1. HCB 3-Chap 19A: All-Air Systems_Single Zone
Chapter 19A: ALL-AIR SYSTEMS: SINGLE ZONE AND SINGLE DUCT Agami Reddy (rev May 2017)
Introduction and common system types
Single zone systems- CAV and VAV operation
CAV systems without reheat- summer design
Air stream heating due to fans
CAV systems with reheat under summer design
CAV systems- winter design
Part-load operation of CAV systems
Part-load operation of VAV systems
Control issues and benefits of VAV systems
HCB 3-Chap 19A: All-Air Systems_Single Zone

2. HCB 3-Chap 19A: All-Air Systems_Single Zone
Introduction
Large number of all-air system variations to fit different requirements for comfort in buildings, process applications and also for special applications requiring close control of temperature and humidity (clean rooms, computer rooms, hospital operating rooms, …)
An all-air system provides complete sensible and latent cooling, preheating, and humidification capacity to the building supply air
No additional cooling or dehumidification required at zone (but heating may be required)
Basically two categories:
(i) single-duct systems: for single zone and multiple zone buildings
all heating, cooling and dehumidification is done within a common duct distribution system
(ii) dual- duct systems: for multiple buildings only (being phased out)
Two ducts- one carrying cold air and another hot air stream
HCB 3-Chap 19A: All-Air Systems_Single Zone

3. Common Types of All-Air Systems
Single duct
(a) Constant Air Volume (CAV): - single space
- multiple-zone reheat
(b) Variable Air Volume (VAV): - variable volume only
- reheat
- induction
- fan powered
2) Dual duct/stream
(a) Dual duct: - constant air volume
- variable air volume
(b) Multizone: - constant air volume
- three-deck or “Texas” multizone
HCB 3-Chap 19A: All-Air Systems_Single Zone

4. HCB 3-Chap 19A: All-Air Systems_Single Zone
Single Zone Systems
Operating principle of CAV and VAV under peak design cooling and heating conditions
Operation of CAV under part load
Operation of VAV under part load
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5. HCB 3-Chap 19A: All-Air Systems_Single Zone
Single Zone Systems
Air from room
is recycled
for energy
efficiency
Fig The three sets of dampers and their locations which are meant to inversely vary the return and exhaust air flows in unison, and also the outdoor air ventilation amount as needed.
HCB 3-Chap 19A: All-Air Systems_Single Zone

6. HCB 3-Chap 19A: All-Air Systems_Single Zone
Single Zone Single Duct CAV Systems - Summer Peak sized so that no reheat is required
Fig Schematic diagram of the basic single zone HVAC system with some of the major energy equipment. The important psychrometric state points (0-9) and the return, supply, exhaust and ventilation flows are also shown.
HCB 3-Chap 19A: All-Air Systems_Single Zone

7. Example illustrating analysis during peak or
design conditions of CAV system
RH=
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8. HCB 3-Chap 19A: All-Air Systems_Single Zone
Fig The air-conditioning process diagram on a psychrometric chart. The state points correspond to those in Fig while the numerical values apply to Example
HCB 3-Chap 19A: All-Air Systems_Single Zone

9. HCB 3-Chap 19A: All-Air Systems_Single Zone
Solution
This allows point 1 to be fixed
HCB 3-Chap 19A: All-Air Systems_Single Zone

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11. HCB 3-Chap 19A: All-Air Systems_Single Zone
Air Stream Heating Due to Fans
HCB 3-Chap 19A: All-Air Systems_Single Zone

12. HCB 3-Chap 19A: All-Air Systems_Single Zone
For spaces with high latent loads- reheat may be required even under peak summer design conditions
Fig Process diagram for single duct CAV system to air-condition a single space with high latent loads. The state points correspond to Fig while the numerical values apply to Example Note that the supply fan reheat is neglected.
HCB 3-Chap 19A: All-Air Systems_Single Zone

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Note:
-substantial increase in supply air flow rate
(from 17,140 lba/h to 26,667 lba/h
-substantial additional cooling
(from 178 kBtu/h to kBtu/h)
- The increase in cooling is equal to reheat = 69 kBtu/h
HCB 3-Chap 19A: All-Air Systems_Single Zone

16. Single Zone Single Duct CAV System Winter Operation
Cooling coil is inactive
Fig. 19.5a Schematic diagram of the complete single-duct single zone CAV system with ducted return. The preheater and the steam humidifier would operate during winter while the cooling coil and the reheat coil would be active when cooling loads are to be met.
HCB 3-Chap 19A: All-Air Systems_Single Zone

