What Is A Balanced System?
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. The result of this will be that the wanted heating/chilling will not be ensured in all parts of the installation. In practice it is not possible to make a correctly 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.
Contents
- What Is A Balanced System
- Typical Design Flow Rates
- Constant Flow
- Variable flow
- Pressure Independent Balancing And Control Valve
- Differential Pressure Controller
- References
An automatic balancing valve uses the latest flow technology to ensure that the design flow rate is achieved at all times irrespective of any pressure changes within 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.
Constant Flow
Until recently, constant-flow systems set the standard in HVAC system design. They allow standard designs to be applied to numerous different projects, typically incorporating fixed-speed pumps sized to match the maximum load of the system.These systems are balanced using a proportional method with manually set, fixed-orifice, double-regulating balancing valves installed to account for and reduce the impact of pressure changes in the system.In such constant-flow systems, the capital costs are, indeed, low. However, the energy usage is high, since these systems and the pumps driving them rarely operate at the 100% load they were designed for, calling into question the effectiveness of balancing this type of system in the first place. Further, the process of proportional balancing to commission the system is long, painstaking and expensive.
Variable flow
Variable-flow systems have risen in popularity primarily because they reduce a system’s energy consumption. They use variable-speed, inverter-driven pumps, the speed of which is changed to match the load. 2-port, motorized control valves are often used to control flows to terminal fan-coil units, or thermostatic radiator valves are used to regulate flow to radiators in heating-only systems. Current solutions in variable-flow systems also include a combination of differential-pressure control valves (DPCVs) which can be used on branches and/or across air-handling units, with double regulating valves to proportionately balance and limit flows to all terminal valves.
DPCVs help maintain a constant differential pressure across a sub-branch, thereby protecting downstream control valves from excessive pressures and nullifying the effects of pressure variations caused by the movement of control valves in other branches. In systems with 2-port control valves, DPCVs can perform the dual duties of maximum-volume control and differential-pressure control, thereby simplifying the commissioning process.Once a sub-circuit has been commissioned, the DPCV prevents the flow and the balance in the sub-circuit being affected by other sub-circuits. This approach is what I term a semi-automatic method of balancing the system.
However, with both variable- and constant-flow systems, fully automatic balancing can enhance the performance of the HVAC system, eliminating any problems caused by high or excessive system pressures, including noise from the valves and, ultimately, poor control of room temperature. The way the system is balanced will also have a major impact on energy usage, by minimising pumping costs. In variable-flow systems the use of combination valves, when fitted with a 0 to 10 V, 3 point or thermal motor, integrates the three functions of motorised control valve, differential pressure control valve and a double-regulating valve commissioning set into one product. Applications for this type of product include the control of fan-coil units, chilled beams and air-handling units in variable-volume heating and cooling systems and the control of secondary flow on plate heat exchangers. This integrated approach significantly reduces installation and commissioning costs, since the three control functions are specifically matched to ensure optimum system performance and only one valve has to be mounted in the system instead of three. The problem of high and varying pressure drops across traditional 2-port control valves is also effectively eliminated, ensuring a control-valve authority of 100%. It also saves on space, an important concern in, for example, air handling unit installations where the available plant room space for valve installation is often severely restricted.
Applying automatic-balancing technology in constant-flow heating and cooling systems brings advantages too.
Primarily, the use of adjustable flow limiting valves is designed to optimize water flow and can prove invaluable. Such valves provide flow limiting, shut-off and adjustment functions, automatically compensating for changes in system pressure to maximize energy efficiency.
The time and cost of balancing is also significantly reduced.
For example, in a typical installation of fan-coil units, manual balancing valves are required at all terminal units, branches, risers and pumps.
In contrast, because of their added functionality, automatic balancing valves are only needed at terminal units, reducing the number of valves used, cutting costs and creating a more flexible HVAC system that can be easily adapted or expanded without having to make any new calculations or re-commission the whole system. Indeed, parts of the system can be commissioned and completed in phases, to allow for partial occupation of the building, if required — which can be especially useful in, for example, hotels and hospitals. These adjustable flow-limiting valves are easy to size and set and provide effective flow limitation at terminal units, irrespective of changes in flow and pressure conditions in other parts of the system
Pressure Independent Balancing And Control Valve
Control Valves In Variable Flow Chilled Water Systems
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. The aim when sizing control valves is 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.
Control valve selection
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) / 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 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 control-ability. This method also relies on having the correct design information. 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.
Control Valve Strategy
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 run time 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 0-7v of the 0-10v signal.
Conclusion
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 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.
Differential Pressure Controller
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)