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

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

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

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

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

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

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

Wind Effect  CIBSE provide K s and a values: BC 3722 HVAC Engineering TerrainKsKs a Open, flat country0.680.17 Country with scattered windbreaks 0.520.20 Urban0.350.25 City0.210.33

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

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

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

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

Wind Effect BC 3722 HVAC Engineering

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

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

Stack Effect BC 3722 HVAC Engineering

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Contaminant dispersion pattern in Indoor Shooting Range BC 3722 HVAC Engineering

THE END BC 3722 HVAC Engineering

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