Optimizing Central Chilled Water Systems

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Optimizing CentralChilled Water Systems

• Kent W. Peterson, P.E.
• P2S Engineering, Inc.

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Everything should be as simple as possible, but
no simpler - Albert Einstein
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Foundation of Design

• Goal -
• Why look outside the plant?
• Understand how loads impact plant operation
• Understand how distribution system will operate
• Understand how temperature differential will be
  effected by dynamics of system
• Apply findings to chilled water system operation

Deliver CHW to all loads under various load
conditions as efficiently as possible throughout
the year
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Chilled Water Plant Efficiency

• Target kW/ton achievable in modern plants
  (includes chillers, cooling towers and pumps)
• 0.5 - 0.7 Excellent
• 0.7 - 0.85 Good
• gt1.0 Needs Improvement
• Do you know how your plant is performing?

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Typical Facility Load Information

• Chilled water required for each building is
  determined by the building block cooling load
• Central chiller plant loads depend on the actual
  simultaneous load expected at the central plant
• Central plant chilled water system load diversity
  for multiple building facilities varies between
  55-75

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Understanding Loads

• Estimated annual load profiles must be understood
  to design a chilled water system that will meet
  your performance expectations
• Valuable data to measure and trend
• Chilled water load profiles
• Anticipated load diversity
• Know your facility central plant diversity
• Usually 400-500 GSF/ton

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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 can be very dynamic
• Hydronic system modeling should be used to design
  or troubleshoot complex piping distribution
  systems under various load conditions

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Purpose of Pumping Systems

• Purpose is to move enough water through system at
  a differential pressure that will satisfy all
  connected loads
• CAUTION - Over sizing pumps can cause systems to
  not function as designed and waste considerable
  energy

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Hydronic Fundamentals
Variable Flow System Dynamics
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Hydronic Fundamentals
Variable Flow System Dynamics
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Balancing Considerations
Variable 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
• Options to Consider
• No balancing valves, no balancing
• Automatic differential pressure control valves to
  control differential pressure at close loads
• Pressure-independent control valves
• Options NOT to Consider
• Balancing valves at variable speed pumps

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Hydronic Pumping Conclusions

• Coil heat transfer is easier to control in low
  head (lt50 ft head) systems
• Remote, high head loads can be served more
  efficiently with variable speed series booster
  pumping
• Looped systems can offer redundancy and reduced
  differential pressure in mains

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CHW Temperature Differential

• Lack of chilled water ?T is the largest
  contributor to poor chilled water plant
  performance
• To predict ?T, you must know
• Characteristics of cooling coils in system
• Control valve requirements and limitations
• Control valve control algorithms and setpoints
• Load characteristics on coil

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CHW Temperature Differential

• Cooling Coil Characteristics
• Lower CHWS temperature will typically yield
  higher CHWR temperature assuming load is adequate
• Higher CHWS temperature will typically yield
  lower CHWR temperature
• CHWR temperature will begin to lower when EAT
  gets close to CHWR temperature

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Factors that Degrade ?T
Assuming Coils Are Selected for Desired ?T

• Air economizers and 100 OSA systems
• Excessive distribution differential pressure
• Higher CHWS temperature - collapsing ?T
• Account for heat gain on above grade piping
• Coil control valves (specify close-off pressure)
• Not capable of controlling against differential
  pressure
• 3-way valves or 3-way converted to 2-way valves
• Controls not controlling
• Supply air setpoint cannot be achieved
• Valves not interlocked with AHU, out of
  calibration
• Improper coil selections

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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 is inevitable, therefore, system
  design must accommodate level of degradation
  anticipated

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?T Selection

• What is the optimum ?T for chilled water and
  condenser water systems?
• There is no optimum ?T for all systems,
  however, the following guidelines can lead to an
  answer
• Select chillers at various ?Ts and estimate life
  cycle costs (12-20F)
• If majority of chiller operating hours are at
  reduced load, consider 2 gpm/ton on condenser
  water

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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

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Lift Effect on Part Load Chiller Efficiencies
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VFD Effect on Part Load Chiller Efficiencies
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Chilled Water Plant Design

