1. Chapter 19B: ALL-AIR SYSTEMS FOR MULTIPLE SPACES
Agami Reddy (rev May 2017)
CAV terminal reheat
CAV multizone and dual-duct
VAV terminal reheat
Example of sizing an all-air system
Part-load performance of all three systems- solved example
Fan powered VAV systems: primary and secondary air flows
Measures to improve energy efficiency
- Fan control
- Outdoor air economizer
- Heat recovery devices
- Deck reset
8) Advantages of all-air systems
HCB 3-Chap 19B: All-Air Systems_Multizone

2. HVAC Systems for Multiple Spaces
Different zones have different sensible and latent loads and need different supply air conditions. Important generic types:
Terminal reheat
Multi-zone system
Dual Duct System
VAV System
Constant CFM, varying DBT and W
Wasteful in energy
Varying CFM with fixed or varying DBT and W
Saves both thermal energy
as well as fan electricity
HCB 3-Chap 19B: All-Air Systems_Multizone

3. CAV Terminal Reheat Systems
CFM to each zone kept fixed, supply temperatures varied with load
Reheat at individual zones can be hydronic or electric
In summer, cooling and humidity control needed
In winter, some zones may still need cooling
Fig (b) Plenum return single-duct VAV system for a two-zone building
HCB 3-Chap 19B: All-Air Systems_Multizone

4. HCB 3-Chap 19B: All-Air Systems_Multizone
CAV Multi-Zone Units
Pro: Simple and low initial cost
Con: High operating cost, lack of flexibility and proper humidity control
HCB 3-Chap 19B: All-Air Systems_Multizone

5. Multi-zone Central Air System
HCB 3-Chap 19B: All-Air Systems_Multizone

6. Fig. 19.16 CAV Multi-Zone Dual-Duct Systems
Pro: Flexibility (compared to multi-zone system)
Con: High operating cost and poor humidity control
HCB 3-Chap 19B: All-Air Systems_Multizone

7. Single duct VAV secondary system sizing (Design or peak conditions)
Example 19.8
Cooling
HCB 3-Chap 19B: All-Air Systems_Multizone

8. HCB 3-Chap 19B: All-Air Systems_Multizone
Other Specifications
HCB 3-Chap 19B: All-Air Systems_Multizone

9. HCB 3-Chap 19B: All-Air Systems_Multizone
Assumptions
HCB 3-Chap 19B: All-Air Systems_Multizone

10. HCB 3-Chap 19B: All-Air Systems_MultizoneEq
Eq IP
HCB 3-Chap 19B: All-Air Systems_Multizone

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17. HCB 3-Chap 19B: All-Air Systems_Multizone
Example Part Load Conditions
(a) Single duct VAV system
HCB 3-Chap 19B: All-Air Systems_Multizone

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(b) Single duct CAV system
HCB 3-Chap 19B: All-Air Systems_Multizone

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Summary table comparing all three HVAC systems
Ideal loads
Single duct CAV
Single duct VAV
Dual duct CAV
Cooling (Tons)
27.6
37.7
29.0
34.5
Heating (Btu/hr) -
93,947
53,726
Fan power (HP)
12.0
8.41
Thermal efficiency
0.61
0.95
0.71
-VAV system outperforms the other two systems, and is very close to the ideal loads. It is the best choice since it requires the least cooling energy, a smaller heating energy and lower fan electricity use.
If we define efficiency in terms of the ideal loads, then for the VAV system it is (27.6/29) = 0.95, while that for the single duct CAV = [(27.6 Tons x 12,000 Btu/h-ton) / (37.7 Tons x 12,000 Btu/h-ton+ 93,947)= 0.61.
The two CAV systems are similar thermodynamically, and the differences in energy use determined are partly due to the manner we selected the hot deck temperature, the way we adjusted for the supply fan being upstream, and also due to slightly different resulting humidity levels in the space which affect the latent loads.
HCB 3-Chap 19B: All-Air Systems_Multizone

