The Conical Vacuum Dryer: Still an enigma!

Recently a client contacted us seeking clarity on the pros and cons of the features being offered by the OEMs of the conical vacuum dryer, aka, the Vrieco-Nauta vacuum dryer, a modern day evolution of the original Dutch cattle feed mixer developed in the 1930’s. Essentially a conical vessel with a turning screw mounted on a slow-revolving ‘orbital arm’, the powder being dried is given a 3-dimensional motion and brought in contact with the heated side walls while maintaining a partial vacuum inside the vessel. Since drying is by contact with the heating surface (the side-walls), heat transfer from the side walls as well as the duration of contact and the level of vacuum maintained in the chamber, can make a huge difference in the drying time, especially during the initial stages of the drying cycle. Drying is often the slowest leg of a process and can present a bottleneck, sometimes spanning days.

The client was confused with the numerous features of a conical vacuum dryer being presented by two competing OEMs, each claiming their design as ‘better’. Given the high price tag of these vacuum dryers, it is important to know what the truth is, and what it is you are paying for. Below are 4 major areas of consideration when you are weighing the pros and cons of these vacuum dryers:

1. The Vessel: usually a sheet of metal (stainless steel, hastelloy, etc.) rolled in one or multiple sections and welded in the form of a cone, the thickness varies from OEM to OEM. Some use lighter gauge with stiffening rings to maintain structural integrity (and meet Code), but may lose out on the dimensional aspect, requiring greater clearance between screw and vessel. While the conductive heat transfer coefficient for a thicker gauge may be lower compared to a thinner gauge with stiffeners, the sturdier design is usually the better choice. Also, be mindful of the cone angle as not all OEM’s design are the same. Everything else being the same, a dry powder will tend to discharge better out the bottom when the cone angle is steep. Another aspect of the vessel that you might keep in mind is the design of the jacket (usually offered either as a full jacket or a welded half-tube with multiple zones).

2. The Cover: this is what takes the weight of the rotating assembly and motors, as well as allows for the transfer of forces back from the rotating screw to the mounting structure for the dryer. As such, a sturdy cover that won’t buckle or warp is critical to the efficient and trouble-free operation of a vacuum dryer. These are either flat plates with stiffeners for structural rigidity, or spun-formed into a dome shape, and may be welded or flanged to the conical vessel below. While the cover does not usually contact the product and therefore does not contribute to the drying process, it may have areas of ‘cold spots’ where the vapors inside can condense. So, trace heating of the cover is something to consider. ‘Radial arm’ heating jacket is also a design option from some OEMs, albeit a less desirable one.

3. The Internals: the design and manufacturing of the screw plays a huge role in the drying efficiency. The screw may be in the form of a cylinder or it may be slightly tapered toward the bottom, which has many advantages. It may be fully welded to the shaft or it may have small gaps where the screw blade and the shaft come together. The screw and the vessel may be machined to a close tolerance, so that clearance between the screw and the wall is small along the entire length of the screw, which accords many advantages and should be taken into account in your comparison.

4. Power transmission from drive motor to the screw: there are two prevalent modes of power transmission, either (a) cogged belt-drive or (b) bevel-gear assembly. The cogged belt design has higher efficiency, quieter operation and oil-free construction. The bevel-gear design transmit the power from the top-mounted motor to the screw using a system of gears immersed in an oil bath. Also, in some cases the speed ratio between the screw and the orbital arm may be fixed allowing for less flexibility in dryer operation. Each system has a variety of sealing elements, with mechanical seals usually being the most expensive.

While all choices and considerations have to be weighed against the extra price for these features, usually the initial trade-off of the less expensive options comes with a steeper operational price tag over the life of the dryer, and sometimes compromised drying efficiency which could carry a hidden price tag. Given the 7-figure investment required of a medium-size vacuum dryer, its best to carefully weigh all your options before making a decision.

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