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Filtration in Detail

Filtration is essential for removing contaminants from compressed air and protecting equipment and products.

The Contamination Problem​

Contaminants in Compressed Air​

  1. Water - Moisture condensed during compression
  2. Oil - From lubricated compressors
  3. Particles - Dust, rust, pipe scale
  4. Microorganisms - Bacteria, mold

The Compressor Multiplies Contaminants​

Important

Liquids and solids cannot be compressed - they multiply in concentration.

Compression Ratio Example:

CR=VsuctionVtank=10 m30.943 m3=10.6CR = \frac{V_{suction}}{V_{tank}} = \frac{10 \text{ m}^3}{0.943 \text{ m}^3} = 10.6

Contaminants are now 10x more concentrated!

Filter Types​

1. Particulate Filters​

  • Air flows from outside to inside the element
  • Remove solid particles
  • Efficiency: 3+ microns
  • Pressure drop: ~0.25 PSI (dry)
  • Material: Pleated cellulose

Applications: First line of defense, coarse particle removal

2. Coalescing Filters​

Coalescing filter diagram Coalescing filters capture oil aerosols and fine particles, draining collected liquids.

  • Air flows from inside to outside the element
  • Remove oil aerosols and fine particles
  • Material: Microglass fiber

Coalescing Filtration Grades​

GradeEfficiencyOil ContentDP (dry)DP (wet)
Grade 299.999%0.001 ppm1.5 PSI4-6 PSI
Grade 499.995%0.03 ppm1.25 PSI3-4 PSI
Grade 699.97%0.08 ppm1.0 PSI2-3 PSI
Grade 898.5%0.2 ppm0.5 PSI1-1.5 PSI
Grade 1095%0.83 ppm0.5 PSI0.5-1 PSI

3. Activated Carbon Filters​

  • Air flows from outside to inside
  • Remove odors, tastes, and hydrocarbon vapors
  • Efficiency: 99%
  • Pressure drop: ~1 PSI
  • Essential for breathing air and food applications

Condensate Separation​

Centrifugal Separators​

5-step centrifugal separator A 5-step centrifugal separator removes 99% of liquids and solids larger than 10 microns.

Separation Principles:

  1. Velocity reduction
  2. Centrifugal action
  3. Impact (collision)
  4. Direction change
  5. Quiet zone (settling)

Filter Selection Guide​

ApplicationRecommended Filtration
Pneumatic toolsParticulate + Coalescing (Grade 6-8)
Spray paintingParticulate + Coalescing (Grade 4) + Carbon
Food processingParticulate + Coalescing (Grade 2) + Carbon
ElectronicsParticulate + Coalescing (Grade 2)
Breathing airFull treatment + Carbon + Monitoring

The Cost of Pressure Drop​

Every PSI of pressure drop costs money:

Example: 50 HP Compressor @ 250 CFM

Annual cost=$28,725 USD @ 100 PSIG\text{Annual cost} = \$28{,}725 \text{ USD @ 100 PSIG} Cost per PSI=$28,725100=$287.25/year\text{Cost per PSI} = \frac{\$28{,}725}{100} = \$287.25/\text{year}
ComponentΔPAnnual Cost
Filter (5 PSI)5 PSI$1,435
Dryer (15 PSI)15 PSI$4,312
Best Practice

Monitor differential pressure across filters. Replace elements before excessive pressure drop wastes energy.

Filter Media Types​

Depth vs Surface Filtration​

Depth Filtration:              Surface Filtration:
(Particles trapped throughout)  (Particles on surface)

    ░░░●░░░░░░░░                ●●●●●●●●●●●●
    ░░░░░░●░░░░░                ────────────
    ░░●░░░░░░░░░                │          │
    ░░░░░░░░●░░░                │          │
                                │          │

Best for: Oil aerosols          Best for: Dry particles
Media TypeMaterialApplicationEfficiency
MicroglassBorosilicate fibersCoalescing99.9999% @ 0.01μm
CelluloseWood pulp fibersParticulate pre-filter95% @ 3μm
PolypropyleneSynthetic fibersChemical resistant99.9% @ 1μm
Stainless meshWoven metalHigh temp, reusable90% @ 10μm
Activated carbonCoconite/coalVapor adsorption99% (hydrocarbon)
PTFE membraneTeflonSterile/pharma99.99% @ 0.01μm

Coalescing Media Structure​

Three-layer construction:

Outer layer: Drainage
├── Coarse fibers
├── Allows liquid drainage
└── Protects inner layers

Middle layer: Coalescing
├── Fine microglass (2-5 μm)
├── Captures and combines droplets
└── Primary filtration zone

Inner layer: Pre-filtration
├── Medium fibers
├── Captures large particles
└── Protects coalescing layer

Beta Ratio (Filtration Efficiency)​

The Beta ratio (β) quantifies filter efficiency for a given particle size.

