The vacuum tubing between the vacuum pump and the chamber does not normally attract much attention. Yet especially in the fine vacuum range below 1 mbar it is very important for the performance of the complete system. In fact, choosing the optimum tubing with a large cross-section and a short length could make it possible to use a smaller and lower priced vacuum pump. The rubber tubing typically employed for chemical processes is hardly suitable for this kind of vacuum installation. Novel flexible corrugated hoses made of PTFE with an almost smooth inside wall are the ideal connection element for many vacuum applications in the chemical industry involving large vapour volumes or vacuum below 10 mbar.
Dr. Juergen Dirscherl
The saying “a chain is only ever as strong as its weakest link“ is also true for vacuum systems, for example in chemical proces-sing. A powerful vacuum pump with low ultimate vacuum is often chosen with the best of intentions to accelerate a vacuum process. However, the influence of the vacuum components, and especially the vacuum tubing, on overall system performance tends to be neglected.
Many processes in chemical and pharmaceutical laboratories or (mini) plants run at a vacuum level above 1 mbar. Among the typical examples are evaporation (e.g. in rotary evaporators), gel drying and drying processes in vacuum chambers. Chemistry diaphragm pumps – nowadays usually equipped with practical accessories such as an inlet separator, an emission condenser and a vacuum controller – are the most suitable and the most widely encountered type for these applications. The vacuum connection elements need to be selected according to the pumped vapour volume (i.e. sufficient tubing cross-section) and chemical resistance. For a small vacuum chamber (and pump), and for vacuum levels above 1 mbar, the popular rubber hoses with an inside diameter of 6 to 10 mm could still be sufficient to connect the pump and process chamber.
However, rubber hoses reduce the effective pumping speed at the process chamber significantly as soon as the vacuum pump capacity exceeds 5 m3/h. Even in this rough vacuum range (above 1 mbar), the use of vacuum connection elements with a bigger cross section is indispensable for larger pumps. In practice, the tubing diameter should not be smaller than the pump inlet.
The vacuum tubing properties are even more important at lower vacuum levels. One typical example of these chemical processes is lyophilisation, which requires a so-called fine vacuum down to 10-3 mbar. Oil-sealed rotary vane pumps are normally used for fine vacuum generation. If large quantities of vapours have to be pumped, a chemistry hybrid pump like the Vacuu-brand RC 6 is a better choice.
Conventional rubber hoses are hardly suitable for vacuum processes below 1 mbar for several reasons. The small cross-section limits the effective pumping speed. If solvents are pumped, the rubber may be softened, causing the hose to collapse under vacuum. Other chemicals could make the rubber brittle, so that it leaks. In addition, the outgassing of these rubber materials is so severe that the attainable ultimate vacuum of the system is often much worse than that of the pump itself.
Examples
Figure 1 shows the pump-down curves of two-stage rotary vane pumps (specified ultimate vacuum: 2 x 10-3 mbar) on chambers with a volume of 50 l. This volume is fairly typical of a freeze drying chamber (lyophilisation). The curves depict the attainable vacuum over time for various combinations of pump sizes and tubing types. The faster a given vacuum is reached, the earlier the process can be started. The steeper the curve at the process vacuum, the more gas can be removed.
The red curve in figure 1a was determined for a medium-sized rotary vane pump with a pumping speed of approx. 9 m3/h. The pump was connected to the vacuum chamber using a short, corrugated, stainless steel tube with a cross-section (nominal width or “NW“) of 25 mm. This cross-section corresponds to the size of the pump inlet flange. The combination of a short tubing length and a large diameter results in the fastest possible evacuation time (for this pump size). Using a longer (150 cm) corrugated, stainless steel tube with a cross section of NW 16 mm, which is more common, produces the dark green curve. The pump-down time is obviously much longer than before (e.g. more than twice the time needed to reach 0.01 mbar).
