By Tom Scozzari, B.Sc.
An indoor pool environment has greater requirements than a commercial building because of the higher humidity. If not controlled, the corrosive condensate that forms will destroy structural components, sometimes with disastrous effects. Further, the high humidity is uncomfortable for occupants and can cause chronic respiratory condition (i.e. lifeguard lung).
To manage humidity, dehumidifiers specifically designed for the pool environment are used to control the excess moisture. But unlike commercial heating, ventilation, and air conditioning (HVAC) equipment, dehumidifiers require more energy to properly operate. Today’s higher cost of energy, and the desire to improve sustainability, has created the need for more efficient technology. One way to achieve this is by using air-side economizers.
Economizers are best associated with commercial building applications, especially those with cooling loads greater than 50,000 BTUs/hr. These types of economizers are designed and configured for cooling only and not moisture content. Office buildings also have a narrow operating range between 21 to 22 C (70 to 72 F), which limits the window of outside air ‘free cooling.’
Economizing can also be used successfully for indoor pool environments where temperature set points are typically 29 C (84 F) with relative humidity (RH) at 60 per cent. This set point allows outside air (OA) to be used more frequently and over a wider range of outside air temperatures as compared to office applications.
Outside air can also be used for dehumidification and to some degree heating as well. The control technology plays a key role in analyzing and comparing outside air temperature and humidity with return air (RA) temperature and humidity and deciding which mode offers the lowest cost of operation to meet set point conditions. When considering a high-end dehumidifier for an indoor pool make sure to include all the necessary hardware and firmware for full economizer operation upon delivery, no additional investment is required to take advantage of these extra features. Many pool dehumidification equipment manufacturers offer economizers as standard or optional equipment, but the majority are for cooling only.
What is economizing?
Often referred to as ‘free cooling,’ an air-side economizer significantly reduces the energy required for cooling commercial buildings by limiting compressor operation. It will exhaust equal parts of warm, moist return air with outside air as exterior conditions permit.
It is also effective in improving indoor air quality (IAQ) through greater quantities of ventilation brought into the building. Because the energy savings are substantial, the International Energy Conservation Code (IECC) requires economizers in any system larger than 54,000 BTU/hr. This excludes extremely hot, humid, and cold climate zones where economizers are less effective.
In their basic form, air-side economizers comprise dampers for the outside air and return air which work in unison to control air flow. When outside air conditions are cool and dry enough, the mechanical cooling is shut off and the system uses outside air to cool and dehumidify the structure.
Air-side economizers come in various arrangements, including temperature-based or enthalpy-based. Economizers operate behind the scenes, and it is difficult to know when they are functioning without expensive monitoring instrumentation.
These systems have typically been more cost-effective for larger buildings, but improvements in mechanical equipment and controls are creating new opportunities in smaller applications. However, economizing is less effective in climates having substantially higher temperatures and moisture levels.
Outside air has gone full circle, and has two practical functions in a natatorium. It removes excess moisture for a more comfortable environment and it improves the IAQ through ventilation, ridding the space of air quality problems like chloramines formed through chlorine off-gassing, and carbon dioxide (CO2).
Before the advent of affordable mechanical dehumidification, outside air was used exclusively to condition the space. The introduction of refrigeration technology allowed better control of temperature and humidity, but at a cost. In addition, dehumidification equipment and control strategies did not often meet expectations, but many equipment manufacturers still relied on costly direct expansion (DX) systems as their primary means of moisture removal. The high cost of energy to heat outside air also deterred its use other than what was required by code. Today’s more efficient equipment can once again use outside air without the penalty of higher cost.
