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THE OLD AND THE NEW
The Story of an Integrated Chilled Water PlantKelley Cramm, P.E.Kelley Cramm, P.E.
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The IRS Processing Facility
• Old Post Office – Offices – 500,000 sq. ft.• New 70,000 sq. ft. warehouse• New processing facility – 600,000 sq. ft.• Approximately 1.2 million total square feet of space
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The TeamA Cast of Thousands
•General Services Administration
•The Developer (under contract with GSA)
•The Architect (a joint venture between 2 firms) – under contract with the developer
•The Construction Manager•The Construction Manager
•The Commissioning Agent
•The Test and Balance Contractor
•The Mechanical Design/Build Contractor –4 MEP consulting firms
•The Electrical Design/Build Contractor –Another MEP consulting firm
•Lots of additional consultants
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DP
VSCHILLER
CHILLERCOOLING COIL
DP
PRIMARY SECONDARY SYSTEM
DECOUPLER
Size decouple pipe for very low pressure drop(It does not need a check valve)
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Controls for Primary/Secondary
• Primary pumps are constant speed – matched to chillers• Secondary pump speed controlled to maintain remote
delta P• Chillers controlled to maintain leaving water temperature• Chillers staged on/off to maintain LWT• Chillers staged on/off to maintain LWT• Excess chilled water will bypass using the decouple pipe• Low delta T can be a problem, resulting in more water
required than what the operating chillers can flow• Consider resetting remote delta P based on coil valve
position
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100%LOAD
100 FT.
75%LOAD67 FT.
50%LOAD42 FT.
25%LOAD26 FT.
SUPPLYRETURN
P: 20 FT.
DIFFERENTIAL PRESSURE TRANSMITTER MUST BE LOCATED AT FAR END OF SYSTEM
PRESSURE GRADIENTS
PRESSURE VARIATIONS IN A CHILLED OR HOT WATER SYSTEM
(COILS AND CONTROL VALVES NOT SHOWN)
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CHILLER
DP
VS
VS
DP
DP
COOLING COIL
CHILLER
DP
VARIABLE PRIMARY SYSTEM
BYPASS VALVE
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Controls for Variable Primary
• Pump speed controlled to maintain remote delta P• Bypass valve controlled to insure minimum chiller
evaporator flow• Use evaporator delta T to determine chiller flow – works
better than a flow meterbetter than a flow meter• Chillers controlled to maintain leaving water temperature• Chillers staged on/off on load (flow + delta T)• Consider resetting remote delta P based on coil valve
position
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Important Design Points
• Size of bypass line and valve– Must flow difference between min. load and min. chiller flow
• Valves– slow chiller isolation valves don’t cause upset– Bypass valve must be actuated open and closed– Bypass valve must be actuated open and closed
• Chiller selection – Select evaporator near peak flow and list the minimum in the equipment
schedule
• Hard wire remote delta P to same controller as pump VFD
• Need tight controls specification
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Advantages/Disadvantages
• Lower first cost• Fewer pumps• Smaller footprint• Lower energy cost• Helps combat low Delta T syndrome
• Operator unfamiliarity
• Helps combat low Delta T syndrome
MUST HAVE GOOD CONTROLS AND SAVVY OPERATORS
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Existing Plant in Old Post Office
• 2 water cooled centrifugal chillers – 500 tons each – converted to R134A• 17 years old when design started (2003)• Constant flow system with 3-way valves at air handlers
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• Needed a total of 2500 tons• Originally planned to replace old chillers• 3 new chillers – variable primary flow• Why replace machines with life remaining?• Can we vary the flow through the old chillers?
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• Flow on old chillers could be reduced to 30% of • Flow on old chillers could be reduced to 30% of original design flow
• Obtained evaporator pressure drop curves• Settled on a minimum of 40% of design flow• Limited rate of change of flow when an old
chiller was running
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• Installed 2 new 850 ton machines• Variable frequency drive on one of the new chillers• Flow could drop as low as 25% of max evaporator flow• Made selections near peak of evaporator flow capability
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Lessons Learned
• Communicate Communicate Communicate
• Just because someone says they did it doesn’t mean it’s done. Double Check
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Communication is a two way streetCommunication is a two way street
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Communication
• A Woman: without her, man is nothing.
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Communication Take 2
• A woman, without her man, is nothing.
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"I know that you believe you understand what you think I said, but I'm not sure you realize that what you heard is not what I meant."
Robert McCloskeyRobert McCloskey