Filtration

Fist and foremost ALUPOR™ is a filtering and coalescing material of volumetric filtration. The unique structure of porous aluminum consists of cavities connected by micron-sized necks. Impurities are trapped in the stagnant zones of these cavities by inertial separation via abrupt flow direction chenges.
The contaminant capacity of the material is 8 times higher than of sintered powder filters due to high volumetric porosity (50-75%). Filtration degree can be chosen from 1 to 200 microns.

The long service life and the possibility of regeneration makes ALUPOR™ the most promising filtering material.

nominal filtration degree (β_x = 20) up to 1 micron
fully controlled and stable structure parameters
high contaminant capacity: 8-10 times exceeds mesh and sintered powder filters
high strength: maximum pressure differential up to 25 bar
custom sizes and configurations
frame-less design
parts with combination of solid and porous sections in a single product
thread cutting on both solid metal and porous sections
service life exceeding 5 years
operating temperature up to 250 °C
possibility of frame-less combination of PLA with filtration membrane, mesh, or paper
environmentally friendly — material recycles as aluminum scrap

Gases only. Maximum efficiency of filtration for inertial mechanism of deposition will be ensured by:
- high density ratio of fluid and contaminant
- non-Darsyan regime of flow for more steep stream direction changes (see video above)
Complex filtration mechanism. Filtration fineness depends on the filtration velocity and the density ratio of the contaminant and the fluid. Individual research needed in every single case
Preliminary filtration of coarse fractions needed. The particle size distribution of the contaminant must be smaller than the size of the neck between pores. Otherwise, the pores on the filter element surface will become clogged, and it will stop working volumetrically. It will function like a conventional mesh element
Regeneration challenges: regular backflushing is required to remove the surface layer of captured filtrate
Not suitable for use in acids and alkalis
Freezing possibility of filter element. When operating at sub-zero temperatures — heating is required.

Parameter	Material type (graded by main pore size range), μm
Parameter	140-315	200-400	315-630	630-1000	1000-1600	1600-3000
Darcy permeability***, m²	4,5×10^-12	5.8×10^-12	8.9×10^-12	10.8×10^-11	17.7×10^-11	26.0×10^-11
Forhheimer permeability***, m	3.3×10^-5	3.5×10^-5	3.6×10^-5	3.8×10^-5	4.2×10^-5	19.0×10^-5
Nominal filtration degree*, μm	10	25	50	75	100	200
Beta ratio β_x (filtration efficiency, %)	20 (95)
Contaminant capacity**, %	34	36	42	42	47	49
Filtration principle	inertial deposition of contaminants in tortuous channels of the material
Regeneration ability	YES
Regeneration methods	reversed impulse blow, impulse washing in solvents, ultra sonic treatment, annealing at 530 C° with following washing
Filtering mediums	Air, natural gas(liq./gas), carbon oxide, carbon dioxide(liq./gas), oxygen (liq./gas), water, liquid carbohydrates

Parameter	Material type (graded by main pore size range), μm
Parameter	140-315	200-400	315-630	630-1000	1000-1600	1600-3000
Mean pore size, μm	245	300	490	700	1300	2300
Mean neck size, μm	67…77	73…88	100…120	135…170	175…225	245…315
Neck per one pore (coordination number)	6.5	6.5	6.5	6.5	6.5	6.5
Pores per unit area, pcs./ m²	11.6×10⁶	6.4×10⁶	2.4×10⁶	1.2×10⁶	0.34×10⁶	0.11×10⁶
Necks per unit area, pcs./ m²	36.4×10⁶	20.9×10⁶	7.9×10⁶	3.9×10⁶	1.1×10⁶	0.36×10⁶
Porosity, %	50...75
Density***, kg/m³	675...1215
Thermal expansion coefficient, 1/°C	23.0×10^-6 (identical to solid metal)
Allowed temperature range, °C	-200...+250

More details on structure of ALUPOR™ see "Structure" page

Parameter	Material type (graded by main pore size range), μm
Parameter	140-315	200-400	315-630	630-1000	1000-1600	1600-3000
Ultimate compressive strength, MPa	105	59	58	49	37	32
Yield point at compression, MPa	39	26	24	20	21	20
Ultimate tensile strength, MPa	29	16	16.5	14	12	8
Yield point at tensile, MPa	26	14	16	13	13	7
Ultimate shear stress, MPa	36	34	30	25	26	-
Young's modulus, GPa	3.1	2	1.9	1.7	1.7	1.6
Elongation, %	0.29	0.38	0.27	0.32	0.3	0.12

More details on mechanical properties of ALUPOR™ see "Properties" page.

