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1-706-884-3006

June 2006

Filtration and Vacuum Specialists since 1976

www.vacuumpumpusa.com




A Scientific Review of Dust Collection - Part 4

Filter Media

Reference material by: Scientific Dust Collectors

Because our newsletters are a service to our valued customers we have decided to share some important and educational information on Scientific Dust Collection. Over the next few months we will be focusing on the use of Dust Collectors. We felt that the extensive information and it's importance in the industry would be very useful in helping our customers make an informed decision on their needs for dust collectors in their businesses. Because the information is extensive we will be spreading it over several months.

Purpose of Filter Media

The main purpose of the filter media is to separate the gaseous air from the solid dust particles in the process air stream by using a membrane material or more commonly referred to as "filter media". The filter media forms a support surface that allows the gaseous air molecules to pass through, while the larger dust particles are captured. A second vitally important capability is for the filter material to easily release the captured dust particles when, for example in a air pulse arrangement, a separate burst of clean air temporarily reverses the flow of the process air stream. The clean air burst has a higher velocity and a greater velocity pressure potential than the process air stream so that the cleaning air is able to overcome he process air flow and thereby release a large percentage of the dust particles, a layer of dust or "dust cake" is generated on the incoming surface of the filter media. As more dust particles arrive at the dust cake which rests on the surface of the filter media, the thickness of the filter cake increases and filter efficiency also rises. During the burst of cleaning air, most of the dust cake will be separated from the surface of the filter media and drop downward into the hopper area.

The are other important capabilities of the filter media for specific application needs that will be briefly listed here and will be discussed in more detail later in this chapter.

  1. Temperature considerations of the process air stream and the filter media with normal upper limits of 200oF for cellulose to 500oF for fiberglass material.

  2. Fire retardant coating which will retard combustian. (note: it is not fireproof.)

  3. Static dissipation properties:

    a) Carbon Impregnation - Applies to wet-laid media (cellulose) and gives excellent static dissipation properties.
    b) Metallized Finish - Applies to polyester media (spun-bonded) and gives an improved dust cake release and excellent static dissipation properties.

  4. Hydro and Oleophobic Finish - Applied into the polyester / media resulting in excellent moisture and mild oil mist tolerance, dust collection efficiency, and material strength.


Process of Collecting Dust on the Surface of the Filter Media and Some General Mathematical Relationships


For illustration purposes, process air contains both dust particles and gaseous air. The goal is to stop dust particles at the incoming surface of the filter media while the air molecules are able to travel through the existing pores or openings in the filter media. In Figure 4-1, a one square foot area of filter media is represented with varying size of holes / openings. The dust particles collect around and in the openings of the filter media to form a dust cake which is helpful in raising the filtration efficiency of the collector. A fan provides the energy to either pull or push the air through the existing media openings. To help further explain this phenomenon, there are some mathematical relationships among the following variable which include static and velocity pressure, area of openings, volumetric flow rate, air velocity, density of air, temperature, and the gas constant for air.

Figure 4-1


Description of Mathmatical Variables

Static pressure (SP) is defined as either a positive or negative pressure that is applied to surfaces which cause the surface to either expand or contract. For example, in a positive pressurized container such as an inflated balloon, the internal pressure keeps the balloon inflated since the internal static pressure is greater than the atmospheric pressure outside. The unit of pressure is usually indicated in inches of water gage ("WG"). For example, 27.68 inches of water equals 1#/in2 (pounds per inch square or psi).

SP as used in dust collection most often refers to the resistance in a duct system to a given volume of air. Air has weight mass. When you move something with weight and mass through duct work, SP is the energy required or resistance that must be overcome.

Velocity pressure (UP) is defined from the kinetic energy equation and is the pressure that is needed to make a change in velocity of the gaseous air molecules.

Velocity (V) is defined as a vector quantity and is the rate of speed of matter which is usually expressed in feet per minute or feet per second.

The density (p) (Greek letter Rho) of air is defined as its mass per unit volume. When using the pound mass ("lbm") unit system, the density of air (p) is expressed as lbm per cubic foot. By using the perfect gas equation (see Example 4.1) which relates pressure, density, and temperature, and the gas constant for air, the air density can be calculated to be .075 lbm per cubic foot at a standard temperature of 70oF, zero water content, and at a standard atmospheric pressure of 14.7 pounds per square inch absolute.



