This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Understanding Chilled Beamand VAV Systems
John Murphy, LEED® AP BD+C Applications EngineerTraneIngersoll RandLa Crosse, Wisconsin
Chilled Beams
• Brief overview of chilled beams• Assess marketed advantages
of chilled beam systems versus VAV• Discuss challenges of applying
chilled beam systems• Review some common applications
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Active Chilled Beams
5Source of images: Halton
active chilled beamsPrimary Air System
primaryair handlerOA
activechilled beamPA
RA RA RA
PA
EA
air handlerOA
RA
6
Primary air must be sufficiently drier than space: • to offset space latent load, and • to keep space DP below chilled beam surface temp
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Induction of Mid-20th Century
Active Chilled Beams• Installed on ceiling rather than w
indo
wunder windows– More coil surface area– Lower air pressure required
• Warmer water temperature– No condensation– More coil surface area– Lower air pressure required
inducedroom airnozzles
7
• Larger ducts– Lower air pressure required– Less noise
7
primary aircondensatedrain connection
floor
Chilled Beams
• Brief overview of chilled beams• Assess marketed advantages
of chilled beam systems versus VAV• Discuss challenges of applying
chilled beam systems• Review some common applications
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
active chilled beamsClaimed Advantage #1
• “An ACB system typically allows for smaller ductwork d ll i h dli it th VAV t ”and smaller air-handling units than a VAV system.”
– Primary airflow < supply airflow due to induction– Shorter floor-to-floor height required?– Less mechanical room floor space?
9
active chilled beamsDetermining Primary Airflow Rate
• Primary airflow (PA) is b d l t f
PAbased on largest of:– Minimum outdoor airflow
required (ASHRAE 62.1)– Airflow required to offset
space latent load (depends on dew point of PA)Airflow needed to induce
RA
SA SA
10
– Airflow needed to induce sufficient room air (RA) to offset the space sensible cooling load
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
active chilled beamsDetermining Primary Airflow Rate
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Classroom (age 9 plus)
Barracks sleeping area
Minimum Outdoor Airflow Required by ASHRAE 62.1
typical ACB primary airflow(0.30 to 0.70 cfm/ft2)
example (0.47 cfm/ft2)
Lobby (hotel, dormitory)
Library
Lecture classroom
Laboratory
Hotel bedroom/living room
Courtroom
Corridor
Conference/meeting room
2
13
0.10 0.20 0.50 0.60 0.70 0.80 0.90 1.0
Retail sales floor
Reception area
Office space
minimum outdoor airflow required, cfm/ft2
(based on default occupant densities)
0.30 0.40
example (0.36 cfm/ft2)
active chilled beamsClaimed Advantage #2
• “An ACB system can typically achieve relatively low d l l ”sound levels.”
– No fans or compressors in or near occupied spaces– Constant primary airflow = constant sound– Depends on air pressure
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
active chilled beamsClaimed Advantage #3
• “An ACB system uses significantly less energy than a VAV t d tVAV system, due to:1. Significant fan energy savings (because of the reduced
primary airflow), and2. Higher chiller efficiency (because of the warmer water
temperature delivered to the chilled beams), and3. Avoiding reheat (because of zone-level cooling coils).”
