Control Valves In Variable Flow Chilled Water Systems
In building services, variable flow systems are essential as we strive for both energy efficiency and lifetime value for our clients. Cost savings on pumps and piping reductions mean the usage of these systems will continue to grow but they require care during the valve sizing process.
Control valves sizing is all too often described as an art. The reason for this is that, as hydronic systems have changed, the valve sizing calculations we make have also changed.
Variable flow systems require new calculations, new terminology and, most importantly, new technology. There is one definitive aim when sizing control valves – to find the perfect valve solution for your system. Finding that perfect valve involves understanding the hydronics of the project and recognising the importance of perfect control flow.
The effects a variable flow system had on the selection of control valves, was not initially realised. 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.
Control valve selection
This is because our Kv calculation
Kv = Flow Rate (m3/h)
Δ P (Bar)was based upon a constant pressure and a constant Kvs, 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 Kvs of the nearest appropriate valve. For example, a Kv calculation of 4.5 M3/h would most likely lead to selection of a valve with a 6.3 M3/h Kvs. 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 as 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‘Hunting’ causes the room temperature to constantly fluctuate, creating a major cost to clients in poor environment quality and increased maintenance. Over three-quarters of complaints to building 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℃ 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 Traditional control valve sizing for constant flow systems involves a Kv calculation, actual pressure drop calculation and a check to ensure minimum valve authority is met. This method is complicated and inflexible, as the changes in design flow, circuit pressure, and required pressure drops can change the required valve, and reduce controllability. This method also relies on having the correct design information.
Pressure independent control valves such as the Danfoss AB-QM 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.
Sizing of a Danfoss AB-QM combined flow-limiter and control valve is done purely on the flow. You first note the flow required to the coil, select a valve that can deliver a higher flow and set the required flow as a percentage of the maximum flow of the valve. This gives the exact design flow to the coil, and eliminates overflow. Changes in design pressures, and required flows can be easily accommodated. AB-QM valves can be set in seconds, without the use of tools.
The ABQM valve is available with or without test points; test points enabling flow verification for commissioning documentation and trouble shooting.
AB-QM valves are extremely compact and are available in 10mm-100mm sizes and handle flow rates from as low as 30 litres per hour to 41,000 litres per hour.
Pressure independent control valves can be used with any control strategy. The actuator options give a choice of thermic, 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 run time limitation. For example, for 70% design flow we give the actuator 70% of it’s total run time. With a modulating actuator, to achieve our 70% example we set the controller to control between 0-7v of the 0-10v signal.
Control Valve Strategy
ConclusionOverflow affects the ability of the control system to achieve the set temperature. It need not be inevitable. Danfoss AB-QM pressure independent control valves enable fan coils and air handling units to have the maximum 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.