1. Chapter 20 B: AIR-WATER AND HYBRID SYSTEMS
Agami Reddy (rev- May 2019)
Traditional air-water systems
- 2-pipe and 4-pipe
- central air for ventilation
Chilled beams (active and passive)
Dedicated Outdoor Air System ( DOAS)
- principle and advantages
- different components
Evaporative cooling: direct-indirect
Desiccant dehumidification systems
Variable refrigerant flow (VRF) systems
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2. HCB 3-Chap 20B: Air-Water_Hybrid
All-water systems do not have proper outdoor air control
Air is allowed to trickle in from outside openings-unreliable
Fig Photo of a room with trickle ventilation along with baseboard heating (from Kolderup)
All-air systems brings in the needed outdoor but this is mixed with supply air.
HCB 3-Chap 20B: Air-Water_Hybrid

3. Traditional Air-Water Systems
Explicit path
for ventilation air
which is different
from all-air systems
Common issue with
water dripping
from unit onto
floor
Fig. 20.2(a) Two-pipe air-water system with local outside air supply and fan coil terminal units. A single circulation pump is used.
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4. HCB 3-Chap 20B: Air-Water_Hybrid
Central Air-Water System
(first attempt to decouple ventilation and room loads)
Individual Heat Pumps for room loads
Outdoor air supply ducting is not shown- has to have its own conditioning system
Common Operational Strategy
Boilers –
- Comes on when loop temperature < 70o F
- Heat pumps do not supply any heat
- All heating supplied by boiler
Cooling tower (no central chiller)
- Cools down loop water by evap cooling
- Comes on when loop temperature > 90o F
- Heat pumps still supply cooling
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5. HCB 3-Chap 20B: Air-Water_Hybrid
Central Air-Water
System contd…
(showing separate ventilation air supply)
Outdoor air is supplied locally (by unit ventilators) or centrally via a constant-volume system (next slide).
-Typical applications:
Exterior spaces of office
buildings not requiring close control of humidity
Primary or
ventilation
air (usually
constant)
Fig Central closed loop individual water heat pump units with a central ventilation system (not shown)
HCB 3-Chap 20B: Air-Water_Hybrid

6. HCB 3-Chap 20B: Air-Water_Hybrid
Outdoor air brought in to meet ventilation needs-
this air needs to be conditioned-
Requires its own heating and cooling equipment
Inside the rooms
Plan view
HCB 3-Chap 20B: Air-Water_Hybrid

7. HCB 3-Chap 20B: Air-Water_Hybrid
Table 20.1 Advantages and Disadvantages of Air-Water Systems
Advantages
Disadvantages
Ideally, combines the best features of all-water and all-air systems
If designed badly, the worst features of all-water and air-systems can result
Low transport energy needed while meeting ventilation comfort requirements
Requires more maintenance because of numerous individual zone units
Air duct size reduced
Controls tend to be more complicated
Central air handler size reduced
Not suitable for high exhaust air requirements
Good for spaces with highly variable loads
Severe damage if pipes leak
Easy to shut off water supply in unoccupied spaces
Cannot shut off primary air to unoccupied spaces
Good for exterior spaces
Humidity control may be difficult
Can simul. heat and cool different spaces
Can be combined with ventilation heat recovery devices (heat wheels)
Can be combined with water loop HP
HCB 3-Chap 20B: Air-Water_Hybrid

8. Dedicated Outdoor Air Systems (DOAS)
Single-zone systems are simple as far as introducing the proper amounts of Outdoor Air (OA)
Multiple-zone recirculation systems (typically VAV) are problematic since it is difficult to guarantee the proper amounts of OA during entire range of climatic conditions. Hence excess outdoor air is drawn- energy penalty incurred
100 % Outdoor Air Systems (or DOAS) are the most reliable system for introducing proper amounts of OA for multiple zones during the full operating range of the system.
HCB 3-Chap 20B: Air-Water_Hybrid

