Improved indoor air quality continues to be one of the major issues facing the HVAC industry.

The air movement industry has been very effective in the areas of human comfort and the removal of particulates. However, to totally "clean" the air, the gases and vapors - the chemical pollution that affects health and productivity - must also be removed.

For HVAC applications, adsorption is the most effective and the most economical process. Activated carbon, known as the universal adsorbent, is the most effective adsorbent.

A variety of products using activated carbon are readily available. Many have been in use for decades and have proven to be effective in eliminating the chemical pollution found in HVAC systems.
 

Why Is It Used?

Theoretically, we can remove almost all particulate matter from the airstream. Now we must face how the gases and vapors - the chemical pollution - should be removed.

HEPA filtration is effective into the 0.3-micron range and will remove some particulates smaller than 0.3 microns. Gases and vapors are 0.01 microns and smaller. Benzene and naphthalene, two compounds that are strongly adsorbed by carbon, are approximately 0.0006 and 0.0007 microns, respectively. One micron equals 1/25,400 of an inch.

Very simply, particulate filters do not and cannot remove material that is this small and is a gas. If particulate means cannot be used to remove gases, how can they be removed?

Adsorption with activated carbon is the process of choice for applications where vapors are present in low concentrations. Examples of these applications are gas masks, radioactive iodine removal (nuclear plants), space capsules, and nuclear submarines.

Are there other processes to remove gases from an airstream? Certainly there are. When higher concentrations are present, incineration, chemisorption, adsorption, and solvent recovery all have their places.

For HVAC applications, where the concentrations are extremely low and the contaminant loading varies, adsorption has proven to be the most effective, most economical process.
 

Absorption Vs. Adsorption

A question that is often asked is, "What is the difference between absorption and adsorption?"

Absorption can be understood if we think of sugar being dissolved in water and mixing evenly throughout, or cream being mixed into coffee. Industrial absorption would be a gas being absorbed (taken into and mixing evenly) by a "scrubbing" liquid.

In contrast, adsorption is the physical attraction and adherence of gas or liquid molecules to the surface of a solid. The attractive force, van der Waals force, is very small and exists between any two bodies, such as between the earth and the moon. Gas molecules are adsorbed by activated carbon.

Activated carbon is truly a unique material. There are no other materials, natural or man-made, that will do all that it can do.

Activated carbon:

  • Has a capacity for virtually any vapor contaminant. It will adsorb some of almost any vapor;

  • Has a large capacity for organic molecules, especially solvents;

  • Will adsorb and retain a wide variety of chemicals at the same time;

  • Has an extremely large capacity to destroy ozone catalytically. Ozone is a major component of smog;

  • Works well under a wide range of temperature and humidity conditions;

  • Adsorbs odors and chemicals preferentially to moisture. It is not a desiccant and will release moisture to adsorb chemicals;

  • Can be used as a carrier of one material to attract and hold or react with another material;

  • Is inert and safe to handle and use; and

  • Is available and affordable.

    Activated carbon is a material that has been treated (activated) to increase the internal surface area of the structure to the range of 950 to 1,150 square meters per gram for gas-phase applications. The internal area is the area that holds the adsorbed chemicals; in effect, this is where the "work" is done.

    Using the number 1,000 square meters per gram, which equals 1,197 square yards, and multiplying 1,197 by 454 (454 grams/pound), results in 543,438 square yards of available surface area per pound of carbon. Compare this to a football field, which is 50 by 100 yards, or 5,000 square yards; this would be the surface area of more than 100 football fields.

    When that 543,438-square-yard internal area is multiplied by 9 (9 square feet/square yard), it equals almost 5 million square feet of available surface area per pound of carbon. Utilizing a maximum working capacity of 33 percent, up to 1.65 million square feet would be available for adsorption.
     

    Activated Carbon

    Many types and grades of carbon can be used for adsorption. There are carbons used for liquid- and gas-phase applications; they are not interchangeable. For the purpose of this presentation, only gas-phase carbons, which are used for general HVAC applications, will be addressed.

    Commercial-quality HVAC carbons being manufactured today are produced from either coal or coconut shells. Petroleum-based carbons are still in use, but since they are all reactivated materials, their usage continues to decline. These materials are interchangeable, as long as the activity level and the average particle size are the same.

    The HVAC industry is currently using 60-percent minimum-activity carbons, which are either 6/8-inch mesh (3-mm) pelletized or 6/12-inch mesh granular. (This activity level helps ensure that the internal surface area is in the range of 1,000 to 1,100 square meters per gram.) I believe there are definite advantages of pelletized over granular materials, but either will do a satisfactory job.

    "Charcoal" is often used as a product descriptor; therefore the question is often asked, "What is the difference between activated carbon and activated charcoal?"