15
Fan Energy Use: ACB vs. VAV
• A zone served by ACB’s may require 60% to 70% less primary airflow, at design cooling conditions…
…but the difference in annual fan energy use will be much closer because the VAV system benefits from:1. Reduced zone airflow at part load2. System load diversity3. Unloading of the supply fan at part load
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
m/
ft2
1.0
0.8
conventionalVAV system
1.0
0.8
exampleZone Primary Airflow at Part Load
e p
rim
ary
air
flo
w,
cfm
0.6
0.4active chilledbeam system
cold-airVAV system
0.6
0.4
30% i i i fl tti
17
designheating load
space load
zon
e
designcooling load
0.2 0.2
0 0
30% minimum airflow setting
System Load Diversity
multiple-zone VAV systemcentral VAV fan sized for “block” airflow
fan airflow = diversity × Σ zone primary airflows
PA PA
EA
OA
RA
variable-volume fan0.72 cfm/ft2
fan airflow = diversity × Σ zone primary airflows
For this example:system load diversity = 80%fan airflow = 80% × 0.90 cfm/ft2
= 0.72 cfm/ft2
RA RA RA
0.90 cfm/ft2 0.90 cfm/ft2 0.90 cfm/ft2
active chilled beam systemcentral CV fan sized for “sum-of-peaks” airflowPA PA
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
typical VAV system supply fanPart-Load Performance
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
supply fan energy useACB vs. Cold-Air VAV
0.8
1.0
00
ft2
0.6
0.4
np
ut
po
wer,
bh
p/
10
0
ACB uses more
VAV uses morefan energy
cold-airVAV
design coolingconditions
activechilled beam
21
supply fan airflow, cfm/ft2
1.00.80.60.40.20
0.2
0
fan
in
80% of VAV supplyfan design airflow
fan energy
Chiller Energy Use: ACB vs. VAV
• Chilled water delivered to the chilled beams must be warmer to avoid condensation…
…but the chilled water delivered to the primary AHU’s still must be cold to dehumidify the building.
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Chiller Energy Use
ACB system• Warm water delivered to
VAV system• Cold water delivered to central
chilled beams• Still needs cold water
delivered to the primary AHU’s for dehumidification
• Typically no DCV• No (or minimal) capacity for
airside economizing
VAV air-handling units
• Commonly implement DCV• 100% capacity for airside
economizing
25
• Waterside economizing (more effective due to warmer water temp)
• Can use waterside economizing, but airside economizing is more efficient
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Pumping Energy: ACB vs. VAV
ACB system• Higher pumping energy use
VAV system• Lower pumping energy use
• Warm water temperatures (58°F to 60°F)
• Small waterside ΔT (5°F to 6°F)
• Water pumped to chilled beams in every space
• Cold water temperatures (40°F to 44°F)
• Large waterside ΔT (12°F to 14°F)
• Water pumped only to centralized mechanical rooms
27
Impact of Reheat EnergyVAV terminal with 40% minimum airflow settingprimary airflow at design conditions = 0.90 cfm/ft2
primary airflow when reheat is activated:= 40% × 0.90 cfm/ft2
0 36 f /f 2
PA
0.36 cfm/ft2
= 0.36 cfm/ft2
cooling provided when primary airflow is at minimum:= 0.36 cfm/ft2 of 55°F primary air
55°F
Reheat is needed to avoid overcooling the space whenthe space sensible cooling load < 40% of design load.
active chilled beamprimary airflow at design conditions = 0.36 cfm/ft2PA
28
primary airflow when CHW valve is fully closed= 0.36 cfm/ft2
cooling provided when CHW valve is fully closed:= 0.36 cfm/ft2 of 55°F primary air
RA
0.36 cfm/ft2
55°F
Heat is needed to avoid overcooling the space whenthe space sensible cooling load < 40% of design load.
