Measurement location on exhaust: why it matters already in the project phase

Why to address the measurement location already when designing technology

In industrial operations, exhausts from paint shops, boiler plants, welding shops, wood-processing operations, dust filters, combustion sources, dryers, chemical technologies or ventilation are often addressed. The designer usually deals with duct route, fan location, roof penetration and coordination with other trades. For emission measurement it is equally important whether safe and representative measurement will be possible on the exhaust in future.

The measurement location has a fundamental effect on result quality. If it is placed immediately behind a bend, behind a fan, behind a damper, behind a branch or in an area with swirling flow, sampling may not be representative. If it is on the roof without safe access, platform or space for measurement equipment, measurement may be practically impossible or unnecessarily expensive.

The measurement location is cheapest to address in the project. Subsequent modifications to ductwork, platforms, ladders, penetrations or measurement ports after completion of construction are usually expensive and often technically awkward.

For sources where operating permit, emission measurement or verification of emission limit compliance is expected, the measurement location should be part of the technical design from the start.

What representative measurement means

Representative measurement means that the sample taken actually corresponds to waste air discharged from the source. It is therefore not enough to make an opening in the duct "somewhere there is space". Flow in the duct must be sufficiently steady and the measurement profile must allow sampling at such points that the result is not distorted.

After bends, fans, dampers, cross-section changes or branches, uneven or swirling flow often develops in the duct. This is a problem especially for measurement of particulate pollutants, where it is necessary to determine not only concentration but also flow rate and sampling conditions correctly.

That is why technical standards for emission measurement address not only the measurement method itself, but also requirements for measurement section, measurement station and measurement plan.

Straight duct sections

Straight duct sections before and after the measurement profile are important so that flow calms after disturbing elements. The more complex the duct route, the greater the risk that air will not be evenly distributed at the sampling point.

In the project it is therefore advisable to design the measurement location in a part of the duct that is as far as possible from bends, fans, dampers, branches, narrowing, widening and discharge heads. It is not a formality. If measurement is carried out in an unsuitable profile, the result may be burdened with higher uncertainty or may be challenged during inspection.

From a design point of view the following should be addressed in particular:

• sufficiently long straight section before the measurement profile,
• sufficiently long straight section after the measurement profile,
• constant circular or rectangular cross-section,
• absence of dampers and control elements in close proximity,
• possibility of installing measurement ports in required number,
• space for probe handling.

If ideal straight sections cannot be maintained, the situation should be consulted in advance. Sometimes an acceptable alternative solution can be found, but it must be clear that the measurement location allows credible measurement.

Accessibility of the measurement location

The measurement location must be not only technically suitable but also accessible. The measurement technician needs to bring equipment safely to the exhaust, install probes, handle them, read instruments, take samples and sometimes work on site for a longer time.

Problems arise for example when the measurement port is placed high on the duct without a platform, on a sloping roof without securing, in a tight space above ceiling, behind technology without handling space or on a facade reachable only by aerial work platform.

Practically important: if measurement is planned, it is not enough to draw a port on the duct. The project must also address how people will walk to it safely and how work will be done there.

Work safety during measurement

Emission measurement is not brief placement of an instrument at an opening. For some methods work is done with sampling probes, heated lines, filters, pumps, analysers, pressure cylinders, electrical supply and sometimes hot or dusty air.

From a work safety point of view height, stability of working surface, guardrails, ladders, access routes, lighting, weather conditions, duct temperature, electrical connection and possibility of safe handling must be addressed.

For outdoor exhausts on the roof it is important to remember that measurement may take place in different seasons. A roof that is "theoretically" accessible in the project may not be safe in rain, frost, wind or when handling measurement equipment.

An unsuitably designed measurement location is not only a technical problem. It can also be a work safety problem and operator liability issue.

Measurement ports are not the same as a drilled hole

In practice there is sometimes an assumption that drilling a hole in the duct before measurement is enough. That can be a fundamental mistake, especially for measurement of particulate pollutants and where sampling must be carried out according to standard procedure in the measurement profile.

Measurement ports must be designed to allow insertion of the probe to required points in the cross-section, safe sealing, handling and repeated use. For larger ducts or rectangular channels several ports in different axes may be needed.

A drilled hole can be problematic for several reasons:

• it is not in the correct measurement profile,
• it does not allow sampling at all required points,
• it cannot be used safely and tightly,
• it is not suitable for probe diameter,
• it does not allow repeated measurement,
• it may not meet requirements of the measurement method.

For new technologies it is therefore advisable to address measurement ports as a routine part of ductwork, similar to inspection openings, dampers or service access.

Problems at building approval and operating permit

The measurement location is often addressed only when emission measurement must be documented after putting the source into operation. That may be too late. If ductwork is complete, technology started and construction nearing completion, every modification means intervention in the implemented work.

