In the ASHRAE Handbook there are three engineered control strategies described for room pressurisation and directional airflow. Two of these control strategies – direct room pressure and volumetric flow tracking apply to hospitals and are described below. Both control methods measure airflow to validate performance of containment.
Room pressurisation & directional flow
Direct room pressure method
This method measures the pressure differential across the room envelope and adjusts the amount of supply or exhaust flow 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 flow rates are difficult to predict
Because direct pressure control works to maintain the pressure differential, the control system may need to react to pesky disturbances such as stack effects, infiltration, and influences of other systems in the facility.
In addition, 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.
Industry practice in Australia is to use direct room pressure control. However continuous measurement of flows for verification as per ASHRAE is rarely implemented.
Volumetric flow tracking method
This method measures both exhaust and supply airflow and controls the differential airflow 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. In fact, engineered leakage in the room of about 75 L/s for differential airflow (offset) is recommended.
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)
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