V Blender - V mixer - Full guide

V blenders can be used to mix dry bulk solids. They are particularly used in the following applications :

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V blenders are widespread in the process industries as they provide an easy and often sufficient way of mixing dry bulks solids (powder, granules, pellets...) in batch . They are simple, reliable and somehow easy to clean, thus constitute a good choice for small and medium size companies working in fields of food processing or pharmaceuticals. They can also be used for pre-mixing in any kind of industries, big or small. This page is giving many pieces of information interesting to plant designers, factory owners or factory operators for chosing a V mixer or optimizing an existing unit : applications, working principle, mixing volume, mixing time.

3. V blender working principle

V mixers are rotary mixers, also called tumble blenders. The whole shell, which has a shape of a V and is made of 2 cylinders welded at 75° to 90°, is rotated allowing to mix the components thanks to a diffusive mixing.

There is no agitators inside the V blender, all the movement of the powder is due to the rotation of the shell which creates an avalanche of product. After a certain number of rotation, an optimum mixing is obtained. Due to the absence of agitator, it must be noted that this kind of mixer will perform satisfactorily with free flowing solids, but less with cohesive ones, cohesive solids cannot be sufficiently dispersed with the rotation movement.

These mixers are usually supplied with 2 opening on top of the "V". The V shape helps to discharge easily the blender at the end of the mixing.

Due to the (slow) tumbling and the absence of agitator, the mixing is usually quite gentle for the components, thus one should expect minimal damage of the ingredients. However, the obligation to run free flowing materials can also be detrimental in terms of demixing, a point that must be controlled when validating a V blender.

Figure 1 : V blender drawing

4. Mixing operating parameters

For V mixers, the mixing time is typically 10-15 min. Common problems leading to a long mixing time or not achieving the right Cv Homogeneity is an overfilling of the blender or using materials that are not free flowing enough, or free flowing but with large particles / density differences. The way to load the mixer has also a very high influence on the mixing time.

The mixer performance, i.e. time to reach a desired homogeneity, is a function of the following operating parameters :

• Mixing batch size : 40%  to max 70% of mixer total volume. It is very important to leave free space in a rotary, free falling mixer. The solids need indeed to have space to roll over and the level of bulk solids should not be too high to allow a good material diffusion in between the 2 "legs" of the V blender. Filling at 70% may already be too high for some mixture, thus trials must be organized to optimize the filling rate of the blender. Some sources are reporting high increase in required mixing time due to an overfill of the mixer : 3 times longer at 60% vs 40% and 2 times longer at 70% vs 50%.
• Mixing speed : one should follow the supplier's recommendation, typical mixing speed for medium size blenders (500- l) is around 10-25 rpm, depending on the mixer size, the bigger the slower, with a Froude number < 1. V blenders have a critical speed, where the centrifugal force is matching the gravity force : the optimum mixing speed is reported in the range of 50% to 80% of the critical rotation speed. It should be noted that the rotation rate does not influence much the mixing rate as long as the mixing speed is far from the critical speed. For free flowing powders, the important parameter is therefore more the total number of rotation that will lead to an homogeneous blend : this allows in some cases to approximate a scale-up considering the total number of rotation equal for both sizes (note : this approach is to be used with precaution as far from being valid all the time). For cohesive powders, on the contrary, the rotation rate is important as the shear rate is promoting the mixing.
• It is very important to load such a mixer symetrically. For example, a very long mixing time will be needed if all ingredients 1 are in the 1st "leg" while all of ingredients 2 are on the other side. It is better to proceed by layers and, in case of small ingredients, to try to tip them in the middle of the major ones. If the level is higher than the junction of the 2 cylinders, one should tip equal amount of powder in both of them to optimize the cycle time.

Figure 2 : V mixer recommended and max mixing volume

• V blenders are very well efficient if the small ingredients represent min 1% of the blend size. Lower amount may lead to higher mixing time.

The power input required for a V blender is quite low, in the range of 1-3 kW/m3.

5. V blender - Detailed specifications

Mixer total volume, useful volume and power

The power input required for a V blender is quite low, in the range of 1-3 kW/m3.

