Stacks
Discharge stacks: once it’s out, keep it out
The stack is the last part of the system, but it decides whether the cleaned air disperses or comes back in through air intakes, windows and the neighbours' homes. Many odour problems come from stacks that are too short or discharge too slowly.
Buildings create a recirculation zone around themselves
When wind blows over a building, it creates an area of swirling air over the roof and another behind the building. If the stack outlet sits inside this zone, the discharge is pulled down to circulate near the roof and walls, and can be drawn back in through the air intakes of other systems.
The size of the recirculation zone depends on the height and width of the building face that meets the wind. The bigger the building, the taller and longer the zone. A stack has to clear this zone, not just "poke out a little above the roof".
Effective stack height
The height that actually counts is not just the physical height of the stack. It is made up of three parts:
- The physical height of the stack
- The extra rise from the momentum of air discharged vertically
- The height lost when outside wind pushes the plume down behind the stack (downwash)
A good stack therefore needs both enough height and the right exit velocity.
Stack exit velocity
| Criterion | Reference value |
|---|---|
| Avoiding downwash | At least 1.5 times the wind speed |
| Commonly recommended velocity | About 15 m/s (depending on the site) |
| Keeping rain out while the fan runs | Above about 13 m/s |
A short stack with its outlet at roof level would need to discharge faster than 40 m/s to escape the recirculation zone, which is rarely worth the power and noise. Raising the stack above the zone is usually the better solution.
Rain caps are not recommended
The familiar cone-shaped rain cap deflects the exhaust back down towards the roof instead of letting it rise straight up, cancelling most of the benefit of the stack's height. Better design practice is to:
- Always discharge vertically upwards.
- Use a stackhead with an offset or overlapping sleeve design and a drain at the bottom of the stack, and include the extra elbow loss in the fan calculation.
- Design the exit velocity high enough that rain does not fall in while the system is running.
Stack position relative to air intakes
Before finalising the stack position, check how the plume will disperse in the prevailing wind, and make sure its path does not cross air-conditioning intakes, supply fans, windows or nearby buildings.
Things to check separately
The engineering principles above help a stack perform well, but they are not legal requirements. Factory emission standards and permit conditions, such as those for a Thai factory operating licence (Ror Ngor 4), must be checked separately against the latest notifications from the relevant authorities.