Skip to main content

Dryers in Detail

Moisture is the enemy of pneumatic systems. Proper drying prevents corrosion, contamination, and equipment failure.

Why Dry Compressed Air?​

The Moisture Problem​

Moisture causes corrosion Moisture removes lubrication and causes oxidation, increasing friction and wear.

  • Ambient air at 70% humidity becomes 100% saturated when compressed
  • As it cools in piping and equipment, water condenses
  • Water causes:
    • Corrosion of piping and equipment
    • Washing away of lubricants
    • Increased friction and wear
    • Product contamination
    • Freezing in cold environments

Moisture Measurement​

MeasureDescription
Relative Humidity (%)Percentage of maximum moisture air can hold
Dew Point (°F/°C)Temperature at which condensation begins

Moisture Content Comparison​

ConditionWater Content
Air @ 50°F dew point11.5 g/m³
Air @ -40°F dew point0.0117 g/m³

Air dried to -40°F dew point contains 1000x less moisture!

Dryer Types​

1. Refrigerated Dryers​

Refrigerated dryer diagram Refrigerated dryer components: heat exchangers, refrigerant compressor, separator, and automatic drains.

How They Work:

  1. Hot, humid compressed air enters
  2. Air-to-air exchanger pre-cools incoming air
  3. Air-to-refrigerant exchanger cools air to ~38-50°F
  4. Moisture condenses and separates
  5. Cold, dry air reheats outgoing air (recovering energy)

Specifications:

ParameterValue
Dew Point35-50°F (2-10°C)
Typical pressure drop3-5 PSI
Energy consumptionLow-moderate

Refrigerated Types:

  • Non-cycling: Refrigerant runs continuously - consistent dew point
  • Cycling: Refrigerant cycles based on load - energy savings at partial loads

2. Desiccant Dryers (Adsorption)​

Use desiccant material to adsorb moisture from the air.

How Adsorption Works:

Moisture molecules are attracted to the desiccant surface due to the vapor pressure difference between humid air (high vapor pressure) and dry desiccant (low vapor pressure).

Regeneration: The desiccant must be periodically regenerated by expelling collected moisture.

Desiccant Types:

  • Activated Alumina (most common)
  • Silica Gel
  • Molecular Sieves

Heatless Regeneration (Pressure Swing)​

ParameterValue
Dew Point-40°F to -100°F
Purge air~15% of rated flow
Cycle time5-10 minutes
No external heat required

Uses a portion of dry compressed air to regenerate the offline tower.

Heated Regeneration​

ParameterValue
Dew Point-40°F to -100°F
Purge air~7% of rated flow
External heatersYes
More efficient in larger sizes

Blower Purge​

ParameterValue
Dew Point-10°F to -40°F
Purge air0% (uses ambient air)
External blower + heaterYes
Most energy efficient for continuous use

Dew Point Selection Guide​

ApplicationRequired Dew Point
General plant air35-50°F
Outdoor piping (no freezing)35°F
Outdoor piping (cold climates)-40°F
Spray painting35-50°F
Food processing-40°F
Pharmaceutical-40°F to -100°F
Electronics manufacturing-40°F
Instrument air-40°F

Common Mistakes​

"I'll Just Drain More Often"​

Myth

Draining removes bulk water but does NOT reduce humidity. The air remains saturated and will continue condensing downstream.

Solution: Proper drying equipment, not just drainage.

Ignoring Pressure Dew Point vs Atmospheric​

A dryer with -40°F pressure dew point @ 100 PSIG will have a much lower atmospheric dew point when expanded. Always specify pressure dew point for compressed air systems.

Energy Consumption by Type​

Dryer TypeRelative Energy Use
Refrigerated (non-cycling)Low
Refrigerated (cycling)Very Low
Desiccant heatlessModerate (purge loss)
Desiccant heatedModerate-High
Blower purgeLow (for desiccant type)
Tip

Match dryer type to actual dew point requirements. Don't over-dry - a -40°F dew point costs significantly more than 35°F if you don't need it.

Dryer Sizing Calculations​

Basic Sizing Parameters​

ParameterSymbolUnits
Flow rateQCFM or m³/min
Inlet pressureP₁PSIG or bar
Inlet temperatureT₁°F or °C
Required dew pointPDP°F or °C
Ambient temperatureTₐ°F or °C

Correction Factors​

Dryer ratings are typically at standard conditions:

  • 100 PSIG (7 bar)
  • 100°F (38°C) inlet temperature
  • 100°F (38°C) ambient temperature

Actual capacity must be corrected:

Qactual=Qrated×CP×CT×CAQ_{actual} = Q_{rated} \times C_P \times C_T \times C_A

Where:

  • C_P = Pressure correction factor
  • C_T = Inlet temperature correction
  • C_A = Ambient temperature correction

Pressure Correction (Refrigerated Dryers)​

Pressure (PSIG)C_P Factor
500.75
750.88
1001.00
1251.10
1501.18
1751.25

Temperature Correction (Refrigerated Dryers)​

Inlet Temp (°F)C_T Factor
801.28
901.13
1001.00
1100.88
1200.77

Sizing Example​

Application:

  • Required flow: 500 CFM
  • Operating pressure: 125 PSIG
  • Inlet temperature: 110°F
  • Ambient temperature: 95°F

Correction factors:

  • C_P @ 125 PSIG = 1.10
  • C_T @ 110°F = 0.88
  • C_A @ 95°F = 1.04

Required rated capacity:

Qrated=5001.10×0.88×1.04=5001.01=496 CFMQ_{rated} = \frac{500}{1.10 \times 0.88 \times 1.04} = \frac{500}{1.01} = 496 \text{ CFM}

Select dryer rated for ≥500 CFM at standard conditions.

