Manual regulating valves are designed to adjust the flow rate through their entire stem travel. Shutoff valves, on the other hand, are not intended for use as regulating valves since they provide most of their regulation in the first turn of the valve from its closed position. Two frequent applications of manual expansion or balancing valves are at the evaporator coils of liquid-recirculation systems and in conjunction with on-off liquid level control valves, as illustrated schematically in Fig. 11.2. In the liquid recirculation system of Fig. 11.2a the function of the valves is to throttle the flow rate to coils whose unthrottled coil-and-piping circuit has a lower pressure drop than others. The liquid supply pressure ahead of the valves is increased, which diverts liquid to those coils that otherwise might be inadequately fed. In liquid recirculation systems, the drop in pressure through the valve is small
relative to that occurring when an expansion valve separates condensing and evaporating pressures.
Valves regulating liquid flow into vessels where the level is controlled are often electrically operated on-off valves. Such solenoid valves are combined with manual control valves, as illustrated in Fig. 11.2b, to prevent wild pressure fluctuations in the vessel as the solenoid valve opens and closes. Pressure drop occurs in both the manual valve and the solenoid valve, even when it is open, but approximately 2/3 of the pressure drop should be taken in the manual valve.