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Optimizing Central Chilled Water
SystemsKent W. Peterson, PE, FASHRAE
P2S Engineering, Inc.
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Presentation Outline
• Foundation of CHW Plant Design
• Hydronic System Design
• Chiller Fundamentals
• Optimizing Plant Performance
• Building Interfaces
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Foundation of Design
• Why look “outside the plant”?• Understand how distribution system will
operate• Understand how CHW ∆T will be effected by
dynamics of the systems connected
GOALDeliver CHW to all loads under various load
conditions as efficiently as possible
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Understanding Loads & Their Impact on Design
• Overall plant capacity is determined by peak design load
• Cooling load profile describes how the load varies over time is needed to design the plant to stage efficiently
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Chilled Water Plant Efficiency
• Operating kW/ton achievable in today’s plants (includes chillers, cooling towers and pumps)
• 0.5 - 0.7 Excellent• 0.7 - 0.85 Good• >1.0 Needs Improvement
• Do you really know how your chilled water plants are performing?
Discussionon Hydronics
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Purpose of Pumping Systems
Move enough water through the piping systemat the minimum differential pressurethat will satisfy all connected loads
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Understanding Hydronics
• The pumping system will be required to operate under various load conditions
• Variable flow system differential pressures throughout the system will be dynamic
• Hydronic systems should be hydraulically modeled to design or troubleshoot complex systems
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Caution
• Excessive pump head can cause systems to not function as designed and waste considerable energy
• Pump Selection
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System & Pump Curves
Total Flow
Tota
l Pre
ssur
e
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Hydronic FundamentalsVariable Flow System Dynamics
VFD
Load
LoadDP
100 GPM5 PSID
100 GPM5 PSID
5 PSID
28 PSID
5 PSID
2 PSID
12 PSID38 PSID45 PSID
PUMP CLOSE LOAD REMOTE LOAD
20
60
50
40
30
10
0
70
PR
ES
SU
RE
PS
IG
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Hydronic FundamentalsVariable Flow System Dynamics
100 GPM5 PSID
0 GPM0 PSID
5 PSID
2 PSID
12 PSID
0 PSID
VFD
Load
Load
DP
12 PSID12 PSID19 PSID
PUMP CLOSE LOAD REMOTE LOAD
20
60
50
40
30
10
0
70
PR
ES
SU
RE
PS
IG
13
Hydronic FundamentalsVariable Flow System Dynamics
VFD
DP
100 GPM5 PSID
0 GPM0 PSID
5 PSID
28 PSID
38 PSID
0 PSID
BAD SENSORLOCATION
Load
Load
38 PSID38 PSID45 PSID
PUMP CLOSE LOAD REMOTE LOAD
20
60
50
40
30
10
0
70
PR
ES
SU
RE
PS
IG
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Hydronic FundamentalsVariable Flow System Dynamics
CONTROL VALVE ∆PAT VARIOUS LOAD CONDITIONS
Case 1Full Flow
Case 275% Flow
Case 350% Flow
Case 425% Flow
Case 510% Flow
Branch Flow (gpm) 100 75 50 25 10
Branch ∆P 38 38 38 38 38
Coil ∆P 5.0 2.8 1.3 0.3 0.1
Balancing Valve ∆P 28.0 15.8 7.0 1.8 0.3
Control Valve ∆P 5.0 19.4 29.8 35.9 37.7
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Balancing ConsiderationsVariable Flow Systems
• Too large a balancing valve pressure drop will affect the performance and flow characteristic of the control valve.• ASHRAE 2003 Applications Handbook, page
37.8
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Hydronic Pumping Conclusions
• Coil heat transfer is easier to control in low head (<50 ft) branches
• Remote, high head loads can be served more efficiently with variable speed series booster pumping
What You Must KnowAbout CHW ∆T
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CHW Temperature Differential
• Poor CHW ∆T is the largest contributor to poor CHW plant performance
• To predict ∆T, you must know:• Characteristics of connected loads• Control valve requirements and limitations• Control valve control algorithms and setpoints• Heat exchanger characteristics
Chilled Water Coil Characteristics
Assumes Constant Load
CHWS Temperature °FCHWS Temperature °F
CH
W ∆
T °F
CH
W ∆
T °F
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Factors that Degrade ∆TAssuming Coils are Selected for Desired ∆T
• Higher CHWS temperature• Poor control valves
• 3-way control valves• 2-position valves on fan coil units
• Controls not controlling• Setpoint cannot be achieved• Valves not interlocked to close if load turns off
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∆T Conclusions• Design, construction and operation
errors that cause low ∆T can be avoided
• Other causes for low ∆T can never be eliminated
• ∆T degradation below design conditions is inevitable, therefore, system design must accommodate the level of degradation anticipated
Chiller Fundamentals
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Understanding Refrigerant Lift
• Lift = SCT - SST
• Saturated Condensing Temperature (SCT) is dependent upon LEAVING condenser water temperature
• Saturated Suction Temperature (SST) is based off of LEAVING chilled water temperature
Percent Loaded
KW
/ton
Centrifugal Chiller without VFD1200T Low Pressure
Centrifugal Chiller with VFD1200T Low Pressure
Percent Loaded
KW
/ton
Percent Loaded
KW
/ton
Centrifugal Chiller without VFD1200T High Pressure
Centrifugal Chiller with VFD1200T High Pressure
Percent Loaded
KW
/ton
Centrifugal Chiller Comparison
Percent Loaded
KW
/ton
Hig
h P
ress
ure
Low
Pre
ssur
e
Constant Speed Variable Speed
Optimizing Plant Performance
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Primary-Secondary vs Variable Primary Flow
