Handbook of air conditioning and refrigeration / Shan K

● To eliminate or to reduce indoor gaseous contaminants such as combustion products, VOCs, and
objectable odors
● According to the ASHRAE Standard 62-1999, to reduce the concentration of the known and
specifiable contaminants in recirculating air to the required concentration level based on indoor
air quality procedures to achieve acceptable air quality
Activated Carbon Adsorbers
GAC Performance
IMPROVING INDOOR AIR QUALITY 24.9
Brown et al. (1994) reported that mean concentrations of VOCs in various office buildings
ranged from 0.18 to 4.15 mg/m 3 with a new building having a total VOC concentration as high as
39.3 mg/m 3 . For each individual compound, its mean concentration was about 0.05 mg/m 3 . Granular
activated carbon (GAC) is one of the most widely used adsorbents for indoor gaseous contaminants.
Adsorption (physical adsorption) is a surface phenomenona. In an adsorption process, the molecule
of the gaseous contaminant
● Must be transported from the carrier airstream across the boundary layer that surrounds the adsorber
granule
● Must diffuse into the pore of the adsorbent and occupy the surface
● Must be bound to the surface
Adsorption is different from absorption in which the gaseous contaminant is dissolved in or reacts
with the absorbing medium, which can be either a porous solid or a liquid.
As discussed in Sec. 15.16, removal efficiency, adsorption capacity, and service life are important
performance parameters for GAC. Toluene has been used as a surrogate for indoor VOCs for the
testing of GAC performance. An increase in relative humidity generally reduces the adsorption of
VOC on GAC. This may affect some compounds more than others, and it becomes more significant
at low VOC concentrations. When the concentration of the contaminant downstream of the adsorber
rises until its concentration is the same as that of the upstream, the penetration at this time is called
breakthrough penetration.
In addition to the adsorption capacity and the removing efficiency discussed in Sec. 15.16, the
performance parameter breakthrough time t b is defined as the elapsed time between the start of
challenge and the time when the penetrating concentration reaches the specified breakthrough fraction,
10 percent breakthrough time t 10% or 50 percent breakthrough time t 50%.
Another performance parameter is called capacity at t b which is expressed as the ratio of the
mass of contaminant collected at a particular breakthrough time to the mass of the GAC bed, in
percentage.
Van Osdell and Sparks (1995) reported the performance of GAC challenged with toluene at various
concentrations at 77°F (25°C) and 50 percent relative humidity as follows:
Challenge concentration ppm 0.44 1.1 9.2 71.7
Challenge concentration, mg/m 3 1.7 4.0 34 270
Carbon bed mass, g 24.1 23.1 23.4 25.8
t 10%, h 625 344 72 11.9
t 50%, h 750 422 88 15.0
Capacity at t 10%, percent 6.6 9.2 16.2 18.8
Capacity at t 50%, percent 7.9 11.3 19.7 23.8