um system is measured in cubic feet per minute (CFM). The machineʼs airflow is inversely pro-portionate to the systemʼs total resistance and directly proportionate to the motorʼs “suction” (low pressure region). Suction — also known as vacuum — is a vacuum cleanerʼs ability to efficiently pick up matter. It is the pressure difference created by the motorʼs spinning fans. There are many ways airflow resistance presents itself. When cleaning carpets, airflow is opposed by carpet fibers, air turbulence within a hose and filtering media due to flow around its fibers. Additional opposition builds up when dirt particles partially block the filter media. This is noticeable when a vacuum clean-er bag fills with dirt. Airflow opposition exists even within the suction motor and vacuum case as the air turns sharply while passing through. A vacuum cleaner takes advantage of a basic environmental science principle. Like fluids, air always flows from a high to a low pressure region. The flow speed depends on the pressure difference (pressure gradient) and flow resistance. A vacuum is the absence of matter in a space. Vacuum cleaners create a partial vacu-um, or an area of reduced air pressure, as air moves outward within the fan. Air pressure is the weight of atmospheric or compressed air (body of gas) on a spe-cific surface area. At sea level, atmospheric pressure is 14.69 pounds per square inch (PSI). A vacuumʼs suction motor creates airflow by reducing air pressure. Airflow is created as air with normal (high) atmospheric pressure moves toward the partial vacuum or the low air pressure. There has been a tendency only to look at pressure (suction) when evaluating vac-uum cleaners. While an important consideration, a vac-uumʼs collection efficiency and bag size also are important. Filter media affects a vacuumʼs ability to retain unwanted matter (fine dusts and allergens) and airflow resistance. Filter components, regardless of type, must be replaced periodically to remain effective. A vacuum cleaner filterʼs total resistance is inversely proportionate to the filter mediaʼs total area. For example, doubling the bagʼs surface area reduces the total airflow resistance by almost 50 percent. Everything being equal, a larger bag makes airflow easier. Determining a vacuum cleaner bagʼs fil-tration efficiency can be frustrating. Claims like “retains dust down to 0.1 µm (micron) in size,” are misleading since the percentage retained at that size may be low. Some marketing materials state a per-centage without indicating the actual effi-ciency percentage at a stated particle size. Vacuum cleaners with a High Efficiency Particulate Air (HEPA) filter often are rec-ommended. The HEPA is an efficiency specification for filters developed by the Atomic Energy Commission in 1946. The specification effectively removed radioactive dust from plant exhausts with-out redistributing. These HEPA filter machines require high-er energy levels and pressure to force air through the deep, dense filter. They have no requirements or significant risk reduction advantages when filtering out common allergens from vacuum cleaner airflow. A HEPA filter must capture 99.97 percent of all particles as small as 0.3 µm from the air flowing through it. “As small as” means if all particles were that size the efficiency would be similar. This is different than “down to,” which may mean a mixture of particle sizes for the stated efficiency. Vacuums properly claiming the name HEPA provide in-depth filtration that may be useful to consumers. There also are feasible alternatives to these vacuums. A collection efficiency of 99 percent at >3 µm and a 95 efficiency at 1 µm is readily achievable with common vacuum cleaners. gas, liquid or dissolved substance or parti-cles adhere to a solid substanceʼs surface. Absorption occurs when molecules assimilate into a solid or liquid. Extraction systems are the surfaces to which things adhere, such as doormats, cleaning cloths, mops, electrostatic clean-ing systems, cleaning powders or particles and sawdust. Unwanted substances are absorbed by or adhere to the surfaces of these cleaning devices before being transported to a suit-able disposal location. Extraction devices vary in cleaning effec-tiveness. Regardless of their ability to contain and remove pollutants, they share a common purpose. Each one aids in removing unwanted substances from within the building enve-lope or the environment being cleaned. What Role Does Airflow Play? Airflow and ventilation are critical to cleaning. Air carries suspended materials as it flows, such as gases and small particles called aerosols. When controlled, air provides for effec-tive cleaning so the materials it carries can be properly disposed. Most cleaning solvents that dissolve greases and particles are liquids containing organic compounds. When a cleaning solvent is applied or delivered to a polluted area, air flows over the solvent releasing the solutionʼs organic compound as a gas. Once the solvent breaks down or dis-solves — a grease spot, for example — parts of the pollutant are released into the surrounding atmosphere as small particles or gases. Airflow must continue over the cleaning site to remove the particles and residual gas released from the solvent or pollutant. Proper and direct ventilation minimizes pollutant and cleaning residue exposure. Airflow is necessary for drying. Drying only occurs when suspended moist air is displaced by dry air on, above and through an environment cleaned with a liquid solvent. Many cleaning problems occur when How Does Adsorption Differ From Absorption? Adsorption occurs when molecules of a 26 CM/Cleaning & Maintenance Management ® • July 2009
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