04 Central Mixers Mixing Units Tools

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4 Central Mixers 30

4 Central Mixers

4.1 Introduction

Central mixers are generally used when mixing fluids with low to medium viscosities.

These standard mixing units consists of a drive unit, an agitator shaft and a stirring unit.

Figure 6 shows a standard unit. The illustration clearly shows the drive unit, agitator

Figure 6: Assembly of an elementary Central Mixer

shaft, stirring device, the control box complete with the control elements, as well as the

quick release coupling for easy removal of the agitator shaft and the necessary safety

protection in open position.

These central mixing units may be fitted with respective seals and bearings to suit any

specific products or mixing tasks.

Centre mounted mixing units are employed amongst others as clamp on mixers, fitted in

vertical stand units, in mobile bowl mixers and standard mixing units.


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4.2 Standard Mobile Mixing Units 31

4.2 Standard Mobile Mixing Units

Smaller mixing units are often fitted with rollers which ensures a high and easy mobility

when operating from varying locations. Figure 7 shows a mobile mixing unit fitted with

a mixing bowl of 200 litre capacity, models of this type are in general used in the phar-

maceutical industry. The surfaces of the mixing bowl and all other parts in contact with

Figure 7: Mobile Mixer HRZ 200

the product are ground and electro-polished. The illustration on the right is a view of an

open mixing bowl which clearly shows the safety grid fitted as a protecting device whilst

the mixer is in operation. Below the grid the centrally located bottom discharge valveand the eccentric located propeller mixer head is visible.

The control box housing all electronic components is, for easy of operation, also mounted

on the mobile mixing unit. The mixing unit is provided with a handle-bar and in combi-

nation with the rollers ensures maximum mobility.

4.3 Mixing Units

It would be neither impracticable nor indefensible to build a mixing unit to be, in its

entirety, completely mobile in combination with high bowl capacities. As a rule units aredesigned where bowl is the sole mobile component and the actual mixing unit is stationary

mounted either to floor or wall etc.

A 200 litres mixing unit with two central mixers is shown on figure 8. The centrally

located anchor mixer and the eccentrically mounted dissolving mixing unit can be clearly


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4.3 Mixing Units 32

Figure 8: Mixing Unit with two central mixers HRV-S 200

seen in the illustration. The anchor mixer is fitted with additional product transport

devices in order to counter act the poorer mixing action of the anchor mixer thereby

improving the mixing effect within the bowl. The high speed dissolver mixer is located

in such a manner that it can operate freely between the slower moving anchor mixer.

The main duty of the anchor mixer is to improve the heat transfer from the jacketed

bowl to the product. Through high shear forces the dissolver initiates the elimination of

agglomerates in the product.

The jacketed mixing bowl is suitable for heating or cooling operation and fitted with asideways located bottom valve.

The complete mixing head can be hydraulically raised. To aid the feeding of larger solid

additives a rectangular flap has been provided.


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4.4 Special Mixers 33

4.4 Special Mixers

4.4.1 Double Dissolver Mixer HRZ-S 20-ex

Figure 9 illustrates a dissolver mixing unit of wall mounted stand construction. The

Figure 9: Double Dissolver Mixing Unit HRZ-S 20-ex

concept of the unit is the operation in an Ex-area 1 for the processing of highly viscose

pastes in the production area of fuel cells.

The anchor stirrer and the eccentrically mounted dissolver is driven by a 3 kW and a 4

kW motor respectively. The anchor stirrer prevents the product movement towards thebowl jacket and at the same time transports it the direction of the centre of the bowl into

the flow of the two dissolver mixing heads.

The double jacketed bowl is connected to an external cooling system in order to dissipate

the heat generated by the mixing process.


