Prepared by the DeFa Engineering Team
Quick answer
When airflow is in m³/h and VOC concentration is in mg/m³, calculate the inlet VOC mass loading as airflow × concentration ÷ 1,000,000. The result is kg/h. For example, 20,000 m³/h at 300 mg/m³ carries 6 kg/h of VOC.
Do not calculate control performance from concentration alone when inlet and outlet gas flow differ. Convert both sides to mass rates, then use (inlet mass rate − outlet mass rate) ÷ inlet mass rate × 100. Confirm that flow and concentration use compatible actual or standard conditions, wet or dry basis, and any required oxygen correction.
Core VOC mass loading formulas
1. Convert mg/m³ and m³/h to kg/h
VOC mass loading (kg/h) = airflow (m³/h) × VOC concentration (mg/m³) ÷ 1,000,000
The divisor comes from 1,000,000 mg per kg. If both inputs use a compatible gas-condition basis, the cubic metres cancel and the result is a mass rate.
2. Calculate the outlet VOC mass rate
Outlet VOC mass rate (kg/h) = outlet airflow (m³/h) × outlet concentration (mg/m³) ÷ 1,000,000
Use the measured outlet flow rather than automatically copying the inlet flow. Combustion air, dilution, leakage, evaporation, condensation or a wet process can change the gas volume across the system.
3. Calculate control-device removal efficiency
Removal efficiency (%) = (inlet mass rate − outlet mass rate) ÷ inlet mass rate × 100
This is the mass-based result for the control device. A simple concentration calculation gives the same answer only when inlet and outlet flow rates and measurement bases are equivalent.
4. Calculate annual captured VOC mass
Annual captured VOC (kg/year) = (inlet kg/h − outlet kg/h) × annual operating hours
Use separate operating cases when the process has different products, shifts, cleaning cycles or seasonal loads. Multiplying one short test result by the entire year can misrepresent the actual duty.
Worked example: from concentration to annual VOC duty
Assume the following measured operating case:
- Inlet airflow: 20,000 m³/h
- Inlet VOC concentration: 300 mg/m³
- Outlet airflow: 22,000 m³/h
- Outlet VOC concentration: 30 mg/m³
- Operating time: 4,000 hours/year
Step 1: calculate inlet mass loading
20,000 × 300 ÷ 1,000,000 = 6.00 kg/h
Step 2: calculate outlet mass rate
22,000 × 30 ÷ 1,000,000 = 0.66 kg/h
Step 3: calculate mass-based removal efficiency
(6.00 − 0.66) ÷ 6.00 × 100 = 89%
A concentration-only calculation would show 90%. The difference occurs because the outlet flow is higher. In some projects the gap is much larger, so mass-based comparison is the safer engineering basis.
Step 4: estimate annual captured VOC
(6.00 − 0.66) × 4,000 = 21,360 kg/year
This annual figure is a screening estimate for this operating case, not a permit inventory by itself. A defensible annual estimate must reflect the full production schedule, downtime, load changes and the approved emissions methodology.
Match the gas-condition and reporting basis
Airflow and concentration must describe the same gas basis. Multiplying actual m³/h by a concentration stated per normal cubic metre mixes two different volumes and produces the wrong mass rate.
| Item | Question to resolve | Why it matters |
|---|---|---|
| Actual or standard volume | Are both values in m³ or both in Nm³ at the same stated reference conditions? | Gas volume changes with temperature and pressure. |
| Wet or dry basis | Has water vapor been included or removed in both flow and concentration? | Moisture changes the reported gas volume and concentration. |
| Oxygen correction | Does the rule require concentration corrected to a reference oxygen level? | Dilution air changes measured concentration without necessarily changing pollutant mass. |
| VOC reporting basis | Is the value compound-specific, total hydrocarbons, as carbon, or as a calibration gas such as propane? | Different reporting conventions are not interchangeable. |
| Sampling time | Were airflow and concentration measured during the same production condition? | A peak concentration paired with an unrelated airflow produces a false load. |
Preserve the basis reported by the laboratory and the applicable permit. Do not silently convert a result merely to make two columns look comparable.
How to convert ppmv to mg/m³
A ppmv value is a molar-volume fraction. Converting it to a mass concentration requires the molecular weight and the gas temperature and pressure. For an ideal gas:
mg/m³ = ppmv × molecular weight (g/mol) × absolute pressure (Pa) ÷ [8.314 × absolute temperature (K)] × 0.001
At 25°C and one atmosphere, this simplifies to approximately mg/m³ = ppmv × molecular weight ÷ 24.45. That shortcut is valid only for those stated conditions. It also requires a known compound or a clearly defined reporting equivalent.
For mixed exhaust, one molecular weight cannot accurately represent every VOC. The EPA's Method 18 addresses measurement of individual gaseous organic compounds by gas chromatography. Method 25A measures total gaseous organic concentration using a flame ionization analyzer. The method required by the applicable rule or permit should control the project calculation.
