Designing Filters for Optimum Performance T.J. Ptak and Chrystal Gillilan Presented at National Air Filtration Association, TECH 2011

Filter performance criteria Scope Introduction Filter performance criteria Is it possible to select filter based several performance characteristics? Filter pressure drop Impact of filter media area on filter performance Validation of filter performance Conclusions

Filter selection and optimization Consumers and filter designer Traditional selection process of filters: Consumer Performance Perceive value Cost Filter designer Based on experience Lacks an objective basis for decision making process

Development of rational and objective basis Filter Selection Development of rational and objective basis Single criterion Deduced from filter performance characteristics Example - CADR Benefits of single criterion Comparison and selection More filtration parameters included Design tool

Filter Performance Characteristics Pressure drop Filtration efficiency Gravimetric (arrestance) Fractional efficiency Capacity (filter life) Dust holding capacity Filter durability tests Flammability tests Compliance to environmental regulations Cost

Filter Performance – Quality Factor Is it possible to combine filter performance characteristics into one criterion? Greater value of the criterion equal to “better” filter Traditional quality factor (figure of merit): E – filter efficiency and ΔPo – filter pressure drop Potential issues: Different ΔPo units Results at different flow rate

Filter Performance - Criteria Example 1: E1 = 80% and ΔPo = 0.40” H2O α1 = - ln (1 – 0.8)/ 0.40 = 1.609/0.40 = 4.02 Example 2: E1 = 80% and ΔPo = 0.30” H2O α2 = - ln (1 – 0.8)/ 0.30 = 1.609/0.30 = 5.36 Example 3: E1 = 85% and ΔPo = 0.40” H2O α2 = - ln (1 – 0.85)/ 0.40 = 1.897/0.40 = 4.74

Filter Performance – Multi-Criteria Process of simultaneously optimizing two or more objectives Single aggregate objective function (AOF) Weighting method, linear AOF: MAX E – filter efficiency; ΔPo – filter pressure drop C – filter cost; DHC – filter dust holding capacity λ – weighting factors; ∑ λi = λ1 + λ2 + λ3 + λ4 =1 λ – weighting factors are assigned based on priority - subjective

Filter Performance – Multi-Criteria Residential filters – Home Depot FPR (Filter Performance Rating) – AOF FPR = λ1 (Efficiency Large Particles) + λ2 (Efficiency Small Particles) + λ3 DHC λ1 + λ2 + λ3 = 0.6 + 0.3 + 0.1 =1 Different weighting factors = different values of FPR Example – DHC priority λ1 + λ2 + λ3 = 0.4 + 0.3 + 0.3 =1

Filter Performance – Multi - Criteria Combination of objectives: Other objectives: Cost, Energy Nonlinear criteria Mathematically more complicated Criteria as designer tool Optimum filter design can be obtained from given AOF criterion if functions describing filter characteristics are known There is no single solution but a set of Pareto points

Filter performance criteria Filter pressure drop Scope Introduction Filter performance criteria Filter pressure drop Does filter pressure drop follow Darcy’s law?

Filter Pressure Drop - Basic Concept Filtration theory – filtration models Efficiency Single fiber efficiency Filter efficiency Pressure drop Darcy law Fiber drag

Filter Pressure Drop - Scope of Filtration Model Characteristics pressure drop, efficiency, filter life, cost, durability basis weight, thickness, permeability, pore size fiber or granule diameter

Filter Pressure Drop – Filtration Models Capillary model Kozeny - Carman Fiber models Single fiber Cell model (Happel, Kuwabara) Fan model (Stechkina) Cylinder array

Filter Pressure Drop – Darcy’s Law Experimental flow of water through bed of sand DP = Uo L / k k - Darcy law constant, permeability coefficient Uo- velocity;  - viscosity; L - media thickness

Filter Pressure Drop – Kozeny - Carman Need to correlate permeability coefficient with parameters of porous media (1927): Model = pores are represented by bundle of capillary tubes Poiseuille’s equation for laminar flow in tube ΔP = Uo µ L / k where: k = ε3/5So5(1 – ε)2 ε – porosity; So – specific surface of particles Limitations Deviation from spherical shape Shapes of pores in real media

