S3 Lec 6 (Air Distribution) Note

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Semester 3
CB303 Ventilation and Air Conditioning
Note
Lec 6 (Air Distribution)

S3 Lec 6 (Air Distribution) Note

  1. 1. SUPPLY DEVICES AND ROOM AIR DISTRIBUTION
  2. 2. SUPPLY DEVICES AND ROOM AIR DISTRIBUTION • Heating, Ventilation and Air Conditioning • Provides comfort for people • Allows humans to exist under adverse conditions.
  3. 3. Comfort • Comfort is primary intent of HVAC systems. • Productivity • Building Durability • Health • Mold
  4. 4. Supply Devices and Room Air Distribution • One way to condition air is to use a fan to move the air over the conditioning space. • The components that make up the forced-air system are :  Blower (fan)  Air supply system  Filter  Balancing damper  Return air system  Grillers  And registers where the circulated air enters the room and returns to the conditioning equipment
  5. 5. Basic HVAC Equipment • Fans / Blowers • Furnace / Heating unit • Filters • Compressor • Condensing units • Evaporator (cooling coil) • Control System • Air Distribution System
  6. 6. Types of fans • The fan or blower as it is sometimes called, can be described as a device that produces airflow or movement. • The fan provides the pressure difference to force the air into the duct system, through grilles and registers and into a room. • Several different types of fans produce this movement such as propeller fan, axial-flow fans and centrifugal fans.
  7. 7. THE PROPELLER FAN • The propeller fan is used in exhaust-fan and condenser- fan applications. • It will handle large volumes of air at low pressure differentials. • The propeller fan can be cast iron, aluminum or stamped steel and is set into a housing called a venturi to encourage airflow in a straight line from one side of the fan to the other(Figure above). • The propeller fan makes more noise than the centrifugal fan so it is normally used where noise is not a factor.
  8. 8. THE CENTRIFUGAL FAN • The centrifugal fan has characteristics that make it desirable for duct work. • It builds more pressure from the inlet to the outlet and moves more air against more pressure. • This fan has a forward curved blade and a cutoff to shear the air spinning around the fan wheel (Figure above). • The centrifugal fan is very quiet when properly applied.
  9. 9. AXIAL FAN • The axial-flow fans have blades that force air to move parallel to the shaft about which the blades rotate. • Axial fans blow air along the axis of the fan, linearly, hence their name. • This type of fan is used in a wide variety of applications, ranging from small cooling fans for electronics to the giant fans used in wind tunnels • Axial flow fans are applied for air conditioning and industrial process applications. • Standard axial flow fans have diameters from 300-400 mm or 1800 to 2000 mm and work under pressures up to 800 Pa.
  10. 10. System Types and Common Terms • Packaged Rooftop Unit • Split System • Heat Pump • Geothermal • Air to Air • Hydronic (water) • PTAC / PTHP Constant Volume Variable Volume Indoor Air Quality Direct Expansion
  11. 11. Packaged Rooftop Units
  12. 12. FURNACE A/C UNIT Ductwork Heating and Cooling Equipment Temperature & Humidity Controls Split System
  13. 13. Heat Pump • Operate on simple refrigeration cycle • Reversing the cycle provides heating • Temperature limitations • Air to air • Water source • Geothermal • Lake coupled
  14. 14. Geothermal Heat Pump Systems
  15. 15. Variable Air Volume T T Variable Speed Return Fan ReturnAir Return Ducts Zone Thermostat Reheat Coil VAV Box Supply Ducts Variable Speed Supply Fan Filters CoolCoilw/TempReset HeatCoilw/Temp Reset HVAC-16
  16. 16. Terminal Units Variable volume: Parallel Constant volume: Series
  17. 17. Hydronic systems • Pumps • Piping • Valves
  18. 18. Control Devices • Thermostats – Manual – Programmable • Optimum Start • DDC Systems • Variable Speed Drives • Automatic Valves and Dampers • Outdoor Sensors
  19. 19. Major Equipment • Chillers • Boilers • Cooling Towers
  20. 20. Economizers Air Side Water Side
  21. 21. Economizers Free cooling source: When available, use cool outdoor air instead of mechanically cooled air. 55 oF 80 oF Minimum supply of outside air Normal Operation Outside air dampers are positioned to provide the minimum outside air Economizer Operation Outside air dampers are fully open. Maximum outside air is provided 80 oF 55 oF and up 85% outside air 85% exhaust HVAC-22
  22. 22. Zoning and Economizers • Economizers provide “free cooling” when outdoor conditions are optimal • Proper orientation & zoning yields comfort & efficiency N S W ECore HVAC-23
  23. 23. Air Distribution • Ductwork – Metal – Flexible – Ductboard • Grilles, Louvers, & Registers • Dampers – Shut off – Fire – Smoke • Sealants • Supports
  24. 24. Return Plenum Problems HVAC-25
  25. 25. Additional Equipment • Energy Recovery Units • Desiccant Systems
  26. 26. Additional Equipment • Heat Exchangers • Humidifiers • Silencers
  27. 27. Mechanical Dehumidification Return air is mixed with ventilation air Cold coil condenses moisture Heat is added back (electric or gas) so that room air is not over cooled- Reheat Filter
  28. 28. Historical Minimum Ventilation Rates (cfm/person) 0 10 20 30 40 50 60 70 1835 1842 1849 1856 1863 1870 1877 1884 1891 1898 1905 1912 1919 1926 1933 1940 1947 1954 1961 1968 1975 1982 1989 1996 Tredgold 1836 Nightengale 1865 Billings 1895 Flugge 1905 Yaglou 1938 ASHRAE 62-73 ASHRAE 62-81 Smoking 62-81 ASH- RAE 62-89 Smoking 62-89
  29. 29. Improved Ventilation Effectiveness • Mechanically provide filtered and dehumidified outdoor air to the breathing space • Vary ventilation based on the number of occupants and process loads - changes in occupancy can be measured by CO2 sensors • Consider designs that separate ventilation and space conditioning • Utilize heat recovery systems to reduce system size and ventilation energy costs
  30. 30. Improved Ventilation Effectiveness • Effective mixing of ventilation air within space • Net positive pressure in the southeast; exhaust from appropriate spaces • Provide clean outdoor air, avoid: – loading docks – exhaust vents – plumbing stacks – waste collection – stagnant water
  31. 31. Additional Information / Resources • ASHRAE – The American Society of Heating, Refrigerating and Air-Conditioning Engineers – www.ashrae.org • Southface Energy Institute www.southface.org • Geothermal heat pump consortium www.geoexchange.org • www.buildingscience.com • www.energycodes.gov

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