ASHRAE Journal - December 2009 - 54

ASHRAE59–2009 Line 1 2 3 4 5 6 7 8 9 YE A R S JOURNAL Masking may reduce the odor sensation but since it does not remove odor molecules, it does not remove whatever additional deleterious properties they may have. The most efficient and economic way to eliminate odor molecules is with an activated-charcoal filter. Because of its unusual microscopic structure, charcoal has the property of adsorbing vast quantities of odor-producing molecules. Each granule of charcoal contains millions of tiny capillary passages. The area of these passages is extremely large (estimated at over 6 million sq ft in a single pound).2 Here in this inner surface area charcoal removes odor molecules two ways: by surface adsorption and by capillary entrapment in the narrowing passages of the charcoal. Surface adsorption is the attraction charcoal has for molecules of gases and vapors. The amount of an odor-producing substance required to create a noticeable sensation is extremely small, often less than 1 in 1 million parts of air. Therefore, odor-removal must be fairly complete to be effective. Starting with an over-powering odor, such as hydrogen sulfide, removing 99 percent of the odor molecules still leaves a definite odor; removing 99.9 percent of the molecules still leaves a perceptible odor. It is by eliminating odors that cost of an air-recovery system becomes compatible with a conventional air-conditioning system. A conventional air-conditioning system with a mechanical filter does not eliminate odors; instead it dilutes them by adding vast quantities of outside air. This outside air must be either heated or cooled to a required temperature, meaning extra heating or cooling capacity. By reducing need for this excess heating and cooling capacity, an air-recovery system can be economically competitive with a conventional system. Conventional Air-Conditioning System Building Area Outside Ventilating Air Required (1957 Chicago Code) Tons of Cooling Capacity Required Cost of Cooling Capacity Cost of Filter Equipment Cost Advantage of Conventional System Operating Cost of Cooling Required Ventilating Air Operating Cost of Heating Required Ventilating Air Operating Cost of Filters 120,000 24,000 60.5 $18,313.35 $8,400.00 $790.00 $687.89 $1,191.36 $900.00 $2,779.25 – Proposed Air-Recovery System 120,000 10,800 27.2 $8,233.44 $19,270.00 – $309.26 $536.11 $1,052.43 $1,897.80 $881.45 10 Total Operating Costs 11 Annual Cost Advantage of AirRecovery System Table 1 Typical operating costs compared. Original Costs To compare cost advantages of an air-recovery system with a conventional installation, assume a 120,000 sq ft office building to be built in the Chicago area. The accompanying figures (Table 1) are averages and may not reflect conditions throughout the country. However, the figures are on the conservative side and are weighed against the air-recovery system. It is probable that an engineer may find definite savings for a given building. The results, of course, will differ, depending on the building, local conditions, area economy and other factors. To determine whether or not an air-recovery system will be beneficial economically must be left to the engineer’s professional judgment. 54 ASHRAE Journal The 1957 Chicago code provides for reduced ventilation requirements when an activated-charcoal filter is used. With a conventional air-conditioning system 0.2 cfm of outside air per square foot of building area would be required for the given office building (line 2) while the proposed air-recovery system would reduce the code requirements by more than half—to 0.09 cfm a sq ft or 10,800 cfm, saving 13,200 cfm of heating and cooling capacity otherwise needed if outside air were used. Summer design conditions for Chicago (from the 1959 ASHRAE Guide, Table II, page 171) are outside temperatures of 95°F dry bulb and 75°F wet bulb and indoor temperatures of 80°F dry bulb and 67°F wet bulb. Cooling the outside air this much takes 30,300 Btu an hr for each 1000 cfm (page 197, 1959 ASHRAE Guide). Since 1 ton of cooling capacity will remove 12,000 Btu an hr (page 5, 1959 ASHRAE Guide) it would take 2.52 ton to cool 1000 cfm of Chicago summer air. The air-recovery system saves 13,200 cfm or 33.3 ton of cooling capacity (line 3). Typical installed cost for a ton of cooling equipment is $302.70 (from the 1959 ASHRAE Guide, Table I, page 728, adjusted by current building cost index, with cooling tower at $54). Since 33.3 fewer tons are required, installed air-conditioning equipment savings are $10,079.91 (line 4). Cost of an electronic air cleaner to handle 120,000 cfm—at 14 cents per 1000 cfm including freight and installation—is $16,800 (based on National Academy of a s h r a e. o rg December 2009

ASHRAE Journal - December 2009

Table of Contents for the Digital Edition of ASHRAE Journal - December 2009