Chiller Plant Design Considerations Jon R. Haviland, P.E., CEM Assistant Vice President Marx/Okubo Associates
[email protected]
z Plant
design options z Equipment options z New plant design considerations z Retrofit design considerations z Retrofit project case study
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Plant Design Options
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Single Chiller System Tower TWP
Chiller
CWP Loads Better Buildings by Design 2005
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Single Chiller System z Advantages – Lower first cost – Simple system for installation and control
z Disadvantages – Inefficient at low load conditions – Lack of back-up and redundancy
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Parallel Chiller System Tower TWP1 CWP1
Chiller 1
TWP2 CWP2
Chiller 2
Loads Better Buildings by Design 2005
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Parallel Chiller System z
Advantages – More efficient at low load conditions – Provides back-up and/or redundancy
z
Disadvantages – Increased first cost – More difficult to control effectively, especially with
different size chillers (should have computerized system) – More equipment to maintain Better Buildings by Design 2005
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Series Chiller System Tower TWP
Chiller 1
Chiller 2
CWP Loads Better Buildings by Design 2005
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Series Chiller System z
Advantages – Easier to control with simple control systems – Effective with different size chillers – Utilize one stand-by pump – Good for large temperature differential systems
z
Disadvantages – Increased pumping requirements for chilled water – Chillers may not be interchangeable
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Primary-Secondary Chilled Water Systems
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Primary-Secondary Chilled Water Systems z Advantages – Uncouples chillers and loads to allow variable flow in – – – – –
loads Can be used with large temperature differential systems Can be used with thermal storage systems Can be used with water-side economizer systems Improved control Reduced operating costs
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Primary-Secondary Chilled Water Systems z Disadvantages – Higher first costs – Requires more, sophisticated equipment – Requires sophisticated control system
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Variable Flow Primary z New
chillers can operate with variable flow z Simplifies system design z Bypass with modulating control valve required to maintain minimum flow z System can be parallel or series design
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Equipment Options
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Electric Chillers z Reciprocating
chillers z Rotary screw chillers z Centrifugal chillers z Air-cooled chillers
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Non-electric Chillers z Absorption chillers – Single-effect absorption chiller z z z
Low pressure steam Hot water Use in CHP applications
– Double-effect absorption chillers z Medium pressure steam z Direct-fired z Heat recovery Better Buildings by Design 2005
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Non-electric Chillers z Centrifugal
or screw chiller without electric
motor – Natural gas engine – Steam turbine – Dual drive – engine and electric motor
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Other Cooling Sources z Air-side
economizer z Water-side economizer z Thermal energy storage
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New Plant Design Considerations z Determine
requirements z Design process z After construction considerations
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Determine Requirements z Capacity
required
– Current load – Potential future loads – Redundancy requirement – Provisions for special loads
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Determine Requirements z Operating
profile
– Hours per day – Days per week – Continuous loads – Load diversity
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Design Process z Schematic
design of possible options
– Chiller sizing – Chiller Performance – Cooling sources – Operating temperatures – Cooling tower selection – Basic plant layout Better Buildings by Design 2005
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Chiller Sizing z Benefits
of different size chillers
– Fewer total operating hours – More operation above 50% load
z Depends
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on load profile
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Even Chillers - 8,463 operating hours
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Uneven Chillers - 7,624 operating hours
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Chiller Sizing z Use
of different size chillers may eliminate need for pony chiller z Variable speed chillers provide improved part load performance and minimum capacity
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Chiller Performance Ratings z American
Refrigeration Institute Standard 550/590-98 z Standard conditions – Full load – Chilled water temperatures - 54 F to 44 F – Condenser water temperatures - 85 F to 95 F
z Efficiency Better Buildings by Design 2005
rating - kW/Ton Chiller Plant Design
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Chiller Performance Ratings z Part
load efficiency
– Integrated Part Load Value (IPLV) z Standard temperature conditions z Weighted average of kW/ton at various loads – Non-standard Part Load Value (NPLV) z Specific application temperatures z Same weighting as IPLV