Safety Factor

Add 10-25% safety factor for flow variations and future expansion.

Purge Air Losses (Desiccant Dryers)​

Understanding Purge Loss​

Desiccant dryers require purge air to regenerate the saturated tower. This air is lost to atmosphere.

Twin-tower operation:

    Tower A (Drying)              Tower B (Regenerating)
    ┌─────────────┐               ┌─────────────┐
    │ ░░░░░░░░░░░ │               │ ░░░░░░░░░░░ │
    │ ░ Dry ░░░░░ │               │ ░ Wet ░░░░░ │
    │ ░ Desiccant │               │ ░ Desiccant │
    │ ░░░░░░░░░░░ │               │ ░░░░░░░░░░░ │
    └──────┬──────┘               └──────┬──────┘
           │                             │
    Wet ───┴──→ Dry              Purge ──┴──→ Atmosphere
    air in     air out           air in      (LOST)

Purge Rates by Dryer Type​

Dryer TypePurge RateComments
Heatless15-18%At rated flow, higher at part load
Heated (internal)7-10%Plus heater energy
Heated (external)2-5%Blower + heater
Blower purge0%Uses ambient air
HOC (heat of compression)0%Uses compressor heat

Calculating Purge Cost​

Example: 500 CFM heatless desiccant dryer

Purge loss = 15% of rated capacity

Purge flow=500×0.15=75 CFM\text{Purge flow} = 500 \times 0.15 = 75 \text{ CFM}

Annual purge cost:

Cost=Purge CFM×kW per CFM×Hours×$/kWh1\text{Cost} = \text{Purge CFM} \times \frac{\text{kW per CFM} \times \text{Hours} \times \text{\$/kWh}}{1}

Assuming:

  • 0.25 kW per CFM (typical compressor efficiency)
  • 8,760 hours/year
  • $0.10/kWh
Annual purge cost=75×0.25×8760×0.10=$16,425\text{Annual purge cost} = 75 \times 0.25 \times 8760 \times 0.10 = \$16,425
Part-Load Purge Loss

Heatless dryers purge at fixed rate regardless of actual flow. At 50% load, effective purge loss doubles to 30% of actual flow!

Purge Reduction Strategies​

StrategySavingsNotes
Dew point demand control30-50%Adjusts cycle based on actual load
Heated regeneration50%Trade electricity for air
Blower purge100%No compressed air used
Heat of compression (HOC)100%Uses waste heat

Dew Point Demand Control​

Without demand control:     With demand control:
(Fixed cycle timing)        (Variable cycle timing)

Dew Point                   Dew Point
    │    ╱╲   ╱╲   ╱╲          │    ╱╲        ╱╲
-40°├───╱──╲─╱──╲─╱──╲───      ├───╱──╲──────╱──╲──
    │  ╱    ╲    ╲    ╲        │  ╱    ╲    ╱    ╲
    │ ╱      ╲    ╲    ╲       │ ╱      ╲──╱      ╲
    └────────────────────      └────────────────────
       Time                       Time

Cycles when not needed      Cycles only when needed

Total Cost of Ownership​

Comparing Dryer Types (500 CFM example)​

FactorRefrigeratedHeatless DesiccantHeated Desiccant
Capital cost$8,000$12,000$18,000
Dew point achieved38°F-40°F-40°F
Annual energy (kWh)8,760015,000
Annual purge loss0$16,425$8,212
Annual energy cost$876$0$1,500
Annual maintenance$500$1,500$2,000
Total annual cost$1,376$17,925$11,712

10-year total:

Refrigerated:$8,000+($1,376×10)=$21,760Heatless:$12,000+($17,925×10)=$191,250Heated:$18,000+($11,712×10)=$135,120\begin{aligned} \text{Refrigerated:} &\quad \$8{,}000 + (\$1{,}376 \times 10) = \$21{,}760 \\ \text{Heatless:} &\quad \$12{,}000 + (\$17{,}925 \times 10) = \$191{,}250 \\ \text{Heated:} &\quad \$18{,}000 + (\$11{,}712 \times 10) = \$135{,}120 \end{aligned}
Selection Guide
  • Need 35-50°F PDP → Refrigerated (lowest cost)
  • Need -40°F, intermittent use → Heated desiccant
  • Need -40°F, continuous 24/7 → Blower purge or HOC

Dryer Installation Best Practices​

Location Requirements​

Proper installation layout:

                          ┌──────────────┐
    Aftercooler          │              │    Dry air
    ────────────→ Filter → │    DRYER    │ ──────────→
                   Pre    │              │    out
                          └──────────────┘
                                 │
                            Condensate
                             drain
                                 │
                                 ▼
                          Condensate
                          management
RequirementReason
Level installationProper condensate drainage
Adequate ventilationHeat dissipation (refrigerated)
Accessible filtersMaintenance access
Bypass pipingFor maintenance without shutdown
Condensate managementEnvironmental compliance

Common Installation Mistakes​

  1. No pre-filter - Contaminates dryer internals
  2. Too close to wall - Blocks airflow (refrigerated)
  3. Hot inlet air - Reduces capacity
  4. No bypass - Forces shutdown for maintenance
  5. Oversizing - Short cycling in refrigerated dryers