Figure4-2
Example 4.1 Calculate the air density at standard conditions (STP) of 70o Fahrenheit and 14.7 PSIA pressure absolute.

This is a large image and will take a little time to load.

Note: When the air is traveling at 4000 feet per minute through any duct size at standard conditions (STP), the velocity pressure is 1" WG. Use equation Q - VA to determine the actual flow rate Q (CFM) through each duct size. Also, there are other line losses and obstructions that contribute to a duct velocity pressure. Use an industrial ventilation manual to predict the required duct and fan pressures.


There is another important relationship called the Frasier Permeability rating for the filter media. It states that the volumetric air flow rate number is determined at a 1/2 inch of water gage pressure and through an area of one square foot of media. For an area of one square foot of standard filter bag media, the Fraiser Permeability number ranges from 20 to 40 CFM at 1/2 inch of water gage velocity pressure. At the same area and velocity pressure, the cartridge filter media has a Frasier Permeability range of 4 to 30 CFM. In most cartridge and baghouse collectors, a magnehelic differential pressure gage measures the pressure in inches of water gage between a port that is the clean air plenum (where the cleaning purge tubes are housed) inside the collector. The value of this velocity pressure differential measurement gives and indication of the working status of the filter cartridge or bag. Typically, a low number such as 1 1/2 to 2" WG indicates a good balance between the collection of dust and the cleaning of the filter bag or cartage. Conversely, a number from 5 to 7" WG indicates an "out-of-balance" system between the filtering of dust on the media and removal of dust by the air pulse from the cleaning system. Some individuals mistakenly relate the differential pressure reading directly to the original Frasier Permeability rating. However, there are other variables that are combined into the pressure reading of the magnehelic gage which include dust cake, orifice in venturi or other openings in the mouth to the bag or cartage.

Selection of Fabric Materials for Dust Collectors


There are many types of fabric materials available that have been developed in order to satisfy the specific requirements of a given application. The basic criteria for selection a specific material are listed below.

  • Temperature of the process air stream in the collector
  • Moisture level inside the collector and / or hydroscopic nature of the dust
  • Electrostatic characteristics of the dust
  • Abrasion of the dust particles on the filter media
  • Acid chemical resistance
  • Alkali chemical resistance
  • Ease of release of the captured dust particles from the media
  • Permeability of the fabric to allow only air to pass through the media
  • Cost of fabric materials
  • Size of the dust particles to be collected

There are a wide variety of media types used in dust collection filters. The most common are:

Baghouse Filters:


  • Polyester: the standard and most widely used baghouse material in the industry.
  • Singed Polyester: used for improved dust cake release.
  • PTFE Membrane Polyester: used for capture of fine particulate where an artificial dust cake is required.
  • Aramid: used for high temperature applications.
  • Fiberglass: has very good performance in acid or alkaline environments where high temperatures are present.
  • Polypropylene: has superior chemical resistance.

Cartridge Filters:


  • Cellulose: the standard and most widely used cartridge material in the industry.
  • Cellulose / Polyester: synthetic fibers blended with cellulose to create a high durability media with very good abrasion resistance.
  • Spun Bonded Polyester: media having good release characteristics with moisture tolerance and excellent abrasion resistance.

Pleated Bag Filters:


  • Spun Bonded Polyester: the standard material in this application.

Two general filter material selection tables are presented below which correlates key application parameters with the various strengths and limitations of the filter media.
Table 4-1
Fiber Generic Name

Trade Name

Cotton

Polyamid

Nylon 66

Polypropylene

Herculon

Polyester®

Dacron®

Recommend continuous operation temperature (dry heat)

.

180oF

82oC

200oF

94oC

200oF

94oC

270oF

132oC

Water vapor saturated condition (moist heat)

.

180oF

82oC

200oF

94oC

200oF

94oC

270oF

94oC

Maximum (short time) operation temperature (dry heat)

.

200oF

94oC

250oF

121oC

225o F

107oC

300oF

150oC

Specific density

.

1.50
1.14
0.9
1.38
Relative moisture regain in % (at 68o F and 65% relative moisture) .

8.5
4.0-4.5
0.1
0.4
Supports combustion

.

yes
yes
yes
yes
Biological resistance (bacteria, mildew)

.

