What Is a Diffusion Pump?

Diffusion pump is currently one of the most extensive and main tools for obtaining high vacuum, usually referred to as oil diffusion pump. A diffusion pump is a secondary pump that requires a mechanical pump as a foreline pump.

Since 1913, Gaede invention diffusion pump has been 101 years, during which technically has been a lot of progress. The pumping speed of oil diffusion pumps produced now exceeds 100,000 L / s and the pumping speed of diffusion jet pumps is 40,000 L / s. Steam flow vacuum pumps play a very important role in vacuum applications. Although there are many other types of pumping methods that can also work in the field of high vacuum, due to the simple structure, convenient operation and maintenance, long service life, and good pumping characteristics for various gases, the diffusion pump has The main extraction equipment for obtaining high vacuum. ^[1]

Gaid applied for a patent in Germany on September 25, 1913. Device for generating high vacuum with mercury vapor stream. He pointed out: High vacuum is obtained through diffusion. Therefore, this pump is known as a diffusion pump. According to reports from the former Soviet Union: Russian scholar Barovik first proposed such a pump at the Petersburg Institute of Physics in 1912, and in 1913 Russian scholar Pavlov used this pump to conduct research on gas-molecular collisions. . Today's modern diffusion pumps are the result of continuous research, improvement and innovation over several generations. But we must not forget Gade's contribution to the diffusion pump. ^[1]

This system is a new large-diameter diffusion pump developed in the 1990s, which has made a significant improvement on the traditional diffusion pump. Its main features are: six pumping speeds, ranging from 3000 to 50000 L / s specification. The pump wall has an upper groove, and the cooling water pipe lies in the groove, which increases the contact area and has a good cooling effect. In 2003, Leibao Company further improved the DIP diffusion pump and changed the water-cooled pipe to a water-cooled interlayer. The whole of the diffusion pump is made of stainless steel. So as to further improve the performance indicators of DIP series pumps. Oil vapor recirculation rate is low. External water cooling and without using baffles common mineral oil, which oil returning rate ^{1 × 10 -3 mg / cm 2} · min; if DC-705 silicone oil, the oil returning rate may be reduced to 1 × 10 ^-4 mg / cm ² · min; ultimate pressure lower than 10 ^-6 Pa. On the pumping curve, the maximum pumping speed is reached when the inlet pressure is 10 ^-1 Pa, and the pumping speed is stable. It is heated by multiple heating rods inserted into the diffusion pump, with high heat transfer efficiency and short warm-up time. For pumps with a pumping speed of 3000 ~ 20,000L / s, the warm-up time is less than 25min; for a pump of 30,000 ~ 50,00000L / s, the warm-up time is less than 30min. The heating rod can be replaced without stopping the pump. Equipped with overheating device to prevent the pump oil from overheating. The quality and quantity of pump oil can be observed at any time through the observation window. The pump is provided with an oil filling port and an oil drain port, and it is not necessary to remove the diffusion pump from the vacuum system when changing oil. The maximum allowable foreline pressure is 60 Pa.

The structure diagram (take DIF800 as an example) is shown in the figure.

The reasons for the unstable pressure of the diffusion pump system are internal and external. The reasons outside the pump are: bubbles from rubber gaskets; pump oil dripping from the baffle plate; high front pressure resistance; defrosting of the well; rupture of the freezing layer in the well. The reasons inside the pump are: the pump oil is boiling or unstable; the oil is boiling outside the diversion tube; the low compression ratio of the light gas; the liquid droplets in the nozzle; the top nozzle becomes cold (steam (Condensation); Leakage near the boiler, the pump body should be checked for leaks.

