The most common system of choice for central outside-air systems — those in which a large volume of fresh air is tempered at a central location, for distribution throughout a building — are desiccant-based enthalpy wheel systems, with two wheels sandwiching a cooling coil. The desiccant requires regenerating heat to function correctly. That is, a very hot air stream is needed to purge the moisture from the desiccant requires; most enthalpy wheels depend on only outside air at the ambient temperature to rid the moisture from the desiccant substrate.
The most trying conditions for the enthalpy wheel are those at warm temperatures, but high humidity. Problems are hard to detect because in most cases if the outside-air system does not work quite right, then the second part of the decoupled system (i.e., the space load is handled by a system operating independently of the outside-air system) does the dehumidifying, and no problems are evident to the user until the enthalpy wheel is irreparably damaged.
Be that as it may, other shortcomings surface when the energy use of the enthalpy wheel system is compared to a flat-plate heat- exchanger system. The flat-plate system uses the plates to exchange sensible heat between the supply and exhaust air streams, and a cooling coil does all the latent cooling to the air stream plus some sensible cooling. This system does not have any purge air (desiccant wheels must have a constant stream, perhaps 10 percent of the total supply air, to filter directly from the supply to the exhaust stream o maintain complete air separation at the seals). Consequently, the total cubic feet per minute that must be moved through the flat-plate system is less, so the fan horsepower is less, and this translates into extra annual energy savings.
Another significant fault in the desiccant system is that performance drops off sharply as the temperature of the exhausted air increases— and so the supply-air humidity escalates. This increase in return-air temperature is typical in any application with a central EMS, as many spaces—often as much as 80 percent—are not conditioned at certain times (e.g., unused classrooms in evening or weekend classes at a community college campus). The result is that the air supplied throughout the facility is more humid than designed, and— in those spaces without an active air-conditioning system—the local units do not circulate air through the cooling coils to dehumidify. This can have disastrous consequences, as the unit pumps humidity into all the building spaces.
The flat-plate system, on the other hand, supplies a consistent temperature and humidity of air at all conditions of supply and return air. When the exhausted air has an increase in enthalpy, the compressor works that much harder. The air always leaves the cooling coils at 55°F dry bulb/54°F wet bulb.
A thorough study of the two systems therefore indicates (1) the flat-plate system needs a larger compressor typically, and spends more energy annually on cooling; (2) the enthalpy wheel system must condition a higher volume of air because of purging, and also has a higher static pressure through the desiccant wheels, so it needs a larger supply- and return-air fan motor; (3) comparing two identical systems, the flat-plate system uses up to 25 percent less energy per year.
Another significant advantage of the flat-plate system is that it weighs about half as much as a comparable desiccant system. Thus extra expense in structural reinforcement is saved. Finally, the flat- plate system typically costs 25 to 50 percent less.
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Excerpt from Energy Conservation Projects.