Take the pressure off vacuum systems

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Take the pressure off vacuum systems

Vacuum service imposes certain unique demands on the design of equipment and systems. That is, sizing and design for vacuum differs from that for equipment operating at atmospheric pressure and higher. This article highlights these differences and provides some proven pointers for successful vacuum system design.

Some differences apply to any vacuum operation; others come into play only at deep vacuum, that is, at operational pressures below 0.1 Torr (mm. Hg. absolute).

There are four important general differences:

1. Pressure drop (“delta P” across the entire system from the user point or user equipment to the vacuum source) is a much greater proportion of the operational pressure. Thus, while a pressure drop of 0.1 in. Hg. is considered acceptable for piping operating above atmospheric pressure, it’s wholly unacceptable if the desired vacuum operation were to be at 0.5 in. Hg.
2. A leak in a vacuum system is the reverse of one in a pressure system — air is leaking in not out. Moreover, the air coming into the vacuum system expands because there’s less than atmospheric pressure. For example, 1 SCFM (standard cubic feet per minute) leaking into a vacuum system operating at 0.1 atm. expands to 10 ACFM (actual cubic feet per minute). The deeper the operational vacuum level, the more the in-leaked air expands. This can greatly raise the required capacity of the vacuum source. In contrast, the same leak from a compressed air system wouldn’t increase; compensating for the loss would require only an equivalent small rise in compressor capacity.
3. A vacuum system can only go down so much in pressure — after all, perfect vacuum is just -14.7 psi. Because there’s a limit on how much differential a vacuum source can produce (how hard it can pull), there’s a limit on the velocity of the pumped vapors. This limit, which is the sonic velocity, means that the volumetric capacity of pipe can only be boosted for larger flow rates by increasing the diameter of the pipe. This is why vacuum piping is always larger than pipe used at atmospheric pressure or higher. Usually, there’s a several-fold increase in pipe size.
4. As the operational vacuum level deepens, heat transfer mechanisms disappear. At atmospheric pressure, three mechanisms transfer heat: convection, conduction and radiation. As operational pressure is lowered, they fall away. By 50 Torr, there’s no convection; by 5 Torr, there’s no conduction, leaving only radiation at deeper vacuum levels. This has a profound effect on sizing, e.g., of condensers and heat exchangers.

Heat transfer only by radiation can provide some practical benefits, such as when a vacuum pump is used with a furnace. The furnace might be operating at 1,000°F but the vacuum pump won’t have problems below 1 Torr if the piping connecting the pump to the furnace is laid out “optically dense” — that is, there’s at least one 90° turn.

Radiation travels in straight lines and must be adsorbed to heat anything. When operating below 1 Torr, the heat can’t get to the vacuum source because the radiation will be adsorbed by the piping. The vapors from the furnace aren’t a heat source as, at 1 Torr and below, they are so dilute the heat quantity carried to the pump is very small (total BTUs are low). Annular pipe arrangements can provide almost “super insulation” if the annular space is evacuated and maintained below 1 Torr.