Can displacement ventilation displace disease?

Can sustainable ventilation approaches be as successful in health care environments as they have been in other buildings?

The obvious concern that differentiates health care facilities from other building types is enhanced hygiene and infection control. Traditional hospital ventilation systems introduce air from near the ceiling, force it downwards with a diffuser, and then suck it back out through an exhaust grille, also at ceiling level.

The good news is that displacement HVAC systems offer a very real possibility to prevent the spread of airborne infections and contaminates, and are ideally suited for semi and non-critical areas such as wards, staff areas, canteens etc, but not for operating theatres.

Displacement ventilation offers the possibility of saving energy while maintaining or improving patient and staff safety, addressing both indoor air quality and energy issues – supplying clean fresh air directly into the occupied area at low level, at the correct temperature. The area’s warm, contaminated air is driven upwards via convection (hence, displacement) for extraction through the exhaust air system.

Particle testing comparing conventional and displacement systems, simulating the effects of airborne pathogens introduced into a space via a cough or sneeze, has proven that displacement ventilation at lower airflow rate is better than conventional ventilation, at higher airflow rates, when considering environmental comfort, ventilation effectiveness, and particle control. This benefit is increased if ceiling heights exceed 2.8m.

As the warm air rises, carrying any particulate contaminants, it is replaced by clean air. Provided that the supply air flow is adequate, the system is entirely self-regulating. Displacement ventilation has been shown to offer a ventilation efficiency of around four times the effectiveness of traditional mixing systems when introducing fresh air into a target zone. This increased effectiveness is consistent, unaffected by whether the space is occupied.

Such a system is not only capable of meeting CO2 requirements; it also provides a suitable room temperature while functioning at relatively low flows. In general, displacement ventilation systems operate at both low velocity and very low pressure. This delivers an immediate additional benefit of low operational noise combined with minimal draught within the occupied area.

With a thermal wheel in the AHU and an evaporative humidifier in the extract, the cooling load required by any chiller or condensing unit can be cut by 50%. This indirect evaporative cooling system not only reduces peak cooling loads, but also significantly increases the amount of time cooling recovery can occur. The energy recovery capabilities are key to minimising energy usage in such a system.

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