For vacuum mixing dryers with rapid temperature changes, metal-bellows-supported mechanical seals and double mechanical seal systems with a controlled barrier medium are especially suitable. For high purity requirements, or where the ingress of a barrier fluid into the product must be excluded, gas-barrier seals or, under suitable operating conditions, gas-lubricated seals may be considered. However, the selection must never be based on the maximum temperature alone. What is decisive is the entire temperature profile, the vacuum level, the rotational speed and axial movement of the mixing shaft, the chemical resistance to the product, solvent and cleaning media, the required tightness, the ATEX and inerting requirements, and the cleaning and maintenance concept.

The dynamic shaft seal is the most demanding sealing point on a vacuum mixing dryer. It must seal the rotating mixing shaft against the vacuum space while simultaneously accommodating relative movements between the shaft, seal housing, bearing and vessel. When the jacket or the mix is heated or cooled rapidly, these components expand to differing degrees and with a time lag between them. The shaft can lengthen axially, the housing can deform radially, and high temperature gradients can arise in the sealing area. An unsuitable seal can therefore tilt, lose preload, wear excessively, or cause leakage.

A metal-bellows mechanical seal is particularly suitable for such applications. The metallic bellows takes up the axial movement and provides the preload for the sliding faces. This allows it to accommodate thermal changes in shaft length without requiring dynamically sliding elastomer O-rings on the shaft. It is precisely these dynamic elastomer O-rings that can become problematic under strong temperature changes, aggressive solvents, product build-up or deposits. They can swell, shrink, harden, or be impeded in their movement by friction and deposits. Metal-bellows seals largely avoid this risk and are therefore used for vacuum, high temperatures, aggressive chemicals and strongly fluctuating process conditions.

The metal bellows itself must be made of a material that is resistant to the respective medium and to the temperature changes that occur. Depending on the application, highly alloyed stainless steels or nickel-based alloys may be considered. The choice depends, among other things, on acids, alkalis, chlorides, solvents, temperature, moisture, possible stress corrosion cracking, and the expected number of thermal cycles. The choice of sliding faces is equally important. Hard-material pairings such as silicon carbide, tungsten carbide or carbon materials can be used depending on the lubrication condition, abrasiveness, vacuum, temperature and permissible contamination. There is no universally valid material pairing, because suitability depends strongly on the product, rotational speed and operating conditions.

For demanding vacuum mixing dryers, a double-acting mechanical seal is often the most robust overall solution. It consists of two sealing stages arranged in series with a controlled gap between them. This barrier chamber can be operated with a barrier liquid or with barrier gas. The second sealing level improves process safety, makes it harder for ambient air to enter the vacuum process, and limits any possible escape of product into the environment in the event of a fault. This is particularly relevant for toxic, flammable, strongly odorous or high-value products.

In a liquid-buffered double seal, the barrier medium fulfils several functions. It lubricates and cools the sliding faces, removes frictional heat, and can buffer temperature fluctuations in the sealing area. A thermosiphon or an external circulation unit ensures that pressure, temperature, liquid level and, where applicable, flow rate remain within the required range. Particularly with rapid temperature changes, controlled temperature management of the barrier medium can significantly improve the service life of the seal. The heat load on the inner sliding faces is reduced, the secondary sealing elements are spared, and the risk of deformation in the sealing area decreases. Double metal-bellows-supported seals thereby combine the thermal compensation capability of the metal bellows with the additional safety and monitoring function of a barrier system.

The barrier pressure must match the sealing concept and the process pressure. With a pressurised liquid barrier, the barrier pressure is usually kept above the process pressure. Under vacuum operation, it must be checked in particular whether barrier fluid could enter the product in the event of an internal leak. In pharmaceutical, fine-chemical or high-purity processes, the barrier medium must therefore be assessed as part of the containment, cleaning and risk analysis. Depending on the application, a suitable thermal oil, a water-glycol mixture or a pharmaceutically acceptable barrier fluid may be used, for example. The choice depends on what consequence a possible trace ingress into the product would have.

