4-Pipe VAV vs. Active Chilled Beams · Active Chilled Beams Steven T. Taylor, PE ... Alameda, CA...
Transcript of 4-Pipe VAV vs. Active Chilled Beams · Active Chilled Beams Steven T. Taylor, PE ... Alameda, CA...
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Session C5: Ventilation Choices
4-Pipe VAV vs. Active Chilled Beams
Steven T. Taylor, PEPrincipalTaylor Engineering, Alameda, [email protected]
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Learning Objectives
• Explore the pros and cons of using active chilled beams versus 4-pipe VAV systems;
•Become familiar with different lab types as they define which system type is more appropriate;
•Understand the difference between a load-dominated, hood-dominated, and ventilation-dominated lab space; and
•Understand the cost and energy implications of two popular mechanical systems used in laboratories.
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Agenda
• System schematics• 4-pipe VAV (4PVAV) • Active Chilled Beams (ACB)
• Important fundamentals: Lab types• Ventilation-dominated• Hood-dominated• Load-dominated
• Case study: Lab in Oakland, CA• Cooling loads• Energy use
• Other factors• Conclusions
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4-pipe VAV Schematic
Lab Exhaust fans
VFD
VFD
VFD
Air Handler
FSD
Lab air valves -typ
Cond. drain
Change-over valves
Control valves
Sub
-du
ct
Outdoor Air
Min. Flow Bypass
Dampers
Supply Air Duct
Hot Water Coil
No chilled water coil at air handler in “dry” climates
Chilled water coil with condensate drain at zones
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“Dry”: <63F Dewpoint (75F/65%RH) except for a few hundred hours/year
DenverPortland
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Includes most of west coast, southwest, mountain states (ASHRAE B and C Climate Zones)
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Changeover Piping
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Why Changeover instead of two coils?
•Much higher hot water temperature drop ΔT• Improved boiler efficiency with condensing boilers• Smaller piping and HW pumps• Lower HW pump energy
• Less space required• Lower air pressure drop
• Lower fan energy
• Less expensive• Eliminated HW coil• Smaller HW piping
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Active Chilled Beam Schematic
Lab Exhaust fans
VFD
VFD
VFD
Air Handler
FSD
Lab air valves
Active Chilled Beam
Sub
-du
ct
Outdoor Air
Supply Air Duct
Hot Water Coil
Chilled water coil required at air handler
Active chilled beams (changeover piping same as 4PVAV)
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Lab Types
•Ventilation-dominated• Where the minimum ventilation rate (air changes per hour
ACH) is greater than hood exhaust rates and rates required for cooling
•Hood-dominated• Where the hood exhaust rate exceeds those required for
cooling or minimum ventilation
• Load-dominated• Where the airflow needed for cooling exceeds the hood
exhaust and minimum ventilation rates
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Only “load-dominated” labs benefit from ACBs
• ACB induction cooling increases cooling capacity at a lower outdoor air rate• ~2.5 times the sensible load that primary outdoor air can provide alone
• So labs that require 6 ACH to 15 ACH for cooling require only 6 ACH of outdoor air
• Reduces fan energy and energy needed to condition outdoor air
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How prevalent are “load dominated” labs?
