Why do high vacuum pumps require ‘backing’ or ‘regeneration’ by a rough vacuum pump?

If you are trying to achieve a high vacuum, you might wonder why you often need another pump to get started. Ignoring this crucial step can lead to pump damage and failed processes.

High vacuum pumps require a rough vacuum pump for 'backing' because they cannot exhaust directly to atmospheric pressure; they need a pre-evacuated environment to operate efficiently. For 'regeneration', a rough vacuum pump is used to evacuate the system after a high vacuum pump, like a cryopump, has been warmed up and needs to be brought back online.

From my decade of experience in vacuum pump systems, I have seen many people overlook the symbiotic relationship between rough and high vacuum pumps. It is a fundamental principle of vacuum technology. High vacuum pumps, which operate at very low pressures, are simply not designed to handle the high density of gas molecules present at atmospheric pressure.

What is the difference between rough vacuum and high vacuum?

Are you confused by the terms "rough vacuum" and "high vacuum"? Misunderstanding these distinct levels can lead to incorrect pump selection and system failures. I will clarify their differences.

Rough vacuum typically ranges from atmospheric pressure down to about 1 millibar (mbar), dealing with relatively dense gas. High vacuum, conversely, spans from 10⁻³ mbar to 10⁻⁷ mbar, involving much fewer gas molecules and demanding different pump technologies to achieve and maintain these extremely low pressures.

In my journey through the vacuum pump industry, I have learned that knowing these distinctions is essential for designing effective systems. At atmospheric pressure, gas molecules are very close together, colliding frequently. This is the realm of rough vacuum. Pumps designed for this range, like rotary vane or dry screw pumps, use mechanical means to physically displace large volumes of gas. As you move into high vacuum, the gas molecules become far sparser, meaning collisions are much less frequent. Here, the pumping mechanism shifts. High vacuum pumps, such as turbomolecular pumps or diffusion pumps, rely on momentum transfer or cryogenic condensation to capture and move molecules. These pumps would be overwhelmed by the sheer number of molecules at rough vacuum levels, hence their need for a pre-evacuated space. They simply cannot generate the necessary pressure differential to operate when starting from atmosphere.

Vacuum Level Characteristics

What is the difference between a backing pump and a roughing pump?

Do these terms, "backing pump" and "roughing pump," sound like the same thing to you? While often used interchangeably, understanding their specific roles is key. I will explain their distinct functions.

A roughing pump is used to evacuate a chamber from atmospheric pressure down to the rough vacuum range. A backing pump, however, specifically maintains the low exhaust pressure required for a high vacuum pump to operate effectively. In many systems, a single pump can serve as both.

From my background, starting as an employee at a top company and later establishing Elitevak, I can tell you that while a single pump might perform both tasks, their conceptual roles are different. A roughing pump's job is simply to get the system from atmospheric pressure down to approximately 1 mbar. This initial evacuation stage prepares the chamber for the high vacuum pump. A backing pump, on the other hand, is constantly connected to the exhaust side of a high vacuum pump (like a turbopump or diffusion pump). High vacuum pumps cannot exhaust against atmospheric pressure; they need a vacuum on their exhaust side to function. The backing pump maintains this necessary rough vacuum level, removing the gas molecules that the high vacuum pump compresses and sends to its exhaust. For example, a Roots blower (a type of mechanical booster) is often used with a single-stage rotary vane pump. The rotary vane pump acts as the backing pump for the Roots blower, which itself boosts the rough vacuum. For a Roots pump, the compression ratio typically ranges from 10 to 50, which means it can increase the pressure by that factor towards the backing pump.

Pump Roles in Vacuum Systems

What is a regenerative vacuum pump?

Have you heard the term "regenerative vacuum pump" and wondered what it means? It can sometimes be confusing because the word "regeneration" also applies to other pump processes. I will clarify this pump type.

A regenerative vacuum pump, also known as a side-channel blower, operates by accelerating gas molecules in a toroidal path, causing them to be "regenerated" or re-accelerated multiple times within the impeller, providing a relatively high differential pressure at lower flow rates compared to simple fans. They typically achieve rough vacuum levels.

From my practical understanding of vacuum technology, regenerative blowers are quite distinct from high vacuum pumps or typical positive displacement pumps. Unlike a positive displacement pump that traps and moves a fixed volume of gas, a regenerative pump creates a partial vacuum by kinetic means. An impeller with many blades rotates rapidly within a housing that has side channels. Gas enters through an inlet port and is repeatedly propelled outwards by the impeller blades into these side channels, then flows back inwards, creating a series of energy transfers to the gas molecules. This multiple-stage acceleration allows them to achieve pressures lower than simple centrifugal fans. They are primarily used in rough vacuum applications where a moderate vacuum level is needed, combined with high flow rates. For example, in pneumatic conveying, vacuum lifting, or aerating processes. While they are a type of rough vacuum pump, the term "regeneration" in the context of high vacuum often refers to the process of restoring the functionality of pumps like cryopumps, which requires a separate roughing pump to evacuate the system during the warm-up cycle.

Regenerative Pump Characteristics

Why is a large vacuum pump required to remove moisture from a system?

Are you struggling to efficiently dry a system, especially when moisture is involved? You might be wondering why a larger pump is often necessary. I will explain the unique challenge of water vapor.

A large vacuum pump is required to remove moisture from a system because water, upon evacuation, transitions from liquid to vapor (boiling). This vapor occupies a significantly larger volume than liquid water, demanding a pump with a high pumping speed and capacity to efficiently remove the large volume of gas produced during desiccation, especially in the rough vacuum range.

My experience has shown me that moisture is one of the biggest challenges in vacuum systems. When you lower the pressure in a chamber containing moisture, the boiling point of water decreases dramatically. At very low pressures, water can boil even at room temperature. The crucial point is that when water turns into vapor, its volume expands by thousands of times. For example, one gram of liquid water becomes about 1.25 liters of water vapor at atmospheric pressure. At lower vacuum pressures, this volume becomes even greater. Therefore, to effectively remove moisture, you need a vacuum pump that can handle an incredibly high volume of gas. If your pump is too small, it will take an excessively long time to evacuate the vapor, or the system might not reach the desired vacuum level at all because the pump is constantly overwhelmed by the vapor load. This often necessitates a rough vacuum pump with a high pumping speed to quickly remove these large volumes of water vapor during the initial stages of evacuation.

Moisture Removal Challenges

Closing Summary

High vacuum pumps need rough vacuum pumps for backing because they cannot operate against atmospheric pressure. This crucial partnership, combined with understanding pump types and the demands of moisture removal, ensures efficient vacuum processes.