There are a great variety of designs which vary from manufacturer to manufacturer and even within the same manufacturer.  The machine’s design, look and feel are not so important.  This is mainly “curb appeal” for the consumer.  The important consideration for waste containers is the air filtration system through which all air from the airflow through the machine must exit.

The end goal of any vacuum cleaning system is to trap the airborne contaminants in the waste container and not launch any contaminant back into the ambient air of the room.  Some manufacturers are better than others in accomplishing this goal.  At this point in time all manufacturers have the ability to implement HEPA level filtration in the air way exhaust.  Some do, and some don’t.

Lets explore some of the engineering used in airflow management, and the implications of some engineering decisions on how a vacuum cleaning machine might be used.

Filtration of airborne contaminants and their subsequent controlled removal is a multi stage process potentially involving a series of engineering solutions.  In its’ simplest form a vacuum cleaning machine will introduce particulate material into an airflow that subsequently empties into a receptacle for disposal.  The airflow will be interrupted by a waste collection container or bag into which floor contaminants are deposited.

Recall that it is the creation of an air pressure vacuum that is responsible for creating airflow.  Airflow is the process of air of higher air pressure seeking to fill areas with low air pressure.  The low air pressure area in the vacuum cleaning machine is created by the spinning vanes of a fan (light blue).

The vacuum fan can be either before or after the power plant.  As the vacuum fan assembly spins, the blades of the fan cut into ambient air (green arrows) and, with the surface area of the fan blade powered by the electric motor, push the air to the other side of the fan cowling.

The back side of the fan cowling now has higher air pressure (red arrows).  Because of the 2nd law of thermodynamics the high pressure air moves through the vacuum cleaning machine to where air pressure is lower ( Green plus symbols) where the air pressure equalizes.  In a vacuum cleaning machine the waste receptacle can either be located before or after the power plant and fan assembly.

On the path to the waste receptacle, the airway must maintain sufficient airflow to keep airborne contaminates  suspended in the airflow.  This requires that the airway maintain air pressure and good airflow volume.

A waste receptacle that is in-line with the machine airflow will diminish the airflow of the machine as it becomes increasingly full, which in turn reduces the machine’s ability to keep dirt suspended in the air flow.

In the two diagrams shown, one is a front filtered canister style, and the other is a rear filtered canister style.  The front filter style is a common design that traps floor contaminants before they make their way through the machine.  This style of machine has an additional fan to draw ambient air around the power plant motor.

The back side of the fan cowling now has higher air pressure (red arrows).  Because of the 2nd law of thermodynamics the high pressure air moves through the vacuum cleaning machine to where air pressure is lower ( Green plus symbols) where the air pressure equalizes.  In a vacuum cleaning machine the waste receptacle can either be located before or after the power plant and fan assembly.

Some manufacturers have designed their machines with multiple filters in an effort to provide HEPA-like filtration and be able to add this filtration to their marketing position in hopes of selling more units.  A multi filtration airway is costly to manufacturers who must increase the vacuum airflow through either a modified impeller fan design, increased power plant rotational speeds, or both.

Each time a filter is placed in a machine’s airway there is a corresponding decrease in airflow cubic feet per minute (CFM).  The decrease stems primarily from turbulence as air traverses around and through the geometry of the filter medium.  When floor contaminants are introduced to the filter medium the decrease in airflow increases.

With very few exceptions, all vacuum cleaning machines use the same set of physics to trap contaminants.  Differences are found in the filter media, and the waste receptacle design, but not in the physics behind the entrapment of foreign contaminants.  By adjusting the amount of surface area of a filter a manufacturer can alter the time between filter replacement.

When airflow is high, as in a vacuum machine, contaminants are lofted into an airway and directed at an air filter.  At the speeds with which air is moving, much of the particulate matter in the airway would blast right through filter media and not be stopped.  This is just physics.  The airborne contaminants are accelerated from a resting state to a high energy state as they are launched into the vacuum airway.  With all the kinetic energy they have taken on, when they hit the filter media they break through the media fibers by releasing their kinetic energy causing separation or fracture of the media.

