Large chilled-water systems with six or more chillers (Figure below) have different challenges than smaller systems. Examples of these types of systems are commonly found on campuses with multiple buildings, downtown districts, and mixed-use residential and commercial developments.
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 to accommodate the wide range of loads the plant experiences during phased construction.
Another type of large chilled-water system could actually start out as more than one chilled water-system. An existing set of buildings can be gradually added to the central system, or two geographically distant chilled-water systems can be connected.
Operating large chilled-water systems can be different as well. As system load drops, chillers are turned off. Individual chiller unloading characteristics are not as important, because operating chillers are more heavily loaded.
Pipe size
Practical pipe size limitations start to affect the maximum size of a chilled-water system. As the systems get larger, it becomes more difficult to accommodate the increasing pipe sizes, both in cost and in space. Large ΔTs can help reduce flow and required pipe size.
In general, the larger the system, the higher the ΔT should be.
Water
Large systems are almost always water-cooled. Both chilled water (a closed loop) and condenser water (usually an open loop) pipes will have to be filled with water. In some locations, it is difficult to find enough fresh water to fill a very large system with water, especially if the chilled-water system is quite distant from the loads. Cooling towers consume water, which can be significant and difficult to obtain in some locations.
The search for both locally available make-up water and energy savings can lead to the exploration of alternative condensing sources like lake, river, or well water. In rare instances, salt water or brackish water can be applied if the system uses an intermediate heat exchanger, or if the chiller is constructed with special tubes.
Power
Large chilled-water systems can be challenged by site power availability. Transformer size may be dictated by local regulations. On-site power generation may be part of the project, leading to using higher voltages inside the chilled-water system to avoid transformer losses and costs. Alternative fuels for some or all of the chillers may be attractive.
To minimize power, large systems must be very efficient. The upside of a large system is the amplification of energy savings. A relatively small percentage of energy saved becomes more valuable. For this reason, the highly efficient series-counterflow arrangement is popular for large systems.
Controls
The designers of medium and large chilled-water systems are more likely to consider the pros and cons of direct-digital controls (DDC) versus programmable-logic controls (PLC). These platforms deliver similar results, depending on proper design, programming, commissioning, and operation.
One way to think of PLC is “fast, centralized control with redundancy.” PLC has a faster processing speed, with some hot-redundancy features—such as an entirely redundant system processor that is ready to take over if the main system processor fails.
Conversely, DDC can be considered “steady, distributed control with reliability.” DDC controls feature easy programming and user-friendly operation. In the DDC environment, a failure of the system processor results in the lower-level processors defaulting to a pre-determined operating mode.
The speed of the PLC system can be one of its challenges. Controls that are steady and do not overreact to minor changes work very well, even in large chilled-water systems.
FREQUENTLY ASKED QUESTIONS
The optimal chiller configuration for a large chilled water system depends on various factors, including the system’s load profile, piping layout, and available space. A combination of parallel and series configurations may be necessary to accommodate the wide range of loads the plant will experience over its lifetime. A thorough analysis of the system’s requirements and constraints is essential to determine the most efficient and cost-effective configuration.
Blending parallel and series chiller configurations in large chilled water systems offers several benefits, including increased flexibility, improved efficiency, and enhanced reliability. Parallel configurations allow for better load matching and reduced energy consumption, while series configurations provide increased pressure and flow rate capabilities. By combining both configurations, system designers can create a more robust and adaptable system that can efficiently meet varying load demands.
Ensuring efficient operation of a large chilled water system during partial load conditions requires careful planning and optimization. This can be achieved by implementing strategies such as chiller sequencing, load-based pumping, and optimized setpoints. Additionally, advanced control systems and data analytics can help identify opportunities for energy savings and optimize system performance in real-time.
Master planning plays a critical role in the development of large chilled water systems, as it allows for the creation of a centralized system that can accommodate multiple buildings and loads over time. A well-developed master plan takes into account the project’s long-term goals, phased development, and infrastructure requirements, ensuring that the system is designed to meet future demands efficiently and effectively.
Balancing system pressure in large chilled water systems with multiple chillers requires careful consideration of piping layout, pump sizing, and control strategies. Techniques such as pump staging, pressure-independent control valves, and hydraulic modeling can help maintain optimal system pressure and prevent issues like over-pressurization or under-pressurization.
When selecting chillers for large chilled water systems, key considerations include chiller size and type, efficiency, reliability, and maintainability. Chillers should be selected based on their ability to meet the system’s load profile, operate efficiently at partial loads, and provide redundancy and backup capabilities. Additionally, factors such as noise levels, footprint, and environmental impact should also be taken into account.
