Magnetic Mixers: What You Need to Know

Our useful guide on everything you need to know about Magnetic Coupled Mixers.

What are magnetic coupled mixers?

Magnetic coupled mixers, commonly called mag mixers, offer highly effective mixing for a wide variety of applications within the pharmaceutical and biotech industries. When used in a closed tank, a traditional top entry mixer has a mechanical and a shaft/impeller assembly that is mechanically coupled to a drive unit. A mag mixer is mounted on the bottom of the tank, does not require a shaft, and has a magnetic coupling between the impeller and the drive unit. This contactless design ensures complete containment and isolation, which eliminates the risk of contamination. As such, it is the ideal solution for demanding hygienic and sterile applications. Steridose mag mixers are suitable for batch sizes ranging from 10 to 40,000 litres.

How does a magnetic coupled mixer work?

A magnetic coupled mixer is driven by two sets of strong magnets. One set is in the impeller (inside the tank) and the other set is located in the rotor (outside the tank). The magnets are separated by a weld plate that is welded into the bottom of the tank and the two sets of magnets are coupled through the weld plate. A male bearing is installed in the weld plate (inside the tank) and the impeller rotates on the male bearing. A matching female bearing is installed in the impeller.

The drive unit is attached to the outside of the weld plate and is the driving mechanism for the mixer. It turns the rotor, which is magnetically coupled to the impeller, and causes the impeller to rotate. The magnets that form the coupling are aligned radially, resulting in a magnetic field that is strong enough to mix liquids up to 1000 cps.

And when you need to uncouple the magnets? The Steridose Magnetic Mixer offers a unique lowering device that allows the user to break the magnetic coupling and remove the impeller without removing the drive unit from the vessel.

What are the most common applications for magnetic coupled mixers?

Thanks to its aseptic design, the mag mixer is commonly trusted for applications with the most stringent requirements for purity. This hygienic design is most relevant for biotech and pharmaceutical applications but, also has some relevancy in certain aseptic food and beverage applications.

Sterimixer^® is the original magnetic coupled mixer designed for biotech and pharmaceutical applications. Some examples of the most common processes where Sterimixer^® is used include:

• Buffer/media prep and hold
• Product hold/storage
• Filtration and purification skids
• Addition of solids that dissolve easily (salts, sugars, etc.)
• Blending of miscibles
• Heat transfer
• Low viscosity products
• Dangerous liquids that require containment
• Settling/separation

Examples of products that are often manufactured using magnetic coupled mixer include:

• Vaccines
• Antibiotics
• IV solutions
• Cell cultures
• Yeast solutions
• Blood products
• Proteins
• Injectables
• APIs

Which type of magnetic mixer is right for my needs?

Steridose offers a range of impeller options to optimize the mixing process. Depending on your requirements, you can select from the following impeller designs:

SMA

• Closed design impeller
• For low shear, high flow applications
• Generates very low shear forces
• Ideal for applications involving shear-sensitive products, such as blood products
• Must be submerged during cleaning

SMO

• Open design impeller
• For low shear, high flow applications
• Creates slight more shear forces than the SMA
• Typically used in buffer and media applications
• Does not have to be submerged during cleaning

SMMS

• Open design impeller
• Medium shear impeller provides higher shear forces
• No gearbox; mixer can run as high as full motor speed
• Ideal for applications where light or hydrophobic powders must be incorporated
• Does not have to be submerged during cleaning

Why should I choose a bottom mount mag mixer?

Bottom mount mag mixers offer significant advantages to the user over traditional top entry mixers.

1. Isolation and containment:

Nothing outside the vessel can get in (isolation) and nothing inside the vessel can get out (containment). To put it simply: what’s in, stays in. And what’s out, stays out.

2. Minimum product loss:

The bottom mount mag mixer guarantees that you’re mixing to the last drop, resulting in almost no wasted product. It reduces the minimum mixing volume significantly and therefore increases product yields.

3. No baffles needed:

The off-center mounting position eliminates the need for baffles, which are always difficult to clean.

4. Batch size versatility:

Processors are often asked to make small batches or pre-mixes in tanks that were designed for larger batches. This can easily be achieved when the mixer is located on the bottom dish.

5. Cleaning:

The design ensures easy and reliable cleaning using standard CIP and SIP procedures.

6. Maintenance:

No overhead equipment is needed as the mixer is installed safely on the bottom dish. Maintenance is safe and easy thanks to the compact, lightweight design. Additionally, the unique magnet lowering device allows the user to break the magnetic coupling without removing the drive unit from the tank. This results in the simple removal of the impeller and male bearing for easy maintenance.

7. Reduced space constraints on fixed tanks: 

The space above a fixed tank is often congested. In some cases, there’s no room for a drive unit or, an alcove must be built into the ceiling to accommodate the extra space required for a mixer above the tank. The space beneath a tank is often wide open and a bottom mount mag mixer occupies very little space.

8. Increased flexibility on portable tanks: 

On portable tanks, the top-mount mixer is often the highest component off the ground. This can lead to a greater risk of the tank tipping over, a more expensive support structure for the tank, difficulty in passing the tank through a doorway, or even a reduced tank size. All of these problems are alleviated when installing a mag mixer on the bottom of the tank.

What information do I need to properly size a mag mixer?

Sizing a bottom mount mag mixer requires the same information as sizing traditional top entry mixers.

