Membrane Gas Dryers 101: How Hollow Fiber Selectively Removes Water Vapor
The core idea (in plain English)
A membrane gas dryer removes water vapor by letting H₂O molecules pass through an ionomer wall faster than other gases. The driving force is a water-vapor partial-pressure gradient: wet sample gas on one side, a drier “purge” stream on the other. Water migrates across the fiber wall until the outlet dew point reaches your target—continuously, with no moving parts.
What passes vs. what doesn’t
- Passes efficiently: water vapor (H₂O)
- Effectively retained: analytes of interest (SO₂, NOₓ, CO₂, CO, O₂, H₂, CH₄, etc.) under normal use
- Does not remove: liquids/aerosols, oils, particulates (that’s what prefilters are for)
Key point: membrane dryers manage vapor, not bulk liquid. Keep aerosols and oil mists out with a 5 µm prefilter (and coalescer, if needed).
Why choose membranes over desiccant or refrigerated dryers?
- Continuous & fast: no regeneration cycles or cooldowns; outlet dew point stabilizes quickly.
- Analyzer-friendly: low dead volume and predictable pressure drop.
- No consumables: no desiccant to replace, no moving parts to service.
- Compact & modular: easy to fit into crowded racks and cabinets.
- Energy-light: no compressors or chillers required.
Where sunsep™ shines
- Stable dew points across seasonal swings with a tuned purge ratio
- Instrument-grade housings and standard compression fittings
- Direct online purchasing for common SKUs, plus fast engineering support
Typical applications
- CEMS (EPA Part 75/60): Prevents condensation and dilution that invalidate data.
- Lab & analytical (TDLAS/FTIR/GC): Cuts water-line interferences and carryover.
- Fuel cells (BoP): Trims moisture in anode recycle loops; pairs with membrane humidifiers.
- Breath/biomedical & microflows: Gentle, contamination-aware conditioning. (Additional approvals required for medical applications)
Sizing the dryer (what matters)
- Inlet condition: temperature + inlet dew point (or ambient RH/temp if unknown)
- Target outlet dew point
- Flow & pressure at the dryer (normal and peak)
- Temperature deltas along the run (Tinlet vs Tambient)
- Contaminants: confirm prefiltration/oil-mist removal
Quick start: begin with a purge ratio around 10% of process flow and fine-tune while watching outlet dew point.
Mini-glossary
- Dew point: temperature where condensation starts at a given pressure.
- ADP/PDP: ambient vs. pressure dew point—specify which one you need.
- Purge ratio: purge flow ÷ process flow (expressed as %).
Want a right-sized sunsep™ recommendation and a purge-ratio starting point? Request a recommendation!