Good Engineering Practice Stack Height

Air flowing over a building does not close neatly behind it. It separates at the upwind edge and leaves a cavity of recirculating, highly turbulent air downwind, and anything released into that cavity is brought to the ground almost immediately rather than dispersing. Wind-tunnel work through the 1970s put the vertical extent of the disturbed zone at roughly 1.5 times the smaller of the building's height and its projected width, which is where the formula comes from: $H_{GEP} = h_b + 1.5L$. A 30 m building 40 m wide takes $L = 30$ m, the lesser of the two, giving a GEP height of 75 m. Reading L as the greater dimension is the standard mistake and it over-states the answer every time.

The regulatory history matters for understanding what this number is FOR. Through the 1960s and 70s the cheapest response to a ground-level concentration problem was a taller stack, and the tallest reached over 380 m — dispersion used as a substitute for control, and acid deposition exported hundreds of kilometres downwind. The 1977 Clean Air Act amendments closed that door by capping the stack height a source may take CREDIT for in its dispersion modelling. A source may build any height it likes; it simply may not model above GEP. So this equation sets a ceiling on credit, not a required height, and it is the only equation on this site whose purpose is to limit what you are allowed to claim.

Two provisions go with it. There is a floor of 65 m: any source may claim 65 m of credit regardless of what the buildings around it look like, so the operative GEP height is the greater of 65 m and the formula height. And the formula only applies to structures close enough to matter — the influencing building must lie within five times L of the stack, measured along the wind direction under consideration. Since L and the projected width both change with wind direction, a serious downwash analysis works through the full compass, and the governing structure is often not the obvious one. Modern regulatory practice runs EPA's BPIP pre-processor to sort this out and feeds the result into AERMOD's PRIME downwash algorithm rather than applying the formula by hand.

When the stack cannot be raised to GEP, and often it cannot, the alternatives are all about momentum and geometry. Raising the exit velocity keeps the plume out of the cavity for the same physical height. Removing the rain cap is worth more than most people expect: a flat cap deflects the plume sideways into the wake and is the single most common cause of an entrainment complaint, which is why a drain and a rain guard beat a cap. Grouping several small stacks into one taller flue raises both the height and the momentum. And where the receptor is a fresh-air intake on the same roof rather than the fenceline, ASHRAE's dilution method for intake separation is the right tool — GEP is a regulatory dispersion rule and was never meant to site an intake.