9. HCB 3-Chap 20B: Air-Water_Hybrid
DOAS contd…
DOAS units are 100 % Outdoor Air units with the ability to bring 100 % outdoor air to all spaces at all times
Typical DOAS are air-handling units that cool, dehumidify, heat, humidify and filter the outdoor air prior to being introduced to the conditioned space
This approach allows the individual space units to handle only the space cooling and heating loads
It is preferable, however, to introduce the outdoor air at lower humidity ratio than the desired space humidity ratio in order to allow the zone HVAC unit to handle only the space sensible cooling load
Must have a heat recovery system
Significant energy savings are said to occur since generally 20-30% less outdoor air must be conditioned as compared to a VAV system for multizone spaces
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10. HCB 3-Chap 20B: Air-Water_Hybrid
The term DOAS is used differently by different people.
-To some DOAS is the outdoor air conditioning unit (as shown above)
which also meets the room latent load,
To others, for a system to qualify as DOAS it needs to satisfy the
above condition PLUS (next slide):
(a) outdoor air and return air are totally separated,
(b) return air is not recirculated,
(c) exhaust air heat recovery HX used,
(d) efficient room air distribution sensible devices used (chilled beams, radiant panels,…)
HCB 3-Chap 20B: Air-Water_Hybrid

11. HCB 3-Chap 20B: Air-Water_Hybrid
Components of DOAS unit:
Fig General arrangement of the DOAS with state points
1-2 Air cooled and dehumidified by enthalpy wheel
2-3 Supply air passes over a dehumidification coil to cool as necessary but more importantly to dehumidify the air to a state which can meet the latent loads of the space as well. In case the outdoor air is dry enough, the cooling coil need not dehumidify the air.
Advised not to use DX coil, but a chilled water coil because of better control
3-4 Another sensible heat wheel is used to heat the supply air as necessary
HCB 3-Chap 20B: Air-Water_Hybrid

12. Example 20.6 DOAS Design Example
HCB 3-Chap 20B: Air-Water_Hybrid

13. HCB 3-Chap 20B: Air-Water_Hybrid

14. HCB 3-Chap 20B: Air-Water_Hybrid4
HCB 3-Chap 20B: Air-Water_Hybrid

15. HCB 3-Chap 20B: Air-Water_Hybrid
Comments
 In addition to the DOAS cooling coil, the designer has to select a sensible cooling system to remove the sensible loads of the space. Several options are available: chilled beams or radiant cooling panels or DX systems such as mini split units (VRF), or chilled water based fan coils units.
HCB 3-Chap 20B: Air-Water_Hybrid

16. HCB 3-Chap 20B: Air-Water_Hybrid
Fig DOAS with overhead air distribution and parallel radiant cooling system (whose chilled water supply is not shown) – - -
HCB 3-Chap 20B: Air-Water_Hybrid

17. DOAS from underfloor air distribution with Chilled Beams
Separate air system is required for ventilation (outdoor air) and space dehumidification
The separate air system can be a DOAS with dehumidification capability
Often chilled beams because of their limited cooling capacity are used in conjunction with CAV systems (only recently are they being used with VAV systems)
They are also used with under-floor and displacement ventilation
Underfloor
Displacement ventilation
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18. HCB 3-Chap 20B: Air-Water_Hybrid
Typical DOAS include the following major components:
HCB 3-Chap 20B: Air-Water_Hybrid

19. Increasing Popularity: Chilled Beams (Two types: Passive and Active)
Uses convection to cool/heat
In active type, primary air supplied from air handler to increase convection- similar to fan-powered VAV
Passive type: Only cooling coil is used to condition the space
Quiet since no forced air is supplied
Takes time to condition the space
HCB 3-Chap 20B: Air-Water_Hybrid

20. Chilled Beams (can get 4-14 LEED points)
Chilled beams are coils meant to provide
Space sensible cooling- by passing chilled water thru them
Cannot provide dehumidification!
Heat transfer by both convection (primary mode) and radiation
Primarily used for cooling
Much higher cooling capacity (4 – 6 times greater) than radiant systems
Elevated temperature ( around 60o F) chilled water used (to avoid condensation in the space)
Chilled beams can provide better thermal comfort and lower energy cost than traditional HVAC systems
Typical applications: offices and admin areas
Chilled beams can be a cost effective solution for laboratories.
Low noise, significant energy savings, good occupant comfort!
HCB 3-Chap 20B: Air-Water_Hybrid

21. HCB 3-Chap 20B: Air-Water_Hybrid
Passive Chilled Beams
There are two types of chilled beams:
Passive and Active
Passive chilled beams providing the cooling effect by natural convection and flow of the room air thru the cooling coil. Chilled water circulates inside the beam’s cooling coil piping (similar to hydronic terminal units)
There is no primary air supply.
Fig Ceiling mounted passive chilled
beam (a) Picture of a unit, (b) Air flow patterns with interior components
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22. HCB 3-Chap 20B: Air-Water_Hybrid
Max about
400 Btu/hr-ft
Cooling capacity per unit length (W/m) for a 10 K temp. diff
Fig Example of a specification sheet of a passive chilled beam manufacturer
fo free area of bottom perforated cover plate (%),
B width (mm),
H height (mm) and
Z suspension height from lower edge of ceiling to upper edge of chilled beam (mm)
HCB 3-Chap 20B: Air-Water_Hybrid

