Compressed Air Dew Point Conversion

Compressed air dew point has two expressions. One is pressure dew point, one is atmospheric dew point. Difference is what pressure state the air is at when measured. Pressure dew point is condensation temperature measured when compressed air is at system working pressure. Atmospheric dew point is condensation temperature measured after that air is released to atmospheric pressure.

Dew Point

Compressed Air Moisture Control

How much difference between the two values? A lot. In a 100 psig system, pressure dew point of 32°F corresponds to atmospheric dew point of about 0°F. If you see two dryers, one rated pressure dew point -40°F, another rated atmospheric dew point -40°F, the first one's drying capacity is far superior. The second one converted to pressure dew point is only about -8°F.

Conversion formula is Tdp,atm = Tdp,p - K × log₁₀(P/P₀). Tdp,atm is atmospheric dew point, Tdp,p is pressure dew point, P is absolute pressure, P₀ is 14.7 psia, K is empirical coefficient around 32 to 36. 100 psig is about 114.7 psia absolute, log₁₀(114.7/14.7) is about 0.89, times 36 gives about 32°F dew point displacement.

At 100 psig, conversion relationship: pressure dew point +50°F corresponds to atmospheric dew point +18°F, +37°F corresponds to +5°F, +32°F corresponds to 0°F, -4°F corresponds to -36°F, -40°F corresponds to -72°F, -94°F corresponds to -126°F. Pressure changes, conversion relationship changes too. 145 psig system dew point displacement about 36°F, 60 psig system about 25°F.

What Can Refrigerated Dryers Achieve

Refrigerated dryers cool compressed air to near freezing to condense water out. Can't go below 32°F. Water freezing will block the heat exchanger. So pressure dew point lower limit is +35°F to +37°F. Market refrigerated dryers rated pressure dew point from +37°F to +50°F, corresponding to atmospheric dew point +5°F to +18°F.

The +37°F rating has preconditions. Inlet temp 95°F, ambient temp 77°F. Deviate from this condition, performance takes a hit. Summer when shop temp is high, cooling capacity isn't enough, dew point might drift to +41°F or even +45°F. Some plants have dew point meters at inlet, summer afternoons often see alarms.

Refrigerated dryer energy consumption is mainly the refrigeration compressor. About 0.6 to 1.2 kW per 35 CFM processing capacity. Similar order of magnitude as home AC. Maintenance is mainly cleaning condenser fins, checking refrigerant pressure, regularly draining condensate. Refrigerant leaks, dew point goes up. One of the most common failure causes.

Desiccant Dryers

Desiccant dryers use twin tower switching. One side adsorbs, one side regenerates. Adsorbents include activated alumina, silica gel, molecular sieve. Molecular sieve has strongest adsorption but also most expensive.

Heatless regeneration type uses dried compressed air directly to purge regeneration. Regeneration air consumption is 12% to 15% of processing capacity. This loss isn't small. A 350 CFM system throws away 42 to 53 CFM of compressed air every hour. Pressure dew point achievable is -4°F to -40°F, corresponding to atmospheric dew point -36°F to -72°F.

Heated regeneration type adds electric heater. During regeneration, heats adsorbent to over 300°F, moisture desorbs more easily. Air consumption drops to about 5%, drying depth is also better. Pressure dew point can reach -40°F or even -94°F. Trade-off is additional heating power. About 1 to 2 kW heater per 35 CFM processing capacity.

Blower heated regeneration and compression heat regeneration are for large flow systems. Blower heated uses separate blower and heater, doesn't consume compressed air. Compression heat regeneration uses compressor discharge waste heat. Good energy savings but complex system. Suits large flow, continuous operation situations.

Membrane Dryers

Membrane dryers use hollow fiber membrane. Water molecules pass through membrane wall and escape. Dry air flows out from inside the tube. Simple principle. No moving parts. No electricity. Low maintenance. Pressure dew point achievable is -4°F to -40°F. Similar to heatless regeneration desiccant dryers.

Problem is sweep air loss is big. Need 15% to 25% of dried air to sweep moisture that passed through membrane wall. Small flow, this loss is acceptable. Large flow, not economical. Membrane modules also hate oil. Inlet oil content must be controlled below 0.01 ppm. Stricter requirement than desiccant dryers.

What ISO 8573-1 Specifies

ISO 8573-1 divides compressed air moisture into 6 classes. Class 1 requires pressure dew point ≤-94°F, Class 2 requires ≤-40°F, Class 3 requires ≤-4°F, Class 4 requires ≤+37°F, Class 5 requires ≤+45°F, Class 6 requires ≤+50°F.

Refrigerated dryers can only meet Class 4 to Class 6. Reaching Class 2 or Class 3 needs desiccant or membrane dryers. Class 1 only heated regeneration or blower heated desiccant dryers can do.

Measurement and Monitoring

Instrumentation

Dew Point Meters

Sensors

Online Monitoring

Calibration

Reference Standards

Dew point meters include chilled mirror, capacitive, impedance types. Chilled mirror most accurate, directly measures by condensation on mirror surface. Accuracy to ±0.2°F. But slow response, expensive. Usually for calibration. Capacitive and impedance respond fast, accuracy ±2 to ±4°F. Good enough for online monitoring.

Sample point installed downstream of dryer outlet. Sample tube can't be too long, tube wall absorbs moisture, measured dew point reads higher than actual. Low temperature dew point measurement is more trouble. Below -40°F demands high sensor performance. Need calibration every six months to a year.

Online dew point monitoring can set alarms. Dew point drifting up means dryer has problems. Maybe adsorbent needs changing. Maybe refrigerated dryer has refrigerant leak. Maybe regeneration heater failed. Find early, fix early. Better than quality problems at use point then tracing back.

Piping and System

Piping

Insulated Distribution

Zoning

End-Point Treatment

Dryer outlet dew point passing doesn't mean use point dew point passes. Pipe running through cold areas will have secondary condensation. One plant had dryer at compressor station, use point 600 feet away in another shop. Pipe ran outside in between. Winter outdoor temp low, pipe temp dropped below dew point. Use point had water every day. Later added insulation on outdoor section, problem solved.

Zoned supply saves money. Whole plant uniformly configured with Class 2 dryer not necessary. Most use points Class 4 is enough. Few high-demand stations add small desiccant dryers at the end. Central supply handles most needs, end-point drying handles special needs. More economical than whole plant high spec.

When to change desiccant dryer adsorbent? Can't just look at years. Watch dew point changes. Dew point keeps drifting up, regeneration parameters normal, temp and pressure no issues, then adsorbent is aging. Some plants run adsorbent seven or eight years and dew point still passes. Some plants two or three years and it's done. Related to inlet air quality and operating conditions.