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
office buildingExample Energy Analysis• “Baseline” chilled-water VAV system
– ASHRAE 90.1-2007, Appendix G (55°F supply air)
• Active chilled beam systemActive chilled beam system– Four-pipe active chilled beams– Separate primary AHUs for perimeter and
interior areas (with SAT reset and economizers)– Separate water-cooled chiller plants
(low-flow plant supplying primary AHUs)
• “High-performance” chilled-water VAV system48°F l i (d t k t d i d)
31
– 48°F supply air (ductwork not downsized)– Optimized VAV system controls
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Chilled Beams
• Brief overview of chilled beams• Assess marketed advantages
of chilled beam systems versus VAV• Discuss challenges of applying
chilled beam systems• Review some common applications
33
ACB challengesHigh Installed Cost
• Limited cooling capacity = lots of ceiling space– Warmer water temperature requires
more coil surface area– Induction with low static pressures requires more
coil surface area to keep airside pressure drop low
i h (8) i hill d b
34
Example: 1000-ft2 office space Example: 1000-ft2 classroom
four (4) active chilled beams,each 6-ft long x 2-ft wide
eight (8) active chilled beams,each 4-ft long x 2-ft wide
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
System design variable
Impact on installed cost of the chilled beams
Impact on performance of the overall system
2-pipe versus 4-pipe chilled beams
A 2-pipe beam provides more cooling capacity than a 4-pipe beam because more coil surface is available
Using 2-pipe beams requires a separate heating system, otherwise it can result in poorer comfort control because either cold water or warm water is delivered to all zones
Primary airflow rate (cfm)
Increasing the primary airflow rate through the nozzles results in more air being induced from the space which increased
Increasing the primary airflow rate increases primary AHU fan energy use, increases noise in the space and requires a larger primaryinduced from the space, which increased
the capacity of the chilled beam coilsin the space, and requires a larger primary AHU and larger ductwork
Inlet static pressure of the primary air
Increasing the static pressure at the inlet to the nozzles results in more air being induced from the space, which increased the capacity of the chilled beam coils
Increasing the inlet pressure increases primary AHU fan energy use, and increases noise in the space
Dry-bulb temperature of the primary air
Delivering the primary air at a colder temperature means that less of the space sensible cooling load needs to be offset by the chilled beams
Using a colder primary-air temperature may cause the space to overcool and low sensible cooling loads, thus requiring the chilled beam (or separate heating system) to add heat to
35
the chilled beams (or separate heating system) to add heat to prevent overcooling space
Entering water temperature
Supplying colder water to the chilled beam increases the cooling capacity of the beam
Using a colder water temperature requires the space dew point to be lower to avoid condensation, which means the primary air needs to be dehumidified to a lower dew point
Water flow rate (gpm)
Increasing the water flow rate increases the cooling capacity of the beam
Increasing the water flow rate increases pump energy use and requires larger pipes and pumps
ACB challengesNeed to Prevent Condensation
• Primary air system used to limit indoor dew point (t i ll b l 55°F)(typically below 55°F)
• Warm chilled-water temperatures delivered to beams (typically between 58°F and 60°F)
• Start primary air system (chilled beams off) to reduce indoor humidity following shutdown
• Tight building envelope and good building pressure
36
g g g gcontrol to minimize infiltration– Use caution if the building has operable windows or
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
ACB challengesRisk of Water Leaks
• Lots of water piping, pipe connections, and valves b i th b ildiabove every space in the building– Four-pipe systems have twice as much piping and
twice as many connections
37
Example: 1000-ft2 office space Example: 1000-ft2 classroom
ACB challengesNo Filtration of Local Recirc Air
• Chilled beams typically not equipped with a filter– Coils intended to operate dry (no condensation),
lessening concern about preventing wet coil surfaces from getting dirty
– Still concern about removing particles generated indoors or brought indoors
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
ACB challengesLimited Heating Capability
• Active chilled beams have limited heating capacity• Chilled beam systems often use a separate heating
system (baseboard convectors, radiant floor heat)
39
Chilled Beams
• Brief overview of chilled beams• Assess marketed advantages
of chilled beam systems versus VAV• Discuss challenges of applying
chilled beam systems• Review some common applications
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Office Buildings
Why ACB might be a good fit:
• Low sensible cooling loadsWhy ACB might not be a good fit:
• Low ventilation rates lt i i AHU• Low latent loads result in primary AHU
using mixed air• Not friendly for
re-configuring spaces
41
Schools
Why ACB might be a good fit:
• High ventilation rates lt i i AHU
Why ACB might not be a good fit:
• High latent loads require l d i t i iresult in primary AHU
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.
Trane, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. Design of the HVAC system is the prerogative and responsibility of the engineering professional.