23. HCB 3-Chap 20B: Air-Water_Hybrid

24. HCB 3-Chap 20B: Air-Water_Hybrid

25. HCB 3-Chap 20B: Air-Water_Hybrid
Active or Induction Chilled Beams (have conditioned primary air supply)
Max about 600 Btu/hr-ft
Fig Ceiling mounted active chilled beams:
(a) Picture of a unit,
(b) Air flow patterns with interior components
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26. HCB 3-Chap 20B: Air-Water_Hybrid
Active Chilled Beams
Active chilled beams (also called Induction Chilled Beams) provide cooling effect by integrating primary air supply induction effect and supplementary cooling by a built-in cooling coil.
Chilled water circulates inside ceiling cooling coil piping.
An air system supplies primary around 60oF for ventilation and space dehumidification to the Ceiling Induction Diffuser.
The ratio between primary air and total supply air can vary from 1:5 to 1:3
The separate air system can be a DOAS with dehumidification capability
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27. HCB 3-Chap 20B: Air-Water_Hybrid
Evaporative Cooling
Evaporative cooling systems have been utilized for many years for applications such as residential, commercial, and industrial.
They are cost effective in low wet bulb locations.
The process of evaporative cooling is an adiabatic evaporation of water which provides the cooling effect.
Three (3) evaporative cooling mechanisms are typically used:
- Direct Evaporative Cooling
- Indirect Evaporative cooling.
- Indirect/Direct Evaporative Cooling
In many cases indirect evaporative cooling systems used to pre-cool the air and additional mechanical cooling maintains desired conditions.
Special attention is given to water treatment issues.
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28. HCB 3-Chap 20B: Air-Water_Hybrid
Cooling Strategies : Direct and Indirect one stage evaporative cooling
Direct evaporative cooling
Indirect evaporative cooling
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29. HCB 3-Chap 20B: Air-Water_Hybrid
Fig (a) Direct evaporative cooler showing key components, including wetted pads, blower, motor, and water supply system. (b) Typical direct evaporative cooling process plotted on the psychrometric chart
HCB 3-Chap 20B: Air-Water_Hybrid

30. (a) Direct Evaporative Cooling
This is an incomplete adiabatic
saturation process
Model using concept of
“available DBT depression %”
Say, if this is 80%, then
x( )= 82.4
is the exit DBT of air 3
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31. HCB 3-Chap 20B: Air-Water_Hybrid
Cool Towers or Passive Down Draft Towers- examples of direct cooling
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32. HCB 3-Chap 20B: Air-Water_Hybrid
Fig Evaporative cooling media
Fig Example of evaporative cooler
effectiveness fFor different pad thickness and air flows
HCB 3-Chap 20B: Air-Water_Hybrid

33. (b) Indirect Evaporative Cooling
Fig  Indirect evaporative cooler system diagram.
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34. HCB 3-Chap 20B: Air-Water_Hybrid
(c) Indirect-Direct two stage evaporative cooling
Two stage evaporative cooler
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35. HCB 3-Chap 20B: Air-Water_Hybrid
Psychrometrics of Evaporative cooling systems :
Fig  Indirect and direct evaporative cooling processes plotted on a psychrometric chart. Indirect process is at constant humidity ratio whereas direct process is at constant enthalpy
HCB 3-Chap 20B: Air-Water_Hybrid

36. HCB 3-Chap 20B: Air-Water_Hybrid
Actual Building with evaporative cooling (direct) & humidification
HCB 3-Chap 20B: Air-Water_Hybrid
CH2 Building, Australia

37. Active Desiccant Based Dehumidification Systems
Fig Active desiccant based dehumidification system (Courtesy of Munters, DryCool)
HCB 3-Chap 20B: Air-Water_Hybrid

38. Active Desiccant Systems contd…
Desiccant dehumidifier absorbs moisture from the air thru a difference in vapor pressure
As the air dehumidifies in the desiccant its latent heat is converted to sensible heat resulting in increase of the air temperature
Every desiccant unit has two air streams. Process air and Reactivation air (sometimes called Regeneration)
Thermal energy used to reactivate the desiccant
HCB 3-Chap 20B: Air-Water_Hybrid

