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Gas-Phase Air Cleaning

Some HVAC systems may be equipped with sorbent filters, designed to remove pollutant gases and vapors from the building environment. Sorbents use one of two mechanisms for capturing and controlling gas-phase air contaminants-physical adsorption and chemisorption. Both capture mechanisms remove specific types of gas-phase contaminants from indoor air. Unlike particulate filters, sorbents cover a wide range of highly porous materials, varying from simple clays and carbons to complexly engineered polymers. Many sorbents -not including those that are chemically active-can be regenerated by application of heat or other processes.

Some of the most important parameters of gas-phase air cleaning include the following

• Breakthrough Concentration: the downstream contaminant concentration, above which the sorbent is considered to be performing inadequately. Breakthrough concentration indicates the agent has broken through the sorbent, which is no longer giving the intended protection. This parameter is a function of loading history, relative humidity, and other factors.
  
• Breakthrough Time: the elapsed time between the initial contact of the toxic agent at a reported challenge concentration on the upstream surface of the sorbent bed, and the breakthrough concentration on the downstream side of the sorbent bed.


• Challenge Concentration the airborne concentration of the hazardous agent entering the sorbent.


• Residence Time: the length of time that the hazardous agent spends in contact with the sorbent. This term is generally used in the context of superficial residence time, which is calculated on the basis of the adsorbent bed volume and the volumetric flow rate.


• Mass Transfer Zone or Critical Bed Depth: interchangeably used terms, which refer to the adsorbent bed depth required to reduce the chemical vapor challenge to the breakthrough concentration. When applied to the challenge chemicals that are removed by chemical reaction, mass transfer is not a precise descriptor, but is often used in that context. The portion of the adsorbent bed not included in the mass transfer zone is often termed the capacity zone.

Factors affecting gas-phase filter life include removal capacity and sorbent weight, sorbent collection efficiency, airflow rates, and molecular weight and concentration of the contaminant. Filter replacement labor costs depend on the number, size, and type of filters, their accessibility, how they are held in the filter rack, and other factors affecting labor.