Where the ingress of a barrier liquid is undesirable or not permitted, a double gas-barrier mechanical seal can be a sensible choice. Nitrogen is often used as the barrier gas here. The gas stream forms a defined barrier between the process and the environment and can simultaneously contribute to inerting. This is advantageous for oxidation- or moisture-sensitive products as well as for flammable solvents. With rapid temperature changes, however, the gas supply must be regulated especially stably, because the pressure and volume of the gas change with heating and cooling. The barrier gas pressure, consumption, the temperature in the seal chamber and the removal of any leakage flow should be monitored. A dry-running mixer-dryer seal system with a nitrogen or air barrier can use the pressure in the seal chamber as a condition indicator and is available specifically for applications with large temperature changes and thermal expansion.

Gas-lubricated, non-contacting seals, often referred to as dry gas seals, are a special form of gas-barrier technology. Specially structured sliding faces generate a thin gas film under suitable rotation. The sliding faces then barely touch during normal operation. This reduces friction, wear and heat input. Such systems can be attractive under very high purity requirements and suitable operating conditions. However, they require precisely manufactured sealing faces, a stable gas supply, reliable pressure control and adequate operating conditions. Not every gas-lubricated seal is equally suitable for slow-running, high-torque mixing tools. The specific suitability must therefore be assessed together with the seal manufacturer and with consideration of start-up, standstill and emergency operating states.

In addition to the dynamic shaft seal, all static connections must be vacuum-tight. These include manhole covers, inspection doors, sight glasses, flanges, sensor connections, filter housings, heating-jacket connections and discharge units. Various sealing materials are available for these applications. FFKM is suitable for very aggressive chemicals, high temperatures and demanding cleaning media. FKM is a widely used and economical solution for many organic solvents, oils and moderate to higher temperatures. EPDM is particularly suitable for hot water, steam and aqueous cleaning media, but is unsuitable for many oils, greases and non-polar solvents. PTFE and modified PTFE offer very broad chemical resistance and low reactivity, but require suitable structural support because of their cold-flow behaviour. Metallic C-rings, E-rings or spiral-wound gaskets can be sensible at high temperatures, high pressure differentials, or particularly high requirements for dimensional stability, but place higher demands on flange geometry, surface quality and assembly forces.

For static seals, the correct groove geometry is just as important as the material. The seal must remain adequately compressed across the entire temperature range, but must not be loaded so heavily that it is damaged by temperature changes, swelling or permanent deformation. A defined degree of compression, suitable groove depth and width, clean surfaces, even flange bolting and controlled tightening torques are therefore essential prerequisites for lasting vacuum tightness. For critical connections, PTFE-jacketed seals with an elastic core or metal-reinforced seals can combine the advantages of high chemical resistance with additional dimensional stability.

The constructional integration of the seal into the vacuum mixing dryer is at least as important as the seal itself. The sealing area should be tempered as evenly as possible, so that the shaft, seal housing and flange do not operate at strongly differing temperatures. Targeted cooling or temperature control of the seal housing can keep the temperature of the secondary seals, bearings and sliding faces within the permissible range. At the same time, axial and radial thermal expansion should be taken into account in the design. Large mixing shafts can elongate significantly under large temperature differences. Metal-bellows seals are particularly suitable for this, because they can accommodate the axial movement elastically. One example of a seal for horizontal dryers and reactors is explicitly designed to reliably maintain vacuum during large temperature changes and thermal expansion.

A robust sealing concept also requires continuous condition monitoring. For barrier-liquid systems, at least the barrier pressure, temperature, liquid level and, where applicable, flow rate should be monitored. For gas-barrier systems, gas pressure, gas consumption, gas quality and exhaust routing are relevant. In addition, differential pressure between the process, barrier chamber and atmosphere, temperature at the seal housing, bearing temperature, vibration and drive torque can provide important indications of wear, leakage, product build-up or shaft distortion. Alarm limits, shutdown logic, inspection intervals and a preventive maintenance concept should be defined as early as the design stage.

In practice, for a vacuum mixing dryer with frequent and rapid temperature changes, a double metal-bellows mechanical seal with a monitored barrier system is often the preferred default solution. It accommodates thermal shaft movement, avoids dynamically sliding elastomer O-rings, and offers high process safety through the second sealing level. A liquid-barrier design is advantageous where cooling and lubrication of the sliding faces are particularly important and a compatible barrier medium can be tolerated. A gas-barrier design is preferred where barrier liquid in the product is undesirable, or where the process is inerted with nitrogen in any case. For extreme temperature changes, aggressive media and high vacuum requirements, metal-bellows-supported designs are generally more robust than classic pusher seals with dynamically moving elastomers.