Automatic balancing valve
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Automatic balancing valves are utilised in central heating and cooling systems that rely on flow of water through the system. They use the latest flow technology to ensure that the design flow rate is achieved at all times irrespective of any pressure changes within the system.
- 1 Purpose
- 2 Typical design flow rates
- 3 Constant and variable flow systems
- 4 Pressure-independent balancing and control valve
- 5 Differential-pressure controller
- 6 References
A 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. 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. As a consequence, the required heating or cooling will not be ensured in all parts of the installation. Practically, it is not possible to achieve a completely balanced system by manipulation of the piping or alteration of the pipe dimensions only. Only a correct set of balancing valves can ensure the correct distribution of the flow in the system.
Typical design flow rates
In a variable primary chilled-water system, the design flow rate is determined by the water flow velocity in the tube of the coils.
- At typical conditions, 6–7 feet per second (1.8–2.1 m/s)
- Maximum 12 ft/s (3.7 m/s)
- Minimum 1.5 ft/s (0.46 m/s) (based on a Reynolds number of 7500)
- Minimum flow is typically 50% or less of the design flow.
- Chiller manufacturers typically limit the tube velocities in the range from 5.0 to 10.0 feet per second (1.5 to 3.0 m/s).
Constant and variable flow systems
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Automatic balancing systems can be classified as constant-flow or variable-flow.
Pressure-independent balancing and control valve
Control valves in variable-flow chilled-water systems
As hydronic systems have changed, the necessary valve sizing calculations have also changed.
Variable flow systems require new calculations, new terminology and, most importantly, new technology. The aim when sizing control valves is to find the perfect valve solution for the system. Finding that perfect valve involves understanding the hydronics of the project and recognizing the importance of perfect control flow.
The effects a variable flow system had on the selection of control valves, was not initially realized. A control valve was selected by using the same Kv calculation, and the bypass on a 3-port valve blocked, giving a 2-port valve. Unfortunately, it wasn’t that simple. This is because our Kv calculation
- Kv = flow rate [m3/h] / ΔP [bar]
was based on a constant pressure and a constant Kv, delivering a constant flow. 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.
Overflow in a circuit is costly. Unfortunately, traditional control valves make it inevitable. As we size a control valve, the Kv calculated almost certainly will not match the Kv of the nearest appropriate valve. For example, a Kv calculation of 4.5 m3/h would most likely lead to the selection of a valve with a Kv = 6.3 m3/h. This means the valve is capable of delivering 40% more flow than required. As pressure increases in our variable flow system, our valve will deliver this extra pressure flow.
This excess flow will cause the temperature to over-shoot the set-point. Once the room sensor has detected this overflow it will close the actuator, causing a sharp drop in flow. The process will repeat itself in a phenomenon described as "hunting".
Hunting causes the room temperature to constantly fluctuate, creating a major cost to clients with poor environmental quality and increased maintenance. Over three-quarters of complaints to managers are of a thermal sensation nature. These complaints are rarely due to inter-individual differences in preferred temperature but, instead, to increases as temperature deviation widens. The solution that more than two-thirds of building managers use to answer this type of complaint is to change the set-point. By lowering the set-point by an average of 1 °C in a cooling system we increase its energy usage by up to 10%. The solution to the problems of "hunting" and overflow in chilled water systems lies in the use of pressure independent control valves.
Pressure-independent control valves
Pressure independent control valves are used to limit the flow to the fan coil terminal and air handling unit. This flow is not affected by changes in inlet pressure. A diaphragm within the valve keeps the outlet pressure constant, and this delivers a constant flow to the terminal. The added advantage of pressure independent control valves is that, when fitted with an actuator, they replace the manual balancing valve and motorized control valve with a single valve, thus reducing installation cost.
Electronic Pressure-independent control valves
Electronic Pressure-independent control valves use a flow meter or a pressure drop across an orifice to provide flow data to an actuator that works to provide the correct flow down stream. These valves offer much lower pressure requirements as a pressure regulator requires a pressure within a range to achieve operation. They increase flexibility as there settable flow range is often significantly larger than their mechanical counterparts, they also offer improved dirt tolerance thanks to a simplified water path and some the capability to report flow rates to the building management system.
Pressure independent control valves can be used with any control system. The actuator options give a choice of thermal, 3-point control, or modulating control. This will work with building management systems and individual room controls, in the same way as traditional control valves. The actuators can also be used to set the valve by limiting flow. In 3-point control applications, this can be done using a runtime limitation. For example, for 70% design flow we give the actuator 70% of its total run time. With a modulating actuator, to achieve our 70% example we set the controller to control between 0 V and 7 V of the 0–10 V signal.
Overflow affects the ability of the control system to achieve the set temperature. It need not be inevitable. Some pressure independent control valves enable fan coils and air handling units to have the maximal flow set exactly at design flow. Switching a traditional control valve to a pressure independent type should not be seen as only benefiting the mechanical contractor, by reducing installation cost. It benefits the systems integrator and most importantly the client, ensuring both improved comfort levels with reduced energy consumption. Pressure independent control valves are an essential part of the hydronic control in chilled water applications. They are simple to select and easy to set. They enable a steady pressure, a steady flow and most importantly a steady room temperature.
As opposed to having a pressure regulating device on each terminal unit, one larger differential pressure controller can be used when the terminals are in parallel. The Dp controller maintains a constant pressure across the riser and therefore across each terminal unit. This reduces the cost of the system by only having one pressure independent unit and also keeps the advantages of having a manual balancing valve at each terminal (measurement, adjustment, positive shutoff).
- Julian de Bullet, "Variable Flow Chiller Plant Design" Archived 2014-04-16 at the Wayback Machine, ASHRAE, 2012.
- James J. Nonnenmann, "Chilled Water Plant Pumping Schemes" Archived 2013-05-20 at the Wayback Machine, PE (Page 11 of 17).
- Cooper, John (14 April 2006). "Automatic balancing is the key to efficient systems". MBS (Modern Building Systems).