Exploiting the benefits of dynamic balancing
Fig 1. Dynamic balancing greatly simplifies the commissioning of wet systems. This Frese Alpha flow cartridge, for example, has a control range of 7 to 600 kPa.
Not only does the dynamic balancing simplify the commissioning of wet systems, it brings many design, installation and operating benefits. STEPHEN HART explains.Good internal climate control based on wet systems requires a hydraulically balanced HVAC system so that each heating or cooling coil receives enough water flow to generate the required output. The favoured method for balancing HVAC systems in the UK has been to install double-regulating valves (DRVs) at each terminal coil, branch connection, riser connection and pump return and then employ a commissioning specialist to proportionally balance the system. Proportional balancing is a time-consuming, iterative process requiring that each DRV be adjusted several times to achieve the desired flow. Dynamic balancing simplifies the commissioning process and affords many other benefits. Dynamic balancing valves are self acting and can automatically adjust their flow coefficient (Kv)* in response to changes in system pressure. Flow limiters are the most common type of dynamic balancing valve, and they are available with a preset flow cartridge or an external adjustable scale. All flow limiters have a fixed Kv until the start-up pressure for controlled flow is available, at which time the flow through the circuit in which the valve is installed will be limited to a preset maximum. Design flow will be maintained as long as pump pressure at the valve remains within the controlled range and the flow limiter has authority (Fig. 1).
Easy commissioning The benefits of installing flow limiters in HVAC systems are numerous. Most importantly, the need for proportional balancing is eliminated. If variable-speed pumps are installed, the pump speed can be reduced during commissioning until the differential pressure across the index valve is marginally greater than the required start-up pressure. All other valves in the system have more pump pressure available and will limit flow to the preset maximum. The system is immediately in balance and does not require proportional balancing. No further flow regulating valves are required, and all DRVs can be removed from branches, risers and pumps. Flow verification, as part of the commissioning process, can be achieved using a digital manometer and the optional inclusion of metering stations at the head of sub circuits.
Energy-efficient systems Dynamic balancing is most beneficial in variable-flow systems with inverter pumps, pressure sensors and 2-port control valves. At full load, the 2-port control valves will be fully open, and the system will be balanced by the flow limiters. As the 2- port valves close, head losses in the distribution pipe work are reduced, and pump pressure downstream of the sensors increases. Conversely, this partial-load condition leads to reduced pump pressure upstream of remote sensors. If pressure sensors are installed across the index circuit, the pump works just enough to satisfy the index coil, and the flow limiters will protect against any underflows in the rest of the system by reducing their resistance when less pump pressure is available. Variable-flow systems with flow limiters remain in balance even after commissioning and do not need to be rebalanced should the system be modified or extended, for example, when a staged handover is required. Flow limiters and static balancing valves interact with 2-port control valves in the same way. The control valve will have authority when the Kv is lower than that of the static balancing valve or flow limiter. The flow limiter does not need to be at the bottom of its control range to surrender authority to the control valve. If a modulating control valve is to be used, for example, on fan coils or air-handling units, then it should be sized against the available pump pressure at that point in the system, using the start-up pressure of the flow limiter as a fixed resistance for calculation purposes. Care should be taken that the control valve is not exposed to pump pressures that might lead to noise or prevent the valve from closing. If necessary, a differential-pressure control valve (DPCV) should be installed at the head of the upstream sub circuit. DPCVs are also classified as dynamic balancing valves. The Frese EVA is a combination 2-port and flow-limiting valve and can close silently against 400 kPa. It is ideal for on/off control applications such as chilled beams or cooling ceilings (Fig. 2) where control-valve authority is not an issue.
Fig 2. On/off control applications such as chilled beams or cooling ceilings where control valve authority is not an issue can be controlled by a combination 2-port and flow-limiting valve.
Utilising new technology Variable-flow systems with inverter-driven pumps, remote pressure sensors, 2-port control valves and flow limiters maximise energy savings and optimise system performance. However, it is still necessary to size 2-port modulating valves correctly. If system pressures are not available, an upstream DPCV can simplify the sizing process by fixing a known pump pressure point. The final class of dynamic balancing valve is the pressure-independent control valve (PICV). By controlling pressure across its own balancing and thermostatic control components, a single combination valve can control flow, pressure and temperature —removing the need for additional regulating valves in the system or the sizing of separate 2-port control valves. The Frese Optima PICV (Fig. 3) maintains 100% authority irrespective of fluctuations in system pressure and ensures the full modulating stroke is utilised irrespective of any preset maximum flow. It is only necessary to specify the maximum flow to select the correct valve. This new generation of dynamic balancing valves will help to accelerate the uptake of variable-flow systems in the UK.
Fig 3. A single valve such as the Frese Optima pressure-independent control valve can control flow, pressure and temperature — removing the need for additional regulating valves or the sizing of separate 2-port control valves.
Dynamic balancing valves simplify the design, installation and commissioning of HVAC systems whilst maximising the energy savings to the end user. A dynamically balanced system remains in balance throughout its operation, thereby ensuring good internal climate control.
Stephen Hart is managing director of Frese Ltd, Trafalgar House, Union Street, Southport PR9 0QE.
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