Isolation by Laminar Flow Air (a basic guide)
As anyone using a fireplace knows, hot air from the flames rises, pulling smoke up the chimney, away from us, so we can breathe easy. This happens in a slow, steady laminar flow way, because hot air rises and the fireplace flue is full of it.
This laminar air motion up the flue will carry smoke, carbon monoxide and excess heat away, protecting the inside space from becoming a choke zone. This principle has been used in heating living spaces for thousands of years, and laminar air flow air wash principle based on it is behind fume hoods (fume cupboards for Brits), clean rooms and biological safety cabinets world-wide. When it comes to handling flames, smoke and massive quantities of other particulates, this old standby is the best way to remove the problem from living/working space. It is so effective, that it has become the standard way to address nearly all air-born hazards and irritants:
“the solution to pollution is dilution”
This is the rule at the core of more standards, rules, laws and operating principles than nearly anything else in safety and environmental protection. While this is a powerful principle, it is important to apply it correctly, in ways and situations where it will do the most good. That starts with understanding how it works.
How to wash your air
Laminar air flow is the constant, gentle breeze that flows into the front of fireplaces and fume hoods, so steady and constant, you may not know it is there. If you hold a smoking match near it, you will know, because the smoke will be pulled into that airflow and up the flue or exhaust duct in a steady, continuous stream. The laminar air flow that works this way flows like a gentle stream of water, or olive oil pouring out of a bottle; it flows in one direction only, and the edges of the flow path move more slowly, compared to the more rapid center of the path. In between the edge and the center, there are layers of the flow, “lamina” that travel at speeds that vary from slow near the edge to faster near the center. When dust, smoke or a small feather fall into this flow, they are trapped between the lamina or within one of them, and they “go with the flow” up the stack.
Laminar flow air washes contaminants away reliably, because it is slow, steady and constant; in a word, “laminar”. The opposite of laminar flow is turbulent flow, which is violent, unpredictable and goes in multiple, changing directions, like the air from an open window in a car going 60 miles/hour. Turbulent flow will also capture dust and contaminants, but we don’t know where they will go; they could be stuck in an eddy current for minutes, go right out a window, or sent right into your face. The main tricks to keeping laminar flow air wash working reliably are:
- Keep the flow constant at about 100 feet per minute (slow enough to stay laminar, but fast enough to be effective at washing away contaminants)
- Maintain a smooth, uncomplicated flow path for the stream that avoids large obstacles, sharp edges and tools or fixtures that and not “streamlines”
- Keep your motions, actions and processes in the laminar flow path slow and steady, avoiding movements that disturb air lamina
- Maintain fans, filters and flow paths to ensure that clean air supply, motion and exhaust are reliable and constant, and that everything in the air supply path and work zone is cleaned and sterilized after there is an interruption, power failure or shut down
None of this is rocket science, but it does require training, vigilance, planning, infrastructure and a lot of energy to keep it operational. There are enough critical elements to such systems that risk management is essential. This is all justified, when laminar air wash is the best control method for the process.
When to wash your air
As important as how to use laminar flow is deciding if, when and where to use it. Too often it is used without considering the carbon footprint, operating cost or effectiveness for the application, and that can lead to serious performance issues. Because laminar flow hoods often exhaust all or much of the air flow out of the building, the Heating, Ventilation and Air Conditioning (HVAC) costs can be great. In fact, compared to sealed or pressurized containment, energy use for a fume hood or biological safety cabinet can be 5000% higher (50 times). Also, much of this laminar air flow equipment must be tested and re-certified 2 to 4 times annually or more, to ensure that it is actually working and providing the protection required. To avoid problems and ensure effective application, we recommend the following process when planning a project that may require laminar flow:
- If the process produces large amounts of smoke, gas or heat that must be removed, laminar air flow is required, so skip to step 4.
- If the process can be fully performed in a simple sealed container or pressurized system practically and legally, proceed to use that simple approach and skip remaining steps
- If standards, practices or laws demand the use of laminar air equipment, attempt to downsize the equipment and the size and duration of the airflow to minimize HVAC, energy cost and operational impact.
- Coordinate the laminar air flow system with the HVAC system to keep both systems operating when, if and as needed, without building air imbalances or inadequate heating or cooling
- Ensure that operating and maintenance access to the source of the contamination can be accomplished in an easy, practical way, with full hood integrity restored for normal operation (otherwise, it may be left disabled)
- If hoods or intakes will be fully or mostly closed some of the time, ensure that ways to clean and/or sterilize them when restarted are in place, and are practical
- Ensure that maintenance, calibration and certification plans, training and infrastructure are in place to keep this equipment operating safely and effectively
- Plan the systems of operation, maintenance and monitoring, to ensure that this technology will continue to function as required
If you proceed with laminar air wash equipment, be sure to audit performance periodically, as isolation in this way is invisible and hard to track without specialized tests. Murphy’s law will be in effect; if operating, maintenance or equipment problems develop, you may not know without an audit to reveal the problem. It is common for such systems to be completely non-functional for years if not properly monitored.