VAV System Theory

1. VAV System Theory • Air Side System Evaluation • General temperature control • Zone temperature selection • Flexibility to adapt to change • Energy efficiency • Ease of maintenance • Fire / smoke control • Indoor air quality • Vibration / noise

2. VAV System Theory General Temperature Control

3. VAV System Theory • General Temperature Control • Issue #1 : Control of Cooling • Modulating control of the valve provides good general temperature control. • Multiple stages of cooling provide fair general temperature control. • On-off control of a valve provides poor general temperature control.

4. VAV System Theory • General Temperature Control • Issue #2 : Sensor Placement • The system is controlled to achieve the setpoint only at the sensor location. • Sensor in the room ensures comfort in that specific room. • Sensor in return air duct reacts to average load; concentrated loads cause problems. • DDC controllers can do averaging, high select or low select of multiple sensors.

5. VAV System Theory • General Temperature Control • Ratings of Systems • Split Unit Fair • WCPU Poor • CAV Good • VAV Excellent

6. VAV System Theory Zone Temperature Selection

7. VAV System Theory • Zone Temperature Selection • Different zones have different loads requiring different setpoints and different amounts of cooling.

8. VAV System Theory • Zone Temperature Selection • The biggest contributors to air conditioning loads are lighting load and solar load. People and equipment loads are smaller. Incoming solarradiation - 100% Solar heatexcluded - 16% Solar heatadmitted - 84% Clear Plate Glass

9. VAV System Theory • Zone Temperature Selection • The cooling load in a zone varies through the day. Because of thermal inertia, the greatest building load occurs about 15:00. S Interior Zone E W

10. VAV System Theory • Zone Temperature Selection • Lighting : Relatively Fixed • Solar : Varies through day • Conduction : Varies through day • Ventilation : Varies with other loads • People : Varies with occupancy • Equipment : Varies with use of space

11. VAV System Theory • Zone Temperature Selection • Any air conditioning system must deal with varying loads. • Distributed systems such as fan coil units and package units allow localized control but are energy inefficient. • Not all zones will call for maximum cooling at the same time. A centralized system can be designed with a diversity factor (20%).

12. VAV System Theory • Zone Temperature Selection • A centralized Constant Air Volume (CAV) system is energy efficient but only responds to the average load, and does not allow individual zones to be controlled. For constant loads, a CAV system is best. • A centralized Variable Air Volume (VAV) system is the most economical way to provide zone control, both in terms of first cost and operating cost.

13. VAV System Theory • Zone Temperature Selection • Ratings of Systems • Split Unit Fair • WCPU Fair • CAV Poor • VAV Excellent

14. VAV System Theory Flexibility to Adapt to Change

15. VAV System Theory • Flexibility to Adapt to Change • One of the key criteria of an “Intelligent Building” is the flexibility to adapt to change. This is because as they way in which we do business changes, so too the way in which we use the office changes. • For example, increasing computerization changes the heat load, power, lighting, communication and LAN wiring requirements in the office.

16. VAV System Theory • Flexibility to Adapt to Change • As partitioning layout changes, the air conditioning system must be capable of cost effectively change the airflow patterns and temperature control requirements in each area.

17. VAV System Theory • Flexibility to Adapt to Change • Ratings of Systems • Split Unit Fair • WCPU Poor • CAV Fair • VAV Excellent

18. VAV System Theory Energy Efficiency

19. VAV System Theory • Energy Efficiency • Centralized systems are far more energy efficient than distributed systems. • Variable Air Volume systems are more energy efficient than Constant Volume Systems as they deliver the minimum amount of cooling required and do not overcool.

20. VAV System Theory • Energy Efficiency • Ratings of Systems • Split Unit Poor • WCPU Poor • CAV Fair • VAV Excellent

21. VAV System Theory Ease of Maintenance

22. VAV System Theory • Ease of Maintenance • The ease of maintenance of a system relates to the availability of skilled labour to service the equipment. • The increased complexity associated with the centralized systems is partially offset by the diagnostic information available from a BAS.

23. VAV System Theory • Ease of Maintenance • Ratings of Systems • Split Unit Excellent • WCPU Good • CAV Fair • VAV Good

24. VAV System Theory Fire / Smoke Control

25. VAV System Theory • Fire / Smoke Control • Only centralized systems can help control the egress of smoke during a fire. • If properly designed, the air conditioning system can work together with the pressurization system to provide a smoke spill system or supplementary pressurization.

