Units also help keep operational expenses down
For the new Ocean Towers at Hammock Beach Resort in Palm Coast, Fla., the owner and engineer wanted equipment that would provide uncompromised comfort and superb indoor-air quality. They also wanted a high-efficiency system that could withstand a harsh and humid coastal environment while meeting, if not exceeding, strict state guidelines requiring positive pressurization and outside air.
Five Pinnacle energy-recovery units (ERUs) from SEMCO Inc., each with packaged direct-expansion units, were chosen to provide 100-percent-outdoor-air ventilation to corridors and common areas. The ERUs provide dehumidified outdoor air to spaces at a neutral dry-bulb temperature of 66°F to 67°F. The centralized system enabled MECA Inc., the firm responsible for designing the mechanical system, to “decouple” the latent load from the sensible load, which is handled by water-source heat pumps in each unit.
Early on, MECA engineers committed to continuous, rather than intermittent, bathroom exhaust. With Florida requiring buildings to be positively pressurized, intermittent exhaust would have led to control issues. Continuous exhaust, on the other hand, can reduce the overall airflow being forced through a system, which reduces fan energy as well as the energy required to condition supply air. With the SEMCO units in place, most of the energy from this continuous exhaust is recovered.
Each of the five Pinnacle units includes a total-energy wheel, a supply-side cooling coil, a passive dehumidification wheel, and a return-air heating coil. Outdoor air enters the unit and passes through the total-energy wheel, where it is preconditioned using exhaust from the units' bathrooms. The cooling coil and passive dehumidification wheel work in concert to further treat the supply air to produce the desired set-point temperature at a much reduced humidity level. This second wheel relies on a larger amount of desiccant material for moisture removal. Also, it rotates at a very slow speed to transfer as much moisture as possible from the supply-air stream to the exhaust-air stream. The driving force of the passive dehumidification wheel is the differential in relative humidity. The cooling-coil leaving air is saturated and cannot hold any more moisture, while the return air is conditioned at 50-percent relative humidity and, thus, can hold more moisture by volume. This differential provides the means of moving moisture from the saturated supply (post coil) to the dryer space air being exhausted. The heating coil is used to further temper the air to the desired discharge temperature, primarily during the heating season.
Supply air is controlled to maintain a constant positive pressure throughout the condominium space to offset the continuous vacuum-assisted exhaust in all of the unit bathrooms.
The system provides 100-percent outside air at a comfortable humidity level without the typical energy penalty. While owners and guests enjoy the quiet comfort of their individual heat pumps, the centralized ventilation system likely is increasing the lifespan of associated equipment, including the heat pumps, which operate less because of the semiconditioned air being supplied by the SEMCO units.
The design is fiscally and environmentally responsible, not only because of its efficiency, but because it reduces the potential for mold.
“We always want to do everything we can to minimize the potential for mold,” Damon Ditch, mechanical engineer for MECA Inc., said. “We knew if we kept these spaces dry and under positive pressure, we would do that.”
Providing pre-conditioned air at a suppressed dew point also is a lot easier on equipment, Tom Rice, Southeastern applied regional manager for SEMCO, explained.
“Space devices don't have to work as hard to meet the sensible load,” Rice said. “This also means less water buildup on interior devices, which reduces the threat of rust. In fact, moisture control can arguably extend the life of mechanical equipment across the board — even ductwork which is known to leak and cause ceiling damage.”
All of this considered, the Pinnacle units serve many more purposes than energy-recovery ventilation. They help keep operational expenses down and may even delay parts and equipment replacement.
Information and photographs courtesy of SEMCO Inc.
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