Exhaust and NOx dispersion studies

Emergency generators, boilers and idling trucks all sit close to the air handlers that feed a building. On most days the wind carries their exhaust away. On some days it carries it straight into an intake, and the building draws diesel exhaust into its air supply.

A dispersion study finds those days before the equipment is placed, and answers:

Does exhaust from a generator, boiler stack or loading dock reach an air intake? At what concentration?
Which wind directions are the problem?
Where should a new intake, stack or generator go, and how tall does a stack need to be?
Does a parapet, a wind screen or exhaust aftertreatment (DEF) bring the numbers under the limit?

Our CFD engineers build the site, run it in the wind cases you care about, and report what reaches each intake. Most of our studies have been for hospitals, where air handlers sit a short walk from generators, ambulance bays and loading docks. The same questions come up at data centers, where generators and chillers crowd the equipment yard.

Same generator, same wind: two stacks

An isometric view of a building with an air handler on its roof and a generator enclosure beside its near corner. Wind blows toward the building from the front of the picture. Exhaust from the generator's short stack, below the roof line, is carried onto the roof and into the air handler's intake. — Diagrams, not results. Where a plume goes depends on the buildings around it, the wind, and the exhaust's heat and speed. That is what a study works out.

Sources we’ve modeled

Emergency generators, permanent and temporary.
Boiler stacks.
Idling and cruising trucks at loading docks, and ambulances at emergency bays.
MRI quench vents, which release helium when a magnet is shut down.
Parked cars near a low intake.

The rest of the site goes in the model too: cooling towers, exhaust fans and the other air handlers all move air, and change where a plume goes.

What you get

A table of concentrations at every intake and louver, for each wind case: NO₂ in parts per billion and CO in parts per million, each against its limit.
The limits, with their source. The NO₂ limit of 100 ppb and CO limits of 35 ppm (1-hour) and 9 ppm (8-hour) are EPA’s National Ambient Air Quality Standards. ASHRAE Standard 62.1 points to them for outdoor air. We quote them as EPA’s.
Maps and pathlines that show where each plume goes and why.
What-if cases: aftertreatment on or off, a taller or moved stack, a parapet or wind screen, a relocated intake.

What we need from you

The site: a plan, CAD or Revit model with the buildings, intakes, louvers and exhaust points.
Each source: its exhaust flow and temperature, and its emission rates from the manufacturer’s data sheet.
The intakes: the airflow each one draws.
The wind: a local wind rose, or the directions you’re worried about. Most studies run two wind cases, such as the prevailing summer and winter winds.

Why CFD, and not a plume model

Regulatory dispersion models such as EPA’s AERMOD are built to predict concentrations at ground level, hundreds of meters to kilometers downwind. They treat nearby buildings as a correction. At an air intake a few dozen feet from a stack, the building is the whole story: its wake, the recirculation over its roof and the gaps between buildings decide where the exhaust goes. CFD models the air around your actual buildings, so it can answer the near-field question a plume model can’t.

How a study runs

You send the site and the sources.
We agree the wind cases and the intakes that matter with you.
Our engineers build the site, run each case and send you a report with the results above.
We walk you through the results.