• Optimize equipment sizing, selection and
  operation to efficiently provide chilled water to
  the loads under various load conditions
• Consider maintenance when selecting and laying
  out equipment
• Select a plant configuration that best suits the
  system requirements
• Do not make control sequences too complicated -
  the plant operator must understand the sequences

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Chiller Selection

• Pick a short list of vendors based on past
  experience, local representation, etc.
• Determine the functional and operational
  requirements by assessing the cooling load and
  load profiles including hours of operation
• Estimate energy usage of options
• Select system (chillers) based on lowest life
  cycle cost considering capital costs, recurring
  costs of operation including maintenance and
  repairs

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Primary-Secondary Variable Flow
Part Load Operation - 1500 Tons
DP
750 tons
750 tons
Off
4000 GPM
VFD
42 F
51 F
Secondary Pumps
42 F
LOAD 1500 TONS
1600 GPM
57 F
FM
2400 GPM
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Variable Primary Flow
Part Load Operation - 1500 Tons
TYPICAL
DP
DP

• Auto isolation valves
• preferred to dedicated pumps
• allows slow staging
• better redundancy
• extends 1 pump operation

750 tons
750 tons
Off
2400 GPM
42 F
VFD
Primary Pumps On VFDs
42 F
LOAD 1500 TONS
NO FLOW
57 F
57 F
FM
FM
2400 GPM
2400 GPM
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Advantages of Variable Primary Flow

• Lower First Costs
• Less Plant Space Required
• Reduced Pump Energy
• Higher efficiency pumps
• Reduced pressure drop due to fewer pump
  connections, less piping

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Variable Primary Flow Design Issues

• Evaporator flow
• Typically minimum 3 fps extended
• Rate of change limits (usually maximum 30 per
  minute)
• Complexity of control
• Not as fail-safe - what if bypass valve fails at
  low flow?
• Must avoid abrupt flow shut-off (e.g. valves
  interlocked with AHUs all timed to stop at same
  time)
• Flow fluctuation when staging chillers on
• CHW TES integration can be difficult

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Thermal Storage

• Chilled water thermal storage is a viable means
  of increasing chiller plant efficiencies to
  reduce KW/ton of delivered chilled water
• Near full load operation during charge
• Ambient relief on tower to reduce condenser water
  temperatures
• Reduction in peak chiller capacity requirement
• Keep it simple

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Control Issues

• Control strategies should consider impact on
  complete system including HVAC systems
• Control strategies should try to continually
  optimize energy consumption for entire system
• Reliable controls are essential
• Remember to keep it as simple as possible

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Building Interface Considerations

• Avoid chilled water tertiary loops whenever
  possible
• Remember cooling coil fundamentals
• A variable speed booster pump should be used to
  boost differential pressure when required

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A Case for Metering

• Most efficiently designed systems are horribly
  inefficient after several years of operation
• Do you really know how your systems are
  operating?
• How can we improve if we dont monitor key
  metrics?
• Load and utility consumption data are essential
  for efficient operations

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A Case for Commissioning

• Commissioning is a systematic process of assuring
  that a system performs in accordance with the
  design intent and the owners operational needs
• Re-commissioning

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Summary

• Start design process outside the plant
• Understand parameters that affect chiller plant
  and overall system performance
• Right size your equipment to deliver CHW to all
  loads under various load conditions as
  efficiently as possible throughout the year
• Commission plant
• Optimize control sequences

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Energy Trend

• World energy demand is forecasted to increase
  170 from 2000 to 2030
• Electrical demand will double
• Fossil fuel will provide 90 of increase in
  demand
• Carbon dioxide emissions will increase 170
• Energy Efficiency is a Must
• What is our responsibility?

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For More Information

• ASHRAE Self Directed Learning Course
  Fundamentals of Water System Design
• ASHRAE 2003 HVAC Systems and Equipment Handbook
• ASHRAE Transactions and Journal
• Hydronic System Design Operation by E.G. Hansen

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Optimizing CentralChilled Water Systems