The machines were completely redesigned to withstand the high operating pressure (up to 20 bar). The solution, developed by a German manufacturer, was a pioneering achievement. The sophisticated shaft seal was developed in close collaboration with EagleBurgmann Industries.

Paddle dryers are normally mounted horizontally with bearings at one or both ends. A shaft with paddles tailored to the specific process rotates inside the cylindrical chamber. The heating medium normally flows through a jacket which surrounds the drying chamber. The shaft is often hollow and is likewise heated with steam. Inside the dryer, the paddles continually circulate batches of material which are heated by the chamber walls and the shaft. Evaporated fluid is suctioned off. Paddle dryers are often used for vacuum drying of pasty and high-viscosity materials such as filter cake or thickened sludge from filter systems or centrifuges. There are also pressurized applications, but not usually higher than 6 bar.

The horizontal paddle dryers discussed in this article have bearings at both ends. They are used at a chemical plant in Germany during production of silica products. The silica gel and silica sol are used, for example, in the production of catalysts, paint, varnishes, cosmetics, pharmaceuticals, as filler for plastic and in the beverage and paper industries.

The chemical reaction takes place in the paddle dryers prior to drying of the end product. This means that only one apparatus is needed for two process steps.

• Vacuum to 20 bar operating pressure on the same batch
• Static test pressure up to 42 bar
• 220°C operating temperature
• Explosion proof
• Zero emissions
• Easy to service 
• Long seal life

The paddle dryer is the first of its kind that was designed for such high operating pressures in the harsh silica product manufacturing environment.

A special discharge port had to be developed which is able to withstand the enormous pressure and thermal stress, and it needed to be both rugged as well as easy to maintain. The port now has a proven track record under harsh production conditions.

The vacuum system also needed to meet demanding requirements, and that presented another challenge to the engineering team; besides high suction power, rugged design and low noise emissions, it had to be compatible with the high moisture content of the vapor which is extracted. Standard off-the-shelf pumps are not up to the job. The German manufacturer solved the problem by producing a new series of the moist-air piston pumps which have been part of the company's portfolio for 50 years and which have performed very well in this application.

The seal has to compensate for deflection of the heated shaft caused by thermal expansion, and that was yet another engineering challenge. Also, the paddle shaft is relatively long and is subjected to frequent changes in temperature. As a result, the seal has to compensate for a significant amount of axial movement. Compensation is provided by a roller bellows at the floating bearing end which is attached to the mechanical seal housing.

The sealing of paddle dryers always poses a significant engineering challenge. The machines are generally installed horizontally so that the seal is partially immersed in the product; this means that the product itself needs to be sealed on the one hand and the gas area on the other.

Dual seals with pressurized buffer medium are frequently chosen for these demanding applications. Compliance with explosion protection regulations is often part of the requirements profile.

Burgmann Industries worked closely with the manufacturer to develop the MR35 shaft seal which fully meets the requirements listed above. Burgmann Industries also provided the TS 1016 buffer-fluid system with cooling coil, circulation pump and refill pump. The seal was designed as a cartridge unit that is supplied ready to mount, which simplifies and speeds up installation by the user. The machine is supported at both ends by a bearing which is integrated into the sliding bearing housing. The seal, which features a proven stationary design with stationary seal face and rotating seat, is well able to withstand media with a high solid content (as in this application).

The seal surfaces which come into contact with the product are particularly smoothly polished to prevent product adhesion. As pressures in this application can reach 20 bar, balanced mechanical seals are used on the outboard side.

Hydrodynamic lubrication grooves are a further special feature of the sealing solution. The grooves improve seal lubrication which in turn increases the life of the seal.

In conclusion, the EagleBurgmann team and German manufacturing company worked closely together to come up with a good solution for this demanding application. The systems fully meet the user’s expectations in terms of operating conditions, service life and ease of maintenance. The paddle dryers have been running since 2006, and the user is completely satisfied.

For more information, contact Eric Vanhie, Technical Specialist  at ebnow@eagleburgmannseals.com.