    There is only one material, and the terms are used interchangeably. Normally, those who are in the carbon business will use the term carbon. Those outside the industry, or just learning, will use the term charcoal. The American Society for Testing and Materials (ASTM) uses the descriptor carbon.
     

    How It Works

    As mentioned previously, activated carbon adsorbs a wide variety of gases and vapors - chemical pollutants. Whether there is one gas molecule and one carbon particle or many of each, the adsorption process is the same.

    The physical process begins with a gas molecule coming into contact with the surface of an activated carbon particle and coming to rest in a large surface pore. Then, due to unbalanced forces on and within the carbon particle, the gas molecule will begin to move "down" into the carbon particle into the smaller pores, where it will finally stop and be held in place. At some point between the surface and the "stopping point," this gas molecule will condense and become a liquid particle.

    For those who would like a more technical explanation: The adsorbate diffuses through the surface film to the macropore structure. Then, due to van der Waals force, the gas molecule migrates into the micropore structure, condensing during this movement, and finally stopping when either the forces become balanced or it becomes physically blocked.

    This molecule, which was an objectionable gas, will remain a liquid inside the carbon until it receives enough energy, in the form of heat, to excite it. If this condition arises, the molecule will begin moving toward the surface. If enough energy (heat) is adsorbed, it will be vaporized, returned to a gas, and be released back into the airstream, (i.e., the process will be reversed).

    In HVAC applications, there is normally not enough heat to excite, or re-energize, adsorbed molecules. With regular carbon changeouts, sufficient capacity always will be available.

    It is interesting to note that the adsorbed gases that condense and become liquid molecules (particles) will "line" the internal surface area, and this lining will be one (and only one) molecule thick.
     

    HVAC Applications

    As a reminder, this article deals only with the removal of gaseous and vapor contaminants.

    The application of carbon to HVAC installations is considerably different when compared to how carbon is applied to solvent recovery systems and other industrial removal applications. For the latter, airstreams with hundreds to thousands of parts per million (ppm) of gas molecules are recovered, or removed, with beds that can be several feet thick and contain thousands of pounds of carbon.

    For HVAC applications, thin-bed filters with comparatively small amounts of carbon are used. Therefore, it is not only important that these differences be recognized, but that the parameters be defined.

    First, air in a structure can be treated as it enters, leaves, is recirculated, or a combination of all these.

    For most HVAC applications, the air to be treated is a combination of air entering (makeup) and air that is recirculating. The gaseous contaminant loading that should be removed:

  • Is always a combination of many odors and pollutants - chemicals;

  • Is normally unknown either as to the chemical makeup, the amount of each pollutant, or the total loading;

  • Varies constantly with changes in occupancy levels, activity within or without the structure, and shifts in the wind;

  • Is around 1 ppm.

    When the loading drops below 1 ppm, down to 0.1, 0.01 ppm, and below, there is no concern. When the loading increases, which it will do for short periods, there are concerns and questions.

    Knowing that the industry has used, and continues to use, thin-bed carbon filters, the following situations must be considered:

    Question: How high are these higher loadings (peaks), and how long do they last?

    Answer: Unless the structure has been studied over a period of time, there is no definite answer. Ten to 20 ppm would be a heavy overload and may be within reason for this application. This condition may last for minutes to hours as opposed to days.

    Question: How effective is the filtration system during peaks?

    Answer: The percentage of these peaks, which exceeds the design capability of the filter or the capacity of the carbon at that time, will pass through and into the inside air. Since the air is being recirculated, these peaks will be adsorbed on future passes.

    As stated earlier, carbon must be changed on a regular basis. This regularity should be established during the first year of operation by utilizing test samples, to ensure that these peaks, typical to that installation, will be quickly adsorbed.

    The old ASHRAE specification of 45 pounds of carbon per 1,000 cfm per year has been used for many years as the design guideline. In all probability the vast majority, if not all, of the carbon V-banks that are being used in airports, sports stadiums, and commercial buildings were designed with these numbers. When this amount of carbon is changed regularly, these filters perform effectively.

    Is this the optimum amount of carbon? From experience with many installations, the answer would certainly be that this has proven to be enough carbon. The only remaining question is, would somewhat less carbon be as effective, and if so, how much less? This question remains unanswered.
     

    Commercial Vs. Industrial

    The carbon adsorption products that have been designed and used for HVAC applications are very effective. When they are misapplied in industrial applications, however, problems can and do arise.

    It is not that the carbon does not adsorb; rather, the concentration exceeds the capability of thin-bed products, and neither the pass efficiency nor the life (capacity) are satisfactory.

