Expansion of working area of ​​diffusion pump

【Abstract】 This paper introduces the exploration of expanding the work area of ​​the high pressure section of the traditional diffusion pump by developing a diffusion booster pump. It introduces in detail the various types of data of the pumping capacity debugging obtained during the development of the diffusion booster pump, and attempts to explore and grasp Various factors that affect the pumping capacity of the diffusion booster pump are needed to meet the need of increasing the pumping capacity.
I. Introduction

Oil diffusion pump wide range of applications, all types of gas can be effectively aspirated, but also has the advantages of low cost, no vibration noise, ease of use and maintenance, but the work area, especially in the high-pressure pumping area , But by a very large area.
The problem is that some of the large amount of deaeration of the vacuum coating equipment such as magnetron sputtering coating machine, winding vacuum coating machine, etc., especially the latter, the working vacuum often lO-1Pa range, and sustained a large amount of deflation . Therefore, the vacuum system vacuum pump matching requirements of the main suction pump in the 10-2 ~ 10-1Pa range to have a larger pumping speed, and the ultimate vacuum to be better than lO-3Pa. The traditional oil diffusion pump generally maintains a maximum pumping speed within a half-magnitude range of 10-3 ~ 10-2Pa, and its constant pumping speed range of the pumping speed curve generally extends to about 7 × 10-2Pa To be worse). If the inlet pressure exceeds the critical range, it will lead to the destruction of oil vapor jet, pumping speed curve began to decay sharply. While the oil booster pump in l0-1 ~ 10Pa about two and a half orders of magnitude with an effective pumping speed, but lO-2Pa range pumping speed is too poor, more lacking is the ultimate vacuum only lO-2Pa level. This makes the design of coating machine vacuum system in the vacuum pump matching, how to take into account the degree of vacuum and pumping efficiency rather hesitated.

Obviously, this problem is not to solve the problem of diffusion pump or oil booster pump alone can only try to take into account their respective advantages in terms of vacuum and exhaust volume is therefore also proposed in the original diffusion pump Based on the expansion of its suction range of the idea of ​​its main means is based on the diffusion pump suction characteristics as a starting point, with reference to diffusion pumps and oil booster pump performance and structural characteristics of each to enhance its high-pressure pumping speed, extending extended exhaust Curve. The improvement of the pump we call it diffusion booster pump.In recent years we have developed and used in large-scale coating equipment for large-caliber amplification pump (KZ series) are Ф600, Ф630, Ф800 three, in the performance tuning Get more useful data, but also accumulated some experience.

Second, the design concept of the expansion pump

The so-called extended high-pressure section pumping speed, in practice, is two goals: 1. Do not drop the pumping speed of the high-pressure section sharply to zero, forming a descending pumping speed curve; 2 Minimize the falling slope of this curve.

Analysis of its characteristics, based on the original diffusion pump modified, consider three aspects: 1. Improve the nozzle structure. To remove the high-pressure section of the large amount of gas, the traditional diffusion pump nozzle structure is clearly not competent. In addition to changing the nozzle throat area, expansion rate and other parameters to meet the boiler pressure increase variable, but also the original straight-shaped jet nozzle to Laval nozzle and the expansion of the tube with the match (see Figure 1) to Produce a strong positive shock, greatly enhance the gas port at the portability; 2. Increase the heating power. The purpose of increasing the power is to increase the boiler pressure so that at the same expansion rate there is a higher oil vapor pressure at the exit of the nozzles at all levels to form a stronger jet of oil vapor. For pump fluid. The replacement of pump fluid is attempting to greatly increase the saturated vapor pressure of the oil vapor in the boiler at the same heating temperature but the current restriction is still limited due to the fact that the alternatives are only booster pump oil and ordinary diffusion pump oil. Saturated vapor pressure is too high, the ultimate vacuum is limited, even if the effect of fractionation measures are not good, can not enter the 10-3Pa. The latter vacuum is high, but the high temperature vapor pressure is too low, the boiler pressure is not ideal, high pressure section Speed ​​to be subject to great losses. The ideal is saturated vapor pressure at room temperature than diffusion pump oil an order of magnitude higher, but the high temperature oil vapor pressure multi-directional oil booster pump oil move closer, both types of pump oil characteristics of oil. Such products abroad to have some options, the domestic is expected to spread pump oil manufacturers early development and introduction of new products.

Regardless of the pumping mechanism or the specific structure of the pump, the expansion pump is equipped with a diffusion-based diffusion pump and a viscous carrier-based oil booster pump. In its wider working range, two different sections of the pumping speed curve can characterize this characteristic.

