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Optimizing Central Chilled Water

SystemsKent W. Peterson, PE, FASHRAE

P2S Engineering, Inc.

kent.peterson@p2seng.com

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Presentation Outline

• Foundation of CHW Plant Design

• Hydronic System Design

• Chiller Fundamentals

• Optimizing Plant Performance

• Building Interfaces

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

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

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

Purpose of Pumping Systems

Move enough water through the piping systemat the minimum differential pressurethat will satisfy all connected loads

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

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

Hydronic FundamentalsVariable Flow System Dynamics

LoadDP

100 GPM5 PSID

5 PSID

28 PSID

5 PSID

2 PSID

12 PSID38 PSID45 PSID

PUMP CLOSE LOAD REMOTE LOAD

100 GPM5 PSID

0 GPM0 PSID

5 PSID

2 PSID

12 PSID

0 PSID

100 GPM5 PSID

0 GPM0 PSID

5 PSID

28 PSID

38 PSID

0 PSID

BAD SENSORLOCATION

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

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

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

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

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

∆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

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

Centrifugal Chiller without VFD1200T Low Pressure

Centrifugal Chiller with VFD1200T Low Pressure

Percent Loaded

Centrifugal Chiller without VFD1200T High Pressure

Centrifugal Chiller with VFD1200T High Pressure

Percent Loaded

Centrifugal Chiller Comparison

Percent Loaded

Constant Speed Variable Speed

Optimizing Plant Performance

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

Primary-Secondary Variable Flow

Part Load Operation - 6000 Ton Plant

3000 Ton Load3000 Ton Load58°F

42°F54°F

LoadVFD

6000 GPM 4500 GPM

1500 GPM

1500 tons

Primary-Secondary Variable Flow

Effect of Low CHWR TemperatureLow ∆T Syndrome

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

LoadVFD

3000 Ton Load3000 Ton Load

Variable Primary FlowPart Load Operation - 6000 Ton Plant

CLOSED

4500 GPMFM

1500 tons

Bypass is not needed if minimumflow through chiller is guaranteed 3000 Ton Load3000 Ton Load

Variable Primary FlowEffect of Low CHWR Temperature

CLOSED

6000 GPM

1500 tons

3000 Ton Load3000 Ton Load

Series Arrangement

•In applications with high lift, a series arrangement will improve overall plant performance

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

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

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

CH-1 CH-240°F56°F 48°F

0.50 KW/ton 0.61 KW/ton

7% KW Reductionor

0.555 KW/ton

Series-Parallel

85°F95°F 85°F95°F

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

CHW ∆TOption 1

40°F56°F16°F

10,000 tons15,000 gpm

200 feet head667

38% Pump KW Reductionor

Option 2

38°F58°F20°F

10,000 tons12,000 gpm

146 feet head413

CHWS TempCHWR TempCHW ∆TPlant SizeCHW FlowHeadPump KW

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

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

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

without Heat Exchangers

BuildingLoad Tertiary Loop

BuildingLoad

Boosted Secondary

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

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

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

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

“Everything should be as simple as possible,but no simpler”

“Insanity: doing the same thing over and overand expecting different results”

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

optimizing central chilled water systems...32 inspire • innovate • achieveinspire • innovate...

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