1. Air Conditioning & Refrigeration:
MECH3005 – Building Services
Air Conditioning & Refrigeration:
Air-side Systems
Dr. Sam C M Hui
Department of Mechanical Engineering
The University of Hong Kong

2. Contents Fundamental HVAC Concepts Air Handling Units
Air System Basics
Fan-duct systems
Fan modulation & combinations
Year-round operation & economizers
Fan energy use & outdoor air

3. Fundamental HVAC Concepts
Five subsystems or loops of HVAC
Air-side
Chilled water
Refrigeration equipment
Heat rejection
Controls

4. 33 oC, 28 oC
27 oC
33 oC
25 oC
Conditioned space

5. 33 oC, 28 oC
27 oC
33 oC
25 oC
13 oC
13 oC
Air side system

6. Chilled water system 33 oC, 28 oC 27 oC 33 oC 25 oC 12 oC 12 oC 7 oC

7. Refrigeration equipment 33 oC, 28 oC 27 oC 33 oC 25 oC 12 oC 10 oC

8. Heat rejection 33 oC, 28 oC 27 oC 33 oC 25 oC 12 oC 10 oC 12 oC 49 oC

9. Control Loop 33 oC, 28 oC 27 oC 33 oC 25 oC 12 oC 10 oC 12 oC 49 oC

10. Individual room air-conditioning system

11. Primary air fan coil unit system

12. What type of air conditioning system it is?
Dual duct system

13. Air Handling Units

14. Air Handling Units Terminal unit or device Air handling unit (AHU)
Such as fan coil units, VAV boxes
Air handling unit (AHU)
Primary equipment of the air system
Handle & condition the air, control it to a required state, and transport it
Basic components:
Supply fan, water cooling coil, filters, mixing box, dampers, controls & outer casing
A return or relief fan is optional, so as a humidifier

15. Air Handling Units Types of AHUs: Horizontal or vertical
Draw-through or blow through
Factory-fabricated and field built-up
Rooftop and indoor
Make-up (primary) air
Recirculating

18. Air Handling Units Package units Rooftop package units
A self-contained air conditioner
Equipped with a DX (direct expansion) coil
“Air handler”: the portion inside that handles air
Rooftop package units
Gas heating / electric cooling
Electric heating / electric cooling
Heat pump system

20. Air Handling Units Indoor package units Split package units
Factory-assembly unit installed in a fan room or machinery room
Floor-mounted or ceiling-mounted
Cooling, cooling/heating, heat pump
Heat rejection:
Connected to an air-cooled condenser, OR
Water-cooled condenser
Split package units
Indoor air handler + outdoor condensing unit

21. Air Handling Units Rating conditions & minimum performance
ARI and ASHRAE standards
Rating indices:
Energy efficiency ratio (EER)
Seasonal EER
Integrated part-load value (IPLV)
Heating seasonal performance factor (HSPF)

22. Air Handling Units Coils Indirect contact heat exchangers
Heat transfer between air flowing over the coil and water, refrigerant, steam or brine insider the coil
Fins: extended (secondary) surfaces
Fin spacing and density
Water circuits
Number of water flow passages

23. Air Handling Units Direct expansion (DX) coil
Refrigerant is fed (e.g. R-22 and R-134a)
Air and refrigerant flow:
Usually counterflow and cross flow
Typical evaporating temperature = 3-10 oC
Condensate drain pan (to collect condensation)
Performance factors:
Face velocity, heat transfer coefficients, air-side pressure drop, physical size

24. Air Handling Units Water cooling coils - dry-wet coil
Chilled water flowing at 4-10 oC
Brine or glycol-water at 1-4 oC
Temperature rise (typical) = 7-14 oC
Water cooling coils - dry coil
Sensible cooling (dry); no condensation
Poorer heat transfer coefficient
Steam heating coil

25. Direct expansion (DX) coil
Water cooling coil

26. Water heating coil
Steam heating coil

27. Air Handling Units Air filters Air cleaning and filtration
Operating performance:
Efficiency or effectiveness of dust removal
Dust holding capacity
Initial & final pressure drop
Service life
Types: low-, medium-, and high-efficiency filters + carbon activated filters

28. Air Handling Units Test methods of air filters Weight arrestance test
For low-efficiency air filters
Atmospheric dust spot efficiency
For medium-efficiency air filters
DOP (dioctyl phthalate) penetration and efficiency test
For high-efficiency air filters
HEPA (high-efficiency particle air)
ULPA (ultra-low penetration air)

29. Activated carbon filter HEPA and ULPA filters
Low efficiency (panel-type)
Medium efficiency (bag-type)

30. Air Handling Units Humidifiers Selection of AHUs Steam humidifiers
Air washers
Selection of AHUs
Face velocity < 3 m/s to prevent entrained droplets
Proper size of the equipment (energy efficiency)
Medium-efficiency filter >50% dust spot eff.: improve IAQ, prevent smudging & discoloring
Air economizer: save energy