23. Fan-Powered VAV Terminal Boxes
The basic VAV system with single supply fan evolved into fan-powered VAV boxes with one fan at each box (fan power about 1/3rd to ½ HP) with terminal reheat. The reason was that low supply flows (down to say 0.5 cfm/ft2) compromised IAQ (poorer mixing, more stagnant air spaces, poorer filtration)
Thus, fan-powered VAV maintain higher air circulation thru room at low thermal loads while retaining some of the advantages of VAV systems
Plenum return needed
Fig
Fig Induction VAV system
HCB 3-Chap 19B: All-Air Systems_Multizone

24. Ducted and Plenum Return Air
Supply air is always ducted
Ducted
Plenum
HCB 3-Chap 19B: All-Air Systems_Multizone

25. The two types of fan powered VAV boxes
Fan located within primary
airstream and runs continuously
when zone is occupied
Fan located outside primary
airstream- this allows intermittent
fan operation
Fig (a) Schematic diagrams of series and parallel fan-powered VAV boxes
Series: fan runs continuously
Parallel: fan intermittent
HCB 3-Chap 19B: All-Air Systems_Multizone

26. Fan-Powered VAV Terminal Boxes
The secondary air flow is adjusted so that the total supply air flow is kept at the design flow
Especially used in perimeter zone
In colder climates, terminal heat has to be supplemented by baseboard heaters
Three types of designs evolved which allowed higher room supply flow rates without adversely impacting thermal and fan energy:
- Induction type
- Series fan powered box (consumes slightly more energy)
Parallel fan powered box (consumes less energy)
(good equip manufacturer web site to check is TITUS)
Nowadays VAV terminal fans have ECM (electronically commutated motors) which are very efficient and consume little energy
HCB 3-Chap 19B: All-Air Systems_Multizone

27. Series Fan-Powered VAV
Fan powered VAV boxes allow plenum air (called secondary air)
to be drawn in, thereby increasing supply air flow rate to room
Fig Two zone space conditioned by a series fan powered VAV system
Primary air flow
Secondary air flows
Plenum return needed
HCB 3-Chap 19B: All-Air Systems_Multizone

28. HCB 3-Chap 19B: All-Air Systems_Multizone
Series fan powered
Almost constant air flow to space-
can be used for both perimeter
and interior spaces
Fig Air flow characteristics
Parallel fan powered
Intermittent fan operation-
Primary air modulated in
response to cooling demand
primary air used for perimeter
zones with auxiliary hot water
or electric heat
HCB 3-Chap 19B: All-Air Systems_Multizone

29. All-Air Systems: Energy Efficiency Measures
VAV instead of CAV
Energy efficient operation: fan control
Economizer operation and benefits
Heat recovery devices
Deck (or supply air ) temperature reset
The temperature set points of the heating and cooling coils are changed with changes in outdoor DBT (on which the building loads depend)
HCB 3-Chap 19B: All-Air Systems_Multizone

30. HCB 3-Chap 19B: All-Air Systems_Multizone
Fan Control- Three different commonly used methods of controlling fan speed in VAV systems- the VSD is most energy efficient but most costly
Empirical curve-fit
equations are used
for modeling such
control methods
Figure Part-load fan characteristics for outlet damper, inlet vane, and variable-speed control methods.
HCB 3-Chap 19B: All-Air Systems_Multizone

31. Traditional HVAC System Variants
Similar to a reheat systems but w/o requiring heat - Control is a little more difficult
Humidity control may be poor
Fig Recycle air bypass
- Used in the 60-70’s
- No longer used
- Replaced by fan-
powered VAV
systems
Fig HVAC system modifications
(a) Mixed air bypass (b) face and bypass damper arrangement
HCB 3-Chap 19B: All-Air Systems_Multizone

