ADPI Room Air Diffusion Selection (ASHRAE)
Whether a supply outlet will actually mix the room, by the ASHRAE Air Diffusion Performance Index.
Example
You enter
- Outlet type circular-ceiling
- Room cooling load (Btu/hr-ft²) 40
- Catalog isothermal throw T (ft) 8
- Characteristic length L (ft) 10
You get
- T/L ratio 0.8
- Optimum T/L (max ADPI) 0.8
- Achievable ADPI at this load 88
- Throw to spec for max ADPI 8
- In band Yes
Checked against published worked examples from: Price Industries.
Details, formula, and sources
ScopeThe ratio of the catalog throw T to the room's characteristic length L predicts the ADPI, and each outlet type and cooling load has a T/L for maximum ADPI and a band over which ADPI stays above the comfort threshold. A round ceiling diffuser peaks at T/L = 0.8 (ADPI 88 at a 40 Btu/hr-ft^2 load); a high sidewall grille wants ~1.8 but a heavy 80 Btu/hr-ft^2 load caps it at ADPI 68 no matter the throw - the load, not the outlet, sets the ceiling. Reproduces the ASHRAE Fundamentals selection table (Miller/Nevins, throw per Std 70, ADPI per Std 113). A selection aid; the manufacturer's data and the design engineer govern.
Formula or tableratio = throw_ft / char_length_ft; look up the ADPI table row for the outlet type and nearest cooling load (20/40/60/80 Btu/hr-ft^2); in_band = band_lo <= ratio <= band_hi; target_throw = opt_ratio x char_length_ft.
Source and editionASHRAE Handbook -- Fundamentals, Space Air Diffusion chapter, the ADPI (Air Diffusion Performance Index) Selection Guide table (throw per ASHRAE Standard 70, ADPI per ASHRAE Standard 113; the Miller / Nevins Kansas State research), by name.
Free accessThe ADPI selection method and its throw-to-characteristic-length table are published in the ASHRAE Handbook -- Fundamentals and reproduced across the manufacturer engineering guides (Price, Krueger, Titus); the ratio arithmetic is public. The manufacturer's throw catalog governs the selection.
What governsEstimate. AHJ and licensed professional govern.
API field namesdiffuser_type, cooling_load, throw_ft, char_length_ft, ratio, opt_ratio, max_adpi, target_throw_ft, in_band
- T/L predicts ADPI the throw-to-characteristic-length ratio, looked up per outlet type and cooling load, gives the max-ADPI ratio and the above-threshold bandASHRAE Fundamentals, Space Air Diffusion
- Load caps ADPI a heavier cooling load lowers the achievable ADPI at the optimum throw (e.g. a high sidewall grille reaches only ADPI 68 at 80 Btu/hr-ft^2)ASHRAE Fundamentals, Space Air Diffusion
- Isothermal catalog throw T is the manufacturer's isothermal throw to the outlet terminal velocity (50 fpm most, 100 fpm ceiling slots); cooling mode; light-troffer, heating, and NC are separateASHRAE Standard 70 / 113