Isolation by Purge

Hex screw with wrench & hammerWhen 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.

Face exhaling a cold cloudPurged 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 Roaring fire in a fireplacewas 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, Steel pressurized gas cylindersSterile 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.


6 thoughts on “Purged

  • By Bilalbharde - Reply

    Very great website on Isolation and very informative. I think this website can persuade people a lot on your topic. Has a lot of information and I think that’s great. This is actually my first time ever seeing this topic before. It goes a lot more in depth in talking about the atmosphere which I noticed and it’s really cool. So far really great website!

    • By SteveT - Reply

      Thanks for visiting and sharing. The ocean of atmosphere is what is important when purging with argon gas for welding or clean air for sterile work. We take it for granted since we are built to breathe it, but when doing something that needs a different one, the atmosphere we breathe is a problem that needs solving. Purging it away is one way to do that.

  • By shoval - Reply

    Nice overview of the atmospheric exiles.
    As you said, we are used to breath and in different conditions, we need to protect our health.

    • By SteveT - Reply

      Thanks for joining the conversation, Shoval. I like your term atmospheric exiles – I will use it!

      Yes, purging through a fume hood / fume cupboard or fireplace flue can protect us from breathing polluted air, but much other purging is done to protect sensitive materials or processes FROM our breath or oxygen in the air. The shield gas in mig welding is a good case of protecting the weld process from oxygen in air, the same oxygen we need to breathe.

  • By Jeff - Reply

    Enjoyed this post on, Purging! Wouldn’t it be great if we could find a way to purge all the Green House Gases from the upper Atmosphere? This would be such a great technological advancement to add to the list!
    Regards, Jeff.

    • By SteveT - Reply

      You bet, Jeff – if only we could purge the CO2 and Methane out to space, but gravity will not allow it!

      Our only real options are to recycle the gasses or capture them and tie them up chemically or biologically, or isolate them in deep wells or tanks. Purging can only do so much…

      Thanks for the thought, though. We have to keep exploring options or we are toast!

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