Conductance
VSL, a company that provides solutions to optimize vacuum system design, provides information about vacuum technology.
Hey there, today we're going to talk about one of the key elements of a vacuum system: Conductance ($C$).
What is Conductance ($C$)?
Conductance ($C$) is the ability of the piping to pass gas from the pump through it in a given amount of time. Understanding conductance is a very important factor in understanding and optimizing the performance of a vacuum system.
Which pipe will let more gas through?
In the figure above, the pipe on the left will let more gas through. We can say that the left pipe has good conductance ($C$). The way we express this pipe conductance ($C$) is in units like L/sec, ft³/min, m³/hr, etc., which means the amount of gas passing through the cross-sectional area of the pipe per hour.
A vacuum system consists of a chamber, piping, and a pump. The inside diameter of the piping can be small or large and must be matched to the pump, so even if the pump has an evacuation rate ($S$) of 200 L/sec the effective evacuation rate acting on the chamber will be 100 L/sec if the piping conductance ($C$) is 100 L/sec.
How is conductance ($C$) calculated?
Conductance ($C$) can be calculated by dividing 1) pipes connected in series and 2) pipes connected in parallel.
1) Pipes connected in series
The calculated value in a series of connected pipes is equal to
$1/C_{t\ \mathrm{(total)}}=1/C_{1+1}/C_{2}$
For example, if a 100 L/sec pipe and a 100 L/sec pump are connected in series, the total conductance ($C$) is calculated as follows
$1/C_{t}=1/C_{1+1}/C_{2}=1/100+1/100$
Therefore, $C_{t}=1/(1/C_{t})=50\mathrm{L/sec}$
In a vacuum system, if the pipes are connected in series, the overall pipe conductance ($C$) will have a smaller value than the smallest pipe conductance ($C$). This means that if you use a pipe element with a poor pipe conductance ($C$), the whole is determined by that pipe element.
2) Pipes connected in parallel
On the other hand, for pipes connected in parallel, you can simply add the conductance values of each pipe and the calculation will be
$C_{t\ \mathrm{(total)}}=C_{1}+C_{2}$
When connecting in parallel, pay attention to the difference in capacity between $C_{1}$ and $C_{2}$. If there is a large difference in capacity between $C_{1}$ and $C_{2}$, back pressure may occur and cause the weaker pump to fail.
| Series connection | Parallel connection | |
|---|---|---|
| Calculate conductivity | $C_{t}=1/(1/C_{1+1}/C_{2}+\ldots+ 1/C_{n})$ | $C_{t}=C_{1}+C_{2}+\ldots+C_{n}$ |
| Feature | Overall conductance is lower than the conductance of the lowest pipe | Total conductance is the sum of the conductance of each pipe |
| Conductance determinants | Conductance of the narrowest pipe, longest pipe | Conductance of all pipes |
The importance of piping conductance ($C$) and how to optimize it
Piping conductance ($C$) is greatly affected by the structure and size of the piping: the wider the opening, the shorter the length, and the absence of bends, the higher the conductivity. Therefore, when designing piping, it's important to keep the diameter as large as possible and the length as short as possible. Additionally, piping with bends impedes the flow of gas, so it's best to design it as straight as possible. This allows the pump to effectively move more gas through it.
Conclusion
To optimize the performance of your vacuum system, it is important to understand and apply the correct your piping conductance ($C$). This enables you to design piping with high pipe conductance ($C$) and realize efficient gas movement.