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4.4.2 Twin Shaft Dissolver Mixer HRZ-S 200

The twin shaft dissolver mixer shown in figure 10 is a wall mounted stand which alsois floor supported. The level adjustment of the carriage is hydraulically. The drive has

Figure 10: Twin Shaft Dissolver Mixer HRZ-S 200

an output of 15 kW; the revolutions can be varied by a frequency transformer up to amaximum of 2500 rpm. 200 litres standard bowls with an inner lining serve as mixingbowls. The bowl is fixed by means of a tension belt and there is a safety control viasensors.

The distance between the two dissolver shafts can be changed by means of a hand crankand in the following the mixer can be adjusted to varying products. Four dissolver arebeing used as stirring tools, i. e. two per shaft. They are arranged in such a fashion thatthey intertwine (without touching) at minimal shaft distance.

Dissolver discs have teethon the circumference, and they result in high shear forces at


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high revolutions on the product.

Dissolvers are used when powdery materials are to be mixed homogenously into a liquid,whereby the high shear forces have an desagglomeration effect.

Dissolver mixers are often in use in the paint industry besides the adhesive industry. Theymay also be found in the chemical and the food industry.

4.4.3 Container Mixing Units

Figure 11 shows a central mixing (stirring) unit for containers. The mixing unit is fitted

Figure 11: Container Mixing Unit HRZ-C 1000

with fork cut-outs for fork tucks which provide the means of lifting the unit onto a 1000l container. The folding mixer-head is sized such as to allow for a trouble free fitting

through the small container opening. When in operation the mixer-head will unfold toits full size due to the centrifugal forces (see illustration bottom right). A frequencytransformer for speed control purposes is fitted directly onto the motor.


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4.4.4 Barrel Roll System HFR 200-ex

At times ready mixed products require re-mixing for further processing. If these are de-

livered in barrels and are to be homogenized the so called Barrel Roll System, as shown

in figure 12, thereby eliminating any additional product transfer. The barrel roll system

Figure 12: Barrel Roll System HFR 200-ex

allows for two barrels to be rotated simultaneously. The rotational speed is adjustable

via a frequency transformer.

It is of advantage that a repackaging of the product is not necessary before mixing.

This method of homogenizing presupposes that the product is suited for homogenization,

achieved by drum rotation only, without any additional mixing tools. This is normally

the case for liquid products of low or middle viscosity and free-flowing powder mixtures.

4.4.5 Magnetic Mixing Units

With magnetic mixers the torque transfer takes place via a magnetic field, thus eliminat-

ing the necessity of providing a shaft penetration through the bowl jacket which would

normally be placed eccentrically into the bowl bottom.

Magnetic mixers are employed for mixing low viscosity liquids, which in most cases are

mobile bowl mixing units.


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4.5 Mixing Tools 37

4.5 Mixing Tools

Figure 13 shows several typical mixing tools for central mixers.

Figure 13: Standard Mixing Tools for Central Mixers

Due to the wide range of its jet stream the propeller stirrer is being used to suspend

and homogenise low viscosity liquids. It is suitable for a centric as well as for an ex-centric

installation and to circulate the contents of a large vessel by means of even low powered

engines.

The dissolver or gear disc permits through its shape high local energy inputs and is con-

sequently used in dispersing processes of low and middle viscous products and furthermore

to disagglomerate powdered solids. Due to its low output, other stirring processes re-

main unsatisfactory. In order to improve the product circulation the dissolver disc is often

combined with a second stirrer device.

The trapezium stirrer consists of two blades, reduced at their ends, arranged in a 24

degree angle, which mainly produce an axial and radial stream. It is mostly applied for

homogenising of low and middle viscous liquids.

The oblique blade stirrer is being used mainly with 4 or 6 blades which are fixed at 45

degree and resulting in an axial as well as radial stream. It is being applied for dispersing

as well as homogenising.

The disc stirrer effects a locally concentrated energy supply and this is why it is mainly

used to disperse non-mixable liquids. Its vertical blades produce mainly a radial stream

with two large whirls underneath and above the blades.


