CEMS moisture control & EPA Part 75

Why moisture control is a compliance issue (not just a nuisance)

EPA’s Part 75 requires continuous monitoring and reporting of SO₂, NOₓ, CO₂, and flow data from affected units—typically using CEMS—with strict certification and ongoing QA/QC requirements. US EPA+1
These requirements underpin programs like the Acid Rain Program and interstate transport rules for power plants, so data quality and availability matter. US EPA

When sample lines or analyzers see condensation, you risk dilution, adsorption, and instrument faults that translate into invalid data, missing-data substitution, and failed tests—each with real costs.

How membrane gas dryers (sunsep™) stabilize CEMS data

sunsep™ hollow-fiber membrane dryers remove water vapor by diffusing H₂O through the membrane wall, driven by a water-vapor partial-pressure gradient. There’s no refrigeration or desiccant to regenerate; the process is continuous and fast, so dew point at the analyzer stays stable even as humidity swings. (Engineering note: we’re controlling water vapor only—bulk gas composition remains effectively unchanged for the target analytes.)

Where they fit in the conditioning train

• Upstream of analyzer, downstream of probe/filters, before condensable formation risk.
• After particulate and oil-mist removal to protect the membrane.
• With a tuned purge ratio to reach the target outlet dew point over expected temperature/flow ranges.

A quick refresher on Part 75 QA/QC touchpoints

• Initial certification (e.g., 7-day calibration drift, linearity, RATA, etc.), then ongoing QA set by Part 75 (see Appendices). eCFR
• Ongoing QA/QC and timely reporting are mandatory; administrative timelines and notifications are spelled out by EPA. US EPA

While Part 75 doesn’t prescribe a specific dryer technology, it demands reliable, accurate, and continuously valid data—keeping moisture out of the analyzer is foundational to meeting those bars.

Sizing and selection (what we’ll ask you for)

1. Inlet dew point (or ambient conditions) and target outlet dew point
2. Flow & pressure at the dryer
3. Gas temperature along the run (T-inlet vs T-ambient)
4. Contaminants (oil mists, particulates) and available pre-filtration
5. Purge strategy (initial ratio, venting/abatement)

Not sure on the numbers? We’ll back-solve with a short worksheet and recommend a model that hits your dew point without over-purging.

Field checklist to avoid re-work

• Keep T-inlet > T-ambient to reduce in-line condensation risk before the dryer
• Install a 5 µm prefilter and oil-mist separator where compressors are involved
• Verify purge ratio at startup and after seasonal changes
• Log dew point during linearity/CGAs and RATA windows to correlate stability

CTA: Need a right-sized dryer for your certification or QA window? Request a recommendation!

Regulatory note: For definitions, scope, and certification/QA specifics, see EPA’s Part 75 overview and the eCFR text/appendices. US EPA+2eCFR+2