RU2789162C1 - High vacuum system for industrial and laboratory installations - Google Patents

Abstract

FIELD: vacuum technology.

SUBSTANCE: invention relates to vacuum technology and is intended for the manufacture and sealing of electrovacuum devices (EVD) and other products. The high-vacuum system of industrial and laboratory installations consists of a series-connected vacuum chamber with heaters, equipped with vacuum gauges, a turbomolecular pump, a valve, and a fore-vacuum pump. A vacuum accumulator equipped with a vacuum gauge is installed at the outlet of the turbomolecular pump behind the valve. An electric valve is installed between the vacuum accumulator and the valve.

EFFECT: increasing reliability, simplifying the design of the vacuum system.

5 cl, 3 dwg

Description

The invention relates to vacuum technology and is intended for the manufacture and sealing of electrovacuum devices (EVD) and other products.

The most time-consuming, complex and responsible technological process that forms the electrical and vacuum characteristics of vacuum devices of all types is pumping and thermal outgassing. As a rule, this technological operation is quite long and can last several weeks, which imposes a high load on the high-vacuum pumping system of the process equipment.

Known high-vacuum system based on a diffusion pump (E.P. Sheshin Vacuum technology: Tutorial /EP Sheshin - Dolgoprudny: Publishing House "Intellect", 2009. P. 412-428. Table 6.1, p. 2). The high-vacuum system includes a vacuum chamber, a diffusion pump, and a mechanical fore-vacuum pump (FVP) connected in series.

The disadvantage of such a high-vacuum system is the contamination of the vacuumized product with oil vapor. There is also an increased loss of oil from the diffusion pump. Conditions are created for intensive oil oxidation and gumming of the pump steam lines. All this reduces the operating time between successive preventive cleanings of the installation, which is critical for long-term technological processes and increases the complexity of maintenance, while reducing the service life of pumping equipment. Failure of a mechanical fore-vacuum pump during a long technological process entails a violation of the technological process with a possible loss of vacuum in the vacuum chamber and an irreparable defect of the evacuated and degassed products.

The closest in technical essence is a high-vacuum system based on a turbomolecular pump (E.P. Sheshin Vacuum technologies: Textbook / E.P. Sheshin - Dolgoprudny: Intellect Publishing House, 2009. P. 412-428. Table 6.1, item 1). This oil-free pumping system guarantees the absence of heavy hydrocarbons in the residual gas spectrum, which allows obtaining a high vacuum and improving the performance of the EVP. The vacuum system includes a series-connected vacuum chamber with heaters equipped with vacuum gauges, a turbomolecular pump, a valve, a fore-vacuum spiral or other mechanical oil-free pump.

The disadvantage of such a system, especially with long-term technological processes lasting more than two weeks, is the continuous operation of the FVN due to the duration of the technical process. Long-term continuous operation of dry mechanical FVN, for example, spiral ones, leads to their intensive wear and failure. The failure of the FVN leads to a violation of the technological process and the failure of an expensive EVP. Repair of failed FVN leads to high material costs. Long-term operation of the FVN entails a high consumption of electricity.

The technical result of the invention is to increase the reliability, quality of long-term pumping and evacuation of the EVP, saving the life of the FVN, reducing the irremovable defects, simplifying the design of the vacuum system, and reducing the cost of the process.

The specified technical result is achieved by the fact that it is proposed:

1. A high-vacuum system of industrial and laboratory installations, consisting of a series-connected vacuum chamber with heaters, equipped with vacuum gauges, a turbomolecular pump, a valve, a fore-vacuum scroll or other mechanical oil-free pump, characterized in that a vacuum accumulator equipped with a vacuum gauge is installed at the outlet of the turbomolecular pump behind the valve , and an electric valve is installed between the vacuum accumulator and the valve.

2. High-vacuum system of industrial and laboratory installations according to claim 1, characterized in that the working volume of the vacuum accumulator is selected from the ratio:

,

,

Where

- рабочий объем аккумулятора вакуума, м³;

- working volume of the vacuum accumulator, m ³ ;

- длительность технологического процесса, с;

- duration of the technological process, s;

In ₁ - leakage through the leaks of the system AB + TMN + working volume, Pa m ³ / s;

B ₂ - the flow of the evacuated gas through the turbomolecular pump (TMP), Pa m ³ /s;

P _TMN - allowable pressure at the outlet of TMN, Pa;

P _n - the value of the initial pressure (vacuum) in AB, Pa.

3. High-vacuum system of industrial and laboratory installations according to claim 1, characterized in that a valve is installed between the electric valve and the foreline pump.

