Source: Robert M. Rioux, Ajay Sathe & Zhifeng Chen, Pennsylvania State University, University Park, PA
Vacuum is required for a number of laboratory procedures. This is most routinely achieved in the laboratory by the use of vacuum pumps. In addition to working at low pressures, vacuum pumps can also be used to enable rapid changing of the atmospheres in a reactor or flask by evacuation and backfilling.
Vacuum is useful for a variety of purposes in the lab. For example, vacuum lowers the boiling point of liquids and promotes the vaporing process, which is used for vacuum ovens, degassing equipment, and freeze drying. Besides, vacuum generated a pressure difference compared to atmosphere, which is used for filtration and pipettes. Ultra-high vacuum removes air to achieve chemical inertness, which is used for electron beam welding, maintaining a clean surface and chemical or physical vapor deposition. A vacuum pump is a device that helps evacuate a sealed chamber in order to attain a pressure lower than atmospheric pressure. The most commonly utilized pumps in the laboratory are turbomolecular pumps, oil pumps, dry scroll pumps, or water aspirators.
Turbomolecular pumps are often used in laboratory instrumentation, such as inside a mass spectrometer, and can achieve vacuum levels of 10-10 Torr. These work by rapid spinning to collide with air or vapor molecules to impact momentum toward the direction of exhaust. The high vacuum levels have a pump suitable for lots of ultra-high vacuum applications. However, air is too dense for a turbomolecular pump to work, and therefore these pumps need a secondary pump to drop the atmosphere pressure down to 1 Torr to enable the turbomolecular pump to work.
Oil pumps are most often used in the lab and typically achieve a vacuum of 10-3 Torr. This meets most of the general lab applications, and they are easy to operate. Oil is used to lubricate and seal the pump, which helps to achieve deep vacuum. However, the use of oil also brings the problem of oil change and waste oil disposal.
The dry scroll pump, which has the ability to achieve an ultimate vacuum level of 10-3 Torr, is one of the most common dry pump technologies used in the laboratory setting. The dry scroll pump works with two interleaved spiral scrolls moving eccentrically and compressing air and vapor towards exhaust. This pump doesn't need oil, and also pumps with a faster rate, which is attractive for some applications like a glove box. However, tip seals are needed to keep vapors in the correct channel, but these tip seals are wear parts and needed periodic maintenance.
Water aspirators, which are also called water jet pumps, are usually attached to the lab sink faucet and could achieve a vacuum level of 10-15 Torr. These work by utilizing fast flowing water to create vacuum in the side arm. Due to their low costs, these were historically popular to achieve deep vacuum. However, water is wasted and the vacuum level is not high.
The choice of the type of pump is dictated by the end application and the quality of the vacuum ultimately required. Irrespective of the pump used, the generation of vacuum leads to the possibility of implosion or explosion hazards. The following protocols are outlined to minimize the risks associated with the use of vacuum equipment and to ensure safe working conditions.
1. Use of Personal Protective Equipment
- Safety glasses, lab coats, and face shields must be utilized when working with or near a vacuum apparatus.
- A blast shield must be utilized to prevent flying glass or debris resulting from a sudden change in pressure.
2. Use of Proper Tubing and Equipment
- Always use tubing, glassware, and other equipment that is rated for use with vacuum. Improper use can result in material failure and cause explosion/implosions.
- Check the glass and tubing regularly for defects/cuts, as these can easily crack/break under vacuum.
- The exhaust of the vacuum pump must be connected to a fume hood or a scrubbed building exhaust. This is particularly crucial if the vacuum is utilized on a system utilizing corrosive or toxic chemicals.
- Depending on the experiment and extent of vacuum involved, a guard or protective barrier between the operator and vessel under vacuum should be employed. These can be the same type of barriers used to isolate operators from high-pressure equipment.
- Always use a trap between the vacuum source (pump) and the apparatus that utilizes the vacuum. The trap protects the expensive vacuum source from damage in case of accidental leaks or material back flow into the vacuum line.
- The traps also help to prevent vapors/odors from being emitted into the exhaust of the pump.
- The traps are usually cryostatted using dry ice or liquid-nitrogen baths. Extreme care must be taken while utilizing such cryogenic temperatures, and proper PPE must be used to transfer the coolants in and out of the traps.
- Since the use of cryogenic fluids (i.e., liquid nitrogen) for cooling purposes can lead to the liquefaction of oxygen, vacuum systems must be under vacuum before and during operation. After achieving the desired vacuum level, the dewar containing the oil trap can be subsequently filled with liquid nitrogen. After completion of the experiment requiring vacuum, while still under vacuum, remove the Dewar flask from the liquid nitrogen filled trap, let the trap warm to room temperature, and slowly open the system to atmospheric pressure.
- The Dewar flasks themselves are under vacuum and must be handled with utmost precaution as they can instantaneously implode. Always use proper PPE when transporting or working with Dewar flasks.
4. Bleed lines
- The vacuum lines must be slowly bled before disconnecting from the traps and vacuum source. A sudden change in the pressure stresses the materials and can cause premature fracture and explosions.