Device efficiency is not overall system efficiency
A treatment device can only control the VOC that reaches it. EPA defines capture efficiency as the percentage of emissions captured and vented to the control device. Hoods, enclosures, ducts and fans therefore affect the result alongside the RTO, carbon bed or other treatment stage. See the EPA capture-system guidance.
Overall system efficiency = capture efficiency × control-device efficiency
Convert the percentages to decimals before multiplying. If capture efficiency is 90% and device efficiency is 95%, overall system efficiency is 0.90 × 0.95 = 0.855, or 85.5%. Reporting the device's 95% result as the performance of the entire production line would overstate total control.
EPA Method 204 contains criteria and procedures associated with capture-efficiency evaluation. The correct test approach depends on the enclosure and applicable regulatory requirements; it should be agreed with qualified testing and permitting personnel.
How VOC mass loading affects treatment-system sizing
Airflow determines duct, fan and equipment cross-section. VOC mass loading determines the amount of pollutant that must be destroyed, captured or recovered. Two sources with the same airflow can therefore need very different systems.
| Technology | Why mass loading matters | Additional data needed |
|---|---|---|
| Activated carbon | Captured kg/day helps estimate media duty and replacement or regeneration frequency. | Compound-specific working capacity, humidity, temperature and breakthrough limit |
| RTO | VOC mass and composition affect heat release, auxiliary fuel demand and thermal controls. | Heating value, concentration peaks, inlet temperature, operating hours and heat recovery |
| Condenser | Recoverable mass affects condensate rate and the value of solvent recovery. | Vapor-liquid behavior, cooling conditions, water content and solvent mixture |
| Scrubber | Absorbed mass affects liquid circulation, reagent use and blowdown load. | Solubility, reaction chemistry, gas-liquid equilibrium and wastewater capacity |
| Concentrator plus oxidizer | The mass balance should close across adsorption, desorption and final destruction. | Concentration ratio, desorption flow, media compatibility and peak load |
Read the industrial VOC treatment guide for the complete technology shortlist. For the two common options, the RTO versus activated carbon guide explains how airflow, loading and operating schedule change lifecycle cost.
Carbon projects should continue with the activated carbon breakthrough and replacement guide. Thermal projects can use the RTO working-principle guide to connect VOC loading with heat recovery and monitoring.
Build a measurement plan that represents production
A calculation is only as good as the measurements behind it. Record startup, normal production, maximum throughput, product changeover, cleaning and shutdown when those states affect emissions. Batch plants should capture short peaks rather than relying only on a shift average.
EPA Method 2 and related methods are used for stack or duct gas-velocity and volumetric-flow measurements. EPA notes that Method 2 flow results are used with pollutant measurements to determine pollutant emission rates. The agency's promulgated test-method index lists the applicable flow, moisture, oxygen and organic-compound methods.
A useful measurement record includes:
- Sampling location and test method
- Production line, product, throughput and operating state
- Airflow, temperature, pressure, moisture and oxygen basis
- Inlet and outlet concentrations with the reporting equivalent
- Start and end time for each run
- Control-system setpoints and alarms during the run
- Any bypass, leakage, maintenance or abnormal condition
VOC treatment RFQ data checklist
- Normal, minimum and maximum airflow with actual or standard basis
- Normal and peak VOC concentration with wet/dry and reporting basis
- Compound list, approximate fractions and safety data
- Calculated normal and peak VOC mass loading in kg/h
- Temperature, humidity, oxygen, pressure and co-pollutants
- Daily and annual operating hours by production case
- Required outlet concentration, mass limit or removal efficiency
- Capture-efficiency information for the hood or enclosure
- Applicable test method, averaging period and permit conditions
- Future production expansion and credible upset cases
Need a VOC load review before equipment selection?
DeFa Environmental Equipment supplies industrial waste gas treatment equipment. Send the measured airflow, concentration profile, compound list, gas conditions, operating schedule and outlet requirement. The engineering team can compare treatment concepts using one consistent mass-balance basis.
Frequently asked questions
How do I convert mg/m³ to kg/h?
Multiply concentration in mg/m³ by airflow in m³/h, then divide by 1,000,000. Confirm that both values use compatible actual or standard, wet or dry conditions.
Can I calculate VOC removal efficiency from concentration alone?
Only when inlet and outlet flow rates and reporting bases are equivalent. Otherwise, calculate inlet and outlet mass rates separately and compare those values.
Are m³/h and Nm³/h interchangeable?
No. m³/h generally describes the gas at actual operating conditions, while Nm³/h refers to stated normal conditions. The precise reference temperature and pressure must be identified before conversion.
Why is overall VOC control lower than device efficiency?
The device treats only captured emissions. Overall performance combines capture efficiency with control-device efficiency, so losses at the hood or enclosure reduce the result for the complete production source.
