Astec’s Fiberbed Mist Collector

Step 1.

The gas stream slows rapidly as it enters the pre-filtration area and solid particulate matter settles out of the air flow. 1

Step 2.

The gas stream makes a 90-degree turn into a bank of prefilters.

Step 3.

Prefilters collect solid particulate matter and large liquid particles.

Step 4.

The gas stream passes downward through openings in the tubesheet into the oil collection chamber. 2

Step 5.

The gas stream is pulled upward through the main coalescing filters in the collection chamber. 3

4

Step 6.

Collection Occurs. (See Figure 6)

Step 7.

Captured particles coalesce into droplets large enough for gravity to pull through the coalescing filters.

Step 8.

Droplets collect beneath the tubesheet and can be removed for recycling or combustion via a valve.

Figure 6.

Inertial Impaction

Inertial Impaction = Particles continue along their original path until colliding with a fiber, which removes them from the gas stream. 5

6

7

8

Interception

Interception = Particles brush against the edge of a fiber, which removes them from the gas stream.

Diffusion

Diffusion = Random collisions with air molecules allow particles to move independently of the streamline until they make contact with the media fibers and are removed from the gas stream.

TThe fiberbed mist collector from Astec Industries, Chattanooga, is designed to control emission opacity by removing hydrocarbons from the incoming gas stream. Here’s how it works:

Gases enter a pre-filtration area at the inlet of the fiberbed. Here the gas stream rapidly decreases in velocity and solid particulate matter settles out of the airflow. The gas stream then makes a 90-degree turn into a bank of prefilters for further collection of solid particulate matter and large liquid particles.

After the pre-filtration compartment, the gas stream is directed downward through openings in the tubesheet into the oil collection chamber.

Next, the gas stream is pulled upward through the main coalescing filters where liquid droplets collect on the inner media surface. Gravity pulls accumulated droplets through the coalescing candle filters into the oil collection chamber. Clear air exits the fiberbed and is exhausted through a stack atop the system fan.

As the gas stream flows through the coalescing candle filters, air molecules flow around obstacles—the individual media fibers—in their paths. However, hydrocarbon particles in the 5-micron range and larger, will not. Inertia forces these large particles to continue their original path until colliding with a fiber, which removes them from the gas stream. This is referred to as impaction.

Hydrocarbon particles in the 1- to 3-micron range are collected via interception when they graze against the media fibers.

Particles below 1-micron in diameter follow the streamline as it flows between the micro-glass fibers. They exhibit random motion, which is known as Brownian diffusion. This phenomenon is due to collisions with air molecules within the gas stream. The random movements allow particles to move independently of the streamline itself. Particles tend to migrate from areas of high concentration to areas of lower concentration. Those particles that contact the media fibers are removed from the gas stream. Particle concentration near the fiber surface will go to zero thus continuing to drive the diffusion/collection process.

Captured particles coalesce into droplets, which gravity pulls through the filters to accumulate in a collection chamber located beneath the tubesheet. The bottom of the chamber slopes toward the inlet of the unit to facilitate removal of the collected hydrocarbons through a valve. The collected compounds are primarily oils, so they may be recycled or combusted, as required by local regs and permit conditions.

For more information, visit www.astecindustries.com or call Catherine Sutton Choate at (423) 827-1387.