GETTING AROUND THE G FACTOR
FLOCCULATION IS the main mechanism in removing turbidity from water. The floc growth is affected by several factors but is largely dependent on intermolecular chemical forces and the physical action induced by agitation.Having progressed from horizontal gates or rakes to vertical rakes, the technology now becoming commonly employed for agitation purposes is high-efficiency impellers. The majority of flocculation specifications call for a process that is based on the G factor, where:
G factor = k. \/ power/vol
This equation represents the shear rate. The G factor is applied to the whole tank, which gives an average shear rate. What it does not give, however, is information about the shear rate distribution in the tank. The upshot of this is that different types of impeller will produce different levels and types of performance at the same G factor.
With shear being the prominent concern in the flocculation process, different impellers display different shear-rate constants, which mean different operating speeds. The speed of the impeller tip is largely of no consequence unless, however, the shearing or breaking up of agglomerated particles that have been formed does not occur.
Looking at the flow velocities, or mean velocity gradients, emanating from a given impeller shows that shear rates are a function of the impeller geometry and these shear rates indicate the relative velocities from one liquid layer to another, and will vary for different impellers.
It is essential for some velocity gradients to be present, as particles would never catch up with each other to form larger floc particles. For example, if one particle has a velocity of 1m/s and another has a velocity of 1.3m/s, they will eventually meet up as they flow throughout the basin.
The important factor is to maintain the fluid gradients below a critical or threshold shear rate that might disrupt or break up floc particles that have been formed.
To evaluate impellers other than the typical horizontal rakes and paddles, it is necessary to review the flow characteristics of various impellers. By reviewing mixing concepts, it becomes clear that it is necessary to analyse the flocculation process from a mixing point of view as well as from the chemistry occurring in the tank. A basic concept is that power is proportional to the flow multiplied by the head (P=Q x H). For a given power, a certain amount of its energy will be used in developing a quantity of primary flow within the tank, with the remaining energy being dissipated as fluid shear.
Certain impellers have much better flow characteristics than others, and each impeller has its specific purpose, depending on the particular process results required. Applications such as dispersing and breaking up gas bubbles in submerged aeration require high shear to optimise the process.
Similarly, basic blending and solid suspension applications require flow rates to generate the mixing contact that is necessary to achieve the desired uniformity or degree of suspension. Therefore, the impeller must be selected on the basis of the particular process requirements.
SPX Process Equipment has applied its Lightnin Laser Doppler Velocimeter to obtain extensive data on the flow and shear characteristics of various types and designs of impellers.
Analysis of the data collected has enabled the company to obtain very detailed profiles of many different impellers and determine primary pumping capacities, total flow generation and relative shear rate profiles.
By scanning the vessel, it is possible to document the parameters that define the impeller's mixing characteristics. This is a very precise method of evaluating the relative performance of impellers.
The work undertaken by SPX Process equipment in its Lightnin Mixers laboratories reveals that the average shear rates of various impellers are proportional to the operating speed. This proportionally constant is called the shear rate constant (see Table 1). These constants illustrate the relative shear imparted by a given impeller. By comparing the relative shear rates, it is possible to operate low-shear impellers, such as the Lightnin A510, at tip speeds more than three times higher than the Rushton Turbine R100 impeller and produce less shear. The rakes, gates and paddles once used extensively in the sewage treatment process bear similarities both to the Anchor R400 and Rushton Turbine, with vertical blades pushing through the liquid.
These create high levels of shear in the vessels, and it is only when limited to very low rotational and tip speeds that they produce reasonably acceptable results. In order to evaluate flow and shear for different impellers, two basic mixing requirements are used:
Where Np=power number
There are two important concepts to be noted from these equations. Firstly, each impeller has unique flow number and power number. Where the power number of a Lightnin A510 in water is 0.3, for the Rushton Turbine it is 5 so at the same speed and diameter a Rushton Turbine will draw more than 15 times the power of the Lightnin A510. Even with a flow number lower than a Rushton Turbine, the A510 will generate the same flow at considerably less power due to its power and flow characteristics.
The second concept is that, for a given impeller design at a constant power level, the flow may be increased by employing a larger diameter impeller. To maintain the power level, the speed must be reduced.
However, as the speed reduces for a given power level, the torque and cost increase and a point will be reached where it is no longer cost effective or practical to keep increasing the size of the impeller due to the tank size.
In the flocculation process, the emphasis is on gentle agitation, where the shear is minimised and flow maximised for every unit of energy imparted to the liquid. By applying the equations set out, it becomes possible to illustrate the relative characteristics of the high-efficiency axial flow impeller versus the Rushton Turbine. The flow or pumping capacity of the impeller will control the suspension and distribution of the particles in the basin.
Axial flow impellers, rakes and gates do not necessarily produce the correct flow regime to achieve good particle distribution throughout the basin. The total flow in the basin will exceed the impeller's primary pumping capability by a factor of two to four times due to the induced flow resulting from the effects of viscosity.
However, a correctly designed system will ensure sufficient flow to utilise the maximum volume of the basin and distribute effectively the incoming flow.
Table 1: Average shear rates of various impellers are proportional to the operating speed
|Impeller designation||Type||Shear rate||Relative tip speeds m/sec
|A200||Pitch blade turbine||5||2.74|