Trane SYS-APM001-EN manual System Configurations, Accurate flow measurement, Bypass locations

System Configurations

Accurate flow measurement

The success of a variable-primary-flow installation depends on the quality of the flow-measuring device that controls the system bypass valve (and perhaps also indicates the plant load). Some practitioners use a flow meter16 to directly detect the flow rate (C in Figure 37); others use a differential pressure sensor (D) that monitors the change in water pressure across the chiller evaporator and then correlates the pressure differential to a water flow rate.

Select a flow measurement device with accurate and repeatable measurements. Regardless of which type of device you use, the flow meter or differential pressure sensor must be of high quality; that is, the device must provide accurate and repeatable measurements. For the plant to operate well, the device also must remain calibrated and perform reliably over time. Purchase prices vary widely, but the adage “you get what you pay for” typically holds true. In our experience, the cost of a suitable flow- measuring device is closer to $1,000 USD than to $100 USD. Put simply, don’t compromise on accurate sensing devices when negotiating potential cost reductions during the “value engineering” phase of a project.

One further caveat about measurement accuracy: Proper installation is critical to ensure accurate readings. If the manufacturer states that at least 10 pipe diameters of uninterrupted flow are required both upstream and downstream of the sensing device, then make sure that the piping layout complies.

Select an accurate proof-of-flow device for each chiller. Flow reductions through chillers in VPF systems often cause paddle-type flow switches to flutter or open altogether, which shuts down the chiller. To provide accurate, reliable confirmation of flow, select a sensitive pressure-differential switch (or other high-quality device) and install it properly, piping it across the evaporator.

Bypass locations

A bypass is required whether the primary flow is constant or variable. In a primary–secondary system, the decoupler allows excess primary water to be bypassed. In the VPF system, the bypass allows excess flow only when needed to maintain the chillers' minimum flow requirements.

There are three common locations for the VPF bypass line:

• Place the smaller bypass required for a VPF system in the same position as the bypass in a “decoupled” system. A variable-speed drive on the pump located near the chillers reduces flow and allows substantial energy and operating cost savings. One drawback is that the valve must work against higher pressures—possibly causing wear and lack of controllability.

	• Use three-way valves at some of the system coils. While this approach
	ensures minimum chiller flow, it reduces the pump operating cost-
	savings, due to the increased system flow and decreased return-water
	temperature.
62	Chiller System Design and Control	SYS-APM001-EN