Take the pressure off vacuum systems

Proper design requires attention to often underappreciated issues.

By Henry H. Hesser, Busch, LLC

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Pressure drop

The effect of delta P across the piping system is best viewed by looking at the mass flow through a system. A simple material balance across any vacuum system tells us the mass flow out of the equipment must be matched by the mass-handling capacity of the vacuum source.

Per the ideal gas law, the mass equals the product of the operational pressure and the pumped gas volume. So, for a piece of vacuum equipment operating at 10 Torr with 100 ACFM of vapor being removed by the vacuum system, mass flow is 1,000 Torr-CFM (10 Torr × 100 ACFM). The vacuum source would have to have an equivalent mass-handling capacity, 1,000 Torr-CFM. However, it would have to handle that mass of gas at the vacuum level of the user equipment plus the pressure drop across the piping connecting the equipment to the vacuum source. In our example, if there were a 1 Torr pressure difference due to the design and sizing of the pipe, the vacuum source would have to handle the flow at 9 Torr (the operational pressure of the equipment minus the pressure drop) because the pressure would have to be lower at the vacuum source for flow to be in the direction of the vacuum source.

The same mass flow, 1,000 Torr-CFM, handled at 9 Torr produces a vacuum capacity requirement of 111 ACFM, not 100 ACFM (1,000/9 = 111). So, the 10% pressure drop increases the size of the vacuum source by 11%.

The generally accepted criterion of a well-designed vacuum system is that the pressure drop across the entire system not exceed 10% of the operational pressure.
 

Air in-leak

The expansion of in-leaks makes the requirement of the tightness of a vacuum system much more critical than in a compressed air system. There’s debate in the vacuum industry over the acceptable amount of in-leak. Of course the deeper the level of vacuum used the less in-leak that can be tolerated.

Remember that, besides its impact on the vacuum system, an in-leak also may ruin the product. For example, in the semiconductor industry, oxygen from an air in-leak can chemically react with silane to produce sand, SiO2, not the desired semiconducting solid.

Piping practices suitable for pressure operations often aren’t sufficiently tight for vacuum operation. One common example is the use of threaded connections (Figure 1). While they work well for applications at atmospheric pressure and above, they aren’t good for vacuum because it pulls apart the mating thread surfaces responsible for sealing. The sidebar provides some more tips about piping design, while Figures 2–4 illustrate some proper practices.

Figure 1. Threaded connections shouldn’t be used in vacuum systems unless properly sealed.

Figure 1. Threaded connections shouldn’t be used in vacuum systems unless properly sealed.

Figure 2. For pumps in high vacuum systems, always install a seal disk, as shown above.

Figure 2. For pumps in high vacuum systems, always install a seal disk, as shown above.

Figure 3. Employ a metal bellows connector to take pipe stress out of a system with a piece of rotating equipment.

Figure 3. Employ a metal bellows connector to take pipe stress out of a system with a piece of rotating equipment.

Figure 4. Gap is a tip-off to use of raised face or pressure flanges, which will require “high durable” gasket for vacuum service.

Figure 4. Gap is a tip-off to use of raised face or pressure flanges, which will require “high durable” gasket for vacuum service.

High vacuum design

As the operational pressure deepens all these effects continue to magnify. At really low levels of vacuum (below 0.1 Torr), what may seem like minor things begin to be very important in design:

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