There is an option for partial restoration of the flow characteristic of contaminated filter elements.
On the operating site, this can be done by:

Backflushing directly in the housing (if technically possible)
Backflushing with superheated steam directly in the housing (if technically possible)
Washing in a solvent with filter removal from the housing

ALUPOR™ is able to be treated with 530°C annealing without mechanical loads and ultrasonic and pulse treatment. But this requires special equipment. We offer our services for restoring the flow capacity of your filter elements using special equipment:

Pressure differential at fixed flow rate test on contaminated element
Thermal annealing at 530°C. This removes organic contaminants.
Backflushing in clamping fixture with superheated steam
Washing in clamping fixture with water
Ultrasonic/pulse treatment in special unit
Pressure differential at fixed flow rate test on regenerated element

The inertial filtration mechanism is based on the principle when flow direction in pores changes abruptly, but contaminant particles continue to move by inertia separating from the main flow. Contaminant deposits on the pore walls in the stagnant zones. This is the primary mechanism for capturing contaminants in ALUPOR™.
To achieve this effect, filtration must occur in transitional or turbulent mode. In laminar flow (Darcy regime), the flow core does not reach the pore walls and particles move with the flow: the effect of inertial deposition is weaker. We ensure the maximum difference between pore size and neck size to increase the size of stagnant zones in the pores during casting process by applying maximum impregnation vacuum.

Symbols definitions

Constants

R=8,314 462 618 153 24 [J/(mol×K)] - gas constant;

Physical properties

c [J/(kg×K)] - specific heat capacity;
λ [W/(m×K)] - heat conductivity;
μ [Pa×s ] - dynamic viscosity;
ρ [kg/m³] - density;
σ [Sm/m] - electrical conductivity.
M [kg/mol] - gas molar mass;

Physical values

F_filt [m²] - filtration surface;
j [kg/(m²×s)] - specific mass flux;
Q [J] - heat quantity;
T [K] - absolute temperature;
t [°C] - temperature;
V [m³] - volume;
v [m/s] - velocity;
v_filt [m/s] filtration velocity;
τ [s] - time.

ALUPOR™ structure parameters

Π [dimensionless] - porosity;
S_min [dimensionless] - ratio of minimal solid cross-section and filtration area (same as minimal void cross-section of packed bed);
d_p [μm] - mean pore size;
S_sp [m²/m³] - specific surface area;
k_v [m²] - Darcy permeability;
k_i [m] - Forhheimer permeability.

Subscripts

#_f value related to fluid ;
#_s value related to solid (metal matrix);
#_p value related to pore volume(example: velocity in pores);
#_n value related to necks between pores (example: velocity in necks);
#_Me value related to solid metal;
#_AP value related to ALUPOR™;
#_eff effective property of composite (example:"ALUPOR™-air" system);

Hydrodynamics in porous media

Filtration velocity is a physical quantity that characterize the movement of liquid or gas through a porous medium. It is artificial value: filtration velocity is defined as the ratio of fluid flow rate to the filtration area of filter element. Mathematically this is expressed by the formula:

v_{f i l t} = \frac{Q}{F_{f i l t}}

Velocity at pores. It is important to distinguish between filtration velocity and the actual velocity of fluid in the pores. The velocity in pores is determined through the porosity:

v_{p} = \frac{v_{f i l t}}{Π}

Velocity in necks. In the case of AUPOR™ we can introduce the concept of velocity in the necks between pores. This is determined by dividing the filtration velocity by the clearance of the necks.

v_{n} = \frac{v_{f i l t}}{ψ_{n}}

Clearance of the necks ψ_n is defined as:

ψ_{n} = n_{n} F_{n} = 0.5 N n_{p} F_{n} = 0.125 N n_{p} π {d_{n}}^{2}

N [pcs] - coordination number: necks per one pore ratio;
n_p [pcs/m²] - specific quantity of pores on surface of ALUPOR™;
n_n [pcs/m²] - specific quantity of necks on surface of ALUPOR™. n_n= 0.5Nn_p;
F_n [m²] - neck cross-section;
For more details on structure of ALUPOR™ please refer to "Structure" page

Reynolds number
Concerning specifics of ALUPOR™ structure we are implementing two variants of Reynolds number:

Reynolds number at pores: $R e_{p} = \frac{ρ_{f} v_{p} d_{p}}{μ_{f}}$
Reynolds number at necks: $R e_{n} = \frac{ρ_{f} v_{n} d_{n}}{μ_{f}}$

Unlike flows in pipes, the transition from laminar to turbulent filtration in porous material occurs smoothly. The critical Reynolds number value is taken when the dependence of the hydraulic resistance coefficient ξ on velocity deviates from linear by 5%.

In the literature on sintered porous metals, the range is given as Re_crit=4.5...10 (calculated for the narrowest section of the sintered material). At the same time, lower values refer to materials with a larger difference between the narrowest and widest sections of the pore channel. As a first approximation for AUPOR™, it can be taken as Re_{n, crit}=4.5, since the difference between pore size and neck size in ALUPOR™ can reach significant values and is always greater than that of sintered materials.

