How Many Blades for Possible Agitation?
Mixing is a big part of the reaction process. The right type of agitator can impact numerous parameters, including product quality, yield, heat transfer, bath time, etc. A tank agitator needs to be perfectly suited to the type of reaction you’re influencing, or it will leave a negative influence on the operation.
Generally speaking, an industrial agitator can be tweaked and optimized for your process. One way of tweaking the agitator is to vary the number of blades, design, angles and spans. Other factors include the location of tiers and running speeds; each variable can be tweaked and adjusted, which will help to improve the performance of the operation. Naturally, NASA is interested in understanding what will influence and have the most impact on agitation. That led them to conduct a study mainly testing various blades and the number of blades needed for ideal fluid transfer. NASA’s findings are interesting, and at the very least, hint at how an industrial tank agitator should be designed for optimal performance.
Note: The impact of using two, three, and four blades were studied using DEM. The study shows that particle velocities are directly influenced by the number of blades in the mixing system.
The Two Blade System
NASA’s study found the two-blade system to be ideal in the sense that it results in a much higher radial and vertical velocity of the liquid’s particles. One and two blade mixers also showed more pronounced three-dimensional recirculation patterns. In other words, the mixing was a lot more thorough.
The solid fraction analysis using the one and two-blade mixer system showed that dilation of the particle bed was the highest when two blades were used. Data from the contact force network and the blade particle force calculations clearly showed that varying the number of impeller blades had a pretty noticeable effect on how force was distributed.
Apart from NASA’s finding, it is interesting to note that the two-blade system is also one that costs the least to manufacture. That’s why our tank agitators rely on using two blades designed to generate a great deal of force.
The 3 and 4 Blades Agitator System
NASA’s study of the 3 and 4 blades found that it resulted in much higher tangential velocity than a two-blade agitator. The data showed that using two or three impeller blades offered optimal mixing performance compared to adding another blade, i.e. the fourth blade based on the relative standard deviation and lacy index of systems.
Interestingly the study found that the granular temperature and the particle diffusivities from the two and three-bladed systems were much higher than for systems that used either one blade or four blades. However, in terms of the cost of manufacturing, the three-bladed system costs more to produce. Three blades also require more power to drive, which does not make it as cost-efficient as the two-blade system. That said, it (the three-blade system) might be ideally suited for some applications.
The four-blade system is deemed to be entirely unnecessary. It does not optimally agitate and but adds significantly higher cost to manufacturing. Plus, it requires significantly more power to drive four blades than two, but it produces far less agitation per dollar spent on power. That’s why most applications don’t require four blades, though they may appear sleek in advertising and promotional materials.
Improving Industrial Tank Agitator Performance
NASA’s research clearly shows that two and, at best, three blades are all that’s needed for optimal performance for most types of agitators. However, other factors influence an agitators’ performance. We’ll quickly go through three of the most common and yet important ones below:
Running It Below Blade Level
Depending on the type of industrial tank agitator being used, it is generally unacceptable to run the mixer when the container’s content is below blade level. It shouldn’t be run at full speed, in particular. When the fluid is around the same level as the blades, the fluid could cost the shaft to skate, resulting in additional stress and finally running out towards the upper end of the agitator or maybe its seal area. That’s what commonly leads to seal failure in most industrial agitators. In some cases, there may even be glass damage to the agitator.
The solution is always to make sure that the container is filled to the right volume before turning on the agitator. If it is a regular problem, then you might want to change the size of the reactor.
When Emptying Out the Vessel
Agitators shouldn’t be run, especially at full speed when the vessel is being emptied out, or it is in the process of slow draining. It is of particular concern if the liquid is at blade level. That said, if extra care is taken to keep the volume at acceptable levels, the agitator can run at full speed. However, make sure that the viscosity does not increase and once the level of liquid drops by around 20% above blade level, the agitator should be slowed to 35% of its max speed.
We are aware of misconceptions regarding the operation of mixing systems when filling. Usually, it is considered normal to start and then run the agitator as it is being filled, provided that the vessel is being filled quickly and the liquid isn’t at blade level.
The agitator should be run slowly when liquid is added, but speed can be increased as the liquid level in the container rises.
Knowing how many blades are optimal for an industrial agitator or any other type is important because it helps cut through all the jargon being used in the industry. Advertisers may have you believe that more blades are better, but the science shows otherwise. Any questions on your mixing or blade needs? Call us today for a free consultation!