To capture contaminants manufacturers must slow down the airflow and decrease the amount of kinetic energy of each particle.  This is accomplished by increasing the capture area.  The airway is blasting air through a 1.5 inch diameter tube.  When that 1.5 inch tube is expanded 10 times (or more) it’s diameter, the airflow slows dramatically and the kinetic energy of the particles is reduced correspondingly.  With a lower velocity and lower kinetic energy, particles are more easily intercepted by filter media.  The expanded area in most vacuum cleaning machines is the waste receptacle that can contain the air filter or filters if more than one is present.

The three mechanisms responsible for causing a particle to “stick” to fiber material are: Diffusion, Interception, and impaction.  There are other methods, but they seem to all have these three mechanical methods in common.

Diffusion is a method that is governed by particulate size, filter media fiber size, and air turbulence.  When very small particles (diameters below 0.1 micrometers) are introduced to a filter they are buffeted by air turbulence created by air moving in and around the filter media fibers.  The manner by which they travel through the filter follows the Brownian motion of gas particles in air.  The Brownian motion is a zigzag-like path through the filter.  Eventually these small particles come in contact with the filter fibers and lodge against the fibers.  Diffusion is very effective at low airflow velocities.  High velocities defeat this filtration method.

Interception is similar to diffusion, but has less reliance upon the Brownian behavior and more reliance upon physical proximity to filter fibers.  Interception is a filtration method that applies to larger particulates.  As these particles are introduced to the filter fibers, their pathway is governed by air turbulence that causes them to carried through the filter until they become surface attached to a fiber.  This method of filtration is also sensitive to airflow speed.

Impaction is another mechanical filtration method.  When particles are larger they retain much of their inertia and kinetic energy.  When these particles are introduced to the air filter they slam into filter fibers and physically lodge into the structure of the fiber.  They are too large to be affected by the turbulence surrounding filter fibers.  So, they do not slide past a fiber in the airflow.

Manufacturers use a wide variety of filter geometries that are closely tied to the type of machine and the type of particulate matter their machine is designed to pick up.  Commercial machines tend towards canister styled pleated fabric.  This is a very durable filter design used by most air-carriers because of its durability.  It is often found in wet/dry vacuums because of its mechanical durability.

Other styles are matched to the machine and the intended use of the machine.  Toner vacuum cleaning machines will often use HEPA or ULPA filters that are able to trap contaminants as small as 0.1 micrometers in diameter.  However, these machines become surprisingly HOT after extended use.  This is due to the restricted airflow caused by the HEPA and ULPA  filter technology.  These little machines are working hard to push air through.

Manufacturers have recently begun introducing a reticulated (net-like) flexible polyurethane foam technology which is characterized by a skeletal strand structure and open voids (windows) between adjacent cells providing between 90% to 97% open space.  The increase in void space allows for increases in permeability.

The foam is measured in terms of Pores Per Inch (PPI) and has a wide range of permeability available in 4 to 100 PPI.  A PPI of 30 will allow for 18 CFM.

The PPI rating of a manufacturer’s filter can vary widely and it depends on the manufacturer’s other filter as to what the permeability of the foam filter will be.

Some manufacturers use foam as a pre-stage filter for trapping large particulates.  If there is a pre-stage filter, the second stage filter is typically a HEPA filter.  Cleaners using foam as the filter are well suited for larger materials like sawdust or large fiber pickup.  Keep in mind that unlike basket filters that are tossed away with the dirt they contain, foam filters typically remain in place for months and in many cases years.

Foam filters will absorb odors quickly on the surfaces of the foam cells through adhesion.  The mechanical force behind the airflow slams odor bearing particulates against the cell structure where the oils continue to release their volatile compounds giving an odor to the air exiting the vacuum cleaning machine.  While most odors can be washed out of the foam, a remnant of odor always seems to remain.  Foam filters have a shelf-life.   The solvents in foam do evaporate over time  shrinking the foam and making it increasingly brittle.  Try to replace the foam filter annually if not more frequently if you have pets.

