那位高手知道Pressure-Independent Control Valves是指什么？

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The flow-ta-position response of valves (called valve authariry) is characterized by ASHR,4E as either quick opening,linear or equal percentage. Almost all valves used for temperature control systems are specified as equal percentage.Chapter 42 of the ASHRAE Handbook-HVAC Systems and Equipment shows how flow in different valve styles reacts to changes in position.

It also has a chart illustrating a phenomenon called authority distortion. The chart shows that any valve will only perform as advertised if the pressure differential across the valve stays constant, which almost never happens. From full load to part load, pressure differential across a control valve can vary significantly. For example, 50％ valve position may equal 10％flow at design differential, or it may equal 50% flow at part load (authority distortion=0.1，see 542.9, Figure 19).
A pressure-independent control valve automatically and immediately responds to variations in pressure. It is a relatively recent innovation for hydronic applications. A perfect analogy is the variable-air-volume (VAV) box. Years ago, when VAV was in its infancy (a great idea but inconsistent space control), the invention that made VAV the smart choice for most modern buildings was making the VAV boxes pressure independent The addition of a flow ring in the box inlet meant that the box could maintain constant airflow into the space regardless of anything that was happening on the upstream side. A pressure independent control valve has a similar device at the inlet.