Mitigating The Risk From Airborne Infections
In the ASHRAE Handbook there are three engineered controls described for room pressurisation and directional airflow. Two of these controls – direct pressure and volumetric flow tracking apply to healthcare and are described below. Both control methods measure airflow to validate performance of containment.
Room Pressurisation & Directional Airflow
Direct Pressure Method
This method measures the pressure differential across the room envelope and adjusts the amount of supply or exhaust air in the room to maintain the required room differential pressure. The challenges for this method include:
- maintaining a well-sealed room for high room pressures
- maintaining the pressure differential when the room door is open
- finding suitable sensor locations
- obtaining and maintaining accurate pressure measurements
Engineering parameters relating to the room construction integrity and associated airflow rates are difficult to predict.
Because direct pressure control works to maintain the pressure differential, the control system may react to pesky disturbances such as stack effects, infiltration, and influences of other systems in the facility.
Entry/exit doors may need a reed switch to disable the corrective action of the feedback loop and maintain balance when the doors are open.
Volumetric Flow Tracking Method
This method measures both exhaust and supply airflow and controls the differential airflow called offset to maintain a desired room pressure. Flow tracking can provide more stable control by ignoring pesky disturbances explained before and is independent of room door operation.
Engineering parameters can be easily pre-set, and extremely tight construction is not necessary.
The flow offset as a minimum must be greater than the accuracy of the flow measurement and associated control error.
Advantages of volumetric flow tracking include the following:
- Low pressurisation level (normally -2 to -10 Pa)
- Less tight construction required
- Simple set of relative pressurisation levels (one or two levels)
Now is the time to rethink engineering controls in ventilation systems to better mitigate the risk of infection in our hospitals.