Specific mass flux
Specific mass flux is the mass of substance passing through a unit area of material per unit time. It is measured in kilograms per second per square meter [kg/(s·m²)].
This parameter is particularly convenient for calculating gas filtration processes, becouse it takes into account both the density of the medium and its velocity through the porous material:

j = ρ_{f} v_{f}

Darcy-Forhheimer law
Since our material mostly should works at transition regime we prefer to use quadratic porous flow ralation - Darcy-Forhheimer law. It is universal and takes into consideration both viscous and inertial aspects of porous flow.
We have for liquid (using mass flux j):

\frac{p_{i n} - p_{o u t}}{l ρ_{f}} = \frac{μ}{k_{v}} j + \frac{1}{k_{i}} j^{2}

For gas we modify above formula with avereged gas density (

\bar{ρ} = 0.5 (p_{i n} + p_{o u t}) M / R T

\frac{({p_{i n}}^{2} - {p_{o u t}}^{2}) M}{2 R T l} = \frac{μ}{k_{v}} j + \frac{1}{k_{i}} j^{2}

here in above relations l [m] is filter element wall thickness.

Hydraulic loss coefficient
To determine pressure losses the following formula is also used.

\frac{p_{i n} - p_{o u t}}{l} = ξ_{p} \frac{ρ_{f} {v_{p}}^{2}}{2 d_{p}}

Hydraulic loss coefficient ξ_p has to be determined experimentally from "flow-pressure drop" data in following form:

ξ_{p} = C (1 + b R {e_{p}}^{n} Π^{m}) / R e_{p}

here
С, b, n, m - empiric coefficients.
Experimental values of ξ_p are obtained by treating "flow-pressure drop" data using following relation:

ξ_{p} = \frac{2 (p_{i n} - p_{o u t}) d_{p}}{ρ_{f} {v_{p}}^{2} l}

In practice, hydraulic loss coefficient is more convenient way to compare hydraulic of porous materials

Comparing the hydraulic properties of porous materials based on the values of coefficient ξ_p at identical values of the Reynolds number Re_p
Predicting the hydraulic properties of new types of porous materials based on their porosity, average pore sizes, and porous structure type

The accuracy of such predictions is relatively low (up to 30-40%), but it is quite sufficient at the stage of developing new types of porous permeable materials. See below an example of such correlation for sintered bronze:

ξ_{p} = \frac{152}{R e_{p}} (1 + 5.56 \cdot 10^{- 3} Π^{- 1.72} R {e_{p}}^{0.9}) R e_{p} = 0.085 . . . 2300

Filtration parameters

Filtration degree
Absolute and nominal filtration degree are most common characteristics of filter and filter element performance. Usually absolute and nominal filtration degree means contaminant particle size with 0.999 and 0.95 filtration efficiency correspondently. For instance, ALUPOR 350-630 is arresting 95% of particles with 25 μm or grater size, hence it has 25 μm nominal filtration degree .

Bata ratio β_x (ISO 16889:2022)

β_{x} = \frac{N_{1}}{N_{2}}

where x — subscript, standing for contaminant particle size ranging from x and larger; N₁ — particle count from filter upstream of x size and larger; N₂ — particle count from filter downstream of x size and larger.

Fractional efficiency E_x (EN 779:2012)

E_{x} = \frac{N_{1} - N_{2}}{N_{1}} = 1 - \frac{1}{β_{x}}

Penetration coefficient P_M

P_{M} = \frac{N_{2}}{N_{1}}

Penetration coefficient of a filter element equals the percentage ratio of particle concentration after the filter N₂ to the particle concentration before the filter N₁.

Contaminant capacity C
Contaminant capacity is the mass of artificially introduced contaminant of a specified particle size distribution retained by a clean filter element during the time until the maximum pressure differential is reached at nominal fluid flow rate.
For ALUPOR™, it is possible to quantitatively estimate the contaminant capacity by knowing the average pore size and neck size, as well as the specific gravity of the contaminant. Let’s represent a pore as a sphere with a diameter equal to the average pore size. The average number of necks per pore (coordination number N) is determined by:

N = \frac{Π_{0} + 3 + \sqrt{{Π_{0}}^{2} - 9 Π_{0} + 10}}{Π_{0}}

where Π₀ is porosity Π₀=1-Π

Let’s consider the simulation: each neck operates as a nozzle in the Darcy-Forchheimer regime. Turbulent flow "tubes" are formed by each neck and they are connecting in the central mixing zone. The areas between these tubes are stagnation zones where contaminants will accumulate.

