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System Configurations
Chiller sequencing in decoupled systems
Given the amount and direction of flow in the bypass line, chillers can be added or subtracted.
Adding a chiller
When there is deficit flow in the bypass line, the system is receiving water at a temperature above the desired supply water temperature. At this point in time, a chiller and pump may be added. Many operators sense deficit flow for a particular amount of time (for example, 15 minutes) to ensure that the deficit flow is not a result of some transient condition. This reduces the chances of cycling a chiller; that is, turning on a chiller and then turning it off after a short time.
Subtracting a chiller
A chiller may be turned off when enough surplus water is flowing through the bypass line. How much is enough? Enough so that the chiller does not need to cycle on again in a short time period. Many system operators compare the amount of surplus flow with the flow rate of the chiller they are considering turning off. If this ratio is 110 to 115 percent, they turn the chiller off. Let’s look at an example:
Chiller 1 can make 960 gpm [60.6 L/s] of 40°F [4.4°C] chilled-water, while Chiller 2 can make 1,440 gpm [90.8 L/s]. At present, there is 1,100 gpm [69.4 L/s] of surplus flow in the bypass line.
•The surplus bypass flow is presently 115 percent of Chiller 1’s flow. If we turn Chiller 1 off, we will have 140 gpm [8.8 L/s] of surplus flow left.
•Note that the surplus bypass flow is presently only 76 percent of Chiller 2’s flow. If we turn Chiller 2 off, we will have 340 gpm [21.5 L/s] of deficit flow. It is clear that we would have to cycle that chiller back on soon.
In this case, we could turn Chiller 1 off and leave Chiller 2 on for the most efficient use of the chillers.
Multiple chilled-water plants on a distribution loop
When decoupled systems are used on large campus-type systems, added loads are often located some distance away from the original loads. Yet, planners like the idea of somehow hooking the new loads to the existing system. The double-ended system shown in Figure 36 is one way of handling this requirement. A second production facility is placed at a convenient location in the new part of the campus. Its distribution plant is laid out as a mirror image of the original piping, and connects to it at the ends of each system. Each production facility has its own bypass.
Both production loops feed into the now common distribution loop. Depending on the flows from the production facility distribution pumps, loads could be served by either plant.
52 | Chiller System Design and Control | SYS-APM001-EN |
Contents
May
Page
Chiller System Design and Control
Preface
Contents
100
Primary System Components
Chiller
Primary System Components
Chiller evaporator
Effect of chilled-water temperature
Effect of chilled-water flow rate and variation
Water-cooled condenser
Effect of condenser-water temperature
Effect of condenser-water flow rate
Air-cooled versus water-cooled condensers
Maintenance
Air-cooled condenser
Packaged or Split System?
Low-ambient operation
Energy efficiency
Loads
Air-cooled or water-cooled efficiency
Three-way valve load control
Two-way valve load control
Variable-speed pump load control
Face-and-bypass dampers
Chilled-Water Distribution System
Chilled-water pump
Pump per chiller
Distribution piping
Manifolded pumps
Pumping arrangements
Constant flow system
Primary-secondary system
Condenser-Water System
Cooling tower
Variable-primary system
Effect of ambient conditions on cooling tower performance
Condenser-water pumping arrangements
Effect of load on cooling tower performance
Single tower per chiller
Chiller control
Unit-Level Controls
Recommended chiller-monitoring points per Ashrae Standard
Centrifugal chiller capacity control
Centrifugal chiller with AFD
AFD on both chillers
Application Considerations
Small Chilled-Water Systems 1-2 chillers
Variable flow
Application Considerations Constant flow
Condensing method
Parallel or series
Application Considerations
Number of chillers
Part load system operation
Managing control complexity
Mid-Sized Chilled-Water Systems Chillers
Preferential vs. equalized loading and run-time
Large chilled-water system schematic
Large Chilled-Water Systems + Chillers, District Cooling
Pipe size
Power
Water
Chiller Plant System Performance
Chiller performance testing
Limitations of field performance testing
Controls
SYS-APM001-EN
SYS-APM001-EN
System Design Options
Guidance for Chilled- and Condenser-Water Flow Rates
Standard rating temperatures
Chilled-Water Temperatures
System Design Options
Chilled- and Condenser-Water Flow Rates