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4.5 Mixing Tools 38

The anchor stirrer belongs to the slow motion stirrer devices and its asset is the inten-sive renewing of the wall bordering layer which results in a good heat transmittance. Itsmain stream direction is tangential with whirls at the anchor bars. There are zones withrather low stream speeds between the anchor bars and there is only a fairly weak mixingresult. However, this may be cured by means of additions.

Of course, also product related special designs can be applied. A helical mixer is shownon figure 14. The bottom section of the mixing shaft of the outer helical mixer is fitted

Figure 14: Helical Stirrer for central mixing units

with spirally mounted flutes which ensures an upwards flow of the product, in additionPolyamide wall scrapers can be fitted if so required. The illustrated mixing unit has beenspecially designed for cylindrical bowls fitted with a conically shaped bottom.

Design criteria of the various mixing tools are shown in table 5.

Note the power factor Ne is not a constant, the indicated value refers to a turbulent flowpattern (see fig. 3 on page 18).


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4.6 Application Examples 40

Figure 15: Vacuum Central Mixer HRZV-S 1-2 with rotating bowl

4.6.2 Vacuum Central Mixer HRZV-S 3-12-ex

The central mixer shown in figure 16 is designed for the use in ex-zone 1. This mixingunit is provided with two mixing bowls of varying capacity (3 & 12 litres).

The top illustration shows a 3 litre bowl; the larger bowl (see the bottom illustration) hasa capacity of 12 litres.

In order to permit vacuum operation of both sized bowls an adaptor is provided on thesmaller (3 litre) bowl.

The elevation of the mixing head is adjusted hydraulically. The bottom illustration showsthe hydraulic aggregate which, together with the main control cabinet, is located outsideof the ex-proof area. An ex-proof control cabinet housing the necessary controls for themixer operation is located in close vicinity of the mixing unit.

From the illustration it can be seen that the mixer speed is adjusted by means of amechanically operated variable speed gear box.

The mixing bowls are secured to the base plat, and the fastenings are being scanned for

safety reasons.


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Figure 16: Vacuum Central Mixer HRZV-S 3-12-ex

Top: used with a 3 l bowl

Bottom: used with a 12 l bowl


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4.6.3 Centrally mounted Mixing Unit HRZ-S 4 with rotary mixing bowl

This unit (see fig. 17) is generally used in the product development area of the cosmeticindustry. The internal bowl is rotary pivoted and is easily removed from the outer bowl

Figure 17: Centrally mounted Mixing Unit HRZ-S 4 with rotary mixing bowl

jacket which is fitted with in/outlet connection for an external heating/cooling system,thus providing temperature control means of the product in the inner bowl.

The rotary motion of the inner bowl can be effected either manually or electrically.Through the rotational movement a mixing condition similar to that achieved of a plan-etary mixing unit; a wall scraper is also provided.

4.6.4 Vacuum Central Mixer with Homogeniser HRZV 15-7 HO

This mixer shown in Figure 18 has been designed for the laboratories of two hair cosmeticcompanies. This design is based on the Planetary Mixer HRV 15 HO, however, a 7 litremixing bowl is being used in combination with a circulation Homogeniser HI 35.

Another important difference between the two mixing units is the installation of a centralmixer, this eases the transfer of the laboratory test results to the existing central mixingunits in the production area.

The bowl elevation adjustment manually operated by means of a hand lever.


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Figure 18: Vacuum Central Mixer with Homogeniser HRZV 15-7 HO

A filling funnel is placed into the circulation system of the homogeniser which allowsfor the introduction of flow-able powders into the system. To support the homogeniseroperation a membrane pump may be introduced thereby providing the possibility usingthe homogeniser when processing higher viscous products.

4.6.5 Vacuum/Pressure Central Mixer with Homogeniser HRZVD-S 40 HO

The unit shown in Figure 19 has been designed for the production of low viscous solutionsin the pharmaceutical field. The bowl is designed for a max. working pressure of 10 barand has, among other things, a free dead-space bottom discharge valve.