39. Active Desiccant Systems contd…
Desiccant materials naturally attract moisture from gases and liquids.
The material becomes saturated as moisture is absorbed or collects on the surface; but when heated, the desiccant dries out (regenerates) and can be used again.
Conventional solid desiccants include silica gel, activated alumina, molecular sieves.
Titanium silicate, a class of material called "1M," and synthetic polymers are new solid desiccant materials designed to be more effective for cooling applications.
The desiccant is impregnated or formed in place in an Honeycomb Matrix.
HCB 3-Chap 20B: Air-Water_Hybrid

40. HCB 3-Chap 20B: Air-Water_Hybrid
Fig Desiccant and regeneration system with process (space supply) and return air streams .
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41. HCB 3-Chap 20B: Air-Water_Hybrid
Fig More commonly used (practical) system configuration with separate return air stream and regeneration air stream
HCB 3-Chap 20B: Air-Water_Hybrid

42. Active Desiccant Systems contd…
Typical applications are:
- DOAS for hotels, schools, dormitories
- Dehumidification in Supermarkets, Ice Rinks, & industrial facilities where the desired humidity level is lower then typically achieved by mechanical cooling
(Mechanical cooling can provide around
50 grains/lb or lb/lb)
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43. HCB 3-Chap 20B: Air-Water_Hybrid
Variable Refrigerant Flow (VRF) Systems
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44. HCB 3-Chap 20B: Air-Water_Hybrid
VRF Systems
Invented in Japan by Daikin
VRFs use refrigerant directly as the cooling and heating medium.
Conditioning of the refrigerant by a single outdoor condensing unit, and is circulated within the building to multiple fan coil units.
Good application in add-ons, historical buildings, retrofit situations, office buildings, strip malls, banks, schools, hotels, etc due to compartmentalized comfort conditioning.
VRF systems are enhanced versions of ductless multi-split systems, permitting more indoor units to be connected to each outdoor unit and providing additional features such as simultaneous heating and cooling and heat recovery.
VRF technology uses smart integrated controls, variable speed drives, refrigerant piping, and heat recovery to provide products with attributes that include high energy efficiency, flexible operation, ease of installation, low noise, zone control, and comfort using all-electric technology.
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45. HCB 3-Chap 20B: Air-Water_Hybrid
Provides individualized air conditioning to different rooms as needed
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46. HCB 3-Chap 20B: Air-Water_Hybrid
VRF HVAC systems are DX heat pump technology platform built on standard reverse Rankine vapor compression cycle.
Salient Features of a VRF system:
a) Zoning System
b) Operates as Heat Pump
c) Simultaneous heating and cooling
d) Multiple indoor units- up to 50 indoor units per system
e) Connected Diversity- up to 150%
f) Inverter Technology/ Variable speed compressor.
System Types : There are 3 basic types of VRF systems
1) Cooling only
2) Heat Pump
3) Heat Recovery
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47. HCB 3-Chap 20B: Air-Water_Hybrid
Advantages of VRF Systems
Installation Advantages: Lightweight and modular. Modularity enables staged or floor by floor installation. Easy retrofitting.
Comfort: Many zones possible each with individual set point control.
Capacity Modulation: VRF systems enable wide capacity modulation and bring rooms to the desired temperature extremely quickly and keep temperature fluctuations to minimum. Precise temperature control.
Energy efficient: Ductless system, therefore no losses.
Simultaneous heating and cooling to different zones are possible, each individual indoor unit can be controlled by a programmable thermostat.
Separate billings can be generated by VRF systems which makes individualizes billings easier.
Variable speed compressors with 10 to 100% capacity range can be used by VRF systems that provide unmatchable flexibility for zoning to save energy.
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48. HCB 3-Chap 20B: Air-Water_Hybrid
Outcomes
Knowledge of the different types of traditional air-water systems and how they differ from all-water and all-air systems
Understanding the operational principles of the traditional air-water system types
Understanding the advantages and disadvantages of air-water systems
Knowledge of the two different types of chilled beams and their operational principles
Be able to design chilled beams for rooms given manufacturer’s tables
Understanding the unique features of DOAS and the dvnatges they provide
Be able to design DOAS systems for simple cases
Knowledge of different types of evaporative cooling systems and understanding their operating principle
Knowledge of active desiccant system configurations, and understanding their operating principles
Familiarity with VRF systems and their advantages
HCB 3-Chap 20B: Air-Water_Hybrid