The unstable boiling of the pump oil in the boiler will cause the density of the ejected steam jet to fluctuate, thereby causing changes in pumping speed and light gas compression ratio. The boiler may overheat for a while, so that it has a bump time. The remedy is to use a winged boiler in modern oil diffusion pumps to increase the internal surface area, reduce the heat flux density, and increase the power to minimize bumps. Or eliminate. In a vacuum, a strong bump can cause self-detonation to rise, cause instant blockage of the nozzle, and cause steam flow fluctuations. Adding an explosion-proof plate in the pump can prevent the impact of bumping. Because of the low compression ratio for light gases, it can cause pressure instability. It is advantageous to increase the power and the number of pump stages. The side outlet jet nozzle and diffuser are set in the diffusion pump to minimize this instability of the diffusion pump. Over-cooling the top nozzle cap will also cause discontinuous steam flow, resulting in pressure instability. Try to keep the temperature of the top nozzle from falling and not make it too cold. Bubbles from the rubber seal, sometimes the oil traps the gas, leaks out through the sealing rubber ring, and the oil film ruptures into the system after the bubble has a certain pressure. The pump oil accumulated in the sealed tank should be able to drain, which can reduce the instability, and it is better to use metal gaskets. A well-designed cold cap prevents oil droplets from evaporating on the hot surface of the top nozzle. When pumping with hydrocarbons, improper operation may cause explosion of the diffusion pump system. The regulated filament is the source of ignition in most cases. The daily operation of the pump should be standardized, and the quality and quantity of the pump oil should be checked regularly. ^[2]

(1) At present, there are many manufacturers of domestic diffusion pumps, the models and sizes are not uniform, the larger the pump is, the larger the smaller pumps are rarely produced. Moreover, there is no assessment index for the overall performance of the pump. The industry should conduct regular inspections to improve performance and quality, organize manufacturers to do more improvement work, and make it convenient for users.

(2) In terms of structure of the pump: To reduce the size and weight, to ensure the processing accuracy of the parts, the number of stages can be increased to increase the pumping capacity at 4 × 10 ^-1 Pa. The pumping amounts of several commonly used pumped gases are given.

(3) Add attachments. Such as setting oil cap, overheat protection device (on-line measurement and automatic protection of cooling water temperature, water pressure and pump oil temperature. It can be automatically cut off and reset). Monitoring of outlet pressure.

(4) Specify the maximum and minimum oil quantity and fuel consumption (such as 0.5 ~ 0.7 g / h, etc.). Observe the oil level to facilitate users to fill the oil in time. Look at the deterioration of the pump oil through the observation window for timely replacement.

(5) Give the appropriate foreline pump capacity.

(6) In order to save energy, the heating power must be adjustable, and the pump must be started quickly and stopped quickly to increase the operating rate of the equipment.

(7) Give the performance curves of various pumped gases, and it is best to give the compression ratio of some gases (easy for users to choose.

(8) Microcomputer-controlled operation and fault diagnosis.

(9) Choose a high-quality rubber ring with a low outgassing rate and low air permeability at the seal.

(10) Give the limit value of oil return rate of various pumps to further reduce the oil return rate and expand the use of oil diffusion pumps. The oil diffusion pump has a wide working pressure range (less than 1 ~ 10 ^-5 Pa). It has higher reliability and lower cost than other high vacuum pumps. Only a few parts need to be repaired. There are no moving parts in the pump. The heater can be replaced during work. Vapor pressure oil or the polyphenylene ether is 10 ^-9 ~ 10 ^-10 Torr, using suitable baffles or obtain the well 10 ^-10 Torr or lower pressure at room temperature.

Disadvantage is that diffusion pump oil return problems, coupled with the cap and the slinger and the correct operation of the cold trap, which the oil returning rate can be reduced to ^{1 × 10 -5 mg / cm 2} min point. ^[2]

The fatal disadvantage of the oil diffusion pump is that the pump oil backflow pollutes the pumped system. After a lot of research, due to the addition of measures such as oil caps, baffles and cold traps in modern diffusion pumps, the oil return rate of the diffusion pump is very low.