17. HCB 3-Chap 19A: All-Air Systems_Single Zone
Example 19.1.
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Fig. 19.5
Schematic
diagram
(b) Psychrometric
process
HCB 3-Chap 19A: All-Air Systems_Single Zone

19. HCB 3-Chap 19A: All-Air Systems_Single Zone

20. HCB 3-Chap 19A: All-Air Systems_Single Zone4. 5.
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21. HCB 3-Chap 19A: All-Air Systems_Single Zone6.
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22. HCB 3-Chap 19A: All-Air Systems_Single Zone
Alternative system:
Not requiring a
steam boiler
Fig. 19.8
(a) Schematic of the winter heating and humidification system with injection of liquid water. The cooling coil is not shown.
(b) Psychrometric process diagram with state points.
HCB 3-Chap 19A: All-Air Systems_Single Zone

23. HCB 3-Chap 19A: All-Air Systems_Single Zone
Part-load operation of CAV
(Example 19.1)
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Fig. 19.1
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Note:
Energy penalty
due to reheat 6.
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27. HCB 3-Chap 19A: All-Air Systems_Single Zone
Variable Air Volume
(VAV) Systems
Fig (a) Terminal reheat configuration for a VAV system for a single zone ducted return system for a small laboratory
Great energy savings compared to CV systems under part-load operation:
VAV systems modulate air flow as load reduces while keeping supply air
temperature constant at cooling coil leaving air temperature. This reduces both
cooling load on coil, fan power and avoids reheat
Supply air flow modulation is achieved by having terminal boxes
at each zone AND having a variable speed supply fan
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28. HCB 3-Chap 19A: All-Air Systems_Single Zone
Part-load operation of VAV System
Example 19.5
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For CAV = 7.03 T
For CAV = 3.24 T
For CAV =10.27 T
No reheat needed
HCB 3-Chap 19A: All-Air Systems_Single Zone

31. HCB 3-Chap 19A: All-Air Systems_Single Zone
Comments:
In the above example, the reduced airflow has eliminated the need for reheat but has somewhat compromised occupant comfort due to the slightly elevated humidity in the space (RH increased from 45% to 49%).
The flow reduction of ( /17,140) = is right at the lower end of the norm allowed.
Had it been lower, the proper operating strategy would be to provide a small amount of reheat at the entrance to the space
HCB 3-Chap 19A: All-Air Systems_Single Zone

32. Different types of VAV Operation: Cooling only
Fig Modulation of room supply air flow for a single zone VAV system as space loads vary over the year. Note the three modes of operation: (a), (b) and (c).
Design flow: range cfm/ft2
Different types of VAV Operation:
Cooling only
VAV reheat (because minimum supply air flow reached)
Heating only
HCB 3-Chap 19A: All-Air Systems_Single Zone

33. Single Zone CAV
Pros:
Used for simple rooms with similar conditioning requirements over year (interior rooms)
Simple to build and operate
Lower initial cost
Used in:
Small commercial
Strip malls
Elementary
Small health clinic
Auditoriums
Theaters
Cons:
Higher energy costs during operation
Not energy efficient for rooms whose loads vary over year
HCB 3-Chap 19A: All-Air Systems_Single Zone

34. Single-zone VAV
Can be used for single zone (many rooms)
Pros:
Used for simple rooms whose conditioning requirements are similar but varies during day
Simple to build (inexpensive) and operate
Used in:
Small commercial
Strip malls with different types of stores
Small health clinic with different specialization
Cons:
Requires reheat if rooms with different conditioning requirements since supply air cannot be reduced beyond a certain amount
Total amount of outside air may be difficult to maintain due to variable volume
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35. HCB 3-Chap 19A: All-Air Systems_Single Zone
Fig Cutaway drawing of a packaged air-handler unit (AHU).
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36. HCB 3-Chap 19A: All-Air Systems_Single Zone
Outcomes
Understanding of the common all-air system types used for air distribution in buildings
Understanding the operation of the basic CAV and VAV systems for single zone buildings
Knowledge of air stream heating across fans
Familiarity with different configurations of CAV and VAV systems
Be able to analyze single zone CAV with reheat, CAV without reheat, and VAV under summer and winter design conditions
Be able to analyze the performance of CAV and VAV under part-load operation
Understanding of the various control issues and energy efficiency benefits of using VAV system
HCB 3-Chap 19A: All-Air Systems_Single Zone