4. The high vacuum system of industrial and laboratory installations according to claim 1, characterized in that, when using a rotary vane pump or other pump with an oil seal as a fore pump, a cryogenic vacuum trap is installed between the electric valve and the fore pump.

The invention is illustrated by drawings, where figure 1 schematically shows a high-vacuum system with a mechanical dry FVN; figure 2 schematically shows a high vacuum system with a mechanical FVN with an oil seal and a cryogenic trap.

To substantiate the principle of operation of the vacuum accumulator (AV), a design diagram (figure 3) is shown, consisting of a working volume (vacuum chamber) (VK), a vacuum accumulator (AV), a turbomolecular pump (TMN), a fore vacuum pump (FVN), a valve ( IN).

As AB, a sealed vessel is taken, the volume of which is greater than the working volume VC. A fore vacuum was initially created in the VC and AV by means of the FVN, then a working vacuum was created in the VC by means of the TMN. After that, valve B is closed. Thus, an isolated vacuum system is considered, consisting of a VC with a working vacuum and an AW with a fore vacuum. Both of these vessels are separated from each other by a working TMP.

There are two sources of gas leakage into the AB:

1. Through TMP, which pumps gas from the VC and transfers it to the AB.

2. Leakage of air from the atmosphere, due to leakage of the AB.

1. To calculate the contribution of TMP to the increase in pressure in the AW, the equation of state of an ideal gas is used:

(1)

(1)

where P - gas pressure, Pa;

V is the volume of gas, m ³ ;

- количество вещества (газа), моль;

- amount of substance (gas), mol;

R is the universal gas constant, J/(mol K);

T - gas temperature, K.

At a high vacuum in the VC and a moderate vacuum in the AB, the gas will obey the ideal gas law with great accuracy.

Therefore, from equation (1) it is possible to obtain the amount of gas pumped out by the pump from the VC to the AV:

(2)

(2)

where V _n - the volume of gas that has passed through the pump, m ³ ;

T ₁ , P ₁ - temperature [K] and pressure [Pa] of the gas in the VC;

R is the universal gas constant, J/(mol·K).

V _n, in turn, is equal to the product of the TMP pumping speed and the time of its operation:

(3)

(3)

where v_n - volume of gas passed through the pump, m³;

- скорость откачки ТМН, м³/с;

- TMP pumping speed, m ³ /s;

- время работы ТМН, с.

- operating time of TMP, s.

According to Dalton's law, for the gas evacuated from the working volume, the equation of state (1) can be written:

(4)

(4)

- парциальное давление газа в АВ, откаченного из ВК, Па; Where

- partial pressure of gas in the AW, pumped out of the VC, Pa;

T ₂ - gas temperature in AB, K;

V ₂ - volume AB, m ³ ;

- количество газа, откачиваемого насосом из ВК в АВ, моль;

- the amount of gas pumped out by the pump from the VC to the AB, mol;

R is the universal gas constant, J/(mol·K).

Knowing the amount of gas (2) that got through the TMP into the AB, and taking into account (3), we obtain its partial pressure:

(5)

(5)

- скорость откачки ТМН, м³/с;Where

- TMP pumping speed, m ³ /s;

R is the universal gas constant, J/(mol K);

T ₁ , T ₂ - gas temperature in VC and AB, K;

V ₁ , V ₂ - the volume of VC and AB, m ³ ;

P ₁ - gas pressure in the VC.

Having the dependence of the pressure change in the AV on time, the value of the flow of the pumped gas through the TMP is found. By definition, the amount of leakage is:

(6)

(6)

where V ₂ - volume AB, m ³ ;

ΔP is the change in pressure in the volume AB over time Δt, Pa.

Substituting (5) into (6), we have the final formula for calculating the gas flow entering the AW through the TMP:

(7)

(7)

where ΔP is the change in pressure in the volume AB over time Δt, Pa;

T ₁ , T ₂ - gas temperature in VC and AB, K;

V ₁ , V ₂ - the volume of VC and AB, m ³ ;

- время, с;

- time, s;

P ₁ - gas pressure in the VC, Pa;

- скорость откачки ТМН, м³/с.

- TMP pumping speed, m ³ / s.

2. The contribution of air leakage from the atmosphere, due to the leakage of the AW, is determined experimentally for each specific case.

The total gas leakage (B) into the AB, as noted above, is
from the leakage of atmospheric air through connections, seals, possible defects in the casing of the AB itself (B₁) and the flow of the pumped gas through the TMP (B₂):

(8)

(8)

where B ₁ is the leakage of air from the atmosphere into the AB, due to the leakage of the AB, Pa m ³ / s;

B ₂ - the flow of the pumped gas through the TMP, Pa·m ³ /s.

