As we continue the exploration of design issues for forensic facilities, we will address room pressurization and air flow direction, improving facility operation and maintenance, and saving energy and resources. It is important to understand how and why architecture and HVAC system design must work together to develop a successful project.
As we continue the exploration of design issues for forensic facilities, we will address room pressurization and air flow direction, improving facility operation and maintenance, and saving energy and resources. It is important to understand how and why architecture and HVAC system design must work together to develop a successful project. In this article, Ken Mohr and Lou Hartman explore ideas and solutions for your engineering and architectural questions.
Maintaining a good indoor environment: The direction of airflow can be more important than the rate of airflow.
Let’s understand the basics. The total airflow rate for a laboratory is determined by how much air leaves (exhausted) vs. how much air comes into the lab (supplied). Airflow rate is usually measured by cubic feet per minute (cfm). If the total cfm is greater coming into a room, then the room is positively pressurized, or more air is being supplied than exhausted. If you opened the door to this room you would feel the air leave the room. If the opposite existed—more air is exhausted than supplied—then the room would be negatively pressurized. Again, if you opened the door to this room you would feel the air come into the room. Exhausted air could be through a fume hood device or an exhaust grill in the ceiling. Supplied air could be through a supply grill in the ceiling or through the door into the room. The amount of cfm needed for a room is determined by the number of exhaust devices (fume hoods, snorkels...), the amount of cooling (supplied air) required to help offset internal heat gains from people and equipment, or industry space standards of minimum ventilation rate requirements due to the activities in the room.
Any laboratory that uses chemicals or compressed gases requires a non-recirculating air supply system. The system is also known as a 100% outside air supplied system. This means all of the air that comes into the room is used and then exhausted; all of it. The opposite of this system would be a recirculating air supplied system—not all of the room air supplied would be exhausted, but reused in the same room or maybe by an adjacent room. Each room would be evaluated separately to understand the hazards present. As you can imagine, providing a recirculating air supply system would be less expensive to install and to operate. A space commonly overlooked for a recirculating system in a forensic lab is the Computer Forensics Unit.
Industry space standards tell us that minimum airflow rates are generally in the range of six to ten air changes per hour when the space is occupied. However, some spaces may have minimum airflow rates established by specific activities in the room or by internal facility management policies. A guideline often followed is one established by the National Institutes of Health recommending a minimum ventilation rate of six air changes per hour for an occupied laboratory. For laboratories where the amount of ventilation is determined by either equipment cooling requirements or exhaust device rates, it is common to recognize that all exhaust devices or equipment are seldom used at the same time. This creates a “usage factor” which is helpful in determining the entire size of the building’s HVAC system. Applying a “usage factor” in the design process of a forensic facility must be carefully analyzed to avoid under-sizing the ventilation system in the hopes of saving a few dollars.
The exhaust system must be controlled and coordinated with the supply air system to maintain the proper pressurization in each room as it relates to the surrounding rooms. The laboratory HVAC systems can either be constant volume or variable volume. A constant volume system is just that; the air is always moving at the same speed while a variable volume system has air moving at different speeds. This is best explained by the night set back. During normal facility hours of operation, the HVAC system is in one mode and at night the system is throttled back, still safe, but the demand is clearly reduced. Fume hoods may have their own exhaust fan, or may be manifolded and connected together to one or more common central exhaust fans. In general, a variable volume HVAC system with manifold exhaust is more flexible (ability to add exhaust devices), requires less ductwork, involves fewer pieces of control equipment, reduces the number of roof penetrations, and creates an opportunity for energy recoveryand savings.
Care needs to be taken when choosing the right external building exhaust and air intake locations to avoid drawing exhaust effluent into the fresh air supply and contaminating the building. The height of the exhaust stack must extend ten feet past the highest point on the roof, and the discharge velocity must fall between 3,500 cfm to 4,500 cfm to properly dissipate the exhaust plume.