5. Glassware coating
- Glassware larger than 250 mL that is utilized with vacuum equipment must be shrouded with either tape, netting, or a plastic coating to reduce the chance of flying debris, in case of an explosion. This includes traps, dewars, rotary evaporators, and any other glassware maintained under vacuum.
Vacuum pumps are employed in a wide array of laboratory procedures. Common examples include filtration, drying, degassing, evaporative coating, and mass spectrometry.
Pump equipment must be maintained and operated safely to prevent equipment failures, explosions and chemical release. This video will introduce several common pump designs, discuss common precautions to be observed when setting up vacuum equipment, and demonstrate operational safety.
Let's begin by exploring various pump designs.
In rotary-vane pumps air and other gases are drawn through an inlet by a rotor. The gases are forced via an oil-sealed exhaust, which prevents backflow, to the outlet leaving the system. Rotary vane pumps can generate vacuums of ten to the negative three Torr. These pumps are self-lubricating, but require oil changes and are vulnerable to corrosion by water vapor.
In scroll pumps air passes through an inlet between two eccentric spiral scrolls, one fixed, the other orbiting. The motion compresses the air and pushes it toward the outlet. Vacuums of ten to the negative two Torr can be achieved. Scroll pumps are "dry" mechanisms - they do not require oil or water, but the scrolls must be periodically replaced as they wear down. Scroll pumps and rotary-vane pumps are suitable for distillation, filtration, and degassing.
A water aspirator is another type of pump often found in laboratories. In this type of pump water enters through an inlet to a high-speed nozzle, and exits as a low-pressure fluid jet. The gases are drawn in through a side port and forced to the outlet. Water aspirators produce vacuums of only 10 Torr. Although they easily connect to ordinary sink faucets, they require large amounts of water. Water aspirators are frequently used for drying and extraction.
Lastly, turbomolecular pumps produce ultrahigh vacuum. Air is forced in through alternating stator and turbine blades that drive the gas molecules through the outlet connected to a roughing pump. Turbomolecular pumps can produce vacuums as low as ten to the negative ten Torr, but require another pump to first lower the pressure to 1 Torr. Turbomolecular pumps are used for electron microscopy, crystal growth, and evaporative coating.
Now that you're familiar with the designs, let's examine personal protection and safety measures that should be observed before operating these vacuum pumps.
If possible, operate all vacuum equipment inside a fume hood with the sash lowered. Wear safety goggles and a face shield. These provide protection against chemicals and debris in case the glassware implodes under the vacuum.
Use glassware and equipment rated for use with the expected level of vacuum. Check the glassware and tubing for cracks or other defects. Defective or inappropriate equipment can easily implode under vacuum. Wrap glassware larger than 250 mL in tape, netting, or plastic, as a further precaution against flying debris.
If the procedure is known to generate corrosive vapors, select a pump that can withstand those vapors. Ensure the pump is clean and free from corrosion. For oil pumps, check the oil level and change the oil periodically.
Ensure the pump is level and balanced. Connect the pump outlet to the fume hood exhaust. Securely place tubing inside the hood to prevent the release of chemicals. Ensure all tubing is unrestricted, and that there are no leaks, especially near the flanges.
Now that the vacuum pump is set up, let's examine safety considerations during and after pump operation.
Connect the pump inlet to the glassware via a cold trap. A cold trap is a glass container that protects the pump by freezing volatile organics evacuated from the apparatus.
During the procedure, the cold trap is submerged in dry ice or a Dewar of liquid nitrogen. Use cryogenic protective equipment when handling these coolants.
A potential hazard is the condensation of oxygen in the cold trap to yield highly explosive liquid nitrogen. To prevent its formation, start the vacuum pump and evacuate the apparatus before submerging the cold trap in liquid nitrogen. Never allow the cold trap to contact liquid nitrogen if not under vacuum, and never open the vacuum line to air with the cold trap in place.
Check the cold trap for condensed solvents and liquid oxygen regularly. If necessary empty the cold trap to prevent solvents entering the tubing and vacuum pump. If liquid oxygen, a light blue fluid, is visible, terminate the procedure and call for assistance, but do not stop the vacuum or remove the liquid nitrogen.
Once the procedure is complete, withdraw the cold trap from the coolant and then switch off the pump. Bleed the vacuum lines slowly before disconnecting the cold trap and pump, to prevent sudden pressurization.
You've just watched JoVE's introduction to lab safety for vacuum-based equipment. You should now be familiar with different types of vacuum pumps, their potential hazards, and precautions to be observed to ensure safe operation. As always, thanks for watching!
Applications and Summary
Operations requiring vacuum have multiple hazards associated with them. Vessel implosion can lead to flying glass and other materials, the release of chemicals to the working environment, and potentially fire due to the condensation of liquid oxygen. Vacuum operations should be set up properly and only operated after potential risks have been identified and properly mitigated.
- NRC (National Research Council). Prudent Practices in the Laboratory. Handling and Management of Chemical Hazards. National Academy Press: Washington, DC, 2011.
- Laboratory Vacuum Pump Buyers' Guide, 2012.