Filter Pressure Drop – Fibrous Media Flow through fibrous media: Modification of Kozeny equation Davies (1952) – experimental correlation ΔP = Uo µ L /k where: k = D2/ 64 (1- ε)1.5{1 + 56(1 – ε)3} ε – porosity; D – fiber diameter Limitations 0.6 < ε < 1 Reynolds number, Re = ρ U D / µ < 1

Filter Pressure Drop – Fibrous Media Theoretical calculations: Single fiber theory – widely used in calculations Cell models – include porosity of media ΔP = 16Uo µ L(1- ε) / D2ξ ξ - hydrodynamic factor: Lamb - ln Re + 2.0002 Kuwabara - 0.5 ln (1-ε) - 0.75 Happel - 0.5 ln (1-ε) - 0.50 ε – porosity; D – fiber diameter; Re – Reynolds number Linear relationship between ΔP, U, L and µ

Filter Pressure Drop – Filter Media and Filters Filter media – Fibers + Binders: Glass fiber media pleatable - fiber diameter 3-7 µm wire backed – fiber diameter 1- 4 µm Synthetic media coarse – fiber diameter 18 - 22 µm fine – fiber diameter 2 - 8 µm

Filter Pressure Drop – Filter Media and Filters Experiment: Filters: Dimensions 20 x 20 in. Depth 1; 2 and 4 in. Type minipleats with 4; 6; 9 and 12 PPI wire back with 18 to 45 pleats Testing Initial pressure drop and efficiency Dust holding capacity

Filter Pressure Drop – Filter Media Experimental results for various media High velocity application, MERV 7- 8 Linear function of velocity ΔP = AV

Filter Pressure Drop – Filter Media Experimental results for various media Low velocity application, MERV 10-14 Linear function of velocity ΔP = AV

Filter Pressure Drop – Filters Pressure drop for 20 x 20 in. wire backed filters Filter media – coarse synthetic fibers Polynomial curve ΔP = BV + CV2

Filter Pressure Drop – Filters Pressure drop for 20 x 20 in. minipleat filters Filter media – glass and synthetic fine fibers Polynomial curve ΔP = BV + CV2

Filter Pressure Drop – Filters and Filter Media Pressure drop for filters and flat sheet media Filters with 55 ft2 of media area; minipleat Filter media – glass and synthetic fine fibers ΔPFILTER/ ΔPMEDIA ~ 1.6 – 4 at 25 fpm

Filter Pressure Drop – Filters and Filter Media Pressure drop for filters and flat sheet media Filters with 36 ft2 of media area; minipleat Filter media – glass and synthetic fine fibers ΔPFILTER/ ΔPMEDIA = 1.3 – 2.2 at 25 fpm

Filter Pressure Drop – Filters and Filter Media Pressure drop for filters and flat sheet media Filters with 16.2 ft2 of media area; wire backed Filter media – synthetic coarse fibers ΔPFILTER/ ΔPMEDIA = 4 – 5 at 75 fpm

Filter Pressure Drop – Filters Correlation between flat sheet media and filters Flat filter media ΔP = AV Flow is perpendicular o the media Filters ΔP = BV + CV2 Filter media reduced due to pleat deformation and glue beads resulting in higher media velocity Impact of frame on media area and flow pattern Inertial loses due to change of flow direction Flow through media not perpendicular

Filter Pressure Drop – Filters and Filter Media Flow pattern through pleats is not perpendicular to the filter medium Real flow pattern Assumed perpendicular flow

Filter Pressure Drop – Filters and Filter Media Pleat collapsing and pleat deformation Collapsing Deformation

Filter performance criteria Filter pressure drop Scope Introduction Filter performance criteria Filter pressure drop Impact of filter media area on filter performance Is filter media fully utilized?