26. VAV System Theory • Fire / Smoke Control • Ratings of Systems • Split Unit Poor • WCPU Poor • CAV Excellent • VAV Excellent

27. VAV System Theory Indoor Air Quality

28. VAV System Theory • Indoor Air Quality • The issue of Indoor Air Quality (IAQ) and Sick Building Syndrome started to become an issue in the west in the early 1980s. • The energy crisis of the 1970s resulted in energy efficient office buildings with sealed envelopes to minimize fresh air intake.

29. VAV System Theory • Indoor Air Quality • At the same time, new types of building materials (carpets, partitions, etc.) became popular. The new building materials used formaldehyde-based adhesives which gave off a irritating odour. • Chlorinated Hydrocarbons and Volatile Organic Compounds (VOCs) were also introduced into the space from cleaning solutions and other sources.

30. VAV System Theory • Indoor Air Quality • Legionnaire's disease increased awareness of bacteria, fungus and mould which lives in air conditioning systems. • Peoples’ jobs became more sedentary as personal computers became more common in the workplace.

31. VAV System Theory • Indoor Air Quality • Reduced fresh air ventilation • + • Increased number of pollutants in the office • + • Less activity in the job • = • Complaints (and litigation!) • Complaints are of headaches, irritated eyes, nose and throat and flu-like symptoms.

32. VAV System Theory • Indoor Air Quality • ASHRAE increased ventilation requirements in an office environment. • ASHRAE recommends measuring CO2 as an indicator of ventilation effectiveness. People generate CO2 and the ventilation system disperses it. Office CO2 should be <1000 ppm (ambient is 400 ppm).

33. VAV System Theory • Indoor Air Quality • VAV systems have been linked to indoor air quality problems. With a conventional VAV system, at part load the amount of fresh air introduced is reduced. • This can be countered by modulating the fresh air dampers or increasing the off-coil temperature setpoint but these approaches add to first cost or to operating cost.

34. VAV System Theory • Indoor Air Quality • Indoor air quality has not become an issue with local owners or tenants. It is unlikely to become a serious issue locally. • Local “air quality” problems are usually related to improper placement of fresh air intakes (i.e. next to a durian stand or beside a toilet exhaust).

35. VAV System Theory • Indoor Air Quality • Ratings of Systems • Split Unit Poor • WCPU Poor • CAV Excellent • VAV Good

36. VAV System Theory Vibration / Noise

37. VAV System Theory • Vibration / Noise • Localized units are inherently more noisy than centralized system. • Conversely, the ductwork can distribute noise from the fan or noise resulting from airflow. • The box in a VAV system provides additional acoustic buffering.

38. VAV System Theory • Vibration / Noise • Ratings of Systems • Split Unit Poor • WCPU Poor • CAV Fair • VAV Excellent

39. VAV System Theory VAV System Components

40. VAV System Theory Return Air Cooling Coil Fan Outdoor Air VAV Box Chilled Water Valve Mechanical Components Ceiling Diffusers

41. VAV System Theory • VAV System Components • Return Air • Typically from ceiling plenum • Represents average of all zones • Outdoor Air • Used to dilute contaminants in Return Air • Cooling Coil • Cools air stream by exposing it to coil with chilled water passing through

42. VAV System Theory • VAV System Components • Chilled Water Valve • Controls the water passing through coil and thus controls off-coil air temperature • Fan • Controls the amount of air in the system • VAV Box • Controls the amount of air going to a zone

43. VAV System Theory Room Temperature Sensor T Actuator Off-coil Air Temperature Sensor P Sensor Static Pressure Sensor Valve Actuator Variable Speed Drive Control Control Components

44. VAV System Theory • VAV System Components • Valve Actuator • Controls Chilled Water Valve • Off - Coil Temperature • Used to control valve actuator • Variable Speed Drive Control • Controls fan speed • Static Pressure Sensor • Used to control fan speed

45. VAV System Theory • VAV System Components • VAV Box Actuator • Controls air flow through box • P Sensor • Used to measure air flow through box • Temperature Sensor • Used to determine amount of air required

46. VAV System Theory VAV Control Theory

47. VAV System Theory • VAV Control Theory • Room temperature sensor determines amount of cold air required • P sensor measures actual amountof cold air • Actuator controls cold air to matchrequired amount

48. VAV System Theory • VAV Control Theory • When VAV box actuators open or close, the static pressure sensor detects the change • The variable speed drive control changes the fan speed to maintain a static pressure setpoint

49. VAV System Theory • VAV Control Theory • The off-coil sensor measures the air temperature • The chilled water valve is controlled to achieve a desired off-coil temperature

50. VAV System Theory VAV / CAV Comparison