Definition​

βx=Particles upstream ≥ x μmParticles downstream ≥ x μmβ_x = \frac{\text{Particles upstream ≥ x μm}}{\text{Particles downstream ≥ x μm}}

Efficiency Conversion​

\text{Efficiency (%)} = \frac{β - 1}{β} \times 100 = \left(1 - \frac{1}{β}\right) \times 100
Beta RatioEfficiencyMeaning
β₂ = 250%Half of 2μm particles pass
β₂ = 1090%1 in 10 pass
β₂ = 10099%1 in 100 pass
β₂ = 100099.9%1 in 1,000 pass
β₂ = 10,00099.99%1 in 10,000 pass
Beta Ratio Requires Size

A "β = 1000" rating is meaningless without particle size. Always specify: β₃ = 1000 means 99.9% efficient at 3 microns.

Test Standards​

StandardMethodNotes
ISO 12500-1Oil aerosol efficiencyIndustry standard
ISO 12500-3Particulate efficiencyMulti-pass test
DIN 24550Older European standardBeing phased out

Filter Placement Strategy​

System Layout​

Optimal filter placement:

                   Point-of-Use
                   Filter
                      │
    Compressor → Wet Tank → Dryer → Dry Tank → Distribution → Application
         │           │        │        │              │
         │      Separator  Pre-filter  After-filter   Final filter
         │           │        │        │              │
         ▼           ▼        ▼        ▼              ▼
    Aftercooler   Bulk      Protect   Polish      Process
    + separator   liquid    dryer     air         specific
                  removal   media     quality     needs

Pre-Dryer Filtration​

Purpose: Protect dryer from oil and particles

Dryer TypePre-Filter Requirement
RefrigeratedParticulate filter (5μm)
DesiccantCoalescing (Grade 6) + particulate
MembraneCoalescing (Grade 4) + particulate
Desiccant Dryer Protection

Oil contaminates desiccant permanently. Always install coalescing filter upstream of desiccant dryers.

After-Dryer Filtration​

Purpose: Remove any desiccant dust or particles from dryer

ApplicationAfter-Filter
General plantParticulate (3μm)
InstrumentsCoalescing (Grade 4)
Sensitive processCoalescing (Grade 2)

Point-of-Use Filtration​

Purpose: Final protection for specific applications

ApplicationPoint-of-Use Filter
Pneumatic tools40μm + lubricator
Paint spray5μm + activated carbon
Food contact0.01μm sterile + carbon
Electronics0.01μm + carbon
Breathing airFull treatment + CO monitor

Filter Sizing​

Flow Capacity​

Size filters for actual flow conditions:

Qactual=Qrated×Prated+14.7Pactual+14.7×Tactual+460Trated+460Q_{actual} = Q_{rated} \times \frac{P_{rated} + 14.7}{P_{actual} + 14.7} \times \frac{T_{actual} + 460}{T_{rated} + 460}

Sizing Rules​

GuidelineReason
Size for peak flow, not averagePrevents excessive ΔP at surge
Use 70% of rated capacityAllows for filter loading
Consider future expansionFilters are cheap, ΔP is expensive

Example:

  • Application needs 500 CFM peak
  • Select filter rated for 500 ÷ 0.7 = 715 CFM minimum

Filter Maintenance​

When to Change Elements​

IndicatorAction
ΔP > 10 PSI (coalescing)Replace
ΔP > 5 PSI (particulate)Replace
12 months of operationEvaluate condition
Color changeInspect element
Oil carryoverCheck drain and element

Differential Pressure Monitoring​

ΔP gauge installation:

    Upstream    Filter     Downstream
    pressure   housing    pressure
        │         │           │
        ▼         ▼           ▼
    ┌───●────┬────────┬───●───┐
    │        │░░░░░░░░│       │
    │        │░░░░░░░░│       │
    │        │░░░░░░░░│       │
    └────────┴────────┴───────┘
             │
        ┌────┴────┐
        │   ΔP    │
        │  Gauge  │
        └─────────┘
Element StateTypical ΔP
New (dry)0.5-1 PSI
New (wet/operating)1-3 PSI
Replace soon6-8 PSI
Replace now>10 PSI

Common Mistakes​

  1. Ignoring pressure drop - Wastes energy
  2. Forgetting to drain condensate - Re-contaminates the air
  3. Wrong sequence - Filters before dryer
  4. Undersizing - Causes excessive pressure drop
  5. Using wrong element - Voids warranty, poor performance
  6. Cleaning and reusing - Damages media structure