Thus, with a known total leakage into the AW, the minimum necessary volume of the AW (V _min ) is determined to ensure the conditions for the correct operation of the TMP during one technological cycle. Thus, according to (6) and (8):

(9)

(9)

where B ₁ is the leakage of air from the atmosphere into the AB, due to the leakage of the AB, Pa m ³ / s;

B ₂ - the flow of the pumped gas through the TMP, Pa·m ³ /s;

Δt - duration of the technological cycle, s;

ΔP - change in pressure in AV, equal to the difference between the allowable pressure at the outlet of TMP (P _TMN ) and the initial pressure (initial forevacuum) in AV (P _n ), Pa.

The general calculation formula for determining the required volume of AB is:

(10)

(10)

where Δt is the duration of the technological cycle, s;

B ₁ - leakage of air from the atmosphere into the AB, due to the leakage of the AB, Pa·m ³ /s;

B ₂ - the flow of the pumped gas through the TMP, Pa·m ³ /s;

P _TMN - allowable pressure at the outlet of TMN, Pa;

P _n - initial pressure (initial forevacuum) in AB, Pa.

For numerical estimates, the operating parameters of a specific vacuum installation are used, for example, the pressure in the VC P ₁ = 10 ^-5 Pa, the pumping speed of the TMP υ _n = 0.685 m ³ / s, the temperature in the working volume of the VC T ₁ = 673 K, the temperature of the medium in the AV T ₂ \u003d 293 K.

So for the value of the flow of the pumped gas (7), in the case of the vacuum installation used, we obtain:

м³/c (11)

m ³ / s (11)

where P ₁ - gas pressure in the VC, Pa;

- скорость откачки ТМН, м³/с

- TMP pumping speed, m ³ / s

T ₁ , T ₂ - gas temperature in VC and AB, K.

м³/cThus, the flow of the evacuated gas through the TMP is

m ³ / s

м³/с. Leakage of air from the atmosphere due to leakage AB, measured by means of a mass spectrometric helium leak detector, for the case under consideration is about B ₂ = 10 ^-5 Pa

m ³ / s.

Since the flow of the evacuated gas through the TMP is an order of magnitude less than the leakage from the atmosphere, it can be neglected in subsequent calculations.

The value of the maximum possible pressure P _TMN at the outlet of the applied TMP model Pfeiffer Vacuum HiPace 700 is 1100 Pa, and the limiting residual pressure FVN P _n of the NVSp-35 model is not more than 1 Pa. Consequently, after pumping out the AW with the help of the FVN to a pressure of the order
1 Pa and overlap of valve B, the pressure inside the AB, due to leakage, should not rise above 1100 Pa.

The duration of one technological cycle is 14 days. Thus, according to (10), the required volume of AB:

м³=12л (12)

m ³ \u003d 12l (12)

where Δt is the duration of the technological cycle, s;

B ₁ - leakage of air from the atmosphere into the AB, due to the leakage of the AB, Pa·m ³ /s;

P _TMN - allowable pressure at the outlet of TMN, Pa;

P _n - initial pressure (initial forevacuum) in AB, Pa.

An example of implementation of the proposed technical solution is illustrated in Fig. 12.

Figure 1 shows a high-vacuum system of industrial and laboratory installations, which contains a vacuum chamber 1 with heaters. The vacuum chamber 1 is equipped with a vacuum gauge 2 to control the vacuum in the vacuum chamber 1 during the process of evacuation and outgassing of the product. A turbomolecular pump 3 is installed on the outlet flange of the vacuum chamber 1. The gas is pumped out from the vacuum chamber 1 by the turbomolecular pump 3 into the vacuum accumulator 4. To control the vacuum in the working cavity of the vacuum accumulator 4, a vacuum gauge 5 is used. 4 serves as a fore vacuum mechanical oil-free fore vacuum pump 6.

As a fore pump 6, for example, a dry scroll pump can be used. For the convenience of a regular start of the technological process of evacuation and outgassing of the product, the high-vacuum system is equipped with valves 7 and 8. To prevent an emergency breakthrough of air into the vacuum accumulator 4 when the fore-vacuum pump 6 is stopped, an electrovalve 9 is used.

To carry out some technological processes, instead of dry foreline pumps 6, mechanical pumps with an oil seal 10, for example, rotary vane pumps (figure 2), can be used. In this case, to reduce the negative impact of the oil seal is cryogenic trap 11, cooled, for example, liquid nitrogen.