V mixers are actually quite standardized among manufacturers, here is a typical range of blenders with the following capacity :

V blender capacity calculation

The capacity of a V blender, in the sense of the throughput in kg/h, is a function of the blender batch size (as explained above, the filling rate is max 40-70% of the water volume of the blender), the mixing time, and the time to load and discharge the blender. A total cycle time can be calculated t=L+M+D, then the throughput can be calculated as m=V*r*ρ*60/t

With :
t = total cycle time in min ; L = loading time in min ; M = Mixing time in min ; D = Discharge time in min ; m = throughput in kg/h ; V = water volume in m3 ; r = filling ratio (0.4 to 0.7) ; ρ = mix bulk density in kg/m3

Loading and discharge

As the whole shell is rotating, the mixer must be disconnected from the rest of the process during mixing. It therefore involves manual operation to load and discharge the mixer. Loading and discharge can be directly done by the operator although this can generate dust emissions. To avoid this, the blender can be located to tipping station and discharge hopper thanks to flexibles, although still requiring some operator's intervention.

Discharge valves

The discharge valve of V blenders is typically a manually operated butterfly valve

Instrumentation

As the mixer is rotating, it is not equipped by many instruments, which don't appear necessary anyway. The control is usually limited to mixing time and rotation speed. To be noted that some suppliers are proposing for NIR sensors to follow the mixing and help optimizing the mixing time. Those sensors are actually in the axis of the mixer.

If the blender is processing powders that can trigger explosions, it must be the object of an ATEX risk analysis. In order to consider the risks associated in the mixer where a risky dust cloud can form but also around it during loading, discharge or in case of powder spill during rotation.

Safety

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The rotation of the blender can constitute a safety risk if an operator is approaching too close. It is necessary to implement a safety cage around the mixer, high enough to prevent people entering the mixing area. The door must be locked when the mixer is rotating, and the mixer should not be started in case the door is opened.

If flexibles are used to load or discharge the mixer, it could be also necessary to have proximity switches that are detecting the connection in order to avoid starting the rotation with the flexibles still on.

6. Special designs of V Blender

Asymetrical design

It is possible to consider 2 cylindrical shells of different length making the blender assymetrical. Such a design can actually help the mixing from one shell to another and thus reduce segregation and decrease the mixing time. This may also be achieved by baffles positionned in the mixer (one should however be careful of the drawbacks of such execution as it can reduce the accessibility and cleanbility of the blender). Note that baffles improve the mixing by reducing the symetry but do not provide additional shear rate.

Intensifier bar

Some manufacturers are proposing to install an agitator along the axis of the mixer and rotating at high speed. This intensifier is bringing shear mixing thus can improve the mixing performance with cohesive powders or mix having the tendency to agglomerate. It must however be noted that having such an intensifier bar will most likely create product breakage, this must be taken into consideration by the operator of the mixer.

7. Common problems with V blenders

V blenders are usually reliable equipment, however a certain number of issues may require some corrections :

Table 1 : common problems with ribbon blenders

Issue Root cause and action Too long mixing time Mixer is overfilled - reduce batch size
Mixing speed is too low - increase mixing speed
Filling sequence is incorrect - make sure the small ingredients are loaded in between majors and that the loading is done by layers see graph above Product damages, breakage Optimize mixing time
Don't use the intensifying bar or reduce its speed

8. V blender buying guide - How to select a V blender

8.1 V blenders for sale : Buying a new V blender

When sourcing a new V mixer for your factory, the following questions need to be asked in order to buy the right specifications :

• What is the expected throughput of the line ? What is the product density to be mixed ? What is the expected mixing time and cycle time ? This will give the size of the V blender to buy. Don't forget that the V blender should not be filled at more than 40-60% of its total volume and max 70%
  
• Is the mixture free flowing ? Or is it necessary to have an intensifier bar to cope with some more cohesive blends ? Was there any trial done to indicate if the use of an asymetrical geometry would be beneficial to the mixing time ?
  
• Is it possible to load the mixer by layer and avoid to have a vertical symetrical loading ?
  
• How fast is the blender to be discharged ? How ? This will give you the inputs necessary for the discharge valve
• Is it in ATEX area ? If yes, the blender must be certified

8.2 Second Hand V blender

Many used V blenders can be found on the market. When looking for a 2nd hand mixer, you should go through the following checks :

• Was the V blender used for a similar application to your needs ?
• Look for damages, scratches on the inside of the shell, hammer marks on the body
• Run the mixer, listen to the bearings, if possible measure vibrations
• If necessary, can the mixer be cleaned ?
• Can the opening doors and outlet valve be modified to accomodate your needs for the fittings
• Test the controls that may be coming with the mixer : mixing timer and mixing speed controller
• Is the mixer ATEX compliant for the area you defined, if not can it be retrofitted

Sources

Experience of the author
Scale Up factor determination of V Blender: An overview, V.S.C. Chopra et al, Der Pharmacia Lettre, , 2(2): 408-433
Scale Up of Powder-Blending Operations, Muzzio and Alexander, Pharmaceutical Technology,

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