Problems may appear for example with operating permit, fulfilment of regional authority conditions, Czech Environmental Inspectorate inspection, building approval or first authorised emission measurement. If measurement cannot be carried out representatively and safely, the whole process may be complicated.

For some sources first emission measurement is carried out after putting the source into operation. If it is found only then that the measurement location is not usable, it may affect the schedule for documenting operating conditions.

What to address in project documentation

Project documentation for sources with possible emission measurement should contain not only exhaust location but also basic data on the measurement location. Ideally it should be clear where the measurement profile will be, what access will be, where measurement ports will be and whether a safe station is available.

For larger or permitted sources it is advisable for the HVAC designer, technologist, stack builder, structural engineer and emission measurement specialist to address the exhaust together. The measurement location can affect duct route, exhaust height, platform location and coordination with other technologies.

Project documentation should state in particular:

1. which exhausts discharge technological emissions,
2. which technology is connected to each exhaust,
3. where the measurement profile is designed,
4. whether straight duct sections are maintained,
5. number and position of measurement ports,
6. height of measurement location above floor or roof,
7. method of safe access,
8. working space for measurement equipment,
9. availability of electrical power,
10. link to future operating permit and emission measurement.

This description need not be long, but it must be specific. It saves much time during implementation, putting the source into operation and subsequent measurements.

Examples from practice

For a paint shop the booth exhaust may be designed correctly for ventilation, but without a suitable measurement section. If TOC or particulate pollutants are to be measured, sampling immediately behind the fan or behind a mixing chamber may be problematic.

In a wood-processing operation the exhaust from a wood dust filter is often addressed. If the measurement location is too close to the filter, bend or discharge head, TSP measurement may be problematic. For dust emissions representative sampling is especially sensitive.

For a boiler plant or diesel generator set the stack may be technically complete, but without measurement ports and safe platform. Subsequent installation then requires intervention in the stack, coordination with operation and sometimes construction modifications.

For a welding shop central extraction may end in an exhaust, but the project does not address whether it is measurable for particulate pollutants. If the operation falls under a listed source regime, absence of a measurement location may appear only when emission measurement is requested.

Most common mistakes

The most common mistake is that the measurement location is not addressed at all. The project contains an exhaust, but not a measurement profile, ports or access. The second common mistake is placing the port in a location that is structurally accessible but technically unsuitable for representative sampling.

The third mistake is underestimating work safety. A port on the roof without a platform or in a place without space for probe handling may be unusable in practice. The fourth mistake is confusing measurement of gaseous substances and measurement of dust. Gravimetric measurement of particulate pollutants has higher requirements for measurement profile, handling and sampling than simple orientational measurement of gaseous components.

Typical warning signs in the project:

• exhaust is drawn without measurement location,
• port is immediately behind bend or fan,
• duct is routed in an inaccessible place,
• measurement would require work from ladder,
• there is no space for probe insertion,
• it is unclear what each exhaust discharges,
• exhaust is confused with ordinary hall ventilation,
• technological and hygiene exhausts are not distinguished.

How to proceed on a new project

On a new project it is best to start with a simple question: which exhausts may in future be subject to emission measurement? This concerns especially listed sources, sources with emission limits, technologies with filtration, combustion sources, paint shops, dust technologies and equipment where the regional authority may set measurement as an operating condition.

It is then advisable to go through the duct route and verify whether a suitable measurement section can be created. If yes, measurement ports, working platform, access and space for measurement equipment are added. If not, it is better to adjust the duct route in the project than to address additional interventions later.

Good rule: an exhaust for which emission measurement is expected should be designed as measurable from the start. Not as ductwork where "somehow an opening will be made" when needed.

Summary

The measurement location on the exhaust is an important part of technology design. It affects representativeness of measurement, work safety, fulfilment of operating permit requirements and practical feasibility of authorised emission measurement. If it is forgotten, problems may arise at building approval, putting the source into operation or during inspection.

For new and modified technologies it is advisable to address measurement profile, straight duct sections, measurement ports, accessibility, working platform and work safety already in project documentation. Subsequent modifications are usually more expensive, more complicated and sometimes technically worse than correct design from the start.

Send us the HVAC project, exhaust schematic, technology description and expected source obligations. We will assess whether exhausts are measurable, whether they meet requirements for representative measurement and what is advisable to add to project documentation.

Factual basis of the article

The article is based mainly on the following sources:

• Decree No. 415/2012 Coll., on permissible pollution levels and their determination,
• Act No. 201/2012 Coll., on air protection,
• ČSN EN 15259 – Air quality – Measurement of emissions from stationary sources,
• ISO 15259:2023 – Air quality — Measurement of stationary source emissions.

Decree No. 415/2012 Coll. states that one-off emission measurement must be representative and credible and that the measurement and sampling method used must reflect actual pollution level as accurately as possible. ČSN EN 15259 and ISO 15259 address requirements for measurement sections, measurement stations, measurement objective, measurement plan and report on emission measurement from stationary sources.