LARGE PARTICLES
IAQ Filtration Performance The Importance of Face Velocity and Its Impact on Filter Parameters and the Role of Inertial Impaction on Larger Particle Size By Dr. Felix Swamidoss Ph.D.
T
he concern over indoor air quality is a pressing issue in today's world, largely driven by the ever-expanding industrial landscape and the increasing use of innovative products and materials. Inhaling air pollution particles exposes individuals to significant health risks, including cardiovascular and respiratory problems that could potentially lead to a shortened lifespan. Its primary role is to mitigate the presence of fine particles, within indoor environments. However, a persistent challenge lies in filtration: the substantial pressure differentials that arise across multiple filter layers, subsequently resulting in elevated energy consumption. The goal in filtration system design is to strike a delicate balance between reducing these pressure differentials, maintaining filtration efficiency, and prolonging the lifespan of filters. This pursuit is fundamental for achieving energy-efficient and costeffective building operations. Filtration hinges on several core mechanisms for trapping particles, including interception, inertial impaction, diffusion, and sieving. The size of particles being filtered is a key
factor in determining which mechanism takes precedence in the filtration process. Yet, it's important to note that each filtration mechanism can be significantly influenced by the face velocity of the filter, an essential parameter with far-reaching impacts on filtration performance. Notably, in the case of nanofiber filters, the pressure differential decreases proportionally with the face velocity, not aligning with Darcy's Law, this is due to the slip flow effect. Researchers have delved into the effects of velocity profiles on pleat geometry, which, in turn, affects the initial local collection efficiencies. Investigations have also explored the determination of filter pleat geometry based on maximum local velocity. Furthermore, studies have assessed how filter media characteristics influence the efficiency of HEPA filters. Notably, it has been observed that efficiency tends to decline as face velocity increases, primarily due to the reduced time particles spend in contact with nanofiber filters, shedding light on the intricate relationship between face velocity and filtration performance. Studies have further revealed that the
specific resistance and density of the dust cake tend to be higher at higher filtration velocities, particularly when the dust concentration remains constant in the case of bag filters. These findings underscore the importance of considering filtration velocity as a pivotal factor in the quest for more efficient and effective filtration systems. This critical factor profoundly affects the overall performance of filtration systems, notably impacting both the pressure differential and efficiency.
The Impact of Face Velocity Over Filter Parameters
Pressure Differential and Face Velocity At the heart of understanding the significance of filter face velocity lies the concept of the pressure differential, often referred to as pressure drop. In filtration systems, this pressure differential is the dominant indicator of performance, and it is intimately linked with the filtration velocity. The basic principle is simple: the higher the face velocity, the greater the pressure drop as seen in Figure 1. As the airflow speed increases, so does the resistance to the movement of air through the filter. This dynamic manifest itself as an essential consideration for anyone seeking to optimize filtration systems for a range of applications.
Efficiency and Face Velocity
p Figure 1: Pressure differential and face velocity.
Efficiency is a central concept in filtration, defining a filter's ability to capture and retain particles from the air passing through it. The relationship between efficiency and face velocity is multifaceted, primarily driven by particle size. In essence, the efficiency of a filtration system can be greatly influenced by the speed at which air moves through it. The interplay between ISSUE 6 2023 FILTNEWS.COM 23