UC Davis Lab Plug Loads (from Labs21 Minimizing Reheat Energy Use in Laboratories)
•Varies significantly from lab to lab•Occupied lab vent
typically 6 to 10 ACH• Equates to ~5.8 to
9.7 w/ft2 total load
• Some labs may be designed for high w/ft2, but seldom operate there
Equivalence of ACH to plug loads assuming a 9 foot ceiling and 55°F supply air temperature, 75°F space temperature
ACHLoads w/ft2
2 1.94 3.96 5.88 7.7
10 9.712 11.614 13.5
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Example Lab
• 55000 ft2 Lab in Oakland, CA
• 6 ACH occupied, 4 ACH unoccupied
• Air-to-water heat pump/chillers• 2-pipe changeover since there is no simultaneous heat/cool
with 4PVAV and not enough hours with ACB system to be cost effective
• Lab mix (which favors ACBs)• 22% ventilation dominated • 78% load dominated labs• 0% hood dominated labs
• EnergyPlus model
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ACB supply air temperature control
• Ideally just low enough to prevent condensation on chilled beams
• Ideal: condensation sensor based reset
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Equipment Sizing vs. Supply air Temperature
ACB 55°F SAT
ACB 58°F SAT
ACB 60°F SAT
ACB 65°F SAT
4-Pipe VAV
AHU (cfm) 39,402 39,402 39,402 39,402 61,492
HP/chillers (tons) 196 185 178 162 105
4PVAV has larger AHU but smaller chiller plant
4PVAV has only HW coil so SAT does not affect loads or energy
~63F SAT warmest to ensure spaces <65% RH
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Site Energy Use Index, Btu/h/ft2
Ideal condensate reset between these two
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Energy Cost Index $/ft2
$0.00
$0.10
$0.20
$0.30
$0.40
$0.50
$0.60
$0.70
$0.80
Heating Cooling Fans Pumps Heat Rejection
Co
st
[$/f
t²]
ACB 55°F SAT + Chilled Beam
ACB 58°F SAT + Chilled Beam
ACB 60°F SAT + Chilled Beam
ACB 65°F SAT + Chilled Beam
4-Pipe VAV
$3.24 /ft²$3.01 /ft² $2.90 /ft² $2.82 /ft²
$2.56 /ft²
$0.00
$0.50
$1.00
$1.50
$2.00
$2.50
$3.00
$3.50
Total
Co
st
[$/f
t²]
ACB 55°F SAT + Chilled Beam
ACB 58°F SAT + Chilled Beam
ACB 60°F SAT + Chilled Beam
ACB 65°F SAT + Chilled Beam
4-Pipe VAV
Ideal condensate reset between these two
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EnergyPlus Results - Heating
•Heating energy is lower for 4PVAV • Literally no reheat• Higher for labs in load-dominated mode
in cold weather due to greater OA conditioning
• But lower for labs in ventilation-dominated mode since ACB dehumidification increases reheat En
ergy
Co
st $
/ft2
AC
B 5
5F
AC
B 5
8F
AC
B 6
0F
AC
B 6
5F
4P
VA
V
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EnergyPlus Results - Cooling
• Cooling energy is lower for 4PVAV • Economizer effect in load-dominated labs
• Many hours/year with Oakland’s mild weather
• ACB could mitigate with waterside economizer but at added cost
• Single temperature chiller plant • No cooling efficiency benefit for lukewarm
CHWST to ACBs
• ACB could mitigate with dual temperature plant or DX for AHU but at added cost/complexity
Ener
gy C
ost
$/f
t2
AC
B 5
5F
AC
B 5
8F
AC
B 6
0F
AC
B 6
5F
4P
VA
V
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EnergyPlus Results - Fans
• Fan energy is lower for 4PVAV • Because ACBs provide higher sensible
loads, all labs become ventilation-dominated so the system is ~CAV
• 4PVAV has cube-law performance of fans at part load• Pressure drop through entire duct
system goes down with square of flow
• Exhaust system also VAV due to high exhaust stacks – savings lower if high exhaust velocity must be maintained
Ener
gy C
ost
$/f
t2
AC
B 5
5F
AC
B 5
8F
AC
B 6
0F
AC
B 6
5F
4P
VA
V
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Why does 4PVAV have lower fan energy?
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Other 4PVAV Advantages:
• Net first costs are usually lower • Very high cost of the ACBs and their installation• Smaller cooling and heating plants• Even smaller piping and pumps due to higher ΔTs• Partially offset by larger AHUs & ductwork, terminal unit condensate drains
• Works for all zones• ACB system only effective in load-dominated labs
• Ventilation-dominated zones are essentially constant volume reheat• ACB cannot be used on some hood-dominated labs due to insufficient
ceiling space to house the ACBs
• More flexible for future remodels• Hoods can be added to load-dominated labs without any modifications • 4PVAV diffusers more readily relocated vs. ACBs
• Lower risk of condensation damage
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Other ACB Advantages:
• Smaller supply air system if many load-dominated labs• Less roof space needed for AHUs• Less ceiling space needed for ductwork
• Possible floor/floor height reduction
•No condensate drains at terminal units
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Conclusions
• 4PVAV Advantages• Usually more energy efficient
• For sure in mild, dry climates• For sure if most zones hood- or ventilation-dominated
• Usually costs less• Unless most labs are highly load-dominated
• More flexible• Works for all lab types• Less expensive to remodel
• ACB Advantages• Less space required for AHUs, duct mains• No condensate drains at terminal units