Condenser-Water Temperatures
Standard rating flow conditions
System Design Options Selecting flow rates
DP2/DP1 = Flow2/Flow11.85
Low-flow conditions for cooling tower Base Case Low Flow
Total system power Component Power kW Base Case Low Flow
System summary at full load
Coil response to decreased entering water temperature
Chilled water system performance at part load
Smaller tower
Entering fluid temperature, F C
Cooling-tower options with low flow
System design
ΔT2 = 99.1 78 = 21.1F or 37.3 25.6 = 11.7C
Same tower, smaller approach
Same tower, larger chiller
Same tower, smaller approach Present Smaller Approach
Retrofit opportunities
Retrofit capacity changes Larger Present Chiller Same tower
Cost Implications
Misconceptions about Low-Flow Rates
Misconception 1-Low flow is only good for long piping runs
KWh
SYS-APM001-EN
System Configurations
Parallel Chillers
System Configurations
Parallel chillers with separate, dedicated chiller pumps
Series Chillers
Series chillers
Primary-Secondary Decoupled Systems
Hydraulic decoupling
Check valves
System Configurations Production
Production loop
System Configurations Distribution
Distribution-loop benefits of decoupled system arrangement
Campus
Common
Tertiary or distributed
Decoupled system-principle of operation
Tertiary pumping arrangement
Flow-based control
Temperature-sensing
Flow-sensing
Adding a chiller
Multiple chilled-water plants on a distribution loop
Subtracting a chiller
Pump control in a double-ended decoupled system
Double-ended decoupled system
Chiller sequencing in a double-ended decoupled system
Variable-Primary-Flow Systems
Other plant designs
Advantages of variable primary flow
Operational savings of VPF designs
Chiller selection requirements
Dispelling a common misconception
Flow, ft.water Flow rate
Managing transient water flows
Flow-rate changes that result from isolation-valve operation
System Configurations
System design and control requirements
Effect of dissimilar evaporator pressure drops
Accurate flow measurement
Chiller sequencing in VPF systems
Bypass flow control
Adding a chiller in a VPF system
Flow-rate-fluctuation examples
Subtracting a chiller in a VPF system
Sequencing based on load
Other VPF control considerations
Select slow-acting valves to control the airside coils
Plant configuration
Consider a series arrangement for small VPF applications
Guidelines for a successful VPF system
Chiller selection
Chiller sequencing
Plant configuration
Bypass flow
Airside control
Condenser Free Cooling or Water Economizer
Heat Recovery
Chilled-Water System Variations
Plate-and-frame heat exchanger
Chilled-Water System Variations
Refrigerant migration
Refrigerant migration chiller in free-cooling mode
Well, river, or lake water
Preferential Loading
Preferential loading parallel arrangement
Preferential loading sidestream arrangement
Sidestream plate-and-frame heat exchanger
Sidestream with alternative fuels or absorption
Chilled-Water System Variations
Preferential loading series arrangement
Sidestream system control
Series-Counterflow Application
Series-series counterflow
Evaporators
Unequal Chiller Sizing
Condensers
Low ΔT Syndrome
System Issues and Challenges
Amount of Fluid in the Loop
System response to changing conditions
System Issues and Challenges
Chiller response to changing conditions
Example
Minimum capacity required
Contingency
Type and size of chiller
Water and electrical connections
System Issues and Challenges Location of equipment
Alternative Energy Sources
Ancillary equipment
Alternative fuel
Plant Expansion
Thermal storage
Applications Outside the Chiller’s Range
Retrofit Opportunities
Flow rate out of range
System Issues and Challenges Temperatures out of range
Precise temperature control
Precise temperature control, multiple chillers
Chilled-Water System Control
Chilled water reset-raising and lowering
System Controls
Chilled-water pump control
System Controls
Critical valve reset pump pressure optimization
Number of chillers to operate
VFDs and centrifugal chillers performance at 90% load
Condenser-Water System Control
Minimum refrigerant pressure differential
Chillers Difference
Condenser-water temperature control
Cooling-tower-fan control
Chiller-tower energy balance
Chiller-tower energy consumption
System Controls Variable condenser water flow
Chiller-tower-pump balance
Decoupled condenser-water system
Effect of chiller load on water pumps and cooling tower fans
CDWP-2
Failure Recovery
Failure recovery
Conclusion
Glossary
Glossary
Pumps system
Glossary
References
Plant. Idea 88th Annual Conference Proceedings 1997
References
Engineering July
102
Index
Ashrae
Index
105
106
Page
Trane