period three
System Variations
notes
Variable-Primary-Flow Systems
variable-flow | | |
pumps | | |
check | | |
| |
| |
valves | | |
control | | |
valve | two-way | |
| |
| valve | |
| optional bypass | |
| withthree-wayvalve | Figure 72 |
| |
Variable-Primary-Flow Systems
One of the reasons that many chilled-water systems are installed using the primary-secondary configuration is that, in the past, chillers could not respond well to varying water flow through the evaporator. Therefore, the production loop was designed for a constant flow through the chillers, and the distribution loop was designed for variable flow to take advantage of the pump energy savings. The system was hydraulically decoupled to meet these two goals.
Alternatively, in a variable-primary-flow (VPF) system, the flow of water varies throughout the entire system—through the evaporator of each operating chiller as well as through the cooling coils. The VPF system differs from the primary-secondary system in that it no longer hydraulically decouples the two loops. The variable-flow pumps move the water through the entire system. The primary benefit of this system is the elimination of the separate distribution pump(s) and the associated electrical and piping connections. There is also a small reduction in operating cost because there is seldom excess water flowing through the bypass pipe.
VPF systems, however, require chillers that can operate properly when the water flow through the evaporator varies. Many of today’s chillers can tolerate variable water flow through the evaporator, within limits. These limits include minimum and maximum flow rates and a limitation on how quickly the flow can vary. Exceeding these operating limits may cause control instability or even catastrophic failure. The VPF system therefore requires a method of monitoring the flow rate through each chiller and a control system to ensure that the flow through the evaporator stays within the limits for the specific chiller. Do not attempt to use a VPF system with chillers that have older, analog electric,
or pneumatic controls that cannot handle variable evaporator flow.