There are a great variety of designs which vary from manufacturer to manufacturer and even within the same manufacturer.  The machine’s design, look and feel are not so important.  This is mainly “curb appeal” for the consumer.  The important consideration for waste containers is the air filtration system through which all air from the airflow through the machine must exit.

The end goal of any vacuum cleaning system is to trap the airborne contaminants in the waste container and not launch any contaminant back into the ambient air of the room.  Some manufacturers are better than others in accomplishing this goal.  At this point in time all manufacturers have the ability to implement HEPA level filtration in the air way exhaust.  Some do, and some don’t.

Lets explore some of the engineering used in airflow management, and the implications of some engineering decisions on how a vacuum cleaning machine might be used.

HEPA stands for High Efficiency Particulate Absorbing (HEPA).  The ISO/U.S. DOE standard specifies that the filter must remove from the air path 99.97% of particles whose diameter is equal to 0.3 μm, with the filtration efficiency increasing for particle diameters both less than and greater than 0.3 μm.  HEPA filters are able to filter out dust, bacteria (0.2-2.1 μm), virus (0.02-0.3 μm), and sub-micron liquid aerosols (0.02-0.5 μm), pollen and metal dusts.

The HEPA filters today are made from a mat of randomly bundled fiber materials of polypropylene, rock wool, or fiberglass with diameters of 0.45 to 2.1 μm.  They stop airborne particles in a variety of ways by reducing the face velocity of a particle causing it to stick to a fiber rather than bounce around in the air way.

Keep in mind that HEPA filters become clogged over time more quickly than other fiber filters.  This is why having a large surface area made from HEPA certified material is of benefit.  A large surface area will not impede airflow through the machine so long as the filter remains permeable.

Some manufacturers have implemented a small filter area of HEPA material along with a high velocity airflow.  These filter types force smaller particles through the HEPA material because of the force associated with high air pressure.  The HEPA filter in this instance only traps the larger particles that become physically entangled in the filter matt.

Many people suffer from allergies (protein sensitivity) and asthma.  These individuals benefit greatly from the use of HEPA filtration material.  Allergens and dust mite feces are asthma triggers and are trapped by the HEPA filtration material.  Be advised that only edge-sealed HEPA canisters are approved for filtering  biologics from air.  These containers do not allow seepage around the filter media, but trap all airflow.

During the COVID pandemic the use of HEPA filters was recommended.  Even though the HEPA filter is only able to physically trap particulates of 0.3 micrometers it was the recommended filter type to use.  The SARS-Cov-2 virus (COVID) is approximately 0.125 µm in size.    This should mean that the virus would slip through the filter fibers.  Ordinarily this would be true.  The piece of information that makes all the difference is that the COVID virons rarely float by themselves.  They are almost always attached to airborne water droplets (mucus) from sneezing or coughing.  Airborne water droplets are much larger than 0.3 micrometers and easily removed by HEPA filters.

[ENGINEERING NOTE]=> Did you know that 40% of airline cabin air goes through HEPA filters and the rest 60% is brought in from outside the aircraft.  HEPA airline filters capture 99.97% of all airborne particulates including virus, fungi, and bacteria.

There is an additional filter technology called the Ultra Low Particulate Air (ULPA) filter.  It is able to trap much smaller particles than the HEPA technology.  The ULPA standard removes an impressive 99.999% of particles as small as 0.12 microns.  This filter technology uses borosilicate glass microfibers just like HEPA filters.

The downside to using this filtration media is that it is expensive and fragile.  In a vacuum cleaning machine the introduction of ULPA filters would significantly reduce airflow and require a much larger power plant and a greater surface area of filter material to handle the increased airflow.  Consequently, this filtration media will not be arriving in the near future.

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