How does a roughing pump and a high vacuum pump work together?

A roughing pump and a high vacuum pump work together in a sequence to achieve and maintain very low pressures, necessary for vacuum furnace operations like those you oversee. Here’s how they work in tandem:

1. Initial Pumping with Roughing Pump: The roughing pump (typically a rotary vane or scroll pump) is responsible for reducing the pressure from atmospheric levels down to an intermediate vacuum level, usually around 10⁻³ to 10⁻⁴ mbar. The roughing pump is not capable of achieving ultra-low pressures by itself, but it efficiently brings the pressure down to a point where a high vacuum pump can take over.
2. Transition to High Vacuum Pump: Once the pressure reaches this intermediate level, the high vacuum pump (such as a diffusion pump, turbo molecular pump, or cryopump) takes over. High vacuum pumps are designed to reach significantly lower pressures, often in the range of 10⁻⁶ mbar or lower. They cannot start directly at atmospheric pressure, which is why the roughing pump is essential for initial pressure reduction.
3. Achieving High Vacuum: The high vacuum pump continues to evacuate the chamber, bringing the pressure down to the ultra-low vacuum levels required for high-temperature and contamination-sensitive processes, such as those used for processing tool steels.
4. Ongoing Support: During operation, the roughing pump often continues to support the high vacuum pump by managing the backing pressure (the exhaust) of the high vacuum pump. This prevents any backflow that could compromise the vacuum quality.

In essence, the roughing pump and high vacuum pump create a “two-stage” vacuum process, with the roughing pump handling the initial load and the high vacuum pump achieving the precise, low-pressure environment needed for applications like heat treatment.