What is vacuum Vacuum pump?

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In our daily lives, we use merchandise that is produced using a vacuum, including electronic equipment (flat panel TVs and mobile phones), medical equipment (MRI), and food products (vacuum dried vegetables and freeze dried instant foods). Vacuum pumps are used in a broad range of development fields and leading technology in academic, official, and commercial laboratories that develop semiconductor products, aerospace equipment, and medical products.

What is a vacuum?

Vacuum means "the state of a space filled with gas with a pressure lower than atmospheric pressure." This means it is necessary to create a state of pressure lower than atmospheric pressure by forcing gas into an enclosed container using a certain method on the ground, except in the outer space. A vacuum pump is used as this method. The state of gas is expressed by temperature and pressure, regardless of the level compared to the atmosphere. The unit of pressure has the dimension of [force/area], which is basically expressed as [N/m2 = Pa: Pascal] in SI unit.*1 While traditional pressure units express the state of a vacuum in units of [Torr = mmHg], units of [Pa: Pascal] are more popular for expressing the degree of vacuum as the standard measure in engineering system units due to a trend of engineering use in outer space. While pressures are often expressed in reference to the state of the atmosphere [pressure = zero] in engineering system units,*2 the expression of absolute pressure as shown in Table 1 is easy to understand in a vacuum system. Molecules of gas in an enclosed space can never be completely eliminated however low the pressure in the space. Therefore, it is almost impossible to achieve zero absolute pressure. Expressing this with a pressure gauge is extremely difficult to understand.
*1 1 kgf = 9.8 N (Newton)
*2 Pressure containers (boiler, gas cylinder)

Vacuum pump

Gas is vented from an enclosed container into the atmosphere by compressing the gas entered from an inlet with rotational or reciprocal motion in the same manner as a compressor (volume exhaust type). Yet, it is necessary to devise a vacuum pump that uses another mechanism to exhaust gas molecules separately to achieve a high level vacuum in which the density of gas is too low for gas molecules to collide (molecular flow region), while the aforementioned method is effective in a low vacuum in which the density of gas is relatively high (viscous flow region) (refer to table 2). This is an oil-diffusion pump or a turbo molecular pump. An oil-diffusion pump heats oil to generate oil vapor, and then compresses it into a high vacuum region by accompanying gas molecules with a high-speed jet flow blown out from a nozzle. A turbo molecular pump exhausts gas molecules by compressing them into a high vacuum region with the impeller rotating at high revolutions per minute (3,000 to 10,000 min-1). Air in a high vacuum exhaust pump is in the low vacuum region, which is lower than atmospheric pressure at the exhaust port, even if it is in high vacuum region at the inlet port. A low vacuum exhaust pump (oil sealed rotary vacuum pump or dry pump) is connected to the exhaust port as an auxiliary pump to eventually vent the gas into the atmosphere.
(Refer to the paragraph of vacuum exhaust unit)
We will discuss the oil sealed rotary vacuum pump, which is the most popular in academic and commercial laboratories here as described below.

Oil sealed rotary vacuum pump

An oil sealed rotary vacuum pump is also called a rotary pump, which is capable of easily generating vacuums up to around the middle level vacuum (105 to 10-1 Pa) from atmospheric pressure. It has been used and has been quite popular for a long time thanks to its high exhausting efficiency among pumps capable of operating in atmospheric pressure, its simple structure and ease of usage, and the ability to obtain high exhausting rate. While the high-speed model with motor direct drive is compact, the low speed model with the belt drive is also used in applications to draw chemical vapors in chemical experiments thanks to its reliability.

The oil sealed rotary vacuum pump consists of three structure styles, largely categorized into the rotating vane type (Gaede type), oscillating piston type (Kinney type), and cam type (Senco type). As shown in the figure, the rotating vane consists of a rotor and two sliding vanes, and the sliding vanes are pressed into the case with a spring. The oscillating piston type consists of a sliding cylinder installed on the outer circumference of the eccentric rotor, and a sliding plate and an intake valve are attached to them. The cam type consists of an eccentric rotor and a sliding plate, which moves up and down following the rotation of the rotor. In either type, gas is drawn, compressed, and exhausted in a cycle following rotor rotation. The rotating vane type exhausts twice and the others exhaust once per one rotation.

