment and its accessories and thus the weight of the equipment is necessary to be included in the specification sheet.