Reducing Ventilation Rates

In the absence of significant indoor emissions, the indoor concentration of airborne particles, and consequently the soiling rate, may be reduced by lowering the rate at which outdoor air enters the museum. The effect of reducing the outdoor air-exchange rate from the nominal value for the conventional mechanical ventilation system design described above is examined below. In adopting this control measure, it is first necessary to ensure that adequate ventilation is provided for the occupants of the building. The recommended lower limit for the

supply of outdoor air to a building is 8.5 m -1 h person , based on the need to prevent accumu- lation of CO 2 produced by the building’s occupants (ASHRAE 1985). In a system in which the

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building is pressurized relative to outdoors, the flow rate of the mechanically induced outdoor air supply must exceed the mechanically induced exhaust flow rate, as, for example, from restrooms. Furthermore, reducing the supply rate will reduce the degree to which the building operates at a higher air pressure than outside. To avoid concomitantly increasing the infiltration rate, it may be necessary to further reduce the leakiness of the building shell.

The occupancy of a museum gallery may be highly variable. With many conventional mechanical ventilation systems, the rate of supply of outdoor air is fixed according to the size of the building to a level sufficient for the maximum expected occupancy. If, instead, the ventilation rate were varied according to the actual occupancy, the average ventilation rate would be much lower, and the rate of soiling could be considerably reduced. Outdoor air-supply rates are generally controlled by the position of dampers at the inlet register. By equipping the inlet register with dampers that are easily adjusted, the ventilation rate could be changed in response to changing occupancy.

Protecting Museum Collections from Soiling Due to Deposition of Airborne Particles

Model predictions of characteristic soiling times indicate the reduction in soiling

rates that can be achieved by reducing outdoor air-exchange rates. Two situations (Cases 3 and 4) are examined in Table 6.1 (p. 101). For Case 3, the baseline retrofit mechanical ventilation system is

retained, but the outdoor air make-up flow rate is reduced to a constant lower level (f 3 ox = 0.167 m

s -1 ). This flow rate corresponds to 8.5 m h person at a design occupancy of 20 persons per 1,000 square feet of floor area. In Case 4, the reduced make-up flow rate stated for Case 3 is used during

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the hours when the museum is open to the public, but it is assumed that the intake dampers are closed at other times of the day. The mechanical ventilation system fans continue to operate, caus- ing air recirculation through the particle filters. Table 6.3 (p. 103) indicates that these measures extend the characteristic soiling times to 0.05–0.1 years for soil dust settling on upward-facing sur- faces and to a range of two to five years for elemental-carbon deposition on the walls and ceiling.

Benefits beyond reduced soiling rates may be realized by implementing this

control measure. Reducing the ventilation rate reduces operating costs by lowering the amount of energy required to control the indoor air temperature and relative humidity. An improvement in the stability of indoor air temperature and relative humidity may also be realized, reducing the mechanical stresses experienced by objects that would otherwise expand and contract as temperature and relative humidity fluctuate. Improved climate control therefore constitutes an important additional benefit toward preservation of museum collections.

Reducing Ventilation Rates