ASHRAE Journal - November 2009 - 25

P Chiller P Back-up Pump Figure 3: Variable primary flow (VPF) system using threeway valves to ensure chiller minimum flow rate. Air-side Coil With ThreeWay Valve to Provide Minimum Flow Rate DP cooling capacity.” This is done by selecting the existing tower and new chiller at higher temperatures (e.g., 88°F to 103°F [31°C to39°C], rather than 85°F to 95°F [29°C to 35°C]). Using this concept may well bring an “over-budget” job back into budget. Variable Flow Considerations Converting Constant-Flow Systems to Variable Flow It may be benef icial to convert a constant-flow system to variable flow. Generally, the most economical way to do so is by using the variable primary flow (VPF) concept, since a VSD can be applied to the pump (provided the pump and drive are compatible). This article only addresses a few issues with respect to applying VPF to existing systems. See the references for more information.9,13,18 While there are many issues that should be considered, three are paramount with respect to the chiller: • Confirm with the chiller manufacturer that the existing unit controls can tolerate variable evaporator flow. If not, consider a control retrofit. • Provide system hardware and controls that keep the evaporator flow rate between the minimum and maximum allowed by the manufacturer. • Ensure that system flow rate changes are below the threshold allowed by the manufacturer. Many new VPF applications provide flow-sensing devices, and a separate byNovember 2009 pass line with a control valve to maintain the chiller’s minimum flow rate. In an existing plant, minimum flow can instead be maintained by using an adequate number of three-way valves to ensure that the chiller’s minimum required evaporator flow rate is always met. In Figure 3, this simple approach reduces pumping costs while providing the chiller with enough chilled water, and it’s simple in retrofit applications whether there is a single chiller or multiple chillers, since the three-way valves are part of the existing infrastructure. Be careful applying VPF to an existing system with chillers differing in size, or chillers with differing evaporator pressure drops. Flow variations may cause operational issues, and sequencing and loading of chillers can be especially challenging. Variable Condenser Water Flow. Some project teams consider varying condenser water flow to reduce system energy use. This concept has been successfully implemented, but is not in widespread use today. The difficulty is that varying condenser-water flow results in chiller and cooling tower performance changes—and those changes are in opposite directions; that is a reduction in cooling tower energy and condenser water pumping energy results in increased chiller power since the leaving condenser water temperature and condenser refrigerant pressure both rise. So, implementation is difficult. While several articles address varying flow rate through the condenser and across the cooling tower, few give specific implementation guidance. Baker, et al.,11 provide a www.info.hotims.com/25210-19 ASHRAE Journal

ASHRAE Journal - November 2009

Table of Contents for the Digital Edition of ASHRAE Journal - November 2009