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Chiller Performance Ratings z IPLV/NPLV
rating scale
– A = kW/ton at 100% load – B = kW/ton at 75% load – C = kW/ton at 50% load – D = kW/ton at 25% load
z ARI
Standard 550-98
– IPLV = 1/(0.01/A + 0.42/B + 0.45/C + 0.12/D) Better Buildings by Design 2005
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Cooling Sources z Use
of different fuel sources provides flexibility and reliability z Types to be considered depend on several factors – – – –
Utility rates and structure Load profile Air quality considerations Maintenance considerations
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Cooling Sources z Economizer
cycle
– Air-side generally most effective – Water-side use z Building configuration does not allow air-side z Cooling for special load requirements z Climatic considerations
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Operating Temperatures z Chilled
water supply temperature
– Lower supply temperature z Reduced air flow requirements z Improved dehumidification z Increased chiller operating cost versus reduced fan operating costs, especially with a VAV system
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Operating Temperatures z Chilled
water ∆T
– Higher ∆T means reduced pipe size and
pumping requirement, increased coil size, and reduced fan energy – Optimum temperature depends on amount of piping in system
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Chilled Water ∆T Comparison
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Operating Temperatures z Condenser
water supply temperature
– Lower supply temperature improves chiller
efficiency – Approach temperature can be 6 – 10 ºF above design wet bulb – Use 0.4% design temperature data
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Operating Temperatures z Condenser
water ∆T
– Normal 10ºF works well in most cases – Larger ∆T common with absorption chillers
due to higher load and should be considered for large systems or when the cooling tower is remote from the chiller plant – Reduced chiller efficiency versus smaller pipes and reduced pumping horsepower Better Buildings by Design 2005
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Cooling Tower Selection z Selection
should be based on required water flow, optimum approach temperature and ∆T z Allowance for potential tenant equipment load z Selection should based on water-side economizer conditions if that is part of system Better Buildings by Design 2005
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Cooling Tower Selection z Increasing
size of the cooling tower is generally the least expensive way to improve the efficiency and provide some safety margin in the system z Use of variable speed drive on the fan motor minimizes any operating cost penalty
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Determine Basic Plant Layout z Select
system that will best fit needs of project z For simpler systems, parallel system with different size chillers is probably best choice
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Determine Basic Plant Layout z Systems
with different occupancy schedules or load requirements need more complex system – Primary-secondary plant offers most flexibility – Variable primary flow works with new electric
chillers with less complexity than a primarysecondary system
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Determine Basic Plant Layout z Large
chilled water ∆T
– Series plant – Primary-secondary with primary flow greater
than secondary flow
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Design Process z Analyze
options
– Opinion of probable costs – Operating cost estimates z Screening measures z Energy simulation – Life cycle costs
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Design Process z Complete
design of chosen option
– Peer review – Operator review – Determine sequence of operations
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New Plant Design Considerations z After
construction activities
– Commissioning – Training
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Chiller Plant Retrofit Considerations
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Don’t Simply Replace; Re-engineer
Chiller Plant Retrofit Considerations z Basic
process should follow same steps as outlined above z Additional considerations – Engineer and contractor should have experience
with this type of project
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Chiller Plant Retrofit Considerations z Additional
considerations
– Engineer needs to familiarize himself with
existing plant z z z
Record drawings Operator interview Observation of plant
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Chiller Plant Retrofit Considerations – Constructability should be considered during
design phase z z
Operator review Contractor review
– Coordination critical during construction phase
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Retrofit Project Case Study
Background z Vacated
single tenant building z 26 years old z Adjacent buildings under same ownership with sale of one or all unlikely z Building to be taken to shell condition and renovated for multi-tenant use z Due diligence based on these assumptions Better Buildings by Design 2005
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Site Plan
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Participants z Owner z Project
manager z Property managers (2 firms) – Contract maintenance staff
z Construction
manager
z Consultants – Architect – Engineer Better Buildings by Design 2005
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Existing Central Plant z z z
