Linden Shuttleworth, Fläkt Woods UK sales manager, asks whether sustainable ventilation approaches can be as successful in health care environments as they have been in other buildings.
Effective ventilation in hospital wards is very important to control respiratory disease transmission through air motion. The very high number of casualties caused by the worldwide outbreak of the Severe Acute Respiratory Syndrome (SARS) in 2003 may have been the result of poor ventilation.

Ventilation systems in hospital wards should be improved to control airborne infectious disease transmission. Many studies have been conducted on ventilation in different hospital facilities. In 2007 a panel reviewed 40 studies and found strong and sufficient evidence of the association between ventilation, air movement in buildings, and the transmission/spread of infectious diseases, such as measles, tuberculosis, chickenpox, influenza, smallpox and SARS.

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. However, displacement HVAC systems offer a real possibility of preventing 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 shifted upwards -- 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 proved 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 2.8 m.

As the warm air rises, carrying any particulate contaminants, it is replaced by clean air. Provided 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 or not.

Additional benefits

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 low pressure. This delivers an additional benefit of low operational noise combined with minimal draught in the occupied area.

In the temperature gradient associated with displacement ventilation, the ceiling-level air temperature is higher than that of the occupied zone. The first effect of this is that the supply temperature can be higher than that of traditional mixing systems. This higher air temperature means longer spells of free-cooling, reducing energy use through the lifetime of the system.

A higher return air temperature also renders the system ideal for use with an energy recovery device within an air handling unit (ahu). There is a consequent dramatic reduction in the heating energy needed to meet room conditions after the energy recovery device. In some cases, particularly if energy recovery is via a thermal wheel, the heating energy requirement can be removed altogether. There are also bonus benefits: the higher supply temperature enables free cooling to be available for much of the year.

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 reduces peak cooling loads, and increases the amount of time cooling recovery can occur. The energy recovery capabilities are key to minimising energy usage in such a system.

Research project

A three-year research project into displacement ventilation has recently been completed in California, and the results have implications for hospitals across the world.

The first phase involved mock-ups and computer modelling. A model was created of the patient's room to examine airflows and to show how the system worked. It proved displacement ventilation performed as well as, or better than, conventional overhead ventilation. The second phase included double-blind testing of displacement ventilation compared with overhead systems, using a numerical computer model and an empirical testing set-up.

The results established that displacement ventilation is comfortable for patients and staff.

M&E contractor Mazzetti Nash Lipsey Burc designed and installed the first displacement ventilation system in a hospital in North America at Kaiser's Modesto facility. It expects this system will show others in the healthcare industry that sustainable design features are safe and effective for patients, staff and the bottom line.