Optimizing Central Chilled Water Systems - Utah ASHRAE
Transcript of Optimizing Central Chilled Water Systems - Utah ASHRAE
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OptimizingCentral Chilled Water SystemsKent W. Peterson, PE, FASHRAEP2S Engineering, Inc.
ASHRAE Utah Chapter
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Learning Objectives
1. Gain a better understanding of the operational dynamics of various load and equipment components in chilled water systems
2. Understand opportunities to provide both functional and energy efficient operation of chilled water systems
3. Develop a logical approach to the performance optimization of chilled water systems
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Presentation Outline
• Foundation of CHW Plant Operation
• Hydronic System Design
• Chiller Fundamentals
• Optimizing Plant Performance
• Building Interfaces
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Foundation of Operation
• 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
• Cooling load “diversity”
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Chilled Water Plant Efficiency
• Operating kW/ton achievable in today’s plants (includes chillers, cooling towers and pumps)
• 0.4 - 0.7 Excellent
• 0.7 - 0.85 Good
• >1.0 Needs Improvement
• Do you really know how your chilled water plants are performing?
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Discussionon Hydronics
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Purpose of Pumping Systems
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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
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0
50
100
150
200
250
300
0 1500 3000 4500 6000 7500 9000 10500 12000
System CurveCombined Pump Curve
Total Flow
Tota
l Pre
ssur
e
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Hydronic FundamentalsVariable Flow System Dynamics
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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
PRES
SUR
E PS
IG
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Hydronic FundamentalsVariable Flow System Dynamics
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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
PRES
SUR
E PS
IG
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Hydronic FundamentalsVariable Flow System Dynamics
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100 GPM5 PSID
0 GPM0 PSID
5 PSID
2 PSID
12 PSID
0 PSID
VFD
Load
LoadDP
12 PSID12 PSID19 PSID
PUMP CLOSE LOAD REMOTE LOAD
20
60
50
40
30
10
0
70
PRES
SUR
E PS
IG
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Hydronic FundamentalsVariable Flow System Dynamics
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CONTROL VALVE ∆PAT VARIOUS LOAD CONDITIONS
CONTROL VALVE ∆PAT VARIOUS LOAD CONDITIONS
CONTROL VALVE ∆PAT VARIOUS LOAD CONDITIONS
CONTROL VALVE ∆PAT VARIOUS LOAD CONDITIONS
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
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• Too large a balancing valve pressure drop affects 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
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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
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Chilled Water Coil CharacteristicsAssumes Constant Load on a Given Coil
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5
10
15
20
25
4042
4446
4850
CHWS Temperature °F
CH
W ∆
T °
F
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Factors that Degrade ∆TAssuming Coils are Selected for Desired ∆T
• Higher CHWS temperature
• Lower entering air temperature (economizer)
• Control valve issues- 3-way control valves
- 2-position valves on fan coil units
- Valves exposed to high ∆P and can’t shutoff
• Controls not controlling- Setpoint cannot be achieved- Valves not interlocked to close if unit 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
• Therefore, system design must accommodate the level of degradation anticipated
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Chiller Fundamentals
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Understanding Compressor Lift
• Temperature 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
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0.2
0.4
0.6
0.8
1.0
25 50 75 100
65 ECWT 75 ECWT 85 ECWT
Percent Loaded
KW
/ton
Centrifugal Chiller without VFD1200T Low Pressure
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Centrifugal Chiller with VFD1200T Low Pressure
0.2
0.4
0.6
0.8
1.0
25 50 75 100
65 ECWT 75 ECWT 85 ECWT
Percent Loaded
KW
/ton
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0.2
0.4
0.6
0.8
1.0
25 50 75 100
65 ECWT 75 ECWT 85 ECWT
Percent Loaded
KW
/ton
Centrifugal Chiller without VFD1200T High Pressure
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Centrifugal Chiller with VFD1200T High Pressure
0.2
0.4
0.6
0.8
1.0
25 50 75 100
65 ECWT 75 ECWT 85 ECWT
Percent Loaded
KW
/ton
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Centrifugal Chiller Comparison
0.2
0.4
0.6
0.8
1.0
25 50 75 100
65 ECWT 75 ECWT 85 ECWT
Percent Loaded
KW
/ton
0.2
0.4
0.6
0.8
1.0
25 50 75 100
0.2
0.4
0.6
0.8
1.0
25 50 75 1000.2
0.4
0.6
0.8
1.0
25 50 75 100
Hig
h Pr
essu
reLo
w P
ress
ure
Constant Speed Variable Speed
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Centrifugal Chiller with VFD1000T High Pressure, Multiple Oil-Free Compressors
0.2
0.4
0.6
0.8
1.0
25 50 75 100
65 ECWT 75 ECWT 85 ECWT
Percent Loaded
KW
/ton
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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 FlowPart Load Operation - 4500 Ton Plant
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Load
58°F
42°F
Load
Load
Load
LoadVFD
∆P
6000 GPM 4500 GPM
1500 GPM
42°F
54°F
1500 tons
1500 tons
OFF
3,000 Ton Load
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Primary-Secondary Variable FlowEffect of Low CHWR Temperature
Low ∆T Syndrome
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44°F
52°F
42°F
6000 GPM 9000 GPM
3000 GPM
Additional chiller will need to bestarted to maintain the secondaryCHWS temperature setpoint if load increases
Loss of CHWStemp control
Load
Load
Load
Load
LoadVFD
∆P1500 tons
1500 tons
OFF
3,000 Ton Load
52°F
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Variable Primary FlowPart Load Operation - 4500 Ton Plant
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42°F
CLOSED
58°F
VFD
4500 GPM
FM
1500 tons
1500 tons
OFF
Load
Load
Load
Load
Load
∆P
Bypass is not needed if minimumflow through chiller is guaranteed 3,000 Ton Load
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Variable Primary FlowEffect of Low CHWR Temperature
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CLOSED
54°F
VFD
6000 GPM
1500 tons
1500 tons
OFF
FM
Load
Load
Load
Load
Load
∆P
42°F
3,000 Ton Load
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Series Arrangement
In applications with high lift, a series evaporator arrangement can improve overall plant performance
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85°F95°F
Series versus ParallelWith High Lift Requirement
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40°F56°FCH-1
CH-240°F56°F
40°F56°F
0.60 KW/ton
Parallel-Parallel Arrangement
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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
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CH-1 CH-2
95°F 85°F90°F
0.52 KW/ton 0.59 KW/ton
0.555 KW/ton
Series-Counterflow Arrangement
40°F56°F 48°F
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Series versus ParallelWith High Lift Requirement
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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 Arrangement
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
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Lowering CWS by from 95°F to 93°F3% Chiller KW Reduction
or
180 KW Reduction on 10,000 ton Plant
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CHW ∆T
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Option 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
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Increased chiller lift would result in 167 KW increase in chiller power
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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!
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Building InterfaceConsiderations
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Building Interface ConsiderationsEnergy 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 Considerationswithout 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 Considerationswithout Heat Exchangers
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BuildingLoad Tertiary Loop
BuildingLoad
VFD
Boosted Secondary
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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 efficiency?
• 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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Control Design Issues
• Control strategies should consider impact on complete system
• Aim to continually optimize energy efficiency for entire system
- Demand control
- Relational control
• Aim for reliability and “elegance”
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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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For More Information
• ASHRAE Self Directed Learning Course “Fundamentals of Water System Design”
• ASHRAE HVAC Systems and Equipment Handbook
• ASHRAE Journal series “Optimizing Chilled Water Plants”
• Hydronic System Design & Operation by E.G. Hansen
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ThankYou