Vacuum pumps are devices that generate low-pressure spaces by drawing gas molecules out of a sealed chamber. A vacuum can be defined as a state in which a chamber has a lower pressure than the ambient atmosphere or adjacent systems. To acquire a better understanding of vacuum systems, we will provide a brief overview of common types and their applications.
Vacuums have varying degrees of pressure, ranging from a low vacuum with an absolute pressure range of 1 to 0.03 bars to a high vacuum with an absolute pressure range of a billionth of a Pascal. High vacuum systems are normally utilized for laboratory applications such as particle reactors and accelerators. Meanwhile, low and medium pressure vacuums are found in industrial systems such as negative pressure ventilation and incandescent bulbs.
To understand vacuum pumps, it is important to become familiar with the three different types of flow: viscous, transitional, and molecular. Viscous or continuous flow occurs within medium to high pressure vacuums. In this flow, the gas has enough density for gas molecules to collide with one another. Transitional flow, on the other hand, occurs when viscous flow changes into molecular flow. Lastly, molecular flow is defined as the random movement of gasses beyond the dimensions of their chamber.
There are two categories of partial vacuums. The first is a gas transfer or gas feeding type, and the second type is an entrapment vacuum pump. This next section will cover gas transfer pumps in depth. Gas transfer vacuums function by mechanically removing gasses via positive displacement or momentum transfer. Positive displacement consists of chambers that alternately expand and contract with check valves to draw and eject flow. Momentum transfer works by accelerating gasses, creating a low-pressure region. Positive displacement pumps in particular are classified by the motion and design of the chamber. There are two main categories: reciprocating and rotary.
Reciprocating vacuum pumps have chambers that expand and contract via a reciprocating or repetitive back-and-forth motion. Additionally, they are equipped with two one-way valves, one for the inlet and the other for exhaust. As the valves open and close, they allow the build-up of vacuum pressure and the ejection of fluid. There are a few types of reciprocating vacuum pumps: piston, plunger, and diaphragm.
Rotary vacuum pumps, on the other hand, create low-pressure regions as the moving components rotate against the pump housing. The surface between the rotor and housing is coated with self-lubricating or low friction materials such as graphite, PTFE, or PEEK. This clearance prevents fluids from leaking into the low-pressure side. More than that, they have lower pulsating delivery so that the flow is more continuous. Classified by the design of the rotor, the most common types of rotary vacuum pumps include rotary vane, liquid ring, rotary piston, screw, and more.
Momentum transfer pumps operate at molecular flow by forcing the movement of gas or liquid molecules via kinetic energy transfer. They are ideal for applications that require a high vacuum. To create molecular flow, low pressure must be present in the system. Keep in mind that exhaust cannot be released into the atmosphere or at pressures where backstreaming is possible. To remedy this, a backing pump can be installed alongside the vacuum pump, allowing for easy discharge into the atmosphere. There are several types of momentum transfer pumps available today including turbomolecular and diffusion variations.
Entrapment vacuum pumps capture gas molecules through condensation, sublimation, absorption, and ionization. Their working principle consists of operating at high vacuum levels without the risk for oil contamination. Unlike the other vacuums, entrapment vacuum pumps do not rely on rotors or other moving parts to operate, making it so they cannot operate continuously. Also, they do not have the ability to remove lighter gasses such as helium and hydrogen. A few of the most common types include cryogenic, sorption, sputter ion, and titanium sublimation.
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