Contamination is a most persistent adversary. It stalks us in the dining room, the kitchen, the bath, in public buildings, factories and laboratories. With an ocean of air full of interesting particles, vapors and living organisms, it is always available to add something to the food we eat, the air we breathe, the products we make and the experiments we perform. It is so relentless, that protection from it is a big challenge, but preventing contamination with positive pressure ventilation is a powerful option. Our home, our vehicle, laboratory or factory and a host of other closed structures can avoid contamination by positive pressure isolation.
When a balloon is inflated, the positive pressure gas inside is retained by the latex envelope that contains that pressurized fluid. We know that the balloon will eventually deflate because balloons leak just a bit as all containers leak. But, that balloon interior is protected from contamination so long as the pressure inside is positive, higher than atmospheric pressure around it.
How positive pressure isolation blocks contamination
Air or helium in a balloon will slowly leak out, but the positive pressure keeps outside air, smoke, microbes or gas from leaking into the balloon. This is true for positive pressure in a balloon, a building, a room, a laboratory glove box or just about anything else. This is a form of positive pressure isolation. The mechanism is pretty simple. Every wall, latex membrane or other solid barrier has leaks in it, as it is made of molecules with lots of space between them, so there are always a few holes, even if they are too small for us to see. If the barrier has good integrity, the holes are very small, so most microbial life, dust or other contaminants are blocked, but a few gas molecules and very small particles or microbes could clear the opening. But if the air or gas inside is positively pressurized, then the leakage flow through any hole will be from the inside to the outside. If the hole is small, no dust particle, gas molecule or microbe can get in, because they can’t swim upstream, against the flow of gas or air. The positive pressure guarantees that the flow direction is outward.
How to use positive pressure in a building or room
When we want to avoid the flow of pollution, cold or hot air or other contamination into a room, a house or a building, we can positively pressurize the structure. This is possible and practical because even a very slight positive pressure will provide isolation protection. A pressure isolated laboratory, house or building is generally kept at a positive pressure of less than 2 inches (5 cm.) of water; a fraction of the pressure we exert to drink through a straw!
This is fortunate, as most buildings have an awfully lot of leaks. They have leaks at doors, windows, pipes, wall panels and other places, so it takes substantial air flow to maintain the positive pressure. This flow is driven by the fan in a heating or cooling system or other ventilation equipment. Fans can propel a substantial flow of air at a small positive pressure that is sufficient to pressurize a room or building. In our post about avoiding air pollution at home, we discuss how to set up such a system.
Using positive pressure isolation in a laboratory or factory
Isolation can be critical in laboratories and factories, as the items handled can be very sensitive and susceptible to contamination. Food processing facilities have been shut down permanently, and companies put out of business because of contamination. In other industries, explosions, fires and other serious problems have been the result of contamination of the materials or processes inside. Clearly many types of manufacturing, processing and lab work must have this protection. Often, the rooms or buildings that require isolation can be pressurized similarly to buildings and rooms discussed above. In some cases, whole rooms or buildings are not an option.
Research experiments and clinical lab tests require high accuracy measurements that are not possible if specimens are contaminated. While some rooms, labs or buildings are isolated this way, often isolation must exclude the people working there, as they would be sources of contamination. Naturally, many researchers and lab technicians use pressurized or purged workstations to ensure clean processing, so they can perform experiments that work, and gather accurate data from them.
In some cases this requires use of highly filtered or treated air, to maintain sterile or ultra-clean work spaces. In other situations, air itself is a contaminant, as oxygen, water vapor, nitrogen or carbon dioxide can attack the materials or processes under study. In such situations the process must be contained in sealed laboratory glassware, tanks or barrier isolation glove box workstations that support controlled atmosphere processing in a non-air special environment. In any of these situations, positive pressure isolation is an important tool, as contamination must be avoided with a secure and reliable protocol.
Even when air is not a contaminant, and super-clean or sterile air is not essential for process integrity, pressurized isolation of an experiment may be critical. Minor variations in air movement, temperature, humidity or other parameters can make a difference in measurements required in almost any test or experiment. When the process being studied is isolated in a positive pressure controlled environment, variation in measurement is reduced by minimizing the mechanical, chemical and thermal “noise” that would otherwise influence experimental data. Pressurized isolation is a powerful tool to reduce variability and improve the integrity of measurements and data.
Alternatively, labs will often use an open hood with laminar air wash to provide clean air without pressurized isolation. While this can work well when the system is properly designed, maintained and operated, it doesn’t provide the fail safe protection of a pressurized barrier isolation approach. This approach uses slow moving (laminar) air that has been filtered to provide protection and consistent conditions.
When configured for positive flow air (flow outward) this system can be used to provide clean and sterile conditions. When work is conducted in a negative or vacuum hood (flow inward) then operator protection is provided from noxious or unhealthy fumes, but the experiment or test is exposed to moving air from the room, so sterile or clean conditions are unlikely. Unfortunately, the vacuum hoods are common in labs, so they are often the cause of contamination and operational noise. This often results in quality and integrity problems with test data.
*NOTE- Some items in this post are provided by sponsors who support this website – see our sponsorship page for details.
Disposable glove bag isolation enclosures can provide full pressurized barrier isolation at a modest cost (some under $50USD). *Glove bags come with gloves bonded into them, a sealable opening for insertion of materials and instruments and a port for connection of a gas line for inflation. These temporary workstations are easy to apply when an Argon or Nitrogen atmosphere is needed, as they can be directly inflated using the pressurized gas.
The unit shown is available for *order: Amazon glove bag link. There are several other sizes available for order, as well as larger, open end glove bags for asbestos removal, nuclear work and other hazardous disposal. While glove bags are economical and offer a full pressurized barrier, they are not as common or popular as glove box enclosures or fume hoods. This is primarily due to the disposable, less stable structure of the units, less clarity for observation though the plastic and greater difficulty in cleaning the interior.
There is an economical, low cost glove box now available that is more costly than the glove bag, but fully reusable, rigid
and stable. This unit is described in more detail in a post on flexible lab glove box isolation. It provides basic positive pressure isolation and delivers atmosphere quality at or below that provided in the pressurized gas used to fill it.
When very extreme atmosphere purity is needed, there are large, heavy wall controlled atmosphere systems available that provide chemical atmosphere processing and circulation to purify chamber gasses beyond quality levels of gas initially injected. These units are more expensive and complex to operate, but are sometimes needed for operations that require oxygen or water vapor levels maintained below 50 parts per billion.
Positive pressure isolation works
The important thing to remember is that when you must avoid contamination of the interior of a house, a room, a building or a work-space, positive pressure isolation makes that possible and practical. Whether the ventilation system is a fan and filter, a pressure tank or a pump, if it is properly controlled and applied, it will keep outside contaminants from leaking into the inside.
Have you used positive pressure isolation, or observed it at work? Have you encountered it in buildings where you had to push hard to get an outside door to open inward (pushing against the positive pressure)? Do you have a question, a story or an idea to share? Please use the form below to leave us a comment – we’d love to hear from you.