31. Steam grid humidifier

32. Air washer

33. Air-side Components
EcoAdvisor: Energy Trainer for Energy Managers: HAVC Module 1 [AV 697 E19]
HVAC components
Distribution equipment
Air-side components
Dampers, filters, coils, fans, ducts and plenums, terminal units, diffusers, humidifiers, dehumidifiers

34. Air System Basics

35. Air System Basics Fan-duct systems
Flow resistance R, pressure drop Δp and volume flow rate V
Duct sections in series:
Duct sections in parallel: o

36. Air System Basics Fan-duct systems Terminology
Primary air (conditioned air or makeup air)
Secondary air (induced space air, plenum air, or recirculating air)
Transfer air (indoor air that moves from an adjacent area)
System curve: volume flow vs pressure loss
System operating point

37. Air System Basics Fan Laws Speed (n) Volume flow (V)
Total pressure loss (Δp )
Air density (ρ)
For air systems that are geometrically & dynamically similar: (D = impeller diameter)

38. Air System Basics System effect Δpts
Its additional total pressure loss caused by uneven or non-uniform velocity profile at the fan inlet, or at duct fittings after fan outlet
Due to the actual inlet and outlet connections as compared with the total pressure loss of the fan test unit during laboratory ratings
Inlet
Outlet

39. Fan system operating point & system effect

40. Air System Basics Modulation of air systems Constant volume system
Volume flow rate remains constant; supply temperature is raised during part load
Variable-air-volume (VAV) system
Volume flow rate is reduced to match part load operation
Modulation curve

41. Fan modulation curve

42. Air System Basics Fan modulation methods
Damper (vary the opening of the air flow passage)
Waste energy
Inlet vanes (opening & angle of inlet vanes)
Low cost; less efficient than following types
Inlet cone (peripheral area of fan impeller)
Inexpensive; for backward curved centrifugal fan
Blade pitch (blade angle of axial fan)
Fan speed (using adjustable frequency drives)
Most energy-efficient; but more expensive

43. Damper, inlet vanes & fan speed modulation

44. Air System Basics Fan combinations in AHUs
Supply and exhaust fan/barometric damper
Used when no return duct or low return pressure loss
Barometric relief damper: to prevent excessive high space pressure
Suitable for systems w/ no air economizer mode & a low pressure drop in return system

45. Supply and exhaust fans

46. Air System Basics Fan combinations in AHUs Supply and relief fan
Relief fan to prevent excessive high space pressure
Usually operated in air economizer mode
Outdoor air damper is fully opened; recirculating damper closed
Suitable when pressure drop of return system < 75 Pa, or there is a considerable pressure drop in relief or exhaust flow passage

47. Supply and relief fans

48. Air System Basics Fan combinations in AHUs Supply and return fan
Return fan at the upstream of the junction of return, recirculating and exhaust flow passage (ru)
Has similar characteristics as supply+relief fan, but a higher total pressure at junction “ru”
Careful selection of return fan is needed to avoid excessive pressure in the duct system
Suitable when pressure drop of return system > 150 Pa, or those requiring a negative space pressure

49. Supply and return fans

50. Air System Basics Year-round operation of a VAV system
Region I: Refrigeration/evaporative cooling
Enthalpy of outdoor > that of recirculating air
Region II: Free cooling & refrigeration
Enthalpy of outdoor ≤ that of recirculating air
Region III: Free cooling, evaporative cooling & refrigeration
Region VI: Winter heating

51. Control diagram of a VAV reheat system for year-round operation

52. Year-round operation Region I: Refrigeration/evaporative cooling
Region II: Free cooling & refrigeration
Region III: Free cooling, evaporative cooling & refrigeration
Region VI: Winter heating

53. Air System Basics Economizer
A device consisting of dampers and control that uses free cooling capacity of either outdoor air or evaporatively cooled water from cooling tower instead of mechanical refrigeration
Air economizer:
Enthalpy-based
Temperature-based
Water economizer

54. Air System Basics Fan energy use For each l/s air supplied:
For system using separate outdoor ventilation:

55. Air System Basics Outdoor ventilation air supply Aims
Provide acceptable indoor air quality
Achieve energy efficiency in the system
ASHRAE Standard (IAQ standard)
Demand-based outdoor ventilation air control using CO2 as an indicator
CO2 concentration < 800-1,000 ppm
A specific indoor air contaminant can also be used

56. Air System Basics Outdoor ventilation air supply
Minimum outdoor air control
Type I: uses a CO2 or mixed gas sensor + DDC controller to control the volume flow rate of outdoor air
The best one but can be expensive
Type II: uses a CO2 or mixed gas sensor + DDC controller to control the ratio of the openings between outdoor and recirculating dampers
Suitable for VAV system

57. Air System Basics Outdoor ventilation air supply
Minimum outdoor air control (cont’d)
Type III: uses a flow sensor or a pressure sensor + DDC controller to control the dampers to provide nearly constant volume outdoor air intake
More complicated & may cause energy waste
Type IV: adjust the opening of outdoor damper manually to provide constant volume outdoor air
Mainly used for constant-volume systems