Chapter 8: Cryo-sorption pumps

Transcription

1 Chapter 8: Cryo-sorption pumps Cryo-sorption pumps offer a clean, quiet, safe, vibration free and inexpensive way to rough pump a vacuum system. They are often used on vacuum systems that are sensitive to oil contamination from mechanical roughing pumps (surface science instruments, for example). Cryo-sorption pumps are a sub category of sorption pumps. All sorption pumps work by gas-capture. Pumped gases and vapors are bound at the active surfaces of these pumps by physical means (Van der Waal's Forces), chemical means ( Chemisorption) or are mechanically embedded in a continuous deposition of material, as in a sputter ion pump (more on this in Chapter 9). Gas capture pumps of these types share a few operational characteristics. With use, they will eventually become "saturated" and will cease to pump- gases effectively. When this occurs, a sorption pump will either need to be "regenerated" or replaced. Theory of operation- Cryo-sorption pumps work by providing a very large surface area of material that is cooled to below the boiling point of most gases. Gas molecules that strike this cooled micro-porous surface become attached and are removed from the gas phase, and are effectively "pumped" from the vacuum system. The active surface area of a cryosorption pump is typically made of zeolite 13X. This alkali alumino-silicate possesses a very high surface area to mass ratio (about 10 3 m 2 per gram). The diameter of pores in this material is about 13Å (1.3*10-9 m) which is approximately the size of a molecule of water, oil vapor and larger gas molecules (nitrogen and oxygen, for example). The pore size is appropriate for capture of the gases most predominant in the atmosphere. Low atomic weight gases, such as hydrogen, helium and neon have molecular diameters smaller than the 13Å pore size of the zeolite, and are captured by this material less effectively. Absorption of gases by a given sorbent is a function of gas specie, sorbent temperature, and gas pressure. As nitrogen gas is cooled, the amount of gas that can be adsorbed by the zeolite per gram increases, as is shown in figure 8.1. Also note in this figure that helium, even when cooled to -195 C is pumped much less efficiently than nitrogen. Another piece of information that may be gleaned from the data presented in figure 8.1 is that in general, as gas pressure decreases, the amount of gas that is adsorbed per gram of sorbent decreases. Page 110 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

2 10+3 Quantity of Adsorbed Gas [Torr-L] per gram of sorbent Nitrogen (-195 C) Nitrogen (20 C) Helium (-195 C) Pressure [Torr] Figure 8.1 Pumping behavior of Zeolite X-13 as a function of pressure. Range of operation Due to the extremely large sorbent surface area, these pumps can begin to trap gases at atmospheric pressure (no roughing pump required), and can achieve pressures of 20 microns or less depending on the gas being pumped, and ratio of the volume of the chamber to the capacity of the pumps. Inspection and First Use Prior to Operation of cryo-sorption pumps it is probably best to inspect a cryo-sorption pump before initial use, especially on a critical vacuum system to insure that the pump contains the correct sorbent, and is filled to the recommended level. If internal hardware (screens,grids, etc,) are used, is it installed and in good condition? Is the pump body sound? How about the vacuum flanges and connections? Do they mate with the vacuum vessel's hardware? Are they in good mechanical condition (no scratches running across sealing surfaces)? Prior to the first use of a new cryo-sorption pump, it should be baked out at 250 C for 24 hours to insure removal of water adsorbed on the zeolite. Page 111 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

3 Pump inlet pressure relief valve Viton Stopper Zeolite screen Viton cuff Dewar Liquid nitrogen Figure 8.2 Typical cryo-sorption vacuum pump. Typical configuration Sorption pumps are usually connected to vacuum chambers in a valved manifold, such as shown in figure 8.3. TC1 Figure 8.3 Cryo-sorption pumps connected to a vacuum vessel. Operation With the valve to the cryo-sorption pump closed, attach the liquid nitrogen dewar to the pump body, and fill the dewar to within 1/2" of the top with liquid nitrogen. Allow 30 Page 112 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

