Air-cleaning devices

List several types of air cleaning devices that can be used to remove airborne particulate matter. Rank these in order of their collection efficiency and typical maximum size particle capture. 

Efficiency, filter The ratio of the number of particles retained by a filter or other air cleaning device to the number of particles entering, expressed as a percentage. 

Self-cleaning An air-cleaning device that has the ability to be mechanically or chemically cleaned. 

Primary collector An air-cleaning device that removes the larger particulate matter before a HEPA filter. 

Where dusts, grits, sawdust or other large particles are being extracted it is normally mandatory to include an air-cleaning device before the air is exhausted. Unless they are toxic or odorous fumes can normally be simply exhausted into the atmosphere at a suitable location well away from anywhere with normal personnel access. 

Air cleaning devices Where possible, air-cleaning devices should be located close to the process to avoid contamination of exhaust ductwork. Bag in/bag out (safe-change) filters or comparable required for all filtration systems... 

Particle size analysis of airborne dust is also necessary in the consideration of appropriate air-cleaning devices for use in a complete air-conditioning system at this site. All components of this air-conditioning system must be installed so that the architectural and artistic character of the tomb is not disrupted or endangered in any way. 

Air cleaning device Particle size range for 90% removal ( xm) Remarks... 

The air cleaning devices (air filters, dust collectors, cyclones, etc.) should be protected against the consequences of dust cloud explosions. This includes measures to prevent propagation of the explosion back through the exhaust ducting to other areas of plant and equipment. 

Minimize accumulations of condensed liquids. Balance airflow without dampers to prevent unwanted change of airflow rates, and eliminate accumulations of condensed liquids and solids. Provide air-cleaning devices (air filters, scrubbers, etc.) with protection measures against the consequences of gas explosions (refer to the section titled Protective Measures ). 

Air purification is a very important issue. Air cleaning devices are needed for respiratory protection (gas masks) and for removing CO, CO2, water vapour and trace amounts of VOCs and ozone from ambient air indoor office space (due to smoking, etc.) and in submarines or space crafts on long missions [2,6,7,18,31,64,71,110,125,126,132,251,260-264,598]. 

ASHRAE. 1999. ASHRAE Standard 52.2 Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. Atlanta, Ga. 

Cyclones are seldom used as the primary means of particulate collection, but often serve as first-stage air cleaning devices that are followed by other methods of particulate collection. Cyclone collection efficiency is probably more susceptible to changes in particulate characteristics than are other types of devices, therefore, care should be taken if used. Cyclone operation is dependant, generally, on physical parameters such as particle size, density, and velocity, as opposed to the chemical nature or properties of the material being collected. 

An air cleaning device to remove light particulate matter from normal atmospheric air. Air Line Breathing Apparatus... 

An air-cleaning device to remove particulate loadings from exhaust systems before discharge to the outdoor environment. 

An air-cleaning device that involves the following steps electrical charging of suspended particulate matter collection of charged particles on a grounded surface and removal of particulates from the collecting surface by mechanical vibration or flushing with liquid. 

Other Losses Not only are there friction losses along pipe walls and dynamic losses at the system entry and at each bend or transition, but there are also losses at filters or other air cleaning devices that are part of the system. Filters placed in the air stream will want to slow the flow and create a friction loss. In summary, Bernoulli s equation (Equation 10-8) applies to air flow in ducts. A related form for the equation is the sum of the static pressure and velocity pressure of a point upstream in a ventilation system is equal to the sum of the static pressure, the velocity pressure, friction loss (FL), and dynamic losses (DL) at a point downstream in the system ... 

Fans, blowers, or ejectors provide air movement in local exhaust systems. Fans are most common. In most cases, fans are downstream of air cleaning devices. That prevents contaminants from affecting fan performance and service life. Blowers are usually upstream of ventilated space and often used in dilution ventilation. 

Very often a local exhaust ventilation system must include a capability to remove contaminants before air moves to the outdoors. Cleaning is essential for recirculated air. There are several types of air cleaning devices. Figure 25-5 illustrates properties of aerosols and related cleaning equipment. 

Efficiency plots or curves help when selecting air cleaning devices for particulates. The curves show the portion of each size of particle actually captured. 

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