• Variable primary flow plants can provide advantages over traditional primary-secondary configurations
• Less plant space required for VPF
• VPF is not conducive to CHW TES
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Primary-Secondary Variable Flow
Part Load Operation - 6000 Ton Plant
Load
3000 Ton Load3000 Ton Load58°F
42°F54°F
Load
Load
Load
LoadVFD
∆P
6000 GPM 4500 GPM
1500 GPM
1500 tons
1500 tons
OFF
42°F
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Primary-Secondary Variable Flow
Effect of Low CHWR TemperatureLow ∆T Syndrome
44°F
52°F
42°F54°F
6000 GPM 7200 GPM
1200 GPM
Additional chiller will need to bestarted to maintain the secondaryCHWS temperature setpoint if load increases
Loss of CHWStemp control
1500 tons
1500 tons
OFF
Load
Load
Load
Load
LoadVFD
∆P
3000 Ton Load3000 Ton Load
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Variable Primary FlowPart Load Operation - 6000 Ton Plant
42°F
CLOSED
58°F
VFD
4500 GPMFM
1500 tons
1500 tons
OFF
Load
Load
Load
Load
Load
∆P
Bypass is not needed if minimumflow through chiller is guaranteed 3000 Ton Load3000 Ton Load
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Variable Primary FlowEffect of Low CHWR Temperature
CLOSED
54°F
VFD
6000 GPM
1500 tons
1500 tons
OFF
FM
Load
Load
Load
Load
Load
∆P
42°F
3000 Ton Load3000 Ton Load
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Series Arrangement
•In applications with high lift, a series arrangement will improve overall plant performance
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85°F95°F
Series versus ParallelWith High Lift Requirement
40°F56°FCH-1
CH-240°F56°F
40°F56°F
0.60 KW/ton
Parallel-Parallel
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7% KW Reduction on Chillersor
450 KW Reduction on 10,000 ton Plant
30 feet head increase on condenser water would result in 230 KW increase in pump power
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Series versus ParallelWith High Lift Requirement
CH-1 CH-2
95°F 85°F90°F
0.52 KW/ton 0.59 KW/ton
0.555 KW/ton
Series-Counterflow
40°F56°F 48°F
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Series versus ParallelWith High Lift Requirement
CH-1 CH-240°F56°F 48°F
0.50 KW/ton 0.61 KW/ton
7% KW Reductionor
450 KW Reduction on 10,000 ton Plant
0.555 KW/ton
Series-Parallel
85°F95°F 85°F95°F
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Optimize Heat Rejection• Oversized cooling towers can decrease
approach to lower chiller lift requirements and improve plant KW/ton
• Approximately 1.5% chiller KW reduction per °F lift reduction
Lowering CWS by from 95°F to 93°F3% Chiller KW Reduction
or180 KW Reduction on 10,000 ton Plant
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CHW ∆TOption 1
40°F56°F16°F
10,000 tons15,000 gpm
200 feet head667
38% Pump KW Reductionor
254 KW Reduction on 10,000 ton Plant
Option 2
38°F58°F20°F
10,000 tons12,000 gpm
146 feet head413
CHWS TempCHWR TempCHW ∆TPlant SizeCHW FlowHeadPump KW
87
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Thermal Energy Storage
• Chilled water thermal storage is a viable means of reducing peak electrical demand and increasing plant efficiency
• Less chiller and cooling tower capacity required
• Keep it simple!
Building InterfaceConsiderations
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Building Interface Considerations
Energy Transfer Stations Using Heat Exchangers
• Heat exchangers designed with lower approaches will typically yield higher CHW ∆T
• Always focus on supplying load with proper CHWS temperature
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Building Interface Considerations
without Heat Exchangers
• Avoid chilled water tertiary loops• Remember cooling coil fundamentals
• A variable speed booster pump should be used to boost differential pressure when needed
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Building Interface Considerations
without Heat Exchangers
BuildingLoad Tertiary Loop
BuildingLoad
VFD
Boosted Secondary
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Control Design Issues
• Control strategies should consider impact on complete system
• Aim to continually optimize COP for entire system
• Reliable industrial-grade controls are essential
• Keep it simple
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A Case for Metering
• Most efficiently designed systems are horribly inefficient after several years of operation
• How can we improve operation if we don’t evaluate the metrics?
• Calibrate regularly
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A Case for Commissioning
• Commissioning is a systematic process of assuring that systems perform in accordance with the design intent and owner’s operational needs
• Retro-commissioning
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Summary• Understand parameters that affect
chiller plant and overall system performance
• Optimize operation through equipment selection and control sequences to deliver CHW to all loads as efficiently as possible throughout the year
• Commission plant
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Albert Einstein
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“Everything should be as simple as possible,but no simpler”
“Everything should be as simple as possible,but no simpler”
“Insanity: doing the same thing over and overand expecting different results”
“Insanity: doing the same thing over and overand expecting different results”
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For More Information
• Join ASHRAE
• ASHRAE Self Directed Learning Course “Fundamentals of Water System Design”
• ASHRAE 2004 HVAC Systems and Equipment Handbook
• ASHRAE Transactions and Journal articles
• Hydronic System Design & Operation by E.G. Hansen