Located in penthouse Two 360-ton single effect absorption chillers Two low pressure steam boilers – Modified to meet AQMD requirements – Also provides heating and domestic hot water
z
Two cell cooling tower – Replaced three years earlier – Capacity for single effect absorption chillers
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Possible Options z Electric
chillers z Absorption chillers z Engine-driven chillers z Thermal storage z Independent or with adjacent building
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Alternatives Considered z Stand-alone
alternatives
– Two 350-ton electric chillers – Two 350-ton absorption chillers – Two 350-ton engine-driven chillers – Electric chillers with 4500 ton-hour thermal
storage
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Alternatives Considered z Integrated
alternatives
– Provide piping between two buildings – Utilize one of the four options above for the
stand-alone plant
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Alternatives Considered z Single
plant alternatives
– Abandon plant in Building 2 and expand plant
in Building 3 with piping between the buildings – Existing Plant in Building 3 z
z
Three 250-ton electric chillers in plant on parking level P-2 Cooling tower on roof
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Alternatives Considered z Single
plant alternatives
– New Equipment possibilities z 600-ton electric chiller z Two 350-ton engine-driven chillers z Two low temperature chillers plus 4500 ton-hour thermal storage
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Constructibility Issues z Building
2 options
– Electric capacity – Cooling tower and structure – Existing chilled water riser size – Noise considerations
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Constructibility Issues z Building
3 options
– Cooling tower capacity – Condenser water riser size – Thermal storage space requirements – Engine exhaust routing – Electrical capacity
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Analysis z Opinion
of probable costs z Schedule z Operating costs – DOE-2 simulations – Nine options considered
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Analysis z Other
considerations
– Electricity riser problem in Building 2 – Impact of utility deregulation
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Economic Analysis z Based
on estimated impact of deregulation z Final options – Integrated system with new chillers in Building
2 connected to Building 3 plant z z
700-tons electric 700-tons absorption
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Economic Analysis z First
Cost
– 700 ton electric z $929,223 z $1,109,273 with potential bus duct repair – 700 ton absorption z $1,395,247
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Economic Analysis z Annual
energy cost
– 700 ton electric - $778,160 – 700 ton absorption - $680,769
z Simple
Payback
– Without bus riser repair – 4.5 years – With bus riser repair – 1.9 years
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Final Choice z Absorption
chillers in Building 2 with connection to Building 3 – Proactive ownership with long term outlook – Support from local natural gas utility – Optimum conditions for new plant in Building
2 – Significant problems with expanding plant in Building 3 Better Buildings by Design 2005
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Chiller Purchase z Chiller
pre-purchased by owner
– Maintain control over final selection and
options – Delivery critical z
z
Potential full building tenant wanted early availability City rigging restrictions – Saturday night only
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Chiller Purchase z Bid
to major manufacturers z Bid specified conditions of capacity, giving cost, efficiency, delivery and willingness to accept penalty clause z Allow manufacturers to suggest alternatives of capacity and/or efficiency and additional cost for this Better Buildings by Design 2005
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New Central Plant z Two
360-ton direct fired absorption chillers with lower supply temperature z One 120-ton rotary screw chiller for off hours use (minimum effective absorption chiller capacity) z Primary-secondary piping system z Piping between the two buildings with transfer pumps and metering Better Buildings by Design 2005
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Single Line Diagram
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Sequence of Operations z Seven
modes of operation identified with written sequence z Independent versus integrated operation z Electric or gas chiller base load z Occupied versus unoccupied
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Phase II z Building
1 central plant
– Originally absorption chillers and boilers – Reciprocating chillers installed in 1987 z Noise complaints z Reliability issues – Small chiller in Building 3 plant for Fitness
Center after hours use with piping to lower level air handling unit Better Buildings by Design 2005
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New Building 1 Plant z Two
centrifugal chillers and new cooling tower – Tower selected for maximum capacity for space
available – Chillers selected to match cooling tower capacity – Use existing connection (increase size to maximize transfer capacity) with new riser Better Buildings by Design 2005
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New Single Line Diagram
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Summary z Project
illustrates choices available and retrofit process discussed z Ownership with long term outlook z Cooperation by all participants z Life is easier with a vacant building
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