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BC 3722 HVAC Engineering Semester A 2003/04

BC 3722 HVAC Engineering Semester A 2003/04. Dr. Richard K K Yuen Department of Building & Construction. BC 3722 HVAC Engineering. Aims of Ventilation System. To remove any undesirable odours or contaminants in a space or living place

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BC 3722 HVAC Engineering Semester A 2003/04

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

  2. BC 3722 HVAC Engineering Aims of Ventilation System • To remove any undesirable odours or contaminants in a space or living place • To ensure the CO2 produced by respiration is being replaced by an adequate supply of O2

  3. 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

  4. 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

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

  6. 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

  7. BC 3722 HVAC Engineering Wind Effect • CIBSE provide Ks and a values:

  8. BC 3722 HVAC Engineering Wind Effect • A non-dimensional coefficient, Cp where p = mean pressure at any point on bldg surface p0 = pressure in the undistributed air stream ur = mean wind speed at a height equal to the bldg height ρ= density of air at temp. of outside air

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

  10. 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 Cd for each opening

  11. 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

  12. BC 3722 HVAC Engineering Wind Effect

  13. 2 ur ti ρi to ρo to ρo 1 BC 3722 HVAC Engineering Stack Effect – Buoyancy flow A1 and A2 = area of openings separate by a height z ρi and ρo= mean density of outside and inside air respectively Δρ = difference in density between outside and inside air

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

  15. BC 3722 HVAC Engineering Stack Effect

  16. BC 3722 HVAC Engineering Stack Effect • Two assumptions can be made with good accuracy: • Density if air at mean of the inside and outside temp, ρ, can be put equal to the densities, ρo and ρi • 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

  17. BC 3722 HVAC Engineering 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

  18. BC 3722 HVAC Engineering Threshold Limiting Value (TLV) • As a time-weighted average concentration for a working day, or working week, to which workers may be subjected without adverse effects • 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

  19. BC 3722 HVAC Engineering Dilution of contaminants • Concentration, C, at any time τ {CiV + Vc – C(V+Vc)}dτ=d(CV)

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

  21. 0 0 BC 3722 HVAC Engineering Special Cases Case 1. No contaminant in incoming air (i.e. Ci=0)

  22. 1 Concentration, C Eqn(1.65) case 4 Co Eqn(1.62) case1 Cc=CiV+Vc /V+Vc Eqn(1.64) case3 Eqn(1.63) case 2 Time, t BC 3722 HVAC Engineering Special Cases

  23. 0 0 0 0 0 0 0 0 BC 3722 HVAC Engineering Special Cases Case 2. Production of contaminants stopped atτ=0 if Ci=0 and c=0

  24. 1 Concentration, C Eqn(1.65) case 4 Co Eqn(1.62) case1 Cc=CiV+Vc /V+Vc Eqn(1.64) case3 Eqn(1.63) case 2 Time, t BC 3722 HVAC Engineering Special Cases

  25. 0 BC 3722 HVAC Engineering Special Cases Case 3. Space is uncontaminated at time τ (i.e. Co=0)

  26. 1 Concentration, C Eqn(1.65) case 4 Co Eqn(1.62) case1 Cc=CiV+Vc /V+Vc Eqn(1.64) case3 Eqn(1.63) case 2 Time, t BC 3722 HVAC Engineering Special Cases

  27. 0 BC 3722 HVAC Engineering Special Cases (Con’t) Case 4. Sudden failure of ventilation system at time τwhen conc. is Co ( =0)

  28. 1 Concentration, C Eqn(1.65) case 4 Co Eqn(1.62) case1 Cc=CiV+Vc /V+Vc Eqn(1.64) case3 Eqn(1.63) case 2 Time, t BC 3722 HVAC Engineering Special Cases

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

  30. 1 Concentration, C Eqn(1.65) case 4 Co Eqn(1.62) case1 Cc=Ci V+Vc/V+Vc Eqn(1.64) case3 Eqn(1.63) case 2 Time, t BC 3722 HVAC Engineering Special Cases

  31. BC 3722 HVAC Engineering 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

  32. 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

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

  34. BC 3722 HVAC Engineering Contaminant dispersion pattern in Indoor Shooting Range

  35. BC 3722 HVAC Engineering THE END

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