Automatic Flow Limiting Valve?

Mon, April 04, 2016

What is an Automatic Flow Limiting Valve?

In a hydronic heating system, overflow can cause overheating, since the additional flow will cause additional heat transfer at the coil. The control valve should sense this based on leaving airside temperature of the coil, and modulate to keep the flow rate in line with the required heat transfer rate. This is good, because the valve is doing its job of controlling heat transfer. If flow is less than design, there will usually be sufficient heat transfer in a heating application, due to the large temperature difference between the mean water and mean air temperatures. While a heating system is quite forgiving, in a chilled water system, flow rates are more critical. Overflowing a coil by 30% does not provide 30% more heat transfer. Since the mean water and mean air temperatures are much closer, the additional flow simply lowers the return water temperature to the chiller. Short differential temperature limit both a chillers efficiency, and output. For this reason, a flow limiting valve may is used to keep the control valve from allowing too much flow to the circuit. As in a heating application, the valve responds to the conditions on the leaving air side of the coil. As the circuit approaches design conditions, the valve may be fully open. Should the water flow increase beyond the design flow, the valve will not react, since it will remain open in order to satisfy the load. The additional flow will not create a significant increase in the rate of heat transfer, and the circuit will be running at flow rates in excess of its needs. The Automatic Flow Limiting Valve is a device that balances a spring force to the force created by water flow across a designed surface geometry. In other words, the flow enters the valve, moves across a fixed orifice, then moves through openings in a cartridge before leaving the valve. The cartridge is counterbalanced by a spring, which maintains a resistance to flow. As flow increases, the cartridge restricts further. The valve must be within its operating range. If the pressure differential across the Flow Limiting valve exceeds the spring rating, the valve will no longer modulate, but will 'peg' itself in a fully open position until the differential decrease enough to allow the valve to modulate. If the pressure differential across the valve is less than the spring rating, the valve will no longer modulate, but will be 'full open' and less than the rated flow can pass through it. There are different spring ranges available for each valve. Ideally, the control valves should be controlling flow by modulation, and the balance valves should be fully open to flow. Unfortunately, we don't live in an ideal world. Control valves may not be properly sized for the flow and pressure differential of the coil/circuit. Pumps may be oversized and creating pressure differentials that force the valves open. Coils may become fouled, with significant reductions in heat transfer. Piping the circuits in a direct return manner may force those valves located close to the main pumps to open. Automatic Flow Limiting Valves are an easy way to keep circuits from overflowing, thus starving other circuits. Ask your Bornquist salesman if the system you are working on is right for Automatic Flow Limiting Valves. Many systems are better served with manual valve, like the B&G Circuit Setter.