understanding chilled beam and vav systems chilled beams

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© 2010 Trane, a business of Ingersoll-Rand

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

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Passive Chilled Beam

perforatedmetal casing

coilwater pipes

ceiling

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Active Chilled Beam

primary air

ceiling

coils

nozzles

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ceiling

induced airinduced air

+primary air

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Active Chilled Beams

5Source of images: Halton

active chilled beamsPrimary Air System

primaryair handlerOA

activechilled beamPA

RA RA RA

PA

EA

air handlerOA

RA

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Primary air must be sufficiently drier than space: • to offset space latent load, and • to keep space DP below chilled beam surface temp

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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

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• Larger ducts– Lower air pressure required– Less noise

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primary aircondensatedrain connection

floor

Chilled Beams




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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?

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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

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– Airflow needed to induce sufficient room air (RA) to offset the space sensible cooling load

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Minimum OA per ASHRAE 62.1(to achieve LEED IEQc2)

C

Example: office space0.085 cfm/ft2(0.085 × 1.3 = 0.11 cfm/ft2)

ACB systemAirflow required to offset space latent load

Airflow needed to induce sufficient room air to offset

ibl li l d

0.085 cfm/ft2 (DPTPA = 47°F)0.11 cfm/ft2 (DPTPA = 49°F)0.36 cfm/ft2 (DPTPA = 53°F)

0.36 cfm/ft2 (55°F primary air)(four, 6-ft long beams)

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space sensible cooling loadVAV systemAirflow needed to offset space sensible cooling load

0.90 cfm/ft2(55°F supply air)


Minimum OA per ASHRAE 62.1(to achieve LEED IEQc2)

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Example: K-12 classroom0.47 cfm/ft2(0.47 × 1.3 = 0.61 cfm/ft2)

ACB systemAirflow required to offset space latent load

Airflow needed to induce sufficient room air to offset

ibl li l d

0.47 cfm/ft2 (DPTPA = 44°F)0.61 cfm/ft2 (DPTPA = 47°F)1.20 cfm/ft2 (DPTPA = 51°F)

0.47 cfm/ft2 (55°F primary air)(eight, 4-ft long beams)

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space sensible cooling loadVAV systemAirflow needed to offset space sensible cooling load

1.20 cfm/ft2(55°F supply air)

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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

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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)


• “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

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• “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).”

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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

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3 U oad g o t e supp y a at pa t oad

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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

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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

EA

OA

RA

constant-volume fan0.36 cfm/ft2

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central CV fan sized for sum of peaks airflow

fan airflow = Σ zone primary airflows

For this example:fan airflow = 0.36 cfm/ft2

RA RA RA

0.36 cfm/ft2 0.36 cfm/ft2 0.36 cfm/ft2

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typical VAV system supply fanPart-Load Performance

80

100

esi

gn

60

40

np

ut

po

wer,

% o

f d

e

VAV l f

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supply fan airflow, % of design100806040200

20

0

fan

i VAV supply fan with VFD

supply fan energy useACB vs. Conventional VAV

0.8

1.0

00

ft2

design coolingconventional

VAV

activechilled beam

0.6

0.4

np

ut

po

wer,

bh

p/

10

0

ACB uses more

VAV usesmore fan energy

conditions

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supply fan airflow, cfm/ft2

1.00.80.60.40.20

0.2

0

fan

in

68% of VAV supplyfan design airflow

fan energy

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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

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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.

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Dedicated Chilled-Water Plants

chillers

42°F 58°F63°F57°F

variable-flowpumps

bypass forminimum flow

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primaryair handlers

chilledbeams

Shared Chilled-Water Plant

mixingvalve

42°F

TT chilled beams

57°F

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primaryair handlers

42°F 58°F

TT

63°F

54°F

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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

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• Waterside economizing (more effective due to warmer water temp)

• Can use waterside economizing, but airside economizing is more efficient

Waterside Economizer

chillers

variable-flowpumps

bypass forminimum flow mixing

valveTT

variable-flowpump

42°F57°F

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primaryair handlers

chilled beamsfrom

cooling tower waterside economizerheat exchanger

42°F 58°F

63°F

54°F

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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

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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

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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.

