Operated by the Marathon Area Swim Association (MASA), the Ray & Marie Goldbach, Marathon Area Swim Center in Marathon, Wis., is a 10,122-sq-ft facility consisting of a 25-yd-long, six-lane-wide main pool; a small instructional pool; locker rooms; and a small administrative area. It opened in 1988.

The original pool dehumidification unit (DHU) used heat of rejection from the refrigeration system to heat pool water and provide hot-gas reheat. In 2003, it began experiencing operational problems. Those problems became severe in early 2004. The temperature controls failed because of corrosion, causing the service contractor to alter the way the system was controlled. The dehumidification-system condenser lost its ability to heat the pools' water. The cooling-coil fins became oxidized and started to fall off. The hydronic heating coil froze and ruptured on several occasions. By 2009, the unit housing had lost integrity, and MASA concluded it needed to replace the system.

Fig. 1. Click on image to view larger.

The system MASA chose (IDECVAV) is a direct-fired, variable-volume, 100-percent-outdoor-air unit equipped with an indirect evaporative precooler used to provide sensible cooling and heating-season energy recovery (Figure 1). The system is installed on the roof of the building, where it is subject to winter design conditions of -25°F.

This article compares the performance of the original and replacement systems. The comparison is based on actual performance. Aiding the comparison is the fact the swim center is a stand-alone facility served by dedicated utilities, rather than a small part of a much larger facility without dedicated utilities or submetering, which can obscure, distort, or dilute actual performance.

Strategies
The DHU introduced minimum outdoor air through the mixed-air path. This concept treats ventilation energy as a tax on the system the owner simply must pay. Refrigeration was used to dehumidify the air. Rejected heat was used to heat supply air and pool water. Recirculation of chloramine-contaminated air caused equipment deterioration and, ultimately, failure. The use of refrigeration for pool-water heating, while more economical than the brute-force use of water heating and ventilation, reduced energy usage, but imposed high parasitic energy losses on the system in the form of compressor energy.

The IDECVAV system is ventilation-based and designed with the following features:

• Outdoor air is free of chloramines and dry under cold weather conditions. The design employs 100-percent outdoor air to maximize air quality. Also, it permits the equipment to be located outdoors, prevents moisture from compromising air-handling-unit insulation during cold-weather operation, allows adjacent areas to be pressurized with fresh air, and limits the potential for corrosion damage.
• Supply-air volume is controlled to maintain space relative humidity with a low limit set to the minimum-outdoor-air ventilation rate.
• The exhaust fan is a direct-drive plug fan that locates the motor outside of the corrosive air stream. Exhaust airflow is piezometrically tracked to supply airflow to maintain mass balance and a pressure consistent with that of adjacent spaces. Secondary-exhaust-fan speed is slaved to primary-exhaust-fan speed to maintain building pressure under all conditions of operation.
• Exhaust-air, secondary exhaust-air, and secondary outdoor-air dampers are modulated as a mixed-air control.
• The system is designed around an indirect evaporative cooler. The air-to-air heat exchanger is of polymer construction to resist corrosion. The housing is American Iron and Steel Institute Type 304 stainless steel for corrosion resistance. Cooling, on the rare occasions it is needed, is provided by cycling the indirect-evaporative-cooler pump on and off.
• Supplemental heat is provided through a direct-fired gas burner with a 40:1 turndown ratio to maximize fuel efficiency.

With the IDECVAV system, outdoor-air ventilation is used to control space relative humidity and always is provided in quantities that meet or exceed minimum code requirements. Energy is conserved by recovering sensible and latent energy from exhaust air to temper outdoor air. Because much of the energy in the exhaust path is recovered and recycled, the need for refrigeration—and the associated high parasitic energy use—is functionally eliminated.

Fig 2. Click on image to view larger.

Figure 2 shows the psychrometric path in heating mode. Both sensible and latent heat (A-B) are converted to sensible heat (X-Y) through energy recovery. Supplemental heat is introduced by a gas burner (Y-Z). Water vapor is absorbed from the space while heat is delivered to offset envelope losses (Z-A). In part because system airflow is controlled from space relative humidity, an 8,760-hr typical-meteorological-year psychrometric analysis was employed not only to predict system economics, but to eliminate uncertainty as to which ambient conditions would dictate design airflow rates and to determine which conditions produce both maximum heating and maximum cooling requirements.

The performance of an air-to-air heat exchanger is amplified when moisture condenses on or evaporates from the heat-exchanger surface. This causes boundary-layer resistance to heat transfer to break down. The increased efficiency can become a serious problem at very low ambient conditions. In this application, however, the potential is much reduced, as long as the air in the exhaust path has sufficient energy to prevent the formation of ice in the heat exchanger. For a pool system, that generally is possible if design space-temperature and relative-humidity conditions are maintained. At low-ambient-temperature conditions, that can be compromised by the use of pool covers.

Performance Comparison
The utility use of the DHU in 2003, the last year the DHU was able to perform as designed, was compared with the utility use of the IDECVAV system from July 2010 through June 2011. Both data sets are imperfect: The DHU experienced a shutdown in March 2003; the IDECVAV system had some unresolved startup issues and experienced an unusually cold spring in 2011. However, by any measure, the performance gains realized by the IDECVAV system were significant.

Figure 3 shows aberrations in the DHU’s natural-gas utility use from January to March 2003, when operational problems began. The IDECVAV system substituted less expensive natural gas for the benefits of the electrically driven refrigerant compressors in the DHU. As a result, natural-gas use for all purposes increased 33.7 percent, from 15,842 therms with the DHU in 2003 to 23,951 therms with the IDECVAV system in 2011.