Example. Designed and manufactured high-vacuum system (Fig.1) for evacuation and outgassing EVP, which includes the following main elements: as a turbomolecular pump 3 is used turbopump model HiPace 700 having a limiting residual pressure of less than 5·10 ^-10 hPa, pumping speed 685 l /s and maximum forevacuum 11 hPa; as a fore-vacuum pump 6, a dry spiral vacuum pump NVSp-35 is used with a speed of 37.0±3.7 m ³ /h and a limiting residual pressure of not more than 1 Pa. The volume of the thermal vacuum chamber is 1·10 ^-2 m ³ . The volume of the vacuum accumulator is 2·10 ^-1 m ³ . After installing the product in the thermal vacuum chamber 1, it is sealed, then the foreline pump 6 is turned on and, then, the valves 7, 8 and the electrovalve 9 are opened. The forevacuum pumping of the thermal vacuum chamber 1 and the vacuum accumulator 4 is performed to forevacuum values of about 10 Pa. The solenoid valve 9 is used as a safety (cut-off), which, in the event of an emergency stop of the foreline pump 6, automatically shuts off the exhaust line. After fore-vacuum pumping out of the thermal vacuum chamber 1 and the vacuum accumulator 4, the turbomolecular pump 3 is turned on and the pressure in the thermal vacuum chamber 1 is increased to 10 ^-4 Pa. After reaching the pre-vacuum, the chamber 1 is heated, the operating temperature is raised to the predetermined values and the vacuum values in the thermal vacuum chamber 1 are raised to 10 ^-5 Pa. When the specified vacuum is reached in the thermal vacuum chamber 1, the electrovalve 9 and the valve 8 are closed. After that, the foreline pump 6 is turned off. From this point on, the vacuum accumulator 4 ensures the operation of the turbomolecular pump 3 as a fore vacuum pump 6 until the end of the process. The forevacuum pumping of the vacuum accumulator 4 is sufficient for the uninterrupted operation of the turbomolecular pump 3 for 14 days, due to the requirements of the technological process.

If necessary, the vacuum accumulator 4 ensures continuous operation of the vacuum system for a period many times exceeding the requirements of the process.

During many months of continuous operation of the high-vacuum system, it is possible, if necessary, to briefly turn on the foreline pump 6, open the electrovalve 9 and valve 8 to improve the forevacuum in the vacuum accumulator 4, followed by shutting off the valve 8, electrovalve 9 and turning off the foreline pump 6.

Thus, during the operation of the high-vacuum system, it was possible to achieve a significant reduction in the operating time and wear of the foreline pump. Its working time was reduced from 14 days to several hours. Due to this, significant savings in electricity and the resource of the fore vacuum pump were obtained, as well as the labor intensity of the technological process was reduced.

In addition, a high-vacuum system for evacuating and degassing EVP was developed and manufactured (figure 2), in which a rotary vane pump of the Pascal 2021SD model with a nominal capacity of 20.7 m ³ /h and a maximum pumping pressure of 1· was used as a backing pump. 10 ^-2 Pa.

A cryogenic trap was used in conjunction with the foreline pump to reduce the negative impact on pump oil seal vacuum cleanliness.

Liquid nitrogen was used to cool the cryogenic trap. On the high-vacuum system, an increase in the reliability and quality of long-term pumping and evacuation of the EVP, saving the life of the FVN, reducing the irremovable defects, simplifying the design of the vacuum system, and reducing the cost of the technological process were also achieved.

Claims (12)

1. A high-vacuum system of industrial and laboratory installations, consisting of a vacuum chamber connected in series with heaters, equipped with vacuum gauges, a turbomolecular pump, a valve, a fore-vacuum pump, characterized in that a vacuum accumulator equipped with a vacuum gauge is installed at the outlet of the turbomolecular pump behind the valve, and between the vacuum accumulator and an electric valve is installed with a valve. 2. High vacuum system according to claim 1, characterized in that the working volume of the vacuum accumulator is selected from the ratio

, Where

- working volume of the vacuum accumulator, m ³ ;

- duration of the technological process, s; In ₁ - leakage through the leaks of the system AB + TMN + working volume, Pa⋅m ³ /s; B ₂ - the flow of the evacuated gas through the turbomolecular pump (TMP), Pa⋅m ³ /s; P _TMN - allowable pressure at the outlet of TMN, Pa; P _n - the value of the initial pressure (vacuum) in AB, Pa. 3. High vacuum system according to claim 1, characterized in that a valve is installed between the electric valve and the foreline pump. 4. High-vacuum system according to claim 1, characterized in that a spiral or other mechanical oil-free pump is used as a foreline pump. 5. High vacuum system according to claim 1, characterized in that a rotary vane pump or other pump with an oil seal is used as a foreline pump, and a cryogenic vacuum trap is installed between the electric valve and the foreline pump.

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