Chiller System Design and Control
Applications Engineering Manual
SYS-APM001-EN
Page
Beth Bakkum, information designer
Susanna Hanson, applications engineer
Mick Schwedler, applications manager
Chiller System Design and Control
Chiller System Design and Control
Preface
2009 Trane All rights reserved
SYS-APM001-EN
Contents
Condenser-Water System Control
Failure Recovery
Chilled-Water System Control
System Controls
Primary System Components
Chiller
Figure 1. Typical vapor-compression chiller
Primary System Components
Chiller evaporator
Figure 2. Flooded evaporator cut-away
Effect of chilled-water flow rate and variation
Effect of chilled-water temperature
Primary System Components
Figure 3. Direct-expansion evaporator cut-away
Primary System Components
Water-cooled condenser
Effect of condenser-water temperature
Effect of condenser-water flow rate
Maintenance
Air-cooled condenser
Air-cooled versus water-cooled condensers
Primary System Components
Primary System Components
Low-ambient operation
Energy efficiency
1212 Midnight midnight
Loads
Primary System Components
dry bulb wetWetbulbBulb
Primary System Components
Three-way valve load control
Two-way valve load control
Heat transferred from the loads can be controlled in a number of ways
Primary System Components
Variable-speed pump load control
Face-and-bypass dampers
Figure 6. Two-way valve
Primary System Components
Chilled-Water Distribution System
Chilled-water pump
Figure 8. Uncontrolled water flow with bypass damper
Pump per chiller
Distribution piping
Primary System Components
Manifolded pumps
Constant flow system
Pumping arrangements
Primary System Components
Figure 12. Constant flow system
Primary System Components
Condenser-Water System
Cooling tower
Primary-secondary system
Effect of load on cooling tower performance
Condenser-water pumping arrangements
Primary System Components
Effect of ambient conditions on cooling tower performance
Unit-Level Controls
Primary System Components
Chiller control
Primary System Components
Centrifugal chiller capacity control
Figure 16. Centrifugal chiller with AFD
The availability of cooler condenser water condenser-water reset
Primary System Components
Application Considerations
Small Chilled-Water Systems 1-2 chillers
Application Considerations Constant flow
Variable flow
Condensing method
Parallel or series
Application Considerations
Number of chillers
Part load system operation
Preferential vs. equalized loading and run-time
Mid-Sized Chilled-Water Systems 3-5 Chillers
Managing control complexity
Figure 18. Mid-sized chilled-water system schematic
Large Chilled-Water Systems 6+ Chillers, District Cooling
Figure 19. Large chilled-water system schematic
Application Considerations
Water
Power
Pipe size
Creating one centralized chilled-water system takes significant foresight, initial investment, and building development with a multi-year master plan. If the initial plant is built to accommodate many future buildings or loads, the early challenge is operating the system efficiently with much lower loads than it will experience when the project is complete. The system may need to blend parallel and series configurations “Series-Counterflow Application” on page 77 to accommodate the wide range of loads the plant experiences during phased construction
Chiller Plant System Performance
Chiller performance testing
Limitations of field performance testing
Controls
Guidelines for system efficiency monitoring
Application Considerations
Guideline 22 includes a discussion of
Energy and economic analysis of alternatives
Application Considerations
1 Full year analysis
Selecting Chilled- and Condenser-Water Temperatures and Flow Rates
System Design Options
Guidance for Chilled- and Condenser-Water Flow Rates
Chilled-Water Temperatures
Standard rating temperatures
System Design Options
Standard rating flow conditions
Condenser-Water Temperatures
Chilled- and Condenser-Water Flow Rates
System Design Options
System Design Options Selecting flow rates
System Design Options
DP2/DP1 = Flow2/Flow11.85
The total system power is now as follows
System Design Options
System Design Options
Coil response to decreased entering water temperature
Example of coil reselection at colder temperature/reduced flow rate
Figure 21. Chilled water system performance at part load
Smaller tower
Cooling-tower options with low flow
System Design Options
Fluid ΔT, F C
ΔT2 = 99.1 - 78 = 21.1F or 37.3 - 25.6 = 11.7C
System Design Options
ΔT1 = 94.2 - 78 = 16.2F or 34.6 - 25.6 = 9.0C
Same tower, smaller approach
System Design Options
Same tower, larger chiller
range
Retrofit opportunities
System Design Options
Reselected tower for 50% more heat rejection with 15F 8.3C
Cost Implications
System Design Options
Misconceptions about Low-Flow Rates
Misconception 1-Low flow is only good for long piping runs
System Design Options
Energy
System Design Options
Consumption
System
System Design Options
Demirchian and Maragareci12, Eley13, and Schwedler and Nordeen11 independently showed that system energy consumption can be reduced by reducing flow rates. It is interesting to note that in the systems studied, three different chiller manufacturer’s chillers were examined, yet the energy savings only varied from 2.0 to 6.5 percent. In all cases, regardless of which manufacturer’s chillers were used, the system energy consumption was reduced. In addition, Demirchian and Maragareci12, and Schwedler and Nordeen11 also noted reduced first costs
System Configurations
Parallel Chillers
System Configurations
Series Chillers
System Configurations
Figure 27. Series chillers
Primary-Secondary Decoupled Systems
Hydraulic decoupling
System Configurations
System Configurations
bypass piping no-flow static head from the building water column, and
System Configurations Production
Figure 29. Production loop
System Configurations Distribution
Distribution-loop benefits of decoupled system arrangement
Common
