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Once the room sensor has detected this overflow it will close the actuator, causing a sharp drop in flow. This excess flow will cause the temperature to over-shoot the set-point. As pressure increases in our variable flow system, our valve will deliver this extra pressure flow. This means the valve is capable of delivering 40% more flow than required. For example, a K v calculation of 4.5 m 3/h would most likely lead to the selection of a valve with a K v = 6.3 m 3/h. As we size a control valve, the K v calculated almost certainly will not match the K v of the nearest appropriate valve. Unfortunately, traditional control valves make it inevitable. However, as areas of the variable-flow system closed down the differential pressure increased, stepping up the delivery flow and causing overflow in the open circuits. Was based on a constant pressure and a constant K v, delivering a constant flow. A control valve was selected by using the same K v calculation, and the bypass on a 3-port valve blocked, giving a 2-port valve. The effects a variable flow system had on the selection of control valves, was not initially realized. Finding that perfect valve involves understanding the hydronics of the project and recognizing the importance of perfect control flow.
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The aim when sizing control valves is to find the perfect valve solution for the system. Variable flow systems require new calculations, new terminology and, most importantly, new technology. Pressure-independent balancing and control valve Control valves in variable-flow chilled-water systems Īs hydronic systems have changed, the necessary valve sizing calculations have also changed. ( March 2019)Īutomatic balancing systems can be classified as constant-flow or variable-flow.
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Minimum 1.5 ft/s (0.46 m/s) (based on a Reynolds number of 7500).At typical conditions, 6–7 feet per second (1.8–2.1 m/s).In a variable primary chilled-water system, the design flow rate is determined by the water flow velocity in the tube of the coils. Only a correct set of balancing valves can ensure the correct distribution of the flow in the system. Practically, it is not possible to achieve a completely balanced system by manipulation of the piping or alteration of the pipe dimensions only.
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As a consequence, the required heating or cooling will not be ensured in all parts of the installation. If the correct balancing of the system is not established, this will result in unequal distribution of the flow, so that there will be a surplus effect in some of the terminals, whereas the effect will be inadequate in others. 4.5 Electronic Pressure-independent control valvesĪ cooling or heating water distribution system is in balance when the flow in the whole system (through the component terminal lines, distributing lines and main distributing lines) corresponds to the flow rates that were specified for the design of the system.4.4 Pressure-independent control valves.4.1 Control valves in variable-flow chilled-water systems.4 Pressure-independent balancing and control valve.