Isolation by Purge
When materials or processes are isolated that is rarely a choice. Generally, isolation is a process requirement that is essential if experimental, forensic, production or other work objectives are to be met when the process and the people have different atmosphere requirements. The type of isolation may be chosen, but the need for isolation is absolute. When we fail to isolate effectively, our results reflect that failure to the peril of the mission or people working to achieve it. When simply sealing processes and materials in a container isn’t the solution, purging the process work zone is often the first isolation choice considered to protect materials processed or professionals doing the processing.
Purged Isolation is a form of atmosphere control where room air users breathe is replaced or purged away from the work zone with an atmosphere required for the process, or contaminated process air is purged away with room air to keep it from harming people doing the processing. Examples of purged isolation include a clean-air purged clean room for handling contamination-sensitive aerospace components, or a Mig welding setup where the welding arc and hot metal are kept inert by a stream of Argon gas flowing over the welding process.
Purge the problem away
The first and most common type of purged system is the fume or smoke hood. This can be a fume hood in a chemical laboratory, a dust hood in a factory or the range hood in your kitchen. Even the fireplace flue works as a smoke hood to remove smoke and combustion gasses from the home, in fact, it was the first purged isolation system in historic times and gave rise to a cardinal rule of purged protection:
“The solution to pollution is dilution”
This approach, using gently flowing air to wash away smoke, dust, pollutants or irritants has evolved into a huge range of hoods, exhaust systems, smoke stacks and environmental controls. The operating principle of the technologies that use this process for worker protection and clean processing is Laminar air wash. When purging is used to remove air by replacing it with another gas, it is laminar gas wash. That is often just described as using a shield gas or purge gas to exclude oxygen.
Depending on the type of process, the purge gas used to wash away normal air can be Helium, Dry air, Sterile air, CO2 or any other target atmosphere the process or user may need. The critical factor in a purged Isolation system is that the air in the outer workplace environment does not support the required process, or the process requires or produces an atmosphere that is not healthy for people. In either case, the problem is addressed by purging away the problem, displacing it with an atmosphere that works for the process and the people.
Purge side effects
Unlike simple sealed isolation systems, Purged isolation can be used to change the process or materials inside by exposure to the atmosphere itself. If the purge gas is water-free, it can dry out materials being processed. If purge atmosphere is high in alcohol or some other solvent, it can saturate, dissolve or chemically react with the contents or process.
These additional process dynamics add power to purged isolation, but they also complicate the situation with risks or side-effects. Unlike simply sealing materials and processes in a container, purged isolation brings new variables into the process system. Those variables must be measured, controlled and maintained within limits dictated by process needs. Otherwise they will damage materials or processes they are used to protect. We want food that is uncontaminated, but not dried out. Our cough syrup should be sterile, but not too thick to swallow.
Purge to exclude
Purged isolation can be used to alter a material or process, but generally it is used simply to exclude a contaminant. The most common contaminants to be excluded are Oxygen, Water vapor and Microbes, but many other materials may also be targets to avoid. It is worth listing the most common atmospheric exiles:
- Oxygen – this is by far the most important and chemically active material in the atmosphere that impacts life, corrosion and most chemical change around us. From welding gas to NItrogen glove box chambers to inert gas pumped into tires, oxygen is the gas that most processes must avoid, thus requiring purge
- Water – H2O, like Oxygen is essential to life and to many organic and inorganic processes. Because water vapor is typically present in air we breathe, it often must be excluded from target materials and processes, because it can facilitate many reactions and changes
- CO2 – critical for photosythesis of plants and other organic and inorganic chemical reactions, Carbon Dioxide must be excluded from many processes to avoid biological or chemical changes that are not tolerated
- Particulates – a wide range of industrial, botanical, biological and inorganic particulates swim in the air we breathe every minute, and many processes have problems with them. A pressurized enclosure filled with pure gas, filtered air or other particulate-free atmosphere can address this problem
- Trace gasses – modern consumer and industrial life mixes thousands of gases and solvent vapors into the air we breathe. Our lungs might tolerate these gasses well, but some processes do not. Nitrogen or Argon are typically used to purge these trace gasses from a process
- Nitrogen – Most of our atmosphere is Nitrogen, which is fine for our lungs and many processes, because it is generally inert, compared to Oxygen. However, for some materials and conditions Nitrogen is chemically active and ready to react. Because it is so common and often slow to chemically react, it is a popular purge gas, but sometimes it is the main atmospheric gas displaced by Argon or Helium used to purge in welding, deep diving and other high temperature or high-reactive work situations.
Essentials of Purged Isolation
Purged isolation is one type of controlled atmosphere, but it may be the only one that is a practical solution for the intended process. In any case, if isolation is to work and support process activity that meets objectives, it demands good process choices. Like any important trip, the journey to a successful process end point can only end well if we know the intended location. Before a purged isolation system can be selected and configured, it must be matched with the process requirements and priorities. This requires defining the limits of materials and contamination tolerance, since
contamination is INEVITABLE, but the degree of contamination depends on isolation choices.
In the final analysis, purged isolation may offer the simplest control of the process atmosphere that requires special isolation, if properly configured. However, other options, such as sealed or barrier isolation with or without purge may be a better choice, so they should be evaluated when laminar purge is not the only practical solution.