Improving Particle Filtration

Indoor particle concentrations may be reduced by improving the particle filters that are other- wise customarily specified for commercial air-conditioning systems. The filters found in many conventional mechanical ventilation systems are designed primarily to protect the ventilation system components, rather than to deliberately control the level of air pollution in the indoor environment. Air filters commonly used in commercial buildings are generally efficient at removing very large particles from the air stream, but have only a modest single-pass efficiency (commonly 20 to 30%) for removing fine particles, such as those containing elemental carbon.

The efficiency of commercial air filters is commonly specified according to one

or more of three rating tests. The “synthetic dust weight arrestance” rating refers to the effectiveness of a filter in removing a suspended mass of coarse artificial dust from an air stream (ASHRAE 1976). The “atmospheric dust spot efficiency” rating refers to the effectiveness of a filter in reducing the opacity of atmospheric particulate matter (ASHRAE 1976). The “DOP efficiency” rating refers to the effectiveness of the filter in removing from an air-stream liquid droplets of dioctyl phthalate having an approximate median diameter of 0.3 µm (U.S. Depart- ment of Defense 1956).

Fibrous air filters may be obtained with a wide range of rated particle-removal

efficiencies. A recent discussion of the efficiency of particle filters used in general ventilation

Protecting Museum Collections from Soiling Due to Deposition of Airborne Particles

applications (Rivers 1988) indicates that the least effective filters commonly employed (home furnace, room air-conditioner) have an atmospheric dust-spot efficiency of 20 to 25%. Rigid, pleated paper filters and pocket bag filters have an atmospheric dust-spot efficiency of 60 to 95%.

High-efficiency particle (HEPA) filters 1 with a 99.97% DOP efficiency rating also are available. An important consideration in selecting an air filter is its resistance to airflow, which generally

increases with increasing filtration efficiency. The design choice involves balancing the goal of removing particles from the air against the additional equipment and operating cost of high filtration efficiency.

In addition to fibrous filters, an electrostatic precipitator may be used to remove

particles from an air stream. These devices operate with much lower pressure drop than a fibrous filter; however, they may be a source of ozone (Sutton et al. 1976, Allen et al. 1978).

The effect on soiling rates of incorporating higher efficiency filters into the base-

line retrofit mechanical ventilation system at the Sepulveda House (along with reduced outdoor airflow conditions) is considered in Cases 5 and 6 (Table 6.1, p. 101). The particle-removal efficiencies adopted in this example—0.99 for coarse-particle removal by the intake filter ( η ox ), 0.95 for coarse-particle removal by the filter in the recirculating line (η ii ), and 0.875 for fine-particle removal by both filters—represent the minimum dust weight arrestance and dust spot efficiency, respectively, recently specified in a draft ANSI standard for the storage of library and archival documents (ANSI 1985, Baer and Banks 1985, Committee on Preservation of Historical Records 1986). To achieve these overall efficiencies, a sequence of several filters of progressively higher efficiency is recommended (ANSI 1985). Results for Case 5 (Table 6.3, p. 103) indicate that the characteristic soiling time associated with soil dust settling on upward-facing surfaces is increased to three months, and that the soiling time associated with elemental-carbon deposition on the walls and ceiling is extended to thirty to forty-five years.

For high-efficiency filtration to be effective, it is essential that the leakage of untreated air directly into the building be reduced to the greatest extent possible. This is demon- strated by comparing the results for Cases 5 and 6. The most significant difference between these cases is that infiltration is entirely eliminated in Case 6. With this combination of high-filter efficiency, no infiltration, and a low rate of outdoor air supply, the characteristic times before soiling is apparent are increased by more than two orders of magnitude relative to the base-case assumptions (Table 6.3, p. 103).

1. A HEPA filter unit with η ii f ii = 300 m 3 h -1 was commercially available for $400 in 1983; replacement filters cost $80, and the unit consumed less than 70 W of power (Offermann et al. 1985). The reader is cautioned that not all devices marketed as air cleaners are effective. Five of the ten devices tested in the same study had effective

cleaning rates (η ii f ii ) in the range 0–12 m 3 h -1 , insignificantly low values for almost all applications. Further- more, ion generators are particularly to be avoided in museum environments: these devices reduce airborne concentrations by electrically charging particles and causing them to deposit more rapidly on indoor surfaces.

Protecting Museum Collections from Soiling Due to Deposition of Airborne Particles

In historical buildings and other sites that lack a mechanical ventilation system, air filtration may still be provided to reduce the rate of soiling due to fine-particle deposition. Unducted console units designed to filter the air in a single room are commercially available. The more effective of these devices combine an efficient air filter with a fan, and are capable of removing 0.45 µm-diameter particles from the air at an effective rate (corresponding to the prod-

3 uct η -1

ii f ii ) of 100 to 300 m h (Offermann et al. 1985). For a building lacking both mechanical ventilation and significant indoor sources of particles, the introduction of a recirculating filtration device will substantially reduce the indoor particle concentration only if the effective filtration rate is significantly greater than the sum of the ventilation rate and the total particle- deposition rate. This condition will seldom be met for coarse particles (because the coarse- particle deposition rate is so high), but may be achieved for fine particles if the infiltration rate is low. Consider, for example, a room having dimensions of 8 m by 12 m, a ceiling height of 3 m,

and an average infiltration rate of 0.3 h -1 . Assume, for the purposes of illustration, that the particle-deposition velocities are near the middle of the range corresponding to typical conditions

3 in Southern California museums. A console filtration unit having η -1

ii f ii equal to 300 m h for both fine and coarse particles is introduced into the room. The concentration of fine particles will be reduced to 23% of the prior value, but the concentration of coarse particles will only be reduced to 86% of the value in the absence of the filtration unit. Thus, the soiling rate of vertical and downward-facing surfaces would be reduced by approximately a factor of four. This improvement is not large relative to the potential improvement of two orders of magnitude for

a high-efficiency, low outdoor-air exchange, ducted ventilation and filtration system. However, employing an unducted filtration unit at least provides some improvement in cases where finances will not permit a complete building retrofit.

Improving Particle Filtration