PARTICLE FILTER SIZE REMOVAL EFFICIENCY
I) Objective Measure the particle removal efficiency vs. particle size for two types of filters or air cleaners: an electrostatic air cleaner and a cartridge-type media filter. II) Background Particulate air filters are used in buildings to remove particles from air streams. Traditionally they were used to reduce the particle concentration in the HVAC system to prolong the life of the equipment and to reduce maintenance costs. Presently, these filters are primarily used to control the particle concentrations in the indoor air for improved indoor air quality. Particles are found in a wide range of sizes. Figure 1 shows typical size ranges in microns for various particles. Those larger than about 10 microns have settling velocities larger than 1 cm/sec. These particles do not remain airborne very long and tend to settle out on the bottom of ductwork and floors of the building. Particles with smaller sizes can remain airborne for extended periods and can be transported within the building along with air currents unless physically removed. Two types of particulate air filters are commonly used in building air handling systems; fibrous media filters and electronic air cleaners. Media filters capture large particles through the mechanisms of inertial impaction and interception when the momentum of the particle causes its path to deviate from the streamline around a fiber. This phenomenon is shown in Figure 2. The small particles have small inertia so they will follow the streamlines. However they also diffuse rapidly toward surfaces with low concentration such as surfaces of fibers where they are captured. Media filters are used in both residential and commercial air handling equipment and are found in a wide variety of forms including replaceable cartridges, bag filters and HEPA (high efficiency particulate air) filters.
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Figure 1: Typical particle size ranges and corresponding settling velocities and diffusion coefficients
Figure 2: Particle collection mechanisms for a fibrous media filter
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Electronic air cleaners capture particles by giving them an electric charge as they pass through an ion field and then capturing them in a series of oppositely charged parallel plates. This arrangement is shown in Figure 3. Large particles are given adequate charge to enable them to be captured very well. Small particles are not charged as efficiently and thus are not forced to the collecting plates as well. Very small particles are collected well because diffusion plays an important role as with fibrous media filters. Although their particle collection mechanisms differ, fibrous media filters and electronic air cleaners have similar collection efficiency vs. particle size performance as shown in Figure 4. The larger particles are captured well because of inertial impaction or good charging efficiency. The smaller ones are also captured well because of diffusion. Particles in the middle size range are not captured well by either mechanism so that the filter collection efficiency exhibits a minimum.
Figure 3: Schematic diagram of a two-stage electronic air cleaner
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Figure 4: Typical particle removal efficiency vs. particle size for fibrous media filters and electronic air cleaners
ASHRAE has developed a standard test method for determining filtration efficiency vs. particle size such as that shown in Figure 4. This is Standard 52.2-2007. In this test method, potassium chloride particles are injected into the air duct upstream of the filter to be tested. A particle counter determines the concentration of particles vs. both size upstream and downstream of the filter under test. The ratio of upstream to downstream concentrations at a given particle size is then used to determine the particle size removal efficiency (PSE) at that particle size. For example, if the upstream concentration of 1-micron particles is Cu and the downstream concentration of 1-micron particles is Cd, the particle size removal efficiency of 1-micron particles is determined as:
C PSE = 1 - d C u where P = III) Procedure
100% = (1 - P ) 100%
Cd is the penetration of particles of that size through the filter. Cu
We will not perform a test according to the ASHRAE standard test method. We will conduct a similar test that can be done in the field, using ambient atmospheric air to provide the particles. 1) Close the indoor air damper and open the outdoor air damper so that all the air entering the air handling unit is outdoor air.
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Baseline 2) Remove the pre-filter screens and collecting plates from the Honeywell unit. Remove the mini-pleat filters from their holder. Remove the pleated filter from the Lennox unit and close the door. 3) Adjust the fan speed so that the average velocity inside the duct near the filters is at the velocity required for isokinetic sampling. You may turn on the reheat coil if the outdoor air is very cold. 4) Measure the static pressure drop between the two aerosol sampling locations. 5) Use the particle counter supplied to measure the particle concentration vs. particle size in the air upstream. Repeat the measurements downstream. These measurements are performed with an isokinetic sampling probe and are taken near the center of the duct. 6) Repeat step 5 twice more (i.e. a total of three upstream/downstream sample pairs). Lennox Unit 7) Insert the pleated filter into the Lennox unit. 8) Repeat steps 3 through 6 with the pleated filter installed. Honeywell Unit 9) Remove the pleated filter from the Lennox unit and insert the collecting plates into the Honeywell unit. 10) Repeat steps 3 through 6 with the Honeywell unit turned ON. Mini-pleat Filters 11) Remove the collecting plates from the Honeywell unit and install the mini-pleat filters....