Book - School of Science and Technology

Control devices 631100DPercent flow50ABCA LinearB Characterised V-portC Equal percentageD Quick opening00 50 100Percentage valve spindle leftFigure 22.11 Control valve characteristicsfunction of valve lift and flow rate; the four main characteristics are shown in Figure22.11.The choice of valve characteristic depends upon the output characteristic of thecontrolled equipment. For steam applications, since heat output is effectively directlyproportional to flow rate, a linear characteristic valve would normally be suitable. Heatexchangers having water as the primary medium, however, have very different characteristics.Figure 22.12 shows heat output to primary heating water flow rate for atypical air heater battery. An equal percentage valve would be most suitable for this typeof application, producing a closer relationship between valve lift and heat output.Valve authority is a function of the pressure drop across the valve and of the pressuredrop across the remainder of the circuit as given byN ˆ P 1 /(P 1 ‡ P 2 )100Flow (percent)5000 50 100Heat output (percent)Figure 22.12 Typical hot water air heater battery characteristic