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BC 3722 HVAC Engineering Semester A 2003/04 Dr. Richard K K Yuen Department of Building & Construction

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Aims of Ventilation System To remove any undesirable odours or contaminants in a space or living place To ensure the CO 2 produced by respiration is being replaced by an adequate supply of O 2 BC 3722 HVAC Engineering

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Types of Ventilation Natural Ventilation No mechanical means of supply and extraction of air Allow air to enter or leave the building due to pressure differences between outside and inside air Open windows, ventilators, shafts Mechanical Ventilation With mechanical means of supply and extraction of air Control the amount of air entering and leaving Fans BC 3722 HVAC Engineering

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Natural Ventilation Two basic driving forces: Wind Effect: Pressure on the face of a building due to rapid deceleration of the wind of the outside surface Stack Effect Buoyancy effect due to the temperature difference between the outside and inside air BC 3722 HVAC Engineering

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Wind Effect Wind speed varies with height above the ground This wind speed is related to wind pressure BC 3722 HVAC Engineering

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Wind Effect Roughness => depends on the type of bldgs From the eqt., z ↑u ↑ However, this is impossible for the wind speed to be infinity. To explain this, basic boundary theory is adopted. Basic boundary theory: When wind speed approaches the ambient wind speed, the height become the thickness of the boundary layer BC 3722 HVAC Engineering

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Wind Effect CIBSE provide K s and a values: BC 3722 HVAC Engineering TerrainKsKs a Open, flat country Country with scattered windbreaks Urban City

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Wind Effect A non-dimensional coefficient, C p wherep = mean pressure at any point on bldg surface p 0 = pressure in the undistributed air stream u r = mean wind speed at a height equal to the bldg height ρ= density of air at temp. of outside air BC 3722 HVAC Engineering

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Wind Effect Volumetric flow rate through opening, BC 3722 HVAC Engineering

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Wind Effect For a number of openings in the same face of bldg, since the pressure difference is the same for each opening, and assuming the same value of C d for each opening BC 3722 HVAC Engineering

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Wind Effect Assume air enters through openings on windward side of bldg and leave bldg through openings on leeward side In steady state, and assume BC 3722 HVAC Engineering

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Wind Effect BC 3722 HVAC Engineering

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Stack Effect – Buoyancy flow BC 3722 HVAC Engineering 12 tiρitiρi toρotoρo urur toρotoρo A 1 and A 2 = area of openings separate by a height z ρ i and ρ o = mean density of outside and inside air respectively Δρ = difference in density between outside and inside air

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Stack Effect BC 3722 HVAC Engineering In steady state, mass flow entering the bldg at section 1 is equal to the mass flow leaving the bldg at section 2

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Stack Effect BC 3722 HVAC Engineering

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Stack Effect BC 3722 HVAC Engineering Two assumptions can be made with good accuracy: (i)Density if air at mean of the inside and outside temp, ρ, can be put equal to the densities, ρ o and ρ i (ii)Ratio Δρ/ρ can be put equal to ΔT/T, where T (in K) is the mean temp. of the absolute temp. of the inside and outside air

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Mechanical Ventilation Air infiltration air leakage through cracks around windows, doors Uncontrolled and undesirable Displacement ventilation Air is supplied at low velocity through large number of openings in ceiling or floor, and push across the cross- section of the space taking all the contaminants with it Dilution ventilation Air is introduced through jets into the room, jet set up a mixing pattern in room, thus diluting the contaminant in space before extract BC 3722 HVAC Engineering

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Threshold Limiting Value (TLV) 1. As a time-weighted average concentration for a working day, or working week, to which workers may be subjected without adverse effects 2. As a maximum concentration to which workers can be subjected for a short time period, say up to 15 minutes * Lower explosive limit: concentration which must never be exceeded at any time BC 3722 HVAC Engineering

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Dilution of contaminants Concentration, C, at any time τ {C i V + V c – C(V+V c )}dτ=d(CV) BC 3722 HVAC Engineering

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Then, the concentration, C, at any timeτis given by, BC 3722 HVAC Engineering Dilution of contaminants (Cont’d)

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Special Cases Case 1. No contaminant in incoming air (i.e. C i =0) BC 3722 HVAC Engineering 00

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Special Cases Eqn(1.62) case1 Eqn(1.64) case3 Eqn(1.63) case 2 Eqn(1.65) case 4 1 C c =C i V+V c /V+V c CoCo Concentration, C Time, t BC 3722 HVAC Engineering

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Special Cases Case 2. Production of contaminants stopped atτ=0 if C i =0 and c =0 BC 3722 HVAC Engineering

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Special Cases BC 3722 HVAC Engineering Eqn(1.62) case1 Eqn(1.64) case3 Eqn(1.63) case 2 Eqn(1.65) case 4 1 C c =C i V+V c /V+V c CoCo Concentration, C Time, t

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Special Cases Case 3. Space is uncontaminated at time τ (i.e. C o =0) BC 3722 HVAC Engineering 0

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Special Cases BC 3722 HVAC Engineering Eqn(1.62) case1 Eqn(1.64) case3 Eqn(1.63) case 2 Eqn(1.65) case 4 1 C c =C i V+V c /V+V c CoCo Concentration, C Time, t

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Special Cases (Con’t) Case 4. Sudden failure of ventilation system at time τwhen conc. is C o ( =0) BC 3722 HVAC Engineering 0

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Special Cases BC 3722 HVAC Engineering Eqn(1.62) case1 Eqn(1.64) case3 Eqn(1.63) case 2 Eqn(1.65) case 4 1 C c =C i V+V c /V+V c CoCo Concentration, C Time, t

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Special Cases (Con’t) Case 5. Equilibrium condition BC 3722 HVAC Engineering

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Special Cases BC 3722 HVAC Engineering Eqn(1.62) case1 Eqn(1.64) case3 Eqn(1.63) case 2 Eqn(1.65) case 4 1 C c =C i V+Vc /V+V c CoCo Concentration, C Time, t

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Design of Ventilation system Ventilation rate By the eqt. Ideal volumetric flow rate Distribution of Air Effects of: Mixing Dilution Removal Cannot be derived from the eqt. shown above BC 3722 HVAC Engineering

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Design of Ventilation system Ventilation can be assessed through Experimental measurement Computational Fluid Dynamics (CFD) Examples: 1. Public Transport Interchange (PTI) 2. Shooting Range BC 3722 HVAC Engineering

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PTI The velocity flow field of a PTI in BC 3722 HVAC Engineering

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Contaminant dispersion pattern in Indoor Shooting Range BC 3722 HVAC Engineering

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THE END BC 3722 HVAC Engineering

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