Using Tribo to Monitor Minimum Transport Velocity for Entrained Dust Particles

Posted by Earl Parker on Apr 26, 2016 10:00:00 AM

No matter what the application accumulations of dust within a duct work system can present a host of problems to industrial facilities. Dust buildup in baghouse ducts can impact operations of the dust collection system, thereby directly affecting production. Malfunctioning dust collection also generates respiratory safety hazards as well as creates a substantial risk for combustible dust fires and explosions. Additionally, poorly performing systems directly impact a plant’s emissions limits, which can lead to heavy fines, sanctions or even forced shutdowns by regulators. 

How can operators and technicians prevent this issue and avoid these serious consequences in their facilities? 

Why Dust Builds Up In Baghouse Duct Worktriboelectric detection can monitor particle velocity to ensure minimum transport velocity for fully entrained dust particles in baghouse dust collection systems

In order for dust particles to flow through a pneumatic conveying system like a dust collector, the collector must create enough vacuum to pull the dust fast enough to remain entrained in the air. This minimum speed is called the minimum conveying velocity or transport velocity. If the airflow in the ductwork ever drops below the minimum transport velocity, the dust will begin to fall out of the airstream and settle on the bottom of the duct. Every material has a different minimum transport velocity, with heavier and larger dust particles generally requiring a higher air speed to stay entrained. Note that the key here is the particle velocity, NOT simply the conveying air velocity.

How Triboelectric Particle Velocity Monitoring Works

Particles encountering the isolated probe generate an electric signal when (1) a particle strikes the probe (DC signal) or (2) when it passes near by the probe (AC signal). Triboelectric monitors from Auburn Systems use an advanced technology to combine both AC and DC signals into a unified signal that then corresponds to particle flow activity in the air stream. 

But triboelectric detection technology can be used to do more than simply measure the presence of particles - it can also be configured to monitor particle velocity. By using two non intrusive sequential probes the system can calculate an accurate particle speed by measuring the time it takes for the particles to travel between the two probes. 

Why Particle Velocity is More Accurate Than Air Speed

Reference works on pneumatic conveying of dry bulk goods (such as the ACGIH Industrial Ventilation Manual published by the American Conference of Governmental Industrial Hygienists) cite standards for conveying speeds in feet per second. However, these speeds refer to air speed not to actual particle speed within the ductwork. Thus in some applications the air speed may sufficiently meet the recommended minimum but due to the unique particle size and distribution the system may not consistently achieve the minimum transport velocity. (Auburn’s particle velocity monitoring technology also has many applications in process industry)

Triboelectric monitoring provide a distinct advantage by directly monitoring particle velocity versus air velocity. This means it can detect changes to particle flow speeds even if the air speed remains constant. For example, a process change may result in larger dust particles entering the system, the larger particles will require more energy to move than the smaller ones the system previously handled. By monitoring the particle velocity (in contrast to just air speed) operators can New Call-to-actioncarefully watch for any drop that may indicate the fan output must increase to compensate. 

Taken a step further, the instruments can be configured to provide output to system fans to increase or decrease fan speed automatically to maintain the minimum transport velocity to keep dust particles fully entrained.

Stopping dust dropouts

By carefully monitoring particle velocity operators can take care to avoid letting anything affect the system performance, namely its ability to maintain the minimum transport velocity within the ductwork. By having accurate velocity data they can keep careful watch over any changes that may occur due to process changes or normal wear and tear on the dust collection system. By doing so, they can prevent the underlaying cause of dust buildup in their duct work systems and thus avoid the many problems that come with it. 

Auburn Systems at 2016 Powder & Bulk Solids Show 

If you would like to see a demonstration of monitoring particle velocity or our complete line of triboelectric monitoring equipment, visit us at Booth 2307.



Topics: Triboelectric Detection, Particulate Monitoring, Process Control, Flow Control