TEG recently completed a hydraulic analysis of the chilled water distribution system at Western Kentucky University. Observations by campus personnel suggested that some buildings have difficulty in maintaining the delta T set-points under certain occupancy or ambient load conditions while providing adequate space conditioning. TEG was retained to review the system hydraulics to determine what the issues might be, where they were occurring and how to resolve them.
The analysis considered several load scenarios for the 24 buildings currently connected and one proposed dormitory. The cooling load diversity was varied between the scenarios based on functional use of the connected buildings. Since actual load data for every building was not available, the peak and simultaneous loads and flow rates were assumed based on the information that was known.
The distribution system is divided into two distinct loops: the north loop and the south loop. Both loops are served by a central chiller plant utilizing four centrifugal, water-cooled chillers in a variable volume arrangement with four primary variable speed-driven distribution pumps. The speed of the pumps is governed by the delivered pressure difference at each building that is remotely sensed and transmitted back to the chiller plant control system. The buildings do not have secondary pumps but each building has a means of automatic control of its own delta T.
The analysis revealed that several of the campus buildings have experienced insufficient flow based on the established temperature differences (delta T’s) at each building similar to field reports.
The analysis further indicated where some of the potential problem areas might be and solutions were presented. These problem areas were primarily located on the south loop.
When problems such as these arise, the cooling issues can usually be resolved by either increasing the flow rate to the individual buildings having the issues or decreasing the pressure drop to the buildings. Adding pumps at critical areas within the system may resolve such issues, but they are often disruptive, expensive (both in capital and operating cost) and may actually create cooling or hydraulic issues at other buildings. In order to reduce the pressure drop to the troubled buildings, distribution piping replacement or upgrades can be implement; however, these solutions are extremely disruptive (requiring long-term outages for multiple buildings) and extremely expensive.
With a district cooling system, changes can easily, and often inexpensively, be applied by making changes at the central plant or to its operation. This hydraulic analysis revealed that the cooling issues could be resolved by changing the sendout pressure (or hydraulic head at the plant) under certain load conditions. For most of the cooling season, all of the campus buildings could be served adequately. Only during the peak load conditions, which occur for only a few hours each year, is any change in the operation necessary.
The recommendations included changing the distribution pressure set-points and adding new control pressure points at buildings not previously used as control points. The new settings could correlate to either ambient outside air temperatures (i.e. increase delta P on hot days), to the total flow rate to the south loop (i.e. increase delta P on high flows) or to the delta T’s at the campus buildings (i.e. if the delta T is too high, increase delta P). These recommendations involved changes to the existing controls that could be implemented with little to no disruption in service and at little cost.
One other solution is to increase the delta T at the buildings so that the overall flow rate is reduced during the peak conditions. Implementing delta T changes is often not as easy as it sounds. Older buildings may have fouling issues at some or all of the building coils or have coils that were originally sized for different delta T’s and approach temperatures that prevent them from providing adequate space conditioning at higher delta T’s. Although cleaning of the coils (water and air side) can improve performance, and regular preventive maintenance and good water chemistry can reduce the re-occurrence of fouling, the coils still may not be able to provide the delta T needed to resolve all of the hydraulic issues at all load conditions.