Frequently Asked Questions Flow Control（常见问题流控制）.pdfVIP

Frequently Asked Questions System and Valve Basics: 1. Q: What is good cooling coil performance? A: The temperature range is controlled within +/- 0.5°F (0.28°C) off the cooling coil at set point or below. Also, the coil delta T exceeds design, even at part load. 2. Q: What is “delta T” and why is it important? A: Delta T is the difference between the water return and supply temperature through a heating or cooling coil. All HVAC hydronics systems have a design delta T; however, this objective is rarely achieved at part load. Delta T is the single most effective indicator of system performance. The higher delta T, the less flow required to serve the load. 3. How fast will the leaving air temperature change when the coil flow rate varies? A: Typically, it changes a lot faster than a conventional valve can respond to. However, there is a delay between the flow change and the LAT (leaving air temperature) change. Stabilizing the flow rate through the coils leads to higher ∆T and better energy performance by stabilizing the flow through the coil, which leads to a more stable LAT. 4. Q: What causes cooling coil performance to decline? A: Hunting valves that cause flow fluctuations, high chilled water supply temperature, dirty coils (air or water side), and improper piping. 5. Q: What makes a good control valve? A: An ideal control valve would be able to precisely match the flow wi th the load at all conditions. This will yield optimum delta T Performance. The valve needs to have high turndown (100:1 minimum), be pressure-independent, sufficient shutoff, and operate over a wide range of differential pressure. Lastly, it needs to be show repeatable performance with a way to verify flow, pressure independence, and pr