RTO Wet to Dry Gas Flow Conversion for Buyers

Direct answer: For a single gas stream under the ideal gas approximation, calculate dry reference flow as Qdry,ref = Qwet,actual × (1 − ywater) × (Pactual / Pref) × (Tref / Tactual). Use water mole fraction, absolute pressures and kelvin temperatures. Define the measurement location and reference conditions before substituting values.

An RTO wet to dry gas flow conversion connects the airflow measured in a humid duct with a dry reporting basis. Buyers need this distinction when a supplier quotes normalized capacity, a laboratory reports dry VOC concentration, and a fan schedule lists actual cubic metres per hour. These numbers describe different volumes and cannot be exchanged without checking their definitions.

This guide provides a transparent calculation and procurement checks. Every numerical input below is hypothetical, with no claim about Yuehua equipment performance or a regulatory limit. Apply the governing measurement method and the project specification when preparing an acceptance calculation.

Specify which gas volume each document describes

Separate actual conditions from reference conditions

Actual wet flow is the volume physically passing the measurement plane per unit time, including water vapor. Dry flow excludes water from the stated gas quantity. Reference flow expresses that quantity at a declared temperature and pressure. Moisture conversion and reference normalization are separate operations, even when software performs them together.

Write a complete flow label: measurement plane, actual or reference volume, temperature, absolute pressure, moisture basis and time interval. Confirm whether an instrument already applies compensation. Correcting a normalized reading a second time can introduce more error than the original measurement uncertainty.

Keep the boundary at one location

The conversion equation compares reporting bases for the same stream. It does not transfer inlet flow to an outlet after chemical reaction, burner operation or dilution. Identify every added or removed gas stream before extending the calculation across equipment. A collection header, RTO chamber and discharge stack can each require a separate balance.

Convert moisture and gas conditions consistently

Let ywater be the water vapor mole fraction of the original wet gas. At sufficiently low pressure for ideal behavior, mole fraction is also volume fraction. Twelve volume percent water is entered as 0.12. It is not twelve percent relative humidity and is not a liquid water mass percentage.

At unchanged temperature and pressure, Qdry = Qwet × (1 − ywater). For reference normalization, multiply by Pactual / Pref and Tref / Tactual. Kelvin temperature equals degrees Celsius plus 273.15. Use absolute gas pressure, calculated from the local barometric pressure and signed gauge pressure when that is how the measurement is supplied.

For a nonideal gas, the complete volume relationship also includes Zref / Zactual, where Z is compressibility factor. The worked example assumes both factors equal one. Do not apply this approximation without review to pressurized or unusual process mixtures. Use the approved method where it specifies a different correction procedure.

Determine water content using a suitable measurement at representative conditions. If the input is relative humidity, obtain saturation vapor pressure at gas temperature and calculate water partial pressure before dividing by absolute pressure. Solvent interference, contamination and temperature gradients can invalidate a convenient humidity sensor reading.

Check a hypothetical wet exhaust calculation

Assume an inlet duct carries 12,000 actual wet m³/h at 60°C and 99.0 kPa absolute, with water mole fraction 0.12. Choose a dry reference basis of 0°C and 101.325 kPa solely for this example. Assume a stable gas stream, representative measurements and ideal gas behavior throughout the basis conversion.

Retain full precision until the final result. Reverse the factors to recover 12,000 actual wet m³/h; this is a useful arithmetic check. The dry reference number is smaller because water has been excluded and the example reference temperature is lower than the actual gas temperature.

If the reference temperature were instead 20°C at the same reference pressure, the dry reference volume would increase by 293.15 / 273.15, or approximately 7.3 percent. The physical stream would be unchanged. Two quoted capacities differing by that amount may simply use different reference definitions.

Pair concentration and flow to preserve mass

At the same temperature and pressure, concentration in dry gas is Cdry = Cwet / (1 − ywater), provided the pollutant remains in the gas phase and sampling does not remove it. Flow decreases by the reciprocal relationship, so Cdry × Qdry equals Cwet × Qwet.

For a separate hypothetical illustration on one shared reference temperature and pressure basis, take 100 mg per wet reference m³ and 10,000 wet reference m³/h at twelve percent water. Dry concentration becomes about 113.64 mg per dry reference m³ and dry flow becomes 8,800 dry reference m³/h. Either matched pair gives 1.00 kg/h.

Multiplying the dry concentration by the wet flow would overstate mass rate. The RTO VOC mass loading calculation guide extends the calculation to species composition and ppmv. Record whether the pollutant is reported as a named compound, total organic carbon or another analytical convention.

Translate reporting flow into equipment duties

A fan moves actual gas at its installed location. It does not physically handle the smaller dry reference volume used in a report. Review temperature, pressure, moisture, composition and density with the fan supplier. Keep the maximum actual flow case visible when comparing motor duty and the available pressure margin.

For chamber sizing, reconstruct the actual hot wet stream inside the defined treatment volume. The RTO residence time calculation guide explains the volume and temperature boundaries. Water formed by solvent and fuel oxidation belongs in the chamber balance where relevant, even when the inlet specification was supplied dry.

Moisture also affects sensible heat demand and possible condensation, but dry reference flow alone does not quantify either. The RTO condensation and dew point control guide addresses cold surfaces, startup and drainage. A reporting conversion is not a prediction of which components will condense.

Resolve measurement gaps before accepting a guarantee

Request synchronized flow, moisture, temperature and pressure records for each operating mode. Averaging matters: average wet flow multiplied by average dry fraction need not equal the average of their product when moisture and airflow vary together. Preserve time alignment and use the averaging procedure specified for the test.

Check sample conditioning. Cooling can remove water and soluble or condensable VOCs, while droplets are a separate liquid phase outside the simple vapor conversion. Record heated line conditions and any condenser or dryer. An algebraic correction cannot recover pollutant already lost in the sampling system.

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Ask each bidder to return a calculation sheet showing original readings, units, reference conditions, moisture source, operating cases and uncertainty assumptions. Mark estimated values clearly. Reconcile the sheet with the process flow diagram before using a normalized capacity as an equipment acceptance criterion.

Frequently asked questions