Airborne infectious isolation rooms protect people from patients with contagious diseases and prevents airborne pathogens from contaminating adjacent corridors.
In the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbook 2016 there are three control methods described for room pressurisation. Two of these methods direct pressure and volumetric flow tracking control are described below.
This method measures the pressure differential across the room envelope and adjusts the amount of supply air from the room to maintain the required 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. In our experience wind can play havoc on pressure control.
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
Volumetric flow tracking is a common method and used in biological containment labs for some time. 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 and ignores 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 differential airflow 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)
Q&A – Our Top 5 Questions
Which of these methods provides better room pressurisation for infection control ?
How important is construction quality of the room envelope?
Our design and construct (D&C) contractor has another control method….how good is it?
What is the on-going service ?
How can I monitor and verify room air changes?
Our goal is to provide you the information to make up your own mind.