The performance of the pump is determined by the pressure and exhaust speed, which are expressed in the units of [Pa] and [L/min], respectively. The vacuum pump is a pump to draw gas, whose exhaust speed should not be confused with the water intake volume of a water pump (or feed volume). For example, assuming a container of 100 L filled with water: the container becomes empty by feeding water for five minutes with a pump with a feed rate of 20 L/min. On the contrary, depressurizing a container of 100 L with a vacuum pump with an exhaust speed of 20 L/min down to 1/10 bar = 104 Pa takes approximately 11.5 minutes. Gas, unlike a liquid, has a compaction property, which causes the gas inside a container to become thinner as it is exhausted. Constant exhaust speeds of the pump to suck this gas result in the ability to exhaust just the volume of pressure inside the container and atmospheric pressure when it is converted into atmospheric pressure. This means that a vacuum pump with performance of 20 L/min in atmospheric pressure is capable of exhausting just 2 L/min converted to atmospheric pressure when the pressure inside the container drops to 1/10 bar (volumetric efficiency is not considered).  Therefore, gas can never be completely emptied from a container as water can; the gas simply keeps reducing its pressure (reducing density), and eventually the pump reaches its limit pressure. This is the big difference from the pump for liquids.

An oil sealed rotary vacuum pump requires lubrication oil, such as called as vacuum pump oil or rotary pump oil. The lubricant covers gaps between the case, rotor, and other sliding sections with oil film to maintain air tightness along with lubrication and to prevent the gas from reversing from the high-pressure section to the low-pressure section. Any inclusion of condensable gas, such as water vapor in the drawn gas results in a residue of gas, which is liquidized and mixed with oil during compression in the pump due to the continuous compressive action in the oil sealed rotary vacuum pump. This corrodes the components and deteriorates the lubrication property of oil that leads to the main reason for a malfunction. In addition, re-evaporation of the liquid mixed during depressurizing causes a significant reduction in the reachable pressure.

The rotation speed of the direct drive type pump is 1,400 to 1,800 min-1 and that of the belt drive type pump is 450 to 700 min-1, so that the torque of the latter is two to four times greater than that of the former when both are using a motor with the same performance. Therefore, the belt drive type pump is less likely to be subject to contamination from rust or fail to rotate due to insufficient lubrication. In addition, the lower rotating speed results in less heat generation and higher durability. Thus, selecting the pump in accordance with the application (usage pressure and inhaling gas) and environment and encouraging frequent changes of vacuum pump oil may prevent problems even when used outside of normal conditions. However, chances are that the pumps are kept operating in most of cases, which causes failures to operate the next time.

Table 1 Display of pressure

Gauge pressure Absolute pressure Number of molecules
Pa.G Kgf/cm² .G mmHg.G atm ※1 Pa Kgf/cm² mmHg ※2
1.013X105 1.033 760 2.0 2.026X105 2.066 1520 5.357X10 22
0.0 0.0 0.0 1.0 1.013X105 1.033 760 2.679X10 22
-1.012X10 5 -1.032 -759 1.316X10 -3 133.29 1.359X10 -3 1.0 3.524X10 19
-1.013X10 5 -1.034 -759.99 1.316X10 -5 1.333 1.359X10 -5 0.01 3.524X10 17

*1: 1 atm = 1 bar *2: 1 mmHg = 1 Torr

Table 2 Type and operating pressure range of vacuum pump

  Low vacuum Middle vacuum High vacuum Ultra high vacuum
Pressure Atmospheric pressure
~ X0.1kPa
X10 -5 Pa ~
Pump type - Oil sealed rotary pump
- Dry pump
- Diaphragm pump
- Water ring pump*1
- Mechanical booster pump
- Turbo molecular pump
- Oil-diffusion pump
- Turbo molecular pump
- Cryogenic pump
- Turbo molecular pump
- Cryogenic pump
- Ion pump

*1 Operating pressure range of water ring pump: Atmospheric pressure (100 kPa) to 2.3 kPa

Figure Structure and intake and exhaust stroke of oil rotation vacuum pump