All oil diffusion pumps have a certain degree of backflow. The returning gas is a small amount of residual hydrocarbons. There are two different sources of backflow: the movement of pump oil molecules towards the top nozzle of the diffusion pump constitutes the main backflow of the pump. The second important reflux is the migration or re-evaporation of the pump oil condensed by the pump wall. The use of a good boiler and nozzle design can minimize the main backflow. A water-cooled oil blocking cap is set above the top nozzle. It has become a water cooling device for the top nozzle to intercept oil return. Due to the continuous improvement of the structure of the cold cap, the main backflow is greatly reduced.

The use of a water-cooled oil blocking cap reduces the pumping speed of the pump a little and reduces the oil return rate. It is allowed for many applications. If some pipes, valves and elbows are added to the system, the backflow of the vacuum chamber can be reduced by 1/10 ~ 1/1000. If a water-cooled optically sealed baffle is used, the main backflow can be restricted, but the pumping speed of the pump can be reduced by about 50%.

Secondary backflow (i.e. pump oil migration and re-evaporation condensed on the surface is a function of surface temperature and pump oil vapor pressure. This backflow to the vacuum chamber depends on the cold trap placed between the pump and the vacuum chamber The geometry and temperature of the liquid nitrogen (77K) cold trap is commonly used to trap oil molecules. Therefore, the pressure in the vacuum chamber is significantly lower than the vapor pressure of the pump oil. The baffle or cold trap must not only be optically sealed, but also must be made into The baffle or cold trap oil molecules collide with the cold surface at least once. For baffles using adsorbents (such as zeolites), staggered adsorbent trays are required to prevent the oil molecules from flying straight. Zeolites are a type of Microporous material with a slightly larger molecular diameter. There is about 5 acres of adsorption area inside 1 ounce (28.35g) zeolite. In a small ultra-high vacuum system, the use of a molecular adsorbent baffle is effective. It can continuously adsorb hydrocarbons and other gases during the week without the need for liquid nitrogen.

In modern oil diffusion pumps, at the pump inlet, the return flow of the pump oil has dropped to a level at which the pump oil evaporates at room temperature. ^[2]

Only by following certain operating procedures during use can the system work normally. The operating procedures of mechanical pumps and diffusion pumps have been described in detail before. According to people's production practice, a set of operating procedures have also been summarized for the exhaust system. Taking the high vacuum exhaust system as an example (other vacuum equipment can be implemented by imitation), its operating procedures are as follows:

Check all parts of the vacuum system, be familiar with the gas circuit structure of the system, understand the role of each component of the vacuum system, and the circuit connection of the operating part.

Close the bleed valve, start the mechanical pump, pump the system, and use a spark leak detector at any time to perform a leak test on each part of the system. If it is an all-metal system, you can close the valve and record the Pt curve with a vacuum gauge reading (for static boost test). If the pressure rises linearly with time, it can be concluded that the system has a leak source, and a leak test should be performed.

When the system vacuum reaches the front pressure of the oil diffusion pump (5 × 10 ^-2 Torr), start the diffusion pump to gradually increase the heating current to the rated (optimal) value. Cooling water must be turned on before starting the diffusion pump.

After the diffusion pump works normally, use a spark leak detector (or thermocouple meter) to determine the vacuum of the measurement system. Ionization vacuum gauges can only be used until 10 ^-3 Torr (the thermocouple gauge is near full deviation or more than 95 divisions). When using, first degas the gauge and then measure the vacuum.

Baking and degassing the system. For glass systems, the baking temperature of hard glass is 400 ° C to 450 ° C, and that of soft glass is 350 ° C to 400 ° C. The degassing time depends on the specific requirements, usually 1 to 2 hours. After degassing, the vacuum still fails to meet the requirements, and the Pt curve should be recorded with an ionizer to determine whether there is a leak. You can also use acetone, alcohol, etc. to paint the suspicious area, see the movement of the hand to find the leak. If it is poorly degassed, it should be further baked and degassed for a long time. New systems typically take longer to reach the desired vacuum. If the vacuum is not getting older, we must consider whether the system design and installation are reasonable, whether there is dead space, whether there are many sources of deflation, whether dirt is brought in during maintenance, whether the pumped container is clean and other factors. ^[3]