As shown in Figure 2, regardless of the amplification pump or diffusion pump, the pumping speed curve can be basically divided into two areas: 4 × 10-2Pa as the cut-off point, the left for the low pressure section, the right for the high pressure section can be seen from the figure, The pumping speed in the two areas showed a completely different trend.

Low-pressure section pumping speed curve range of 10-3 ~ 4 × 10-2Pa, within this range of oil vapor jet at all levels fully expanded, the vapor pressure is relatively low, and has the appropriate thickness, all levels of jet continuous and stable pumping. Being sucked gas can be in the largest pumping area that is, a surface circulation, the full diffusion into the oil vapor jet, which is carried by the removal. Corresponding pumping speed curve at its maximum value showed a more constant state, a slightly curved horizontal line, which is the basic characteristics of the diffusion pump.

The pumping speed curve of the high-pressure section ranges from 4 × 10-2 to 10-1Pa, and the inlet pressure in this range increases continuously. When the pressure is close to 10-1Pa, the pressure of the gas to be drawn is similar to or even higher than the oil vapor pressure at the exit of the first-stage diffusion nozzle. First grade steam jet expansion degree decreases, and finally even compressed. It is not possible to form a steam jet which can effectively evacuate and "seal" the back flow of the evacuated gas. Was pumping gas reflux increased. Pumping rate began to decline. Similarly, with the import pressure continues to rise, a similar situation in the second and third nozzles have emerged. Based on this, 4 × 10-2Pa inflection point after the pumping speed curve is a falling line.

Comparing the pumping speed curve of the amplification pump and the diffusion pump, the pumping speed curve of the diffusion pump is more stable in the low pressure section, while the expansion pump is somewhat similar to the oil booster pump, the pumping speed starts to climb from a lower position, Until 10-2Pa after the diffusion pump, expansion pump, the nozzle exit of the oil vapor jet density is much larger than the diffusion pump, the gas molecules are diffused into the oil vapor jet and the probability of being extracted is lower, and in the more Low pressure at the gas molecules were pumping back the number of small, no advantage of the expansion pump at all, so the pumping speed is lower than the diffusion pump. In the high-pressure section, the result changes in the opposite direction, the inlet pressure gradually increased, not to 10-1Pa, diffusion pump pumping speed has dropped significantly, the amplification pump is more "defensive."

In addition to the pumping speed curve, Figure 2 also shows the pump and the diffusion pump pump exhaust curve, with the exception that the pump with the inlet pressure increases while the amount of linear increase in pumping, to 10-1Pa stabilized. 2 × 10-1Pa, then the emergence of the amount of pumping jumped. Pumping curve, the pump in the high-pressure section of the pumping performance is shown more clearly.

Third, the expansion pump developed pumping speed test

When developing the KZ-600 expansion pump, a series of pumping speed performance debugging has been made for some factors that affect the pumping speed. This article briefly describes as follows:

1. Effect of Diffusion Pump Throat Area on Suction Rate

It can be seen from Fig. 3 that the change of structure parameters of primary diffuser nozzle only has significant effect on the pumping speed of low pressure section: the same jetting angle, the same expansion rate and different laryngeal area have obvious difference in low pressure section, (5 ~ 6 × 10-2Pa Department) at a difference of 30% or more. After the gradual convergence, to 2 × 10-1Pa after tended to be consistent, this section depends mainly on pumping speed nozzle two and three Laval nozzle parameters, consistent with the original design ideas. Test shows that a throat area should not be less than 9cm2.

2. Effect of Pumping Fluid Type on Pumping Rate Pumping fluid with high saturated vapor pressure at high temperature provides higher boiler pressure at relatively low power (without significantly increasing oil temperature and avoiding pump cracking) Steam jet intensity, which in the high-pressure section to improve pumping speed is extremely beneficial. KZ-600 pump with booster pump oil and diffusion pump oil (KS-3) were made pumping speed test. Import pressure is greater than 3 × 10-2Pa, the two began to show different trends, the greater the import pressure, the more obvious advantages of booster pump oil.

Booster pump oil has a much better pumping performance than diffusion pump oil at high pressure, but it is still decided to use diffusion pump oil as the pump pump fluid for better vacuum. This also shows us from another perspective: If you can develop between the two (saturated vapor pressure at room temperature below the booster pump oil, high temperature pump fluid diffusion pump oil) oil. Should be more able to become a pump pump fluid optimization.

