Once you are aware of recommendations for environmental control, and have monitored the environmental conditions in your building to identify problem areas, how do you achieve the desired conditions? What if you have a less-than-perfect building? If compromises in the environment are necessary, how do you provide the best conditions possible given the limitations of your building?
In this section you will learn about strategies for managing HVAC systems, controlling light, and providing cold or frozen storage for collections. You will also explore low-cost options for improving environmental conditions.
A basic HVAC system consists of:
- A heat source (steam, hot water, electric)
- A cooling source (chilled water cooled by some type of refrigeration system)
- Air handling units (for circulating the air, these include fans and dampers—as well as filters and heating/cooling coils)
- Ductwork (through which air moves to and from spaces in the building)
- Equipment for filtering the air
- Control systems (usually computerized)
In very general terms, an HVAC system controls temperature by passing air over a heating coil within the air handling unit. Air is humidified by introducing water vapor into the stream of air before it passes into the space. It is dehumidified by passing air over a cooling coil. The temperature of the cooling coil is lower than the dew point temperature of the air, so moisture condenses on the coil and is then drained away. The air is then reheated to the desired temperature and sent into the space.
Particulates are removed by using various grades of filters within the air handling unit. Electrostatic precipitators can be effective, but should not be used because they generate ozone. Gaseous pollutants are removed by using absorbers within the air handling units (beds or canisters containing granular agents such as activated carbon).
Since filtration is expensive, it is also important to reduce the amount of pollutants that enter the building. Locate air intakes away from sources of pollutants such as vehicles, industrial processes, and laboratories. The smallest possible amount of outdoor air should be used within the HVAC system; as much air should be recirculated as possible. Recirculated air should also be filtered.
Architectural and building design features (such as windows and vapor barriers) and building operation practices (24-hour systems and the availability of heating and cooling) greatly influence the effectiveness of HVAC systems. Some typical problems with HVAC systems stem from the efforts of facilities personnel to save energy, while others may be due to poor maintenance or lack of knowledge. You should be aware of the following:
- Raising the temperature of the chilled water will reduce the ability of the system to dehumidify the air (less water will condense out of the air when it passes over the cooling coil).
- Some systems are not able to dehumidify the air when the outside air is damp, but not warm enough for the air conditioning to go on.
- The use of economizer cycles that bring in 100% outside air when outside conditions are moderate may result in higher pollutant levels.
- Humidifiers may be removed or disabled, perhaps due to their need for frequent maintenance. Be aware that disabling a humidifier may be beneficial, if it was not working properly in the first place.
- Particulate and gaseous filters that are not changed frequently enough will not function properly.
- System sensors that are not calibrated may lead to undesirable levels of temperature and humidity.
- Keeping doors and windows open will keep the HVAC system from maintaining the desired conditions.
Certain specialized collections will benefit from storage at lower temperatures than can be provided by a standard HVAC system; however, cold or frozen storage is only appropriate for materials that are not used frequently. The more often the materials are brought out of cold/frozen storage and acclimated to room temperature, the less long-term benefit cold/frozen storage provides.
Large collections may require insulated rooms or prefabricated chambers, which often use a desiccant dehumidification system, but smaller collections can be stored in refrigerators and freezers. Household or commercial units may be used, provided the proper protocols are followed.
The Image Permanence Institute (IPI) has defined several levels for media storage. Room temperature storage is defined as close to 68°F. Cool storage is defined as close to 54°F. Cold storage is defined as close to 40°F, while frozen storage is defined as 32°F or below. All types of storage should have a stable relative humidity between 30 and 50%.
The lifespan of vulnerable media can be lengthened by freezing or storing at extremely low temperatures. These include motion picture film, photographic film and prints. Be aware, however, that some media may benefit from cold storage, but should not be frozen. These include glass plate negatives, magnetic tape, and CDs/DVDs.
The National Fire Protection Association has guidelines for construction of cabinets and vaults to house nitrate film; these guidelines are complex and are required by some localities. For most small institutions, it is best to store nitrate films in off-site cold storage or transfer them to an institution that is better equipped to care for them. Acetate base and color films also greatly benefit from storage at cold temperatures, but again, this is generally not practical for small institutions with only a few films. The deterioration of audiovisual media is further discussed in Session 6: Media Collections.
If cold or frozen storage is to be successful, you must be able to maintain stable low temperatures with controlled humidity. Humidity control can be managed through the careful use of packaging. Materials must be stored in a moderate relative humidity for several weeks before being packaged into moisture-proof freezer bags (this conditioning is necessary to avoid excessive moisture within the bag). Care must be taken when removing collections from storage so that condensation does not occur; the materials should be acclimatized at least overnight, or longer for a large amount of material, before being removed from the bag.
Since it is not possible to keep collections completely in the dark at all times, staff must compromise to provide the minimum amount of light necessary for the tasks being performed, such as exhibition, research, or searching the stacks. Lighting during research and storage will be considered here; see Session 3: Caring for Collections for more information on exhibition.
Controlling UV light depends on eliminating or filtering sources of light with a high UV component (e.g., natural daylight, and fluorescent, tungsten-halogen, and metal halide bulbs). UV filtering film can be applied to windows. Blinds, shades, or heavy curtains will control UV light (as well as visible light and infrared light). In areas lighted by fluorescent bulbs, low-UV fluorescent bulbs or UV-filtering sleeves should be used. If filtering sleeves are used, they must be transferred to the new bulb when the bulbs are changed.
Natural light should be blocked completely from storage areas. Lights should be turned off when storage areas are not in use, and light levels should be kept as low as possible. Rearrange shelving and collections so that light does not fall directly on collections. Boxing or placing materials in folders and drawers will also protect them. Any reduction of light reduces long-term damage. Each institution should develop guidelines and implement them to control light where collections are used or stored.
How do you deal with a less-than-perfect heating and cooling system? Not every institution has a centralized environmental control system, nor does every institution have the resources to drastically improve its existing system(s). Historic structures are especially prone to antique or defunct systems. If new or expanded HVAC equipment is not feasible, institutions need to implement low or no-cost actions in the meantime. Portable humidifiers and dehumidifiers, as well as A/C units and fans, may be used to moderate the climate in collection storage and usage areas.