Is filter with more media area a “better” filter? YES and NO Filter Media Area Is filter with more media area a “better” filter? YES and NO Impact of pleat density Impact of type of filter media Impact of filter design Media utilization factor, UF UF = Dust Capacity / Media Area Comparison of filters with similar efficiency

Filter Media Area - Impact on Pressure Drop Tested filters MERV 7- 8 - 20 x 20 x 2” wire backed Pressure drop and initial E3 efficiency

Filter Media Area - Impact on Pressure Drop Tested filters 20 x 20 x 2” – glass and synthetic minipleat Pressure drop at 492 fpm

Filter Media Area - Impact on Dust Holding Capacity Tested filters MERV 10 - 14 - 20 x 20 x 2” minipleat Terminal ΔP = 1” and ΔP = 1.4” H2O Glass media with similar thickness and stiffness

Filter Media Area - Media Utilization Tested filters MERV 14 - 20 x 20 x 2” minipleat Media area = 109 ft2 Media area = 55 ft2

Filter Media Area - Media Utilization Tested filters MERV 10 - 20 x 20 x 2” minipleat Media area = 109 ft2 Media area = 55 ft2

Filter Media Area - Impact of Filter Depth Tested filters MERV 7 – 8 wire backed with N = 18 Filter 1 in. 2 in. 4 in. DHC at ΔP = 1 in. H2O 9.2 27.8 37.9 Media utilization, UF 2.14 3.27 2.08

Filter Media Area - Impact of Filter Depth Tested filters MERV 7 – 8 wire backed with N = 18 4 in. filter, UF = 2.08 2 in. filter, UF = 3.27

Filter Media Area - Media Area Utilization Pleat collapsing – limited flow and dust collection Collapsing

Filter Media Area - Optimum Design Tested filters MERV 14 - 20 x 20 x 2” minipleat Pressure drop, Dust capacity, E1 and UF

Filter Media Area - Optimum Design Tested filters MERV 14 - 20 x 20 x 2” minipleat Optimum pleat density (media area) Is it minimum ΔP or maximum DHC? Cost? Impact of other design parameters such as media type Complicated issue

Filter Media Area - Impact of Type of Filter Media Tested filters MERV 14 - 20 x 20 x 2” minipleat Filter media Media area DHC E1 UF Glass fiber 55 ft2 33 79 0.6 Duo layer synthetic 55 ft2 104 83 1.9

Filter Media Area - Impact of Type of Filter Comparison of different filters – 24 x 24” Filter MERV Media Area DHC UF V- cell (3V) 14 Glass 140 ft2 131 g 0.9 V- cell (3V) 15 Synthetic 80 ft2 226 g 2.8 V- cell (2V) 14 Glass 100 ft2 Pocket – 26” 15 Synthetic 69 ft2 173 g 2.5 Pocket – 19” 14 Synthetic 51 ft2 92 g 1.8 Rigid – 12” 15 Synthetic 61 ft2 181 g 3.0 4” Minipleat 15 Synthetic 70 ft2 123 g 1.8

Filter performance criteria Filter pressure drop Scope Introduction Filter performance criteria Filter pressure drop Impact of filter media area on filter performance Validation of filter performance What is variability of test results?

Purpose of filter testing Filter selection Filter comparison Filter must be tested in strict accordance to an acceptable test method Rating Prediction real life performance Filter characteristics must reflect main filter goal

Measurement of Typical Physical Quantities Flow rate Flow sensor – orifice, nozzle, laminar flow element Temperature (density, viscosity) Differential pressure Pressure sensor Mass of filter, membrane Particle concentration Optical particle counter

HVAC Filters – Performance Characteristics Test standard – ASHRAE 52.2 Differential pressure Fractional efficiency for particle size range 0.3 – 10 µm Initial and intermediate Dust holding capacity at specified terminal ΔP Reporting PSE curve after each step of dust loading Develop a composite minimum efficiency curve Report average E1 0.3 – 1.0 µm E2 1.0 – 3.0 µm E3 3.0 - 10 µm MERV - Minimum Efficiency Reporting Value

Round Robin Test sponsored by ASHRAE* ASHRAE 52.2 - Variability Round Robin Test sponsored by ASHRAE* Participating labs 8 Selected filters 4 Type 1 – 24 x 24 x 12” glass (MERV 10-11) Type 2 – 24 x 24 x 4” electret (MERV 8-11) Type 3 – 24 x 24 x 4” cotton/PET (MERV 5-7) Type 4 – 24 x 24 x 12” electret (MERV 15-16) Filters were pre-tested at independent lab Statistical methodology Repeatability – precision of the method within a given lab Sr – repeatability standard deviation Reproducibility - precision of the method when comparing labs SR – reproducibility standard deviation NOTE: * from ASHRAE 1088-RP

