Exhaust Gas Collection System Design for VOC Sources

Ein exhaust gas collection system for VOC sources captures solvent vapor at the booth, flash-off zone or oven, moves it through ducting at a controlled flow, and delivers it to an abatement unit such as an activated-carbon adsorber. Sizing depends on captured airflow and solvent load, not on booth size alone.

What sits between the spray booth and the stack
An exhaust gas collection system is a chain, and each link hands its numbers to the next one:

• Capture device: the booth, enclosure, hood or oven exhaust port. This is the exhaust gas collector in the strict sense, the part at the source.
• Ducting and dampers: they carry the exhaust gas flow and balance it between branches.
• Pretreatment: particulate or mist filtration, and cooling where the stream is hot.
• Fan or fans: they set the flow the capture device actually sees.
• Adsorber: the activated-carbon stage that removes the solvent vapor.
• Monitoring and stack: the sensors that show whether the adsorber is still working, and the point where treated air leaves.

A mistake at the first link, such as pulling more air than the process needs, turns up later as a larger adsorber, a bigger fan and a shorter carbon life. This guide follows the chain in order and stops where sizing decisions get made.

Four emission points on an auto-parts coating line

Senserui’s HAC  lists all four of these process types among its typical applications, alongside chemical and pharmaceutical processes.

Setting the capture flow before choosing any equipment

Capture flow follows from the geometry of the opening and the inward air movement the process needs to hold vapor in. It does not follow from the adsorber. Extract too little and solvent escapes into the workshop. Extract too much and the same solvent mass is spread through more air, which means a bigger adsorber, more fan energy and a weaker inlet concentration for the carbon to work with.
To set the design flow:

1. List every emission point with its operating hours, and mark which ones run at the same time.
2. For each point, calculate the capture flow from the open area or enclosure geometry and the inward velocity your process or EHS specification requires.
3. Add the flows of sources that run simultaneously. Where sources alternate, take the larger duty case instead of the sum.
4. Draw the duct network and record the pressure loss to the farthest branch.
5. Balance the branches with dampers at commissioning and keep the readings as a baseline.
6. Check the total against the rated airflow on the adsorber quotation.

From airflow and concentration to carbon load

The solvent mass entering the adsorber each hour is:
Solvent mass rate (kg/h) = airflow (m³/h) × concentration (mg/m³) ÷ 1,000,000
Illustrative arithmetic with hypothetical values, not a Senserui specification: 20,000 m³/h at 50 mg/m³ delivers 1 kg of VOC per hour, or 8 kg over an eight-hour shift.
Hours until the carbon is saturated then follow from:
Hours = carbon mass (kg) × working capacity (kg VOC per kg carbon) ÷ solvent mass rate (kg/h)

Working capacity is not one number for “activated carbon”. It comes from the carbon supplier’s data for your specific solvent at your temperature and humidity, and a multi-solvent mix has to be assessed species by species.
Two consequences follow. First, doubling the airflow while the solvent mass stays the same does not double carbon consumption. It enlarges the adsorber and the fan, and it lowers inlet concentration, which typically lowers the working capacity the carbon can reach. Second, saturation is the upper bound on the interval, not the target. Solvent starts to appear at the outlet when the leading edge of the adsorption zone reaches the end of the bed, before the whole bed is full. The replacement interval is therefore set by breakthrough.

Protecting the carbon from mist, heat and moisture

Overspray mist that reaches the carbon coats the surface and blocks pores. The result is higher resistance to flow and less capacity, so a filter stage belongs upstream of the adsorber, with its own pressure reading. Heat works against adsorption as well: the warmer the gas, the less solvent carbon holds, so oven exhaust needs conditioning before it enters the bed. Moisture competes with solvent for adsorption sites in some conditions, which matters most in humid climates and with water-borne coatings mixed into the exhaust.
Senserui lists differential-pressure monitoring on HAC. Where the pre-filter stage sits inside a HAC-M layout is not stated on the product page, so confirm that in the quotation.

HAC / HAC-M: what Senserui has published

Senserui’s HAC / HAC-M Modular Exhaust Gas Treatment System is an activated-carbon adsorption unit. This table separates what the product page states from what you should still ask for.

