Using Display Cases and Framing
Using Display Cases and Framing
An alternative means of effectively providing local pollution control in the vicinity of an art object is through the use of display cases and framing behind glass. When analyzed in this con- text, a display case may be considered as a room within a room. Thus, to predict the effectiveness of a display case, one may apply the aerosol dynamics model of Chapter 3, regarding the component concentration within the space of the room as if it were the outdoor particle concen- tration, and the component concentration inside the display case as if it were the indoor concen- tration. Then, as with the control problem for the museum as a whole, the control options within
a display case include reducing the rate of air exchange between the display case and the room, improving particle filtration within the display case, reducing the particle deposition velocity to objects within the display case, controlling the concentration within the room outside of the dis- play case, and eliminating emissions inside the display case.
To consider the potential effectiveness of the use of display cases, additional data from the Sepulveda House will be considered. In recognition of the soiling problem at this site, one of its rooms—a bedroom furnished with historical artifacts—has been retrofitted to be, in effect, a large display case. All identifiable air-leakage paths were sealed. In the room’s one doorway, a glass-walled alcove from the hallway into the room was constructed, entirely sealing the doorway. By standing in this alcove, visitors may examine the entire room, but, except for maintenance activities, no one enters.
Protecting Museum Collections from Soiling Due to Deposition of Airborne Particles
To determine the effectiveness of this approach in reducing soiling, measure- ments of the size distribution and concentration of fine particles were made at two locations: one inside the enclosed bedroom, the other on a stairway landing in a central location in the building.
A pair of optical particle counters (OPCs, Particle Measuring Systems model ASASP-X) were used to take measurements continuously for a day and a half during the spring of 1988. These instruments are capable of detecting particles over a nominal size range of 0.09–3 µm optical diameter and classifying them, according to the amount of light scattered in the forward direc- tion, into 32 size channels (Chapter 4). The air exchange rate between the bedroom and its sur- roundings (outdoors plus the rest of the building) was measured throughout this period by the tracer-gas decay technique (ASHRAE 1985), using sulfur hexafluoride as the tracer.
Figure 6.4 (p. 118) depicts results for a 24-hour segment of the monitoring period: the total volume concentration of fine particles detected by the OPCs as a function of time, and the 24-hour-average aerosol size distributions. By sealing the bedroom, the soiling haz- ard has been substantially reduced. For soot-sized particles of 0.1 µm diameter, the concentration in the bedroom is 32% of the concentration in the core of the building. For particles of 2 µm diam- eter, the corresponding ratio is 8%.
A substantial further reduction in the soiling rate could be achieved through the use of a local particle filtration unit in the room. The average rate of air leakage into this room is
ii f ii ) would reduce the concentration of 0.1 µm particles by a factor in the range of 20–60. The power con- sumption rate of an unducted filtration unit is generally less than 100 W (Offermann et al. 1985); however, some attention must be given to heat dissipation if such a unit is to be used in a small, tightly enclosed space. If heat dissipation or aesthetic considerations dictate that an unducted unit may not be used in a small, enclosed space, it is possible to achieve the same benefit by using
3 -1
3 4.3 m -1 h . Employing an air filter having an effective filtration rate of 300 m h (η
a ducted, local recirculation and filtration system. Using this approach, the heat and noise gen- erated by the fan can be separated from the items in the exhibit.