4 minutes for the sorbent to reach operating temperature. Care should be taken to avoid splashing liquid nitrogen on the skin. See chapter 2 for more safety details in handling cryogenic materials. Regeneration of cryo-sorption pumps-following repeated use, the sorbent material will become saturated with gas molecules, and the pump's ability to remove gas from the vacuum system will rapidly deteriorate. When this occurs, regeneration may be performed by simply valving the pump off from the system, and allowing it to come to room temperature. Gases will be liberated from the zeolite, and will escape the pump body through the pressure relief valve. Make sure that the pressure relief valve is in good operating condition, and is free to operate (no obstructions or blockages. The cork style relief valve may pose a danger in that if the cork's tether is broken, the cork may shoot across the room. In industrial situations it is possible that toxic or explosive combinations of gases may be released on pump regeneration. Be aware! In situations where significant amounts of water vapor are pumped with a cryo-sorption pump, heating at 250 C for several hours is recommended in the regeneration sequence. Performance characteristics-the important quantities for cryo-sorption pumps are the pump's capacity (expressed in Torr-liters), and its operating temperature (which will determine which gas species will be pumped and how efficiently). Pump capacityeach gram of zeolite cooled to liquid nitrogen temperature (77k, or -195 C) approximately 30 Torr-liters of atmospheric gas can be pumped. Remember, at liquid nitrogen temperature, helium, neon and hydrogen gas are not pumped, as they have boiling points below that of liquid nitrogen. Sample problem: 8.1 What temperature would a sorbent material have to be cooled to in order to pump helium, neon and hydrogen? For further reading: Cryo-sorption pumps- High Vacuum Technology, Hablanian, Marsbed, M., Marcel Dekker, INC, New York, New York A User's Guide to Vacuum Technology, O'Hanlon, John F. John Wiley & Sons New York, New York Answers to Chapter 8 sample problems 8.1 Below 10k or -263 C Laboratory Exercise 8.1: Performance of a single cryo-sorption vacuum pump. Page 113 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

5 A. Pump Identification: Who is the manufacturer? What is the pump model number? Locate the manufacturer's literature from the bookcase, and find the appropriate reference information. What is the sorbent? What is the advertised pump capacity? B. Physical Inspection of Cryo-sorption Pump: Inspect the pump for signs of wear or misuse. Are the screens in place? Is the correct amount of sorbent in place? Are the vacuum sealing surfaces in good condition? C. Bake-out of Cryo-sorption Pump: in a safe area, set up a fire-safe area to bake-out your cryo-sorption pump. Bake out the pump for 60 minutes. D. Pumping speed and capacity: once the pump has been regenerated, allow it to cool to room temperature with the pump isolation and relief valves closed. Attach the pump to a vacuum vessel of at least 10 liter volume as shown in figure 8.4. Connect a dewar to the pump body, and fill the dewar with liquid nitrogen. Allow 30 minutes for the sorbent to cool. With the vessel at atmospheric pressure, and the vent valve closed, open the cryo-sorption pump isolation valve, and record pressure versus time for 20 minutes. Close the cryo-sorption pump isolation valve and vent the chamber to atmosphere. Close the vent valve and repeat the experiment. Do this sequence of steps until a noticeable decrease in pumping speed is noted. Plot your data as pressure vs. time and pumping speed vs pressure. Calculate the amount of air pumped in each of the sequential pump-downs. If pure com-pressed gas is available, repeat the TC1 experiment with argon, nitrogen and helium. Figure 8.4 Experimental set-up for experiment 8.1. Page 114 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

6 Run #1 Run #2 Run #3 Run #4 Page 115 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

7 (P1-P 2) [sec] Pressure [Torr] Change in Average Pressure [Torr] Mass Throughput [Torr-L/s] Discussion: How does the pump capacity that you have calculated compare to those listed by the manufacturer for this pressure range? Page 116 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

8 What was the general trend in pumping speed for the series of pumpdowns for each of the gases pumped? Laboratory Exercise 8.2: Performance of multiple cryo-sorption vacuum pumps. A. Using the same vacuum vessel as in the previous experiment, connect two similar cryo-sorption pumps as shown in figure 8.5. TC1 Figure 8.5 Experimental set-up for experiment 8.2. As was done in the previous experiment, bake out the pumps if necessary and measure the pumping speed for two cryo-sorption pumps used simultaneously. Make two plots of your data: pressure vs time and pumping speed vs. pressure. Calculate the total amount of gas pumped in each experiment. If bottled nitrogen, helium or argon are available, repeat the experiment with these gases. A data table is provided on the next page. Page 117 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

9 Run #1 Run #2 Run #3 Run #4 Page 118 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

10 (P1-P 2) [sec] Pressure [Torr] Change in Average Pressure [Torr] Mass Throughput [Torr-L/s] Page 119 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002