A centrally located anchor mixing unit is fitted with product lowering conveying elementson the agitator shaft and on the outer anchor, PTFE-scraper elements for the bowl jacketand bottom are fixed to the anchor. A flange in the bowl cover makes an upgrading withan eccentric dissolver mixing unit possible.

Two separate air-lock vessels (2 and 15 litres) with a swing pipe allow the feeding ofliquids and powdery solids.


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Figure 20: Mobile Bowl Mixing Unit HRZ-M 60-ex

4.6.7 Mobile Centrally mounted Mixing Unit HRZ-S 200 HO

The mixing unit shown in Fig. 21 is generally used in the food industry in the production

of soft cheese. This unit is equipped with a heatable double jacketed bowl, a circulatinghomogeniser and a product pump. The controls are laid out to handle up to 50 different

recipes.

The Anchor design is fitted with forwarding transport elements and wall scrapers of

Polyamide.

The entire mixing head complete with the bowl cover can be lifted and lowered in its

entirety by means of electro hydraulic motor.


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Figure 21: Mobile Centrally mounted Mixing Unit HRZ-S 200 HO

4.6.8 Vacuum Central Mixer HRZV-S 250

Figure 22 shows a floor stand version of a vacuum-central mixer. The double jacketed

bowl is fitted with an additional heat protective insulation and can be heated electrically

up to temperatures of max. 140 degree Celsius. The bowl cover contains, among other

things, a CIP-cleaning head, a large dosage funnel and a connection nozzle for compressed

air. Compressed air eases the discharge operation of the mixing bowl. A pressure relief

valve limits the max. excess pressure to 0,3 bar.

A frequency controlled central mixer with a three-winged propeller mixing tool is used

for the stirring process of low viscous products, i .e. herbal wines etc.


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Figure 22: Vacuum Central Mixer as Floor Stand design HRZV-S 250

4.6.9 Vacuum/Pressure Mixing Unit HRZVD-S 150-300-ex

Figure 23 shows a mixing unit which may be operated with both a 300 ltr. or a 150 ltr.bowl. In order to obtain this flexibility an adapter is provided (see illustration-fitted ontoa 150 ltr. bowl) and the mixing head has to be changed to suit the size of bowl.

To achieve a suitable mixing a slow moving anchor design complete with conveying trans-port elements and a dissolver mixer is used. For the 300 ltr. bowl two dissolver mixers incombination with an anchor mixer are used. A motor is used to drive the two dissolverby means of a cog-belt.

This mixing unit is used for the production of pastel paints. The bowl discharging isassisted by a discharging station fitted with a hydraulically operated discharge plug.


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Figure 23: Vacuum/Pressure Mixing Unit HRZVD-S 150-300-ex

4.6.10 Bowl Mixer Unit HRZ-B 400

Figure 24 shows four 400 ltr. bowl mixer units. These units are used to hold the finishedadhesives in motion by means of an anchor design mixer fitted with forwarding transport

element whilst awaiting further processing. The jacketed bowls are heated/cooled froman externally located heating/cooling system. The bowls are, for safety reasons and tominimise heat losses, provided with heat insulation.


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Figure 24: Bowl Mixer Units HRZ - B 400

4.6.11 Coaxial Mixer HRZ-S 400 KO

A coaxial mixer is shown in figure 25. The design is of a self-contained stand construction

with a hydraulically operated mixer head height adjustment.

The slow rotating scraper mixing tool and the coaxial high speed rotating blade mixing

tool are independently operated by two separate electric motors.

The bowl cover is fitted with a hydraulically operated height adjustment thus allowingthe mixing operation to be carried out at various elevations with a closed bowl cover.


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Figure 25: Coaxial Mixer HRZ-S 400 KO

4.6.12 Mobile Mixing Bowl HRZ-M 500

Figure 26 shows a mobile mixing bowl with a total volume of 500 liters. The gear unit is

fitted with a stainless steel housing to enable an easy cleaning. The shaft sealing for theproduct space is affected by tandem mechanical seals. The mixing tool is a pitched paddle

stirrer with a variable speed control. The insulated bowl jacket is heated with steam

provided by an external steam generator. A magnetic valve steam supply is controlled by

a temperature regulator.

