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

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

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

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:

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

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:

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

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:

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

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:

Product Application

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

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