It would be far more efficient to use a 6 m3/h pump connected to the vacuum chamber by an NW 16 tube (blue curve) with a length of 50 cm instead of the 9 m3/h pump with a tubing length of 150 cm. The two pump-down curves are nearly identical. As the 6 m3/h pump is much more compact and lighter than the big pump, and also runs far more smoothly and quietly, it is possible to position it closer to the application (e.g. on the lab bench instead of underneath it). Shorter tubing can therefore be used. This smaller pump is usually considerably cheaper than a large one, of course.
The black curve in figure 1a was measured with the large 9 m3/h pump connected to the vacuum chamber by an NW 10 mm rubber hose with a length of 100 cm. The pump-down time increases markedly and the ultimate vacuum of 2 x 10-3 mbar specified for the pump is only rarely reached. Achieving a process vacuum of, say, 0.01 mbar takes ten times as long as under optimum conditions (red curve). The hose employed for this measurement is made of high-quality natural rubber designed especially for vacuum applications. Unfortunately, simple laboratory rubber hoses are often preferred for vacuum connections, leading to even worse results as regards outgassing.
In chemistry R+D it is still quite common to use a glass manifold to connect the various applications. The complete system is frequently evacuated by a large pump positioned underneath the bench. Glass manifolds and their hoses sometimes have a cross-section of only a few millimetres. To illustrate the influence of long narrow tubing, the light brown curve in figure 1b shows the pump-down characteristic for the 9 m3/h pump with an NW 8 mm PE tube and a length of 150 cm.
Especially below 1 mbar, the pump-down time increases dramatically (compared with the red curve). It would be even more advantageous to use a 2.5 m3/h pump close to the application instead of this arrangement (9 m3/h pump with long, narrow tubing). The dark brown curve in figure 1b shows the pump-down curve of a small 2.5 m3/h pump using corrugated, stainless steel tubing with a cross section of 16 mm and a length of 50 cm. This very compact, lightweight and much lower priced 2.5 m3/h pump is a better solution than the large pump with long, narrow tubing for any process vacuum below 0.8 mbar.
Even a short piece of narrow tubing can reduce the overall system performance considerably. The light green curve in figure 1c shows the pump-down curve for a 9 m3/h pump with an NW 10 mm rubber hose only 10 cm long. This short piece is already sufficient to bring about a significant deterioration in performance (compared with the red curve). Once again, it would be more advantageous to use the 6 m3/h pump (blue curve).
It should be noted that the vacuum apparatus itself may contain elements that restrict the gas flow, causing the effective pumping speed at the application to be reduced. Moreover, valves etc. often have only a small opening cross-section and need to be included in the overall consideration.
Figure 2 shows the rotary vane pump types used for these measurements and their relative sizes. In spite of its modern compact design, a more powerful pump always needs more space in the laboratory or mini plant. This kind of pump therefore only makes sense for special applications and only if the complete system is designed for it.
Conclusion
To conclude: the vacuum tubing should have a large cross-section but it should also be flexible. If corrosive media are pumped, these requirements can be quite a challenge. Common rubber hoses are hardly suitable for applications below 1 mbar for the reasons described above. Corrugated, stainless steel tubing is often used instead.
However, the pumped media have a tendency to accumulate in the deep corrugations inside the tubing. This can easily lead to corrosion of the thin-walled stainless steel and ultimately to leakage. PTFE is a far superior tubing material when it comes to chemical resistance. Unfortunately, though, conventional PTFE tubes are extremely rigid and vulnerable to kinking, especially if they have a large nominal width. The Vacuubrand product range now includes flexible tubing made of antistatic PTFE with a large nominal width and an almost smooth inside wall (fig. 3). In addition to boasting excellent fluidic properties, it also prevents the accumulation of condensate. This corrugated PTFE tubing is the ideal vacuum connection element for many applications in the chemical industry, especially those involving large vapour amounts and process vacuum below 10 mbar.
cpp 404
Rotary vane pumps from Vacuubrand
American vacuum society
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