Process relevant parameters are printed by a multi-point recorder.

4.6.13 Vacuum Central Mixing Unit with Homogeniser HRZV-S 600 HO

Figure 27 shows a 600 l mixer, whereby the actual mixing unit is designed as a wall

mounted construction; a self-contained floor stand version is also possible. The mixing

unit consists of the following main components - stand, mixing bowl, homogeniser, CIP-

device, vacuum unit and control box.

The elevation of the bowl cover is controlled and adjusted by an electro-hydraulic system.

The bowl is of a triple jacket design whereby the inner jacket is electrically heated.


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Figure 26: Mobile Mixing Bowl HRZ-M 500

Figure 27: Vacuum-Mixing Unit with Homogeniser HRZV-S 600 HO


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The rotor-stator system enables an effective emulsification and, furthermore, it assists thecleansing process of the CIP-system. The control box contains the operational control

equipment as well as the frequency transformer and all the necessary electronic compo-

nents. The programmable control system provides an optimum flexibility for the mixing

of various products.

An anchor stirrer with flexible scraping and additional conveying elements is used as

stirring unit.

4.6.14 Vacuum Central Mixing Unit with Homogeniser HRZV-S 1000 HO

The 1000 litre vacuum-central mixing unit, as shown in Figure 28, is being used mainly

for the production of different cosmetic products, such as lotions etc. It is fitted, among

other things, with an electrically heatable double jacketed bowl, a vacuum unit, a CIP-

cleaning unit, a product thermometer with infrared data transmission, a product pump

and an inline-homogeniser. In addition it is fitted with a rotor-stator-system inside the

mixing bowl, and therefore the mixing unit can not be designed as a planetary mixing

unit. The mixing tool is a low speed anchor mixer with additional conveyor elements.

The design of the mixing unit is a self contained floor stand with hydraulic level adjustment

of the machine head. The bowl is fitted with rollers.

Figure 28: Vacuum Central Mixing Unit with Homogeniser HRZV-S 1000 HO


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4.6.15 Vacuum-Coaxial Mixing Unit HRZV-S 1250 KO-ex

Figure 29 shows a vacuum mixing unit with a 1.250 litres bowl capacity. The unit is used

Figure 29: Vacuum-Coaxial Mixing Unit HRZV-S 1250 KO-ex

in the production of cleaning agents and is fitted with a two motor coaxial drive, which

provides power for a low-speed anchor mixer and a high-speed dissolver mixer.

Furthermore the unit is fitted with a vacuum operation, a CIP-cleaning possibility, a

product pump and an inline homogenizer.

The freestanding support structure has an electro-hydraulic drive for an easy elevation

adjustment of the mixer head. The bowl is fitted with rollers and entry guides.

The complete mixing unit is designed for an Ex-Zone operation, whereby the bowl interioris suitable for an Ex-Zone 0.


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4.6.16 Vacuum-/Pressure Magnetic Mixing Unit HRZVD-B 1300 MA-ex

The pre-mixing vessel for the pharmaceutical industry, which can be viewed on 30, is

Figure 30: Vacuum-/Pressure Magnetic Mixing Unit HRZVD-B 1300 MA-ex

a single jacket 1.300 litres stainless steel bowl with a magnetic mixing unit, which isinstalled eccentrically in the bowl bottom. Power is transmitted by a magnetic field, thusavoiding a shaft penetration, as is the case with conventional central mixing units.

The mixing unit is designed for vacuum as well as for pressure operation and can also beoperated in an Ex-Zone.

The bowl cover of the magnetic stirring unit contains a large lockable dosage flap and

various nozzles.

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04 Central Mixers Mixing Units Tools