Iodine value is an index of micropore development in the carbon. It is a quality indicator, not a guarantee of capacity for any particular solvent, so a figure on its own does not tell you how long a fill will last. Honeycomb carbon is generally chosen to keep resistance to flow low. HAC pressure drop is not published, so read fan power at your flow from the quotation.
Compared with common single-fan adsorber packages, a multi-fan modular layout lets several smaller fans share the airflow. Senserui describes this as demand-based operation for energy management across its systems. HAC-specific energy figures are not published.

Two alarms that look alike: pressure and breakthrough

Differential pressure measures resistance to flow. It rises when mist, dust or condensate clogs the media, and it says nothing about whether the carbon still adsorbs. A breakthrough monitor detects solvent leaving the carbon. The two signals answer different questions, and reading them together separates a maintenance problem from a capacity problem.

A saturated bed can show a low pressure drop, which is why a bed can look healthy on pressure alone while solvent passes through it.
To set the replacement interval from measurement rather than the calendar:

1. At commissioning, record the sizing basis (flow, concentration, operating hours) and the baseline pressure drop with clean carbon and a clean pre-filter.
2. Trend the breakthrough signal weekly, or through the IoT option if fitted, instead of waiting for the alarm.
3. Compare actual solvent consumption from purchase or usage records with the calculated load. A gap means the sizing basis was wrong.
4. Order replacement cassettes when the trend shows breakthrough approaching, allowing for lead time.
5. Rework the sizing basis after any change of coating, solvent or line speed.

Capture layouts compared

Adsorbing room air is usually impractical because the volume is large and the concentration is thin. Hooding the point of release costs more design effort but shrinks everything downstream.

What carbon adsorption does well and where it strains

Carbon adsorption suits streams like booth exhaust: large volume, low to moderate solvent concentration, no fuel or flame required. The cassette design confines maintenance to replacing carbon modules rather than rebuilding a bed.
The limits decide projects more often than the strengths do. Carbon is a consumable, so recurring cost scales with solvent mass rather than with airflow, and a high-concentration stream such as oven exhaust can shorten change-out intervals sharply. Adsorption transfers solvent from air to carbon; it does not destroy it, so spent carbon becomes a waste stream to handle under local rules. Hot or humid streams need conditioning before they reach the bed. For a line dominated by curing-oven load, the design stage should test whether all of that flow belongs on carbon.

Pricing and what to send for a quotation

Price: available on request based on system configuration and project requirements.
Configuration depends on airflow and VOC concentration, so the quotation needs these inputs:

1. Airflow per emission point and which points run simultaneously
2. Solvent list and hourly consumption, from safety data sheets and purchase records
3. Inlet temperature and humidity at the adsorber
4. Whether overspray mist reaches the duct
5. Operating hours and shift pattern
6. Duct layout with approximate run lengths
7. Local outlet limits and any required monitoring or reporting

FAQ

 What is the difference between an exhaust gas collector and an exhaust gas treatment unit?

The collector is the capture device at the source: a booth, hood or oven exhaust port. The treatment unit, such as an activated-carbon adsorber, sits downstream and removes the VOC from the captured stream. Errors in collector design change what the treatment unit receives.

 How is exhaust gas flow decided?

Per emission point, from the opening geometry and the inward air movement your process needs. Flows of sources that run at the same time are added, and the total is checked against the adsorber’s rated airflow.

 How often should activated carbon be replaced?

There is no universal interval. It depends on solvent mass rate, carbon mass and working capacity, and it is best set from the breakthrough trend. Senserui has not published a replacement interval for HAC.

Does Senserui publish an airflow range for HAC?

No. The product page says configurations are adapted to airflow and VOC concentration and gives no fixed numerical range.

 Is HAC ATEX certified or CE certified?

Senserui states company-level ISO 9001:2015 quality management, CE compliance and explosion-protection design based on ATEX principles. No product-specific certificate number is published for HAC, so request the certificates that apply to your delivered unit. The European Commission’s CE guidance places responsibility for identifying applicable requirements on the manufacturer.

Can one HAC unit serve both a booth and an oven?

It depends on whether their schedules, temperatures and concentrations are compatible. Combining them without a configuration review risks feeding hot, concentrated oven exhaust into a bed sized for dilute booth air.