The design process for the mechanical system at this University of Michigan library took more than a year and resulted in a custom designed precision A/C system that safeguards ancient artifacts.

Preserving the Western Hemisphere’s largest papyrus collection - ancient writings on fragile papyrus reeds dating back to 1000 B.C. that the University of Michigan, Ann Arbor, MI, discovered during early 20th century archeological digs - in a constant temperature and humidity is a monumental mechanical engineering task.

The difficulty was exacerbated by an additional quest for a green system, according to the system’s designer, Thomas Girard, manager of mechanical engineering in the University of Michigan Architecture Engineering & Construction Department. Shannon Zachary, head of preservation, University Library – University of Michigan, wrote the operational standard for the room, much of which is based on the Canadian Council of Archives’ “Conservation Environment Guidelines for Libraries and Archives.” Most importantly, Zachary established setpoint conditions of 65°F (± 1°) and 45% rh (± 4%), which are often accomplished via an oversized HVAC system requiring very low discharge air temperature for proper dehumidification (approximately 42°) and a large quantity of reheat.

Designing A Green System

Girard’s design was not only green in many ways; it also outlined many strict tolerances. Girard and project mechanical contractor, James Dornbrock of Altech Mechanical, spent over a year designing the mechanical system, which was then custom manufactured by Ecosaire, a precision air conditioning manufacturing subsidiary of IAQ specialist, Dectron Internationale.

Instead of oversizing a one-compressor system to handle the room at full load, Girard specified a dual 1.5-ton parallel airflow compressor/condenser system that saves upwards of 50% of energy vs. typical archival room systems. “It didn’t make sense to operate a larger system 95% of the time at 50% load with short-cycling or to use hot gas bypass,” said Girard.

Thanks to precise trend data Zachary provided regarding hours of occupancy, necessary temperature/humidity data, etc., combined with his own internal study of the room envelope characteristics, Girard was able to accurately size the main compressor to condition the room 95% of the time when it is unoccupied. In impromptu instances where a semi-monthly class or conference is held and the lighting is on, a second compressor activates to handle the extra load. The second compressor also provides the necessary redundancy. If one compressor fails, the other compressor activates and room access is limited until both compressors are operational again.

“Maintaining these tight tolerances is much easier with a compressor sized exactly to the load that runs 24/7,” explained Girard. “Oversized compressors that kick on two or three times every hour not only make tight tolerances difficult to meet, but they also use much more energy in the process.”

In unoccupied mode, air flowing through the inactive coil essentially serves as free reheat. Less cooling is required since only half the air is sub-cooled for dehumidification. Virtually no additional reheat is required because the capacity of each compressor is closely matched to the continuous cooling load associated with fan heat. 

Girard also used heat recovery to guarantee tighter temperature tolerances. Instead of a more conventional outdoor location where Michigan’s ambient air temperatures have wild swings of -10° to 95°, Girard located the two air cooled condenser coils in the library’s central HVAC penthouse mechanical room, which is conveniently one floor above the eighth-floor papyrus room’s 100-sq-ft mechanical room. This mechanical penthouse shelters general building air handlers and also serves as a return exhaust plenum. Thus, the two condensers are subjected to a relatively constant temperature of 65° to 85° year-round because the penthouse acts as a flow-through relief exhaust plenum.

Waiting For The Weather

Dornbrock’s main challenge was switching out the equipment in only three days during the spring when the outdoor load of temperature and humidity was evenly tempered. The room’s constant temperature and humidity weren’t abandoned during the process, however. Altech Mechanical kept the room at optimum conditions with MovinCool portable air conditioning and an Aprilaire portable humidifier.

The project also includes four critical air purification stages - a 30% pleated filter, 95% pleated filter, gas-phase charcoal filtration, and a post gas-phase pleated filter to catch any charcoal dust, to help eliminate any infiltration of harmful airborne contaminants into the room.

The unit is interfaced with the university’s Siemens Building Technologies campus-wide BAS and monitored. Zachary closely watches the room even when off-campus via a Sensaphone Inc. IMS-4000, which monitors data 24/7 and sends alarms remotely to her home.  ES