32. HCB 3-Chap 19B: All-Air Systems_Multizone
Outdoor Air Economizer operation
Basic concept: Increase the
outdoor air flow intake under
certain outdoor air conditions
so that “free cooling” brought in
can reduce active cooling
Two types of economizers:
- sensible
- enthalpy
In any case, the outdoor air flow has
to be modulated properly, otherwise
the benefit of reduced energy is lost
Fig Enthalpy and temperature economizer operating ranges. The constant-enthalpy line represents the room enthalpy criterion.
HCB 3-Chap 19B: All-Air Systems_Multizone

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About 65 F
FIGURE Outside airflow characteristic for fixed-volume system with economizer.
HCB 3-Chap 19B: All-Air Systems_Multizone

34. HCB 3-Chap 19B: All-Air Systems_Multizone
Outdoor DBT when outdoor mass flow reaches its minimum value:
Example 19.11: Building process—The economizer
- Return air temperature from a zone is 25° C (77° F)
At least 25 percent outdoor air is required at all times to meet ventilation requirements,
design cooling system air supply temperature is to be 13° C
At which outdoor temperature will outdoor inlet damper be at its minimum setting? 25
HCB 3-Chap 19B: All-Air Systems_Multizone

35. Heat Recovery Devices from Exhaust Air
Effectiveness ~ 0.75
Effectiveness ~ 0.6
Fig (a) Rotary total energy exchanger
Fig (b) Plate-type heat exchanger
HCB 3-Chap 19B: All-Air Systems_Multizone

36. HCB 3-Chap 19B: All-Air Systems_Multizone
Example 19.12: Energy benefit of exhaust air energy recovery
Assume a building with 100% outdoor air. The exhaust air temperature is always kept constant at 75oF year-round. The building supply temperature is assumed to be 55oF and 100% saturated during summer and 90oF during winter operation. The ventilation and exhaust air streams are both equal to 10,000 cfm.
 (a) Determine the capacity of the cooling and heating coils without energy recovery under the following peak or design outdoor air conditions:
winter: 40o F and 40% RH
summer: 98o F and 40% RH
(b) Determine the capacity of the cooling and heating coils when an energy recovery unit of effectiveness of 0.6 is included.
Fig Sketch of the HVAC system with energy recovery unit for Example
HCB 3-Chap 19B: All-Air Systems_Multizone

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39. Table 19.4 Advantages/Disadvantages of All-Air Systems
Best choice for close control of zone temperature and humidity
May limit the extent to which energy use can be reduced in low or zero energy buildings
Major equipment is centrally located allowing for nonintrusive maintenance
Duct space requirements add to building height
Providing heating and cooling in different zones simultaneously is easily achieved
Air balancing is difficult
Can handle space churn to some extent
May be more expensive than other secondary systems in first cost and operating cost
Seasonal changeover is easily achieved by HVAC controls
Noise in fan operation may be a problem in certain types of spaces
Well suited for air-side economizer, heat recovery and large outside air requirements
May not be satisfactory for perimeter spaces in cold locations
No drain pipe or power wiring in occupied areas
Difficult to correct indoor conditions in individual rooms if improperly designed initially
Less property damage if air ducts leak
May lead to shoddy construction since air leakage from ducts causes no visible damage and so may be overlooked
HCB 3-Chap 19B: All-Air Systems_Multizone

40. HCB 3-Chap 19B: All-Air Systems_Multizone
Outcomes
Knowledge of the different types and working principles of CAV multizone systems
Be able to solve and analyze problems involving CAV and VAV multizone systems under peak design conditions and under part load operation
Knowledge of the different types, operating principles and features of fan-powered VAV systems
Familiarity with differed ways by which energy efficiency can be enhanced in all-air systems
Be able to analyze the energy benefits of outside air economizer cycles and heat recovery devices
Understanding the advantages and disadvantages of all-air systems
HCB 3-Chap 19B: All-Air Systems_Multizone