3. Influence of water cap on pumping speed

The test of Fig. 5 shows that after adding the water-cooled oil-retaining cap, the pumping speed of low-pressure section increases, but the change of high-pressure section is not obvious. This is due to the addition of water-cooled oil cap, although the exhaust area slightly reduced, but the effective oil retaining effect so that a large number of reflux steam is trapped, played a part of the role of pumping condensable gases, which pumping In the high pressure section, compared with the total amount of gas being exhausted, the proportion of gas is small, the effect of the above analysis is not significant, so the pumping speed curve tends to be consistent. Such as the oil cap with a lower temperature cooling medium, although the paragraph in the low cell plume condensable gas content of the working gas, the effect will be better.

4. Effect of Different Heating Power on Pumping Speed

Amplification pump working area is wide, so the best heating power curve is also different in each section. In the low pressure section, after the heating power exceeds the optimum power, the vapor flow density increases, but the diffusion coefficient of the gas to be drawn decreases and the pumping speed decreases. However, after the high-pressure section increases power, the intensity of the oil vapor stream increases, the capability of carrying the gas to be drawn is enhanced, the pumping speed increases unabated, and the curve drops slowly. Figure 6 shows the different power pumping speed contrast, 10-2Pa area in addition to 18.5kW, other power pumping speed is basically the same. The 10-1Pa area, neatly arranged clearly shows that with increasing power pumping speed slowed down the decline (slope reduction).

Fourth, the structure of the expansion pump features

Differing from the diffusion pump performance characteristics of the expansion pump of course has its own structural features, in addition to the aforementioned nozzle structure improvements, we also made in the development of three structural improvements:

1. Heater improvements

Amplifier pump power than the proliferation of nearly doubled the pump, which brings two problems, first, energy-saving issues, open-type electric furnace is heated to the pump by the heat radiation to the pump, so one-way heat transfer start Slow, more heat loss, low thermal efficiency of the ills. The expansion pump secures multiple bushings to the bottom of the square heating boiler. Another tubular heater penetrates the casing, the entire casing immersed in the pump fluid (see Figure 7), the radial heat transfer, and the heat transfer is solid, both to shorten the heating time, meaning to reduce heat loss. Second, the unit heating power problems, such as still using open-type electric furnace structure, the new heating power heating unit surface heating load greatly increased, there will be pump fluid cracking and bumping. As shown in Figure 7 to improve the heating structure, greatly increased the heating surface area, unit load control within 3.1W / cm2.

2. Water-cooled oil cap device

Amplification pump to increase the heating power, the amount of oil vapor increases, the amount of oil back also increases. A large amount of oil vapor will increase the temperature of the oil cap rapidly and weaken the anti-oiling effect. Therefore, the oil cap on the water-cooled device in a large expansion pump is necessary, can greatly reduce the rate of return. From the test results, the lower temperature of the oil cap on the pumping speed is also beneficial.

3. Pre-stage water-cooled device

The amplification pump not only has a large amount of oil vapor that flows back to the inlet port, but also has a large amount of oil vapor discharged from the exhaust port through the Laval nozzle. After a large amount of oil vapor is thrown toward the front-stage pipe, it can not completely pass through the front-stage pipe wall and the front stage Block the oil trap condensation, non-condensing part of the results by the forepump pumping. This not only makes the pump oil consumption increased, meaning to increase the anterior pump unnecessary load. By the same token, the temperature of the oil deflector should be effectively reduced to allow the oil vapor to condense sufficiently. Therefore, when the design of the expansion pump KZ series in the former plus water-cooled oil device (see Figure 8). Significantly reduce the pump oil consumption.

Five, concluding remarks

The development of an expansion pump is a new attempt to expand the working area of ​​the oil vapor pump. With its convex cavity structure, KZ series expansion pump not only increases the pumping speed of low pressure section but also extends the pumping speed curve downwards and expands the pumping range nearly one order of magnitude after entering 10Pa, and the pumping capacity is increased by one Twice. The pump's commissioning results show that some of the original ideas are feasible.

In traditional performance testing of diffusion pumps, some of the factors that have a significant effect on pumping speed, such as reducing back oil, changing the structure parameters of diffusion nozzles, etc., gradually fade out after the inlet pressure reaches 10-1 Pa, and the main role is to be enhanced Some factors of steam jet intensity, such as heating power, selection of pump type and so on.

In order to achieve the purpose of expanding the pumping performance of the high-pressure section, the amplification pump must make some necessary improvements in the nozzle structure and configuration parameters. In addition, there will be a series of new problems in the upgraded expansion pump such as the heating method, Surface heat load density, back to the oil, the former residual oil steam treatment, etc., these can be solved through structural improvements.

Of course, there are still many problems to be solved, such as appropriate pre-counter pressure to determine the rational selection of pump fluid to improve the unit power pumping capacity and more reasonable choice of heating power, which will be further explored later.