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.
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.
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.
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.
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.