Round Robin Test results ASHRAE 52.2 - Variability Round Robin Test results Precision statistics for pressure drop measurement Repeatability CV 1.8 – 6.7% Reproducibility CV 5.3 – 6.8% Precision statistics for weight gain Repeatability CV 7.5 – 26.5% Reproducibility CV 8.1 – 26.5% MERV summary Type 2 MERV 8 (9); MERV 10 (1); MERV11 (2) Type 3 MERV 5 (1); MERV 6 (10); MERV 7 (1) Type 4 MERV 14 (4); MERV 15 (8)

Round Robin Test results ASHRAE 52.2 - Variability Round Robin Test results Precision statistics for Type 4 filters Repeatability Sr E1 – 1.71; E2 – 0.50; E3 – 0.0 Reproducibility SR E1 – 2.63; E2 – 0.93; E3 – 0.41 Precision statistics for Type 3 filters Repeatability Sr E1 – 1.91; E2 – 2.42; E3 – 3.94 Reproducibility SR E1 – 2.61; E2 – 3.88; E3 – 4.98 Precision statistics for Type 2 filters Repeatability Sr E1 – 2.86; E2 – 4.99; E3 – 2.45 Reproducibility SR E1 – 5.00; E2 – 6.05; E3 – 3.04 Calculated 95% confidence interval for individual results “2 Sr “ = 1.96 Sr “2 SR ” = 1.96 SR

What does it mean for you? ASHRAE 52.2 - Variability What does it mean for you? Pressure drop Filter Type 2 Type 3 Type 4 Average ΔP, [in. H2O] 0.34 0.28 0.76 95% Confidence, 2 SR ±0.05 ±0.03 ±0.11 95% Confidence range 0.29 – 0.39 0.25 – 0.31 0.65 – 0.87 Dust holding capacity Filter Type 2 Type 3 Type 4 Average DHC, [gram] 197 248 138 95% Confidence, 2 SR ±32 ±44 ±72 95% Confidence range 165 – 229 204 – 292 66 – 210

What does it mean for you? ASHRAE 52.2 - Variability What does it mean for you? Efficiency – statistics at indicated level of filtration Filter 20% 50% 80% 95% Confidence, 2 SR 8.1 10.6 6.7 95% Confidence range 12 – 28 39 – 61 73 – 87 Is it as bad as Interlaboratory Testing indicate? Report was published in 2005 Progress was made due to awareness Significant variability still exist

Environmental testing Other Tests Environmental testing Impact of environmental conditions on real life operation, integrity and safety (extreme temperature and humidity) To reveal potential problems area in the filter Durability and mechanical strength ARI – Standard 850 Performance and Rating of Commercial and Industrial Air Filter Equipment Breaching Test – at resistance 50% above the maximum rated Laboratory tests Exposure to high temperature (T = 160oF) Exposure to low temperature (T = 0o F) Exposure to high relative humidity (RH = 85 – 90%) Breaching test at resistance above 400% above rated

Environmental requirements Other Tests Environmental requirements RoHS Directive 2002/95/EC Restriction of the use of certain Hazardous Substances Heavy metals such as: lead, chromium, mercury and cadmium Polybrominated biphenyls and diphenyl ethers Proposition 65 Safe Harbor Levels (known as CA Prop 65) List of chemicals known to State to cause cancer or reproductive toxicity REACH – Regulation for Registration, Evaluation, Authorization and Restriction of Chemicals Manufacturers must identify and manage risks linked to substances they manufacture and market

Pressure drop of filters does not follow Darcy’s law Conclusions Aggregate objective functions (AOF) can be used as filter selection criteria Pressure drop of filters does not follow Darcy’s law Optimal utilization of filter media should be a main goal of filter designers Higher media area does not always result in a “better” filter Current test methods need modifications to address variability issue and filter durability