To estimate the contaminant capacity of a single pore, it is necessary to subtract the volume of flow tubes and mixing zone from the total pore volume. Let’s assume that the mixing zone is a sphere with a diameter 2d_n and stream "tubes" are the truncated cones with base faces of d_n and 2d_n diameters. Lets go ahead and compute stagnation zone volume:

C_{v o l} = Π [δ^{3} (3, 5 N - 8) - 1, 75 N δ^{2} + 1]

where
δ=d_n/D_p;
C_vol [dimensionless] volumetric contaminant capacity (see table above).
Finally, we can estimate contaminant capacity C [g] if bulk density of contaminant is known:

C = 1000 \cdot V_{F E} C_{v o l} γ_{c n t}

V_FE [m³] filter element volume;
γ_cnt [kg/m³] bulk density of contaminant .

Fluid flow measurement

ISO 5167-1:2022 Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full - Part 1: General principles and requirements

ISO 5167-2:2022 Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full - Part 2: Orifice plates

ISO 5167-3:2022 Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full - Part 3: Nozzles and Venturi nozzles

ISO 5167-4:2022. Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full - Part 4: Venturi tubes

ISO 5167-6:2022 Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full - Part 6: Wedge meters

Compressed air quality standards

ISO 8573-1:2010 Compressed air - Part 1: Contaminants and purity classes

ISO 8573-2:1996 Compressed air - Part 2: Test methods for aerosol oil content

ISO 8573-3:1999 Compressed air - Part 3: Test methods for measurement of humidity

ISO 8573-4:2019 Compressed air - Part 4: Test methods for solid particle content

ISO 8573-5:2025 Compressed air - Part 5: Test methods for oil vapour and organic solvent content

ISO 8573-6:2003 Compressed air - Part 6: Test methods for gaseous contaminant content

ISO 8573-7:2003 Compressed air - Part 7: Test method for viable microbiological contaminant content

ISO 8573-8:2004 Compressed air - Part 8: Test methods for solid particle content by mass concentration

ISO 8573-9:2004 Compressed air - Part 9: Test methods for liquid water content

Filter classification and control methods

ISO 5782-1:2017 Pneumatic fluid power - Compressed air filters. Part 1 : main characteristics to be included in supplier's literature and product-marking requirements

ISO 5782-2:1997 Pneumatic fluid power - Compressed-air filters. Part 2: Test methods to determine the main characteristics to be included in supplier's literature

ISO 16890-1:2016 Air filters for general ventilation - Part 1: Technical specifications, requirements and classification system based upon particulate matter efficiency (ePM)

ISO 16890-2:2022 Air filters for general ventilation - Part 2: Measurement of fractional efficiency and air flow resistance

ISO 16890-3:2016 Air filters for general ventilation - Part 3. Determination of the gravimetric efficiency and the air flow resistance versus the mass of test dust captured

EN 779:2012 (deprecated) Particulate air filters for general ventilation - Determination of the filtration performance

EN 1822-1 High efficiency air filters ЕРА. HEPA and ULPA - Part 1. Classification, performance testing, marking

EN 1822-2 High efficiency air filters (ЕРА, HEPA and ULPA) - Part 2: Aerosol production, measuring equipment, particle counting statistics

EN 1822-3 High efficiency air filters (ЕРА, HEPA and ULPA) - Part 3: Testing flat sheet filter media

EN 1822-4 High efficiency air filters (ЕРА, HEPA and ULPA) - Part 4: Determining leakage of filter elements (scan method)

EN 1822-5 High efficiency air filters ЕРА, HEPA and ULPA - Part 5. Determining the efficiency of filter elements

ISO 12500-1:2007 Filters for compressed air - Test methods. Part 1: Oil aerosols

ISO 12500-2:2007 Filters for compressed air - Test methods. Part 2. Oil vapours

ISO 12500-3:2009 Filters for compressed air - Test methods. Part 3: Particulates

ISO 12500-4:2009 Filters for compressed air - Methods of test. Part 4: Water

ISO 16889:2022 Hydraulic fluid power - Filters - Multi-pass method for evaluating filtration performance of a filter element

ISO 3968:2001 Hydraulic fluid power - Filters — Evaluation of differential pressure versus flow characteristics

Contaminant size distribution

ISO 21501-1:2009 Determination of particle size distribution — Single particle light interaction methods - Part 1: Light scattering aerosol spectrometer

ISO 21501-2:2019 Determination of particle size distribution — Single particle light interaction methods - Part 2: Light scattering liquid-borne particle counter

ISO 21501-3:2019 Determination of particle size distribution — Single particle light interaction methods - Part 3: Light extinction liquid-borne particle counter

ISO 21501-4:2018 Determination of particle size distribution — Single particle light interaction methods - Part 4: Light scattering airborne particle counter for clean spaces

FILTRATION

Advantages

Limitations

SPECIFICATIONS

MANUFACTURE ABILITIES

Regeneration

Theory reference

Filtration standards reference list