    Industrial applications, although they will contain many of the same chemicals found in HVAC applications, can be defined as having:

  • Primarily one contaminant (chemical);

  • A chemical that is known, both as to its formulation and quantity (ppm);

  • Either a constant quantity or a constant range; and

  • A loading well above 1 ppm.

    Very simply, this will be, by comparison, a heavy-duty application, and the information concerning the contamination will be known.

    Here is one application that should help crystallize the differences for you:

    A printing company with a large printing press has a solvent-recovery system to recover the solvents from the inks. This system removes the high concentrations of solvent with large amounts of carbon. However, the solvent "leakage" that escapes into the general air and into the air-handling system will be heavily diluted and one of the many other chemicals circulating in the HVAC system. This air recirculates through the offices, lunchrooms, laboratories, etc., and now needs be treated as an HVAC application, with thin-bed products.

    Adsorption may not be the most effective process for all industrial applications. Companies skilled in these fields should be consulted to make the determination regarding the most effective process and system.
     

    Nonvented Areas

    Activated carbon will adsorb without the air being drawn through it (without a fan or blower). This is of importance when rooms or closed areas do not have ventilation ducts, and where the use of a portable air cleaner does not appear to be cost effective.

    Gas molecules disperse (mix) themselves evenly throughout a given area, in accordance with the "Perfect Gas Laws." Activated carbon attracts and adsorbs the odor or pollutant molecules to which it is closest, causing an unequal mixture to result. Whereupon the gases will redistribute themselves, the carbon will adsorb the closest, and the process will continue until the air is cleaned or the carbon reaches its capacity.

    A relatively small amount of carbon can, and will, keep these types of areas odor and pollutant free. An example would be using a small amount of carbon to help keep a refrigerator odor free, thereby eliminating the transfer of odors and tastes among foods and ice cubes.

    For HVAC applications, adsorption products can be divided into two categories: V-banks and extended-surface products or panel filters.

    V-Banks and extended-surface products offer from approximately 30 to 45 pounds of carbon per 1,000 cfm of air. Virtually all units will:

  • Have a surface area of either 24 by 24 inches or 12 by 24 inches;

  • Be prepared so they can be built into filter banks;

  • Use removable trays, or V sections, which are arranged in serpentine (V) configuration.

    Depending on the manufacturer, additional features may be available that offer:

  • The addition of prefilters to either the front of the housing or inserted along with the carbon tray;

  • Housings that allow the trays to be replaced from either the front or the side.

    Panel filters (3/4-, 1-, or 2-inches thick) containing from approximately 1.75 to 5 pounds per square foot, are available using a variety of designs, such as:

  • Two-spaced-apart perforated metal faces, filled with carbon;

  • Perforated metal tubes, filled with carbon and with open spaces between, in a bypass arrangement;

  • Honeycomb-shaped material, which is either partially or completely filled with carbon, contained with a thin netting;

  • Granular or powdered carbon that has been impregnated into nonwoven fibers, available as either flat or pleated products, and with or without a prefilter.

    Product Application

    The application of these products can be divided into two areas: new construction or retrofitting existing installations.

    When a new structure is being designed, the engineer has the option of using any of the above products. When adsorption applications arise in existing installations, panel filters, although they will need to be changed more frequently to offer similar capacities, offer the advantage of interchangeability, thereby eliminating the need to alter the existing air-handling system.

    As stated earlier, activated carbon, the universal adsorbent, will adsorb some of almost any vapor. Detailed lists of gases and vapors - chemical compounds - are available from carbon and equipment manufacturers. The following is a partial list of gases that are of concern in air purification systems and can be removed with carbon:

    Organic compounds - acids, alcohols, aldehydes.

    Chlorinated hydrocarbons - esters, ethers, ketones, mercaptans, amines.

    Inorganic compounds - halogen acids, halogens, sulphuric acid, sulphur dioxide, phosgene.

    Miscellaneous odors - from humans, animals, foods, cooking, and waste processes.
     

    Conclusions

    The removal of chemical contaminants, which affect the health and productivity of many people at work and at play, must and will continue to be addressed by the air movement industry.

    Due to the comparatively low contamination levels found in HVAC applications, adsorption has, and will continue to be, the most economical process.

    Activated carbon has a capacity for virtually all vapor contaminants and will adsorb and retain a wide variety of chemicals at the same time.

    Adsorption products using activated carbon are available from a variety of sources. These products, when properly applied and maintained, can eliminate the vast majority of pollution problems caused by the gases and vapors - the chemical pollution - found in HVAC systems.

    Kasmark is president of D-Mark, Inc., Chesterfield, Mich.; 800-343-3610; 586-949-3610; 586-949-4181 (fax); dmark@bignet.net. Special thanks to Joe C. Enneking, vice president, Nuclear Consulting Services (NUCON), Columbus, Ohio.

    Publication date: 10/06/2003

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