Outdoor Air Ventilation as a Means to Curb Hospital Acquired Infection (HAI)
The healthcare industry is at a crossroads with the increased incidence of hospital acquired infections (HAI) and the HVAC industry has the solution with outdoor air ventilation.
HAI statistics spell a sense of emergency. On any given day, about one in 31 hospital patients has at least one healthcare-associated infection. And in 2015, about 72,000 hospital patients with HAIs died during their hospitalizations.
On any given day, about 1 in 31 hospital patients has at least one healthcare-associated infection. 3% of hospitalized patients in the HAI Hospital Prevalence 2015 Survey had one or more HAI. There were an estimated 687,000 HAIs in U.S. acute care hospitals in 2015. About 72,000 hospital patients with HAIs died during their hospitalizations.
As found on the Center for Disease Control website
To offset the trend, hospitals worldwide are looking to reduce the HAI trend via 100 per cent outdoor air ventilation systems in an air replacement strategy. A minimum of two times per hour, the entire air volume in a hospital can be replaced by introducing 100 per cent outdoor air, and in turn, exhausting interior air along with any of its potential airborne contaminants. Specialty areas, such as surgical suites, may require additional air changes.
Air replacement is a viable strategy, but it is cost-prohibitive to dehumidify, heat or cool a huge volume of air that can surpass one million CFMs in large facilities. Facility engineers walk a tightrope of reducing operational HVAC energy costs while also complying with ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities.
Furthermore, the challenge for hospital management is to maintain code compliance even though relative humidity (RH) levels between rooms, such as a corridor, patient room and surgery suite, can vary. Regardless of safe design precautions, such as controlling air changes and a room’s negative or positive pressurization, moisture will migrate from room-to-room, because of its affinity toward a drier space. Controlling the RH in each space makes it easier to control the various vapour pressures and moisture migration into other spaces.
Energy recovery ventilation (ERV) units have many advantages for cutting the expense of dehumidifying, cooling and heating outdoor air.
ERVs that dehumidify outdoor air, for example, can significantly reduce a building’s air conditioning load. Dehumidification can be accomplished through mechanical, desiccant or a combination of both methodologies. Mechanical refrigeration coils condense moisture from the air whereas a desiccant-based strategy adsorbs moisture with desiccant wheels.
Heat recovery is also an ERV advantage, because of the significant energy cost reduction potential. ERVs can use exhaust air to preheat or pre-cool the outdoor air before it is conditioned for a set point such as 72F. For example, heating -10F outdoor air in January to 72F is a significant expense. However, using 65F exhaust air to preheat the outdoor air will deliver significant savings.
Enthalpy wheels are ideal for heat recovery, but in hospitals, the wheel’s desiccant media choice – typically silica gel or molecular sieve material – is critical for preventing the incoming outdoor air from residual contamination from the outgoing indoor air. Silica gel has a propensity for cross contamination because its pores can adsorb and harbor contaminants from the exhaust air. This can also decrease latent heat transfer. Molecular sieve materials, on the other hand, are not susceptible to contaminant adsorption, because the smaller pore size only allows the transfer of the water vapour from the exhaust to supply air streams.
In winter, when outdoor air is uncomfortably below 30 per cent RH, humidity collected from the exhaust air can be added to the supply air to raise indoor RH levels. Obviously, it is critical to not contaminate outdoor air with indoor contaminants during the humidification process, because of HAI concerns. To further minimize cross-contamination, desiccant surfaces can be specified with anti-microbial coatings to help prevent airborne biological contaminants from harbouring in the media.
Other hospitals are using energy recovery and humidity reduction in critical areas, such as surgical suites, which typically demand very low dew point temperatures 42F and drier air, as per ASHRAE 170 requirements. The trend is to design operating rooms (OR) with their own self-contained system to avoid contact with air from other parts of the hospital.
Conventional enthalpy equipment typically cannot handle such low dew points; however the HVAC industry has recently developed alternative dehumidification strategies that utilize an active desiccant wheel to dry out the air to very low levels. These systems bring in outdoor air and use a cooling coil that reduces the temperature to approximately 55F, and then an active desiccant wheel takes the saturated cooling coil’s moisture and dehumidifies it by adding heat.
The dried air leaves the wheel in the 80F range, but at a 30F to 40F 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 to 30 per cent of the total air volume, so moisture removal is handled energy efficiently.
Many hospitals are currently retrofitting operating room HVAC systems to comply with ANSI/ASHRAE/ASHE Standard 170 requirements for humidity control. This may require 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 improved moisture control combined with energy recovery to control operation expenses and to provide healthier patient IAQ environments.
Modified from the original posted March 2016 in HPACMAG. Written by Steve Ulm, marketing director at Semco LLC. Access, Inc. represents Semco’s unitary air treatment products at our Farmington Hills office.