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example office spaceCold vs. Neutral Primary Air

“Cold” (55°F) primary-air temperature

• four (4) ACBs, each 6-ft long x 2-ft widePA

( ) , g• primary airflow at design conditions = 0.36 cfm/ft2

• total water flow = 6.0 gpmRA

0.36 cfm/ft2

55°F

PA

0 50 f /ft2

“Neutral” (70°F) primary-air temperature

• six (6) ACBs, each 6-ft long x 2-ft widei i fl t d i diti 0 50 f /ft2

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RA

0.50 cfm/ft2

70°F• primary airflow at design conditions = 0.50 cfm/ft2

• total water flow = 9.0 gpm

Heating Energy: ACB vs. VAV

ACB system• Heat added when CHW valve

VAV system• Heat added when damper

closes and primary airflow begins to overcool the space

• Typically no DCV

closes to minimum and primary airflow begins to overcool space

• Commonly implement DCV• Parallel fan-powered VAV

terminals can draw warm air from ceiling plenum

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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)

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– 48°F supply air (ductwork not downsized)– Optimized VAV system controls

(ventilation optimization, SAT reset)– Parallel fan-powered VAV terminals– Low-flow, water-cooled chiller plant

10,000,000

12,000,000

Use

, kB

tu/

yr

Pumps

Fans

Heating

Houston Los Angeles Philadelphia St. Louis

Example Energy Analysis

2,000,000

4,000,000

6,000,000

8,000,000

An

nu

al B

uild

ing

En

erg

y U Heating

Cooling

Plug Loads

Lighting

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Chilled Beams




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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

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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

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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

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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

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g g g gcontrol to minimize infiltration– Use caution if the building has operable windows or

natural ventilation

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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

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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

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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)

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Chilled Beams




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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

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Schools


• 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

with 100% OA• Low sound levels

low dew point primary air• Lack of economizing

capacity

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Hospital Patient Rooms


• High minimum air change t (6 ACH)

Why ACB might not be a good fit:

• No local filtration ( d i t?)rates (6 ACH)

• Low latent loads(code requirement?)

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Patient Room: All-Air SystemVAV terminalwith reheat coil

return airflow

200 ft2 with 10-ft ceiling height

230 cfm supply airflow200 cfm (6 ACH) minimum 67 cfm (2 ACH) outdoor air

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Design space sensible cooling load = 5000 Btu/hrDesign supply airflow (55°F) = 230 cfmMinimum outdoor airflow (ASHRAE 170) = 67 cfm (2 ACH)Minimum supply airflow (ASHRAE 170) = 200 cfm (6 ACH)Airflow turndown before activating reheat = 12%

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Patient Room: ACB Systemactive chilled beam(qty 1, 10 ft long, 4-pipe)

200 ft2 with 10-ft ceiling height

268 cfm (>6 ACH) total airflowprimary air

+induced room air(3:1 induction ratio)

exhaust airflow

67 cfm (2 ACH) primary airflow100% outdoor air

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Design space sensible cooling load = 5000 Btu/hrDesign primary airflow (55°F) = 67 cfmMinimum outdoor airflow (ASHRAE 170) = 67 cfm (2 ACH)Total room airflow = 268 cfm (8 ACH, 3:1 induction ratio)Capacity turndown before activating heat = 70%

active chilled beam systems compared to VAV systems SummaryPotential advantages• Smaller ductwork and

smaller air handlers

Challenges• High installed cost• Need to prevent

– Primary airflow < supply airflow, but likely > outdoor airflow

• Low sound levels• Impact on overall system energy?

– Primary airflow < supply airflow, but constant airflow

– Warm water for beams,

Need to prevent condensation

• Risk of water leaks• No filtration of local

recirculated air• Limited heating capability

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but cold water primary AHU– Increased pumping energy– No DCV, limited airside

economizing

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Additional Resources

• “Understanding Chilled Beam Systems,” TraneEngineers Newsletter ADM-APN034-EN (2009)www.trane.com/engineersnewsletter

• Chilled-Water VAV Systems, Traneapplication manual SYS-APM008-EN (2009)

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understanding chilled beam and vav systems chilled beams

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