A Simple and Inexpensive Method for Determining the Effective Ventilation Rate in a Negatively Pressurized Room Using Airborne Particles as a Tracer

The ventilation rate within a negatively pressurized room is usually determined by measuring the exhaust air flow rate. This method does not account for air mixing factors and gives limited information on ventilation efficiency within the room. Effective ventilation rates have been determined using tracer gases such as sulfur hexafluoride (SF 6 ). The objective of this study was to determine whether artificially generated airborne particles could be used as a tracer to directly measure ventilation efficiency. We monitored the decay of artificially generated particles within negatively pressurized rooms. Separate trials were conducted at air exhaust rates ranging from about 6 to 20 room air changes per hour. Particles were generated to a minimum of 20 times the ambient concentration using a simple ventilation smoke bottle and measured with handheld light-scattering airborne particle counters. Data were obtained for aerodynamic particle size ranges of: 0.5 micron ( w M) and larger, and 1.0 w M and larger. The time rate of decay of particles was plotted after subtracting the background concentrations. Results were compared with simultaneously conducted tracer gas decay analyses (ASTM method E741-95) using SF 6 . Particle concentrations followed an exponential decay (R 2 = 0.98 - 0.99+) and mirrored the decay curve of the tracer gas. The air change rates predicted by the particle count procedure differed from the tracer gas results by a mean of 4.0 percent (range 0% - 12%). The particle count procedure was substantially simpler and less expensive than the SF 6 tracer gas method. Additional studies are needed to further refine this procedure and to explore its range of applicability.

Keywords:

• Tracer Gas
• Ventilation
• Negative Pressure
• Particle Counter
• Sulfur Hexafluoride