Why are you mixing it (agitation requirements):

• Liquid/liquid blending
• Dissolving of powders or other solids
• Solids suspension
• Storage
• Heat transfer
• Mass transfer

Where are you mixing it (vessel and environment):

• Vessel geometry and dimensions
• Materials of construction
• Electrical requirements

What are you mixing (properties of the ingredients and the product):

• Batch size (minimum and maximum)
• Specific gravity
• Viscosity
• Solids (concentration, size, floating, sinking)
• Liquids (miscible, immiscible, shear sensitive)

Steridose’s online sizing tool STIR, lets you size your own mag mixers!

The magnet lowering device looks like a great safety feature. How does it work and can it be retrofitted onto mixers that are in the field?

When the handle is in the lower position, the magnetic coupling is disengaged therefore, the impeller will not spin. When the handle is in the upper position, the magnetic coupling is engaged and the impeller will spin.

The handle is automatically locked in each position. When performing maintenance on the impeller or bearing, there is a specific order of events that you must follow or, you risk damage to the bearings.

For impeller removal, you’d put the handle in the lower position (i.e., disengage the magnetic coupling) first, then remove the impeller and male bearing. If you try to remove the impeller without breaking the magnetic coupling, you’ll be pulling against the strength of the magnets.

For impeller installation, you’d install the male bearing, install the impeller then, raise the handle to the upper position (i.e., engage the magnetic coupling). If you installed the impeller with the magnetic coupling engaged, the magnets will pull the impeller out of your hand and the bearings will crash together.

The lowering device is available for use on standard (white epoxy), stainless steel, explosion-proof, and ATEX drive units. It can be installed on existing mixers in the field that were built as far back as approximately 2008.

Learn more about the magnet lowering device

How does the speed sensor work?

The Impeller Monitor System (IMS) includes a speed sensor, mounting bracket, and cable. The IMS provides a direct measurement of the impeller speed, whereas the screen on the VFD displays the VFD’s output speed.

The bracket is mounted on the drive unit (external to the tank) and the sensor is maintained in the proper position by the bracket. The sensor is aimed directly at the magnets that are inside the impeller, which is located inside the tank therefore, the positioning of this bracket is critical. Each mixer size has its own bracket. As the impeller rotates, the magnets inside the impeller pass by the sensor and are counted. The sensor sends a pulsed signal to a control system or to a digital display (sometimes known as a rate meter) that may be a standalone item or, may be installed in a control panel.

The pulsed signal is converted to a 4-20 mA signal and the actual impeller speed is displayed. For more detailed information on the scaling parameters can be found in the Steridose STIRiverse.

I can read the mixer speed off my VFD – why do I need a separate speed sensor?

In addition to providing the user with the exact impeller speed, the IMS acts as a pass/fail device for the mixer. For example, consider a situation where the impeller has become decoupled and is resting on the bottom of the tank. The display on the VFD would show the speed that the user has selected. The IMS would show the actual impeller speed, which is 0 RPM. Speed sensors that measure the motor speed or the speed of the gearbox shaft have the same shortcoming. The easiest way to ensure that the mixer is running (without opening the manway and breaking sterility) is to use the IMS.

Can I retrofit it onto mixers that are in the field?

The IMS can easily be retrofitted onto existing mixers in the field.

Can we replace the male and female bearing on-site? 

The male bearing is considered to be a consumable part and it can easily be replaced on-site. It simply threads into the weld plate. A PTFE washer is positioned between the male bearing and the weld plate. The male bearing cannot be repaired. If it’s worn or damaged, it must be replaced.

The female bearing is located inside the impeller. It can be replaced however, this procedure is only done at the factory. The user should contact their local Steridose representative or, the factory directly to obtain the proper RMA paperwork. The disadvantage to the field replaceable design is that it typically includes an o-ring that is used as the seal between the bearing and the impeller. Since this o-ring is a wetted part, it provides one additional potential place for contamination to occur. The effectiveness of the procedure is also dependent on the skill of the user’s maintenance staff, which probably does this type of work extremely infrequently.

The disadvantage to the factory replaceable design is the additional shipping cost and additional time waiting for the impeller to be returned. The advantages to the factory replaceable design are one less product contact o-ring and, the work is being done by factory personnel who do hundreds of impeller repairs each year. One other significant advantage is that the factory also repairs scratches, bent blades, etc. then, polishes and electropolishes the impeller, bringing it back to “like new” condition.

Are they a matched set?

The male and female bearings are not a matched set and do not have to be replaced at the same time. They should, however, always be made of the same material. The use of dissimilar materials is not recommended do to the uneven wear that it causes.

Why is Steridose the leading solution in the market?

Steridose is the original innovator of the magnetic coupling mixer and the first manufacturer to apply and fine-tune the technology for the biopharmaceutical industry. There have been copies, but Steridose offers the original tried-and-true product with an excellent reputation.

Steridose knows mixers, mixing, and the markets we serve. We are the only North American manufacturer of mag mixers and are therefore uniquely positioned to serve the world’s largest biopharmaceutical market.

Steridose offers the best pricing in the industry on magnetic mixers made from high alloy materials. We also enjoy excellent relationships with OEMs and engineering firms worldwide.

“We know mixing, mixers, and the market we serve. With technical experts and an excellent rep force, we deliver the best customer service in the industry.” Frank Sequino, Sales Manager