No, if not treated
No Effect
Excellent
No Effect
* Resistance to alkalies

.

Good
Good
Excellent
Fair
* Resistance to mineral acids .

Poor
Poor
Excellent
Fair+
* Resistance to organic acids

.

Poor
Poor
Excellent
Fair
* Resistance to oxidizing agents

.

Fair
Fair
Good
Good
* Resistance to organic solvents

.

Very Good
Very Good
Excellent
Good

* At operating temperatures .

* Not

Recommended

Comments: Based on typical fiber manufacturers published specifications.


Table 4-2

Fiber Generic Name

Trade Name

Aramid

Nomex®

Glass

Fiberglas®

PTFE

Teflon®

Polyphenylenen Sulfide

Ryton®

Recommend continuous operation temperature (dry heat)

.

400oF

204oC

500oF

260oC

500oF

260oC

375oF

190oC

Water vapor saturated condition (moist heat)

.

350oF

177oC

500oF

260oC

500oF

260oC

375oF

190oC

Maximum (short time) operation temperature (dry heat)

.

450oF

232oC

550oF

290oC

550o F

290oC

450oF

232oC

Specific density

.

1.35
2.54
2.3
1.38
Relative moisture regain in % (at 68o F and 65% relative moisture) .

4.5
0
0
0.6
Supports combustion

.

no
no
no
no
Biological resistance (bacteria, mildew)

.

No Effect
No Effect
No Effect
No Effect
* Resistance to alkalies

.

Good
Fair
Excellent
Excellent
* Resistance to mineral acids .

Fair
Very Good
Excellent
Excellent
* Resistance to organic acids

.

Fair+
Very Good
Excellent
Excellent
* Resistance to oxidizing agents

.

Poor
Excellent
Excellent
*
* Resistance to organic solvents

.

Very Good
Very Good
Excellent
Excellent
* At operating temperatures .

*475oF for reverse air & shaker collector
* PPS fiber is attacked by strong oxidizing agents. For example, at 200o F for 7 days
Comments: Based on typical fiber manufacturers published specifications.

Since there are many possible options for some applications, price and availability will also need to be considered before the final filter media material is chosen. Also, there are some different ways of making the filter material optional surface treatments that enhance the properties of the material. The filters can be either natural or manmade and are combined together in various ways as briefly mentioned below:

1.)
Woven or interlacing of fibers is a process to construct a fibrous material which generally consists of the following weaves:

  1. Plain weave is the most basic and consists of the filtering yarn having an over and under construction. By controlling the counts per inch of the interlacing yarn, the weave may be made porous or tight.

  2. Twill weave consists of having the warp yarn pass over two or more filling yarns. Typically, the twill weave has fewer inter lacings than the plain weave; therefore, the twill weave tends to have a greater porosity and is more flexible, and smoother than the other weaves.

  3. Sateen weave has even more distance between the filling yarns than the twill or plain weave which makes it more porous, flexible, and smoother than the other weaves.

2.)
Needled-felt material has the short felt fibers pressed together and mechanically fixed by needle punch machine. The main advantage is the low pressure operation that is coupled with excellent dust collection efficiency and a higher flow rate.

3.)
Singed material is made by a heat process that slightly burns or singes the material surface in order to enhance the surface of the bag material.


Filtration Aid

In some applications where the dust collector contains some moisture, oil and / or very small dust particle sizes, the addition of an inert material or "pre-coat" may be helpful. See Figure 4-3.

Preferably, the pre-coat material is initially applied onto the new, clean surface of the filter media which forms a protective dust cake layer. The benefits are:

  • Aids in dust cake release.
  • Pre-coat material is porous which helps to prevent blinding.
  • Helps to capture the small particles and limits their ability to penetrate the filter media.
  • Increases initial dust collection efficiency to over 99.99 percent.
There are some limitations to the use of pre-coat materials:
  • Frequent applications of pre-coat may be required in order to replenish the protective coating that was partially removed during the pulse cleaning.
  • Recycling dust from the hopper is made more difficult due to the mixing of pre-coat material with product dust.

In making a decision to apply or not apply the pre-coat material, the end user must evaluate his unique application and determine whether the benefits are worthy enough to incur the extra material and labor expense of incorporating the pre-coat material into his system.



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