3.1 Pressure drop features
Pressure drop is an important parameter to evaluate the performance of filters. High pressure drop means high energy consumption, which usually leads to high efficiency. Pressure drop is related to the face velocity, and increases with increasing face velocity linearly. The pressure drop values of the three filter samples are shown in .
Pressure drop as a function of the face velocity for the three filter samples.
Filter A had the lowest pressure drop. In contrast, Filter C had the highest one. Filter B was in the middle and close to Filter C. At the face velocity of 1cm/s, the pressure drop values of Filters A, B and C were 39.2 Pa, 52.3 Pa and 57.8 Pa, respectively. Permeability is related to the pressure drop; higher permeability leads to lower pressure drop. The data in are consistent with the data of filter permeability.
3.2 Efficiency at different face velocities
As air penetrates a filter, the trajectories of particles deviate from the air streamlines due to several mechanisms. As a result, particles may collide with the filter surface and become deposited on them. The important mechanical mechanisms causing particle deposition include diffusion, inertial impaction, interception, and gravitational settling. Because Brownian motion generally increases with decreasing particle size, the diffusive deposition of particles is stronger when the particle size is reduced. Inertial impaction mechanism becomes stronger with increasing particle size and increasing air velocity. Interception and gravitational settling are also related to the particle size. The curve for the total efficiency for all capture mechanisms vs. the particle size takes a typical “v” shape as shown in Brown (1993)
, Hinds (1999)
, Lee and Mukund (2001)
The filtration efficiencies for different particle sizes at different face velocities were measured in the experiments. The efficiency data of Filters A, B and C are shown in , , and , respectively.
Efficiencies of Filter A at different face velocities.
Efficiencies of Filter B at different face velocities.
Efficiencies of Filter C at different face velocities.
As the particle size increases from 10 nm to 300 nm, the efficiency curves demonstrate the typical shape of “v” for all samples. As the face velocity increases from 0.3 to 15 cm/s, the efficiency decreases and the bottom of the v-shaped curve drops. The lowest point of the v-shaped curve is the minimum efficiency and corresponds to the MPPS. At 5.3 cm/s, for Filters A, B and C, the minimum efficiencies are 99.800%, 99.997% and 99.993%, respectively and the MPPS values are 100 nm, 70 nm and 100 nm, respectively. Membrane filters with larger pore sizes allow more penetration of large particles. The data of MPPS are consistent with the pore sizes of the three filter samples. The theoretical model of Lee and Liu (1980)
shows that the MPPS decreases with increasing face velocity. It can be seen from the efficiency curves that the bottom point is moving to the left as the face velocity increases. For Filter A, the MPPS values are 100 nm, 90 nm and 75 nm for the face velocities of 5.3, 10 and 15 cm/s, respectively.
If we compare the efficiencies of the three filter samples, Filter A has the lowest one, Filter B and Filter C have almost the same values, with Filter B slightly higher. This can be seen from , in which the efficiencies of the three filter samples at the face velocity of 5.3cm/s are compared.
Comparison of the efficiencies of samples at 5.3 cm/s.
The quality factor (QF) is a parameter to evaluate the filter performance, which is defined as:
where P is the penetration and ΔP is the pressure drop.
The quality factor changes with the particle size and face velocity. The quality factor curves of the three filter samples at the face velocity of 5.3 cm/s are shown in . Generally, Filter B has higher QF compared to the others. For 50 nm particles, Filter B and Filter C have the same QF; for 100 nm particles, filter B has the highest QF and Filter A has the lowest one; for 300 nm particles, Filter C has the lowest QF.
The quality factor of the three filter samples for different particle sizes at 5.3 cm/s.
3.3 Efficiency for different particle size
The collection efficiency is related to various filtration mechanisms, which depend on the particle size. The data show that for small particles (such as 10 or 30 nm) and large particles (such as 200 or 300 nm), the filtration efficiencies are higher than for the intermediate sizes (such as 50 and 100 nm). An increase in the particle size causes increased filtration by interception and inertial impaction mechanisms, whereas a decrease in the particle size enhances collection by Brownian diffusion. As a consequence, there is an intermediate particle size region where two or more mechanisms are simultaneously operating, yet none is dominant. This is the region where the particle penetration through the filter is a maximum and the filter efficiency a minimum. All filters have specific particle sizes for which the efficiencies are the lowest and the efficiency decreases sharply. The MPPS values for the three samples are between 50 and 100 nm. At low velocities, the efficiency decreases slowly with increasing velocity; but at high velocities, the efficiency decreases sharply.
3.4 Efficiency surface analysis
In order to better describe the filtration efficiency as a function of both the particle size and face velocity, a three-dimensional graph of efficiency surface with the particle size and face velocity is generated, as shown in , and for Filters A, B and C, respectively.
The efficiency surface for Filter A.
The efficiency surface for Filter B.
The efficiency surface for Filter C.
The graphs for the three samples are similar, which indicates that all the three membrane filter samples possess similar characteristics. The shape of the efficiency graph is like a half funnel. The efficiency is relatively high for small and large particles at all face velocities; it is also relatively high at very low face velocities for the particle size range in our study. As the face velocity increases, the efficiency for intermediate particle sizes (50 – 100 nm) is becoming significantly smaller than those for smaller and larger particles. Thus a trough region is formed on the efficiency surface and it becomes deeper as the face velocity increases. The trough region represents the MPPS for different face velocities. The three-dimensional efficiency surface gives a summary of the data and shows the MPPS intuitively.