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Space Air Diffusion

Space Air Diffusion. Fundamentals. Objectives thermal comfort and indoor air quality even space air conditions acceptable air cleanliness acceptable noise level Classification of air jets free or confined isothermal or non-isothermal. Outlet. Air jet. Induced air. Fundamentals.

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Space Air Diffusion

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  1. Space Air Diffusion

  2. Fundamentals • Objectives • thermal comfort and indoor air quality • even space air conditions • acceptable air cleanliness • acceptable noise level • Classification of air jets • free or confined • isothermal or non-isothermal

  3. Outlet Air jet Induced air Fundamentals • Behaviour of free jet • increase in volume of air jet through induction of surrounding air with momentum transfer across the boundary • gradual approach of jet velocity and temperature towards ambient air

  4. Fundamentals • Terminology • throw: axial distance from outlet to where the jet velocity drops to a specified value, e.g. 0.75 m/s, 0.5 m/s or 0.25 m/s; denoted by Tv • entrainment ratio: Qx / Qo • spread: width of jet at a specified jet velocity, usually 0.5 m/s or 0.25 m/s • drop: vertical distance from outlet to where the jet velocity is 0.25 m/s; occurs with jet temperature lower than ambient • characteristic length: usually the horizontal distance from outlet to opposite wall or to mid-plane between two outlets where the jets meet

  5. Fundamentals • Surface effect of confined jet • low pressure region created by high jet velocity leading to induction of ambient air into the jet • jet being pushed against ceiling for further mixing to prevent premature entry to conditioned space

  6. outlet Air Flow Pattern • Mixing flow (induced recirculating flow) pattern • mixing of ambient air with primary air in the jet resulting in conditions of air jet approaching those of the ambient upon entry to the occupied zone • induced reversed air stream covering the occupied zone

  7. Air Flow Pattern • Position of supply air outlet and return air inlet • different combinations possible depending on applications • high level supply generally preferred for cooling

  8. Air Flow Pattern • Position of supply air outlet and return air inlet • low level perimeter type supply generally preferred for heating applications • location of return air inlet not so important as supply air outlet

  9. Supply Air Outlets • Ceiling diffusers • various types (square/circular, different ways of flow) • high induction ratio and good surface effect • particularly suitable for low headroom situations • specification of neck and face sizes

  10. Supply Air Outlets • Linear diffusers • linear in shape with varying no. of slots • adjustable deflecting vanes for flow direction control • excellent surface effect, particularly suitable for VAV systems with flow variation

  11. Supply Air Outlets • Side grilles • various types (fixed blade, single/double deflection) • low entrainment ratio, greater drop, long throw • not suitable for rooms with low ceiling and great depth

  12. Supply Air Outlets • Nozzles • long throw and small spread • for large space volume or for spot (localised) cooling

  13. Return Air Inlets • Different types (e.g. grille, louvre, linear) • Location not so critical • Noise level relatively more important (face velocity ranges from 1 to 4 m/s)

  14. Air Distribution Assessment • Effective draft temperature • combined various factors including air temperature and velocity and their deviation from control conditions •  = (tx – tr) – 8 (vx – 0.15) • for sedentary occupants with comfort • –1.7 oC  1.1 oC and v  0.35 m/s • Air Diffusion Performance Index (ADPI) • criterion: ADPI  80% (for cooling applications)

  15. Air Distribution System Design • Ascertain supply air volume flow rate based on cooling load • Select outlet type to suit actual situations • Air flow intensity for different outlets • Determine no. and arrange location

  16. Air Distribution System Design • Determine characteristic length

  17. Air Distribution System Design • Determine Tv/L ratio

  18. Air Distribution System Design • Calculate Tv by multiplying the Tv/L ratio by L • Determine size of outlet from catalogue by matching volume flow rate and Tv • Check other parameters • supply air velocity (< 4 m/s) • Throw and ADPI for whole flow range of VAV • Drop • Noise • Pressure drop

  19. Sample Diffuser Catalogue

  20. THE END

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