A health care facility’s HVAC system is perceived to control temperature, IAQ, and energy costs, but the often-overlooked factor is relative humidity (RH) levels.
Excessive rh can breed mold and other airborne contaminants and nourish fledging infectious diseases, such as hospital acquired infections (HAI), which may drive up air conditioning energy costs. Controlling moisture will create better air comfort, a healthier environment, and reduce energy costs. Aside from patient wellbeing, rh levels should be kept in compliance with ASHRAE Standard 170, “Ventilation in Healthcare Facilities.”
Therefore, the challenge for hospital management is to maintain code compliance even though rh levels between rooms, such as a corridor, patient room, and surgery suite, for example, can vary greatly. Regardless of safe design precautions, such as controlling air changes and a room’s negative or positive pressurization, etc., moisture will migrate from room to room because of its affinity for a drier space. Thus, controlling the rh in each space will make it easier to control the various vapor pressures and moisture migration into other spaces.
Besides inherently excessive water use and high numbers of respiring occupants, moisture is also introduced through the HVAC system’s outdoor air induction required by ASHRAE Standard 170. Conventional, properly sized air conditioning uses excessive energy to compensate for its technologically limited moisture-removing capacity to reduce hot, humid summer outdoor air to optimal 50 percent rh levels.
In addition to enhancing IAQ, humidity control also decreases HVAC energy costs, which account for nearly half of a hospital’s total energy use, according to the U.S. Department of Energy (DOE) Building Technologies Program. Thus, hospitals should be designed to the worst-case scenario in terms of dew point (the temperature at which water vapor condenses into a liquid at the same rate it evaporates). A 50 percent humidity level throughout the hospital would negate the concern of monitoring vapor pressures for various rooms, except for special needs areas such as operating rooms (OR). Consequently, rooms can be conditioned at 2-5 degrees warmer, yet maintain the same comfort level.
REDUCING MOISTURE THROUGH HVAC
Energy recovery ventilation (ERV) is a growing trend in HVAC hospital retrofits. The ERV system incorporates a true enthalpy desiccant wheel to dehumidify outdoor air. A second wheel dehumidifies (if needed) and recovers energy from warm indoor ventilation air before a portion of it is exhausted outside through the HVAC system in a quantity similar to the inducted outdoor air.
This system helps to greatly reduce chiller and fan energy use and keep the indoor environment comfortable without having to overcool the space. Since conventional HVAC technology typically can’t handle heavy summer humidity loads, many hospitals potentially fall out of ASHRAE 170 compliance. Design engineers compensate for this disadvantage by oversizing the HVAC system, which can sometimes overcool the space in an attempt to lower rh levels. Oversizing carries significantly higher capital equipment and operating costs.
CASE IN POINT
Using enthalpy technology in HVAC for energy recovery and rh control is being employed at the Montreal-based Centre Hopitaliér de l’Universite de Montréal (CHUM), which will be one of the largest hospital additions in North America, encompassing more than 2.5 million square feet when it’s completed later in this decade. The mega-project will use mechanical HVAC system enthalpy wheels to reduce moisture in its 100 percent outdoor air, 2.8-million-cfm design. Traditional hospitals use a minimum amount of outdoor air because it’s energy-intensive to heat, cool, and dehumidify. While most hospitals conform to a minimum outdoor air strategy in favor of recirculating energy-rich indoor air, CHUM will instead supply fresh, 100 percent conditioned outdoor air. Offsetting the significant energy costs of this IAQ strategy will be 47 molecular sieve technology enthalpy wheels that recover heat from the exhaust air, but without cross-contaminating the outdoor air with indoor contaminants. The same enthalpy wheel types will also reduce the cooling load and energy costs by dehumidifying high-rh outdoor air before conditioning it.
Because Montreal’s winter air is typically well below 30 percent rh, indoor moisture recovered by the enthalpy wheels can pretreat outdoor air to raise rh instead of exhausting it outside. This safeguards against low indoor rh levels that can dry out human olfactory systems and make them more susceptible to sickness.
Aside from the anticipated millions of dollars in energy savings over the operational life of the building, the system will help the project attain a Leadership in Energy and Environmental Design (LEED) silver certification, a U.S. Green Building Council (USGBC) designation for green and sustainable facilities. The downsizing of mechanical equipment due to the energy recovery strategy and other sustainable efforts expects to displace 40,000-tons of carbon emissions annually. Furthermore, 55 million liters of cooling tower water will be saved annually due to cooling system reductions.
SPECIALTY CONCENTRATION: SURGERY SUITES
Surgery suites typically demand very low dew point temperatures — 42°F and drier air, per ASHRAE 170 requirements. The trend is to design operating rooms with their own self-contained systems to avoid contact with air from other parts of the hospital. Conventional enthalpy equipment typically can’t handle such low dew point levels; however, the HVAC industry has recently developed alternative dehumidification strategies that utilize an active desiccant wheel to dry air to very low levels. These systems bring in outdoor air and use a cooling coil that reduces the temperature to approximately 55°. An active desiccant wheel then takes the saturated cooling coil’s moisture and dehumidifies it by adding heat. The dried air leaves the wheel in the 80° range, but at a 30°-40° dew point temperature. The air can then be fed to conventional air handlers for the space’s sensible cooling.
These systems deliver dry air on the ventilation portion only and decouple (remove) the humidity from the space with just the outside air portion. Another attribute is no cross-contamination between the indoor and outdoor ventilation during heat recovery. The ventilation is only 20-30 percent of the total air volume, so the hard work of moisture removal is handled easily and efficiently.
Many hospitals are currently retrofitting their operating room’s HVAC systems to comply with ASHRAE Standard 170’s stringent requirements for humidity control. This requires either adding a dedicated outdoor air dehumidifier (DOAS) to the operating room’s air handler or replacing the air handler with a DOAS that performs dehumidification and controls space heating/cooling.
The next decade will see hospitals adding rh control to their management operations to provide healthier patient IAQ and an environment that complies with building codes and cuts operating costs.
Publication date: 8/10/2015