System Configurations
Pumping arrangements
Campus
System Configurations
Decoupled system-principle of operation
Figure 34. Decoupled system supply tee
Temperature-sensing
Flow-based control
System Configurations
Figure 35. Temperature-sensing
Adding a chiller
System Configurations Chiller sequencing in decoupled systems
Multiple chilled-water plants on a distribution loop
Subtracting a chiller
System Configurations
Pump control in a double-ended decoupled system
Figure 36. Double-ended decoupled system
System Configurations
Chiller sequencing in a double-ended decoupled system
System Configurations
Variable-Primary-Flow Systems
Other plant designs
Figure 37. Variable-primary-flow system
Advantages of variable primary flow
System Configurations
Operational savings of VPF designs
Evaporator flow limits
Chiller selection requirements
System Configurations
Dispelling a common misconception
System Configurations
To promote good heat transfer and stable control minimum flow limit
Managing transient water flows
System Configurations
Flow-rate reduction when an isolation valve
Number of operating chillers
System Configurations
System design and control requirements
System Configurations
These descriptions should include control sequences for
System Configurations
Accurate flow measurement
Bypass locations
Chiller sequencing in VPF systems
System Configurations
Bypass flow control
System Configurations
Adding a chiller in a VPF system
System Configurations
Subtracting a chiller in a VPF system
Sequencing based on load
Select slow-acting valves to control the airside coils
Other VPF control considerations
System Configurations
Use multiple air handlers, and stagger their start/stop times
Consider a series arrangement for small VPF applications
Plant configuration
System Configurations
Assess the economic feasibility of VPF for single-chiller plants
Moderate “low ΔT syndrome” by manifolding the chilled water pumps
Guidelines for a successful VPF system
System Configurations
Chiller selection
Bypass flow
System Configurations
Plant configuration
Chiller sequencing
Condenser “Free Cooling” or Water Economizer
Heat Recovery
Chilled-Water System Variations
Plate-and-frame heat exchanger
Chilled-Water System Variations
Refrigerant migration
Figure 41. Refrigerant migration chiller in compression cooling mode
Chilled-Water System Variations
Well, river, or lake water
Figure 42. Refrigerant migration chiller in free-cooling mode
Figure 43. Water economizer piped in parallel with chillers
Chilled-Water System Variations
Preferential Loading
Preferential loading - parallel arrangement
Figure 44. Parallel preferential loading arrangement
Sidestream plate-and-frame heat exchanger
Preferential loading - sidestream arrangement
Chilled-Water System Variations
Figure 45. Sidestream preferential loading arrangement
Recovery
Sidestream with alternative fuels or absorption
Production
Sidestream system control
Preferential loading - series arrangement
Chilled-Water System Variations
Figure 47. Preferential loading - series arrangement
Chilled-Water System Variations
Series-Counterflow Application
Series-series counterflow
Figure 48. Series-counterflow arrangement
Figure 50. Series arrangement of evaporators and condensers
Unequal Chiller Sizing
Chilled-Water System Variations
Evaporators
Amount of Fluid in the Loop
System Issues and Challenges
Low ΔT Syndrome
Required Volume = Flow Rate × Loop Time
System response to changing conditions
System Issues and Challenges
Chiller response to changing conditions
Example
Type and size of chiller
Contingency
Minimum capacity required
System Issues and Challenges
Water and electrical connections
System Issues and Challenges Location of equipment
Alternative Energy Sources
Ancillary equipment
Thermal storage
Plant Expansion
Alternative fuel
System Issues and Challenges
Flow rate out of range
Retrofit Opportunities
Applications Outside the Chiller’s Range
System Issues and Challenges
System Issues and Challenges Temperatures out of range
Precise temperature control
Figure 52. Temperatures out of range for equipment
Chiller System Design and Control
System Issues and Challenges
Figure 53. Precise temperature control, multiple chillers
SYS-APM001-EN
Chilled-Water System Control
Chilled water reset-raising and lowering
System Controls
Chilled-water pump control
System Controls
Number of chillers to operate
Critical valve reset pump pressure optimization
System Controls
Condenser-Water System Control
Minimum refrigerant pressure differential
Table 17. VFDs and centrifugal chillers performance at 90% load
Condenser-water temperature control
System Controls
Cooling-tower-fan control
System Controls
Chiller-tower energy balance
Figure 54. Chiller-tower energy consumption
System Controls Variable condenser water flow
Chiller-tower-pump balance
Decoupled condenser-water system
System Controls
These three energy consumers must be balanced to minimize overall energy use. This makes varying condenser water flow complex, but the strategy below has been implemented on projects
Recirculation Pump CDWP-2 Inverter Chiller CDWP-1 Inverter Chiller
System Controls
Cooling Tower Fan Inverter Cooling Tower Water Sump Tower Water
Figure 56. Decoupled condenser-water system
Failure Recovery
Figure 57. Failure recovery
System Controls
Conclusion
Glossary
condenser water, leaving. See cooling water
cooling tower water. See cooling water
Glossary
pumps system
Glossary
tower water. See cooling water
References
Plant.” IDEA 88th Annual Conference Proceedings 1997
ASHRAE/IESNA Standard 90.1-2007 Energy Standard for Buildings
References
Engineering July
Except Low-Rise Residential Buildings. ASHRAE and IESNA
References
32 Trane Applications Engineering Group. “Thermal Storage - Understanding the Choices.” Ice Storage Systems, Engineered Systems Clinics. Trane, 1991. ISS-CLC-2
Index
Index
Index
Index
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