Book - School of Science and Technology

Control devices 633Selection of two-port valves follows similar criteria as for the three-port variety.However, more detailed consideration is necessary in connection with the remainder ofthe circuit, because as the valves close the flow rate and pressure distribution around thecircuit will vary: this is particularly important since the installation is likely to have manytwo-port valves in the same pumped circuit. As the control valves throttle the flowthrough the controlled circuits the pump, if operated at constant speed, would tend todevelop an increased pressure which in turn would affect the authority of the valves.Added to which, this increase in pressure might affect the ability of the valves to open orclose fully and it may be necessary, therefore, to use either variable flow pumping or flowbypass valves with a system controlled by two-port valves.Steam control is a special application of the two-port valve. As the valve closes, the areaunder the seat decreases causing an increase in steam velocity, which tends to counter thedecrease in volume flow. The limiting condition, for practical purposes, is when thedownstream absolute pressure is 60% of the upstream pressure. When sizing a steamvalve, therefore, the pressure drop at full load is taken as 40% of the upstream absolutepressure, except when this pressure is less than 100 kPa, when impractical conditions arisefrom this rule of thumb and a smaller pressure drop is taken, with resulting loss ineffective control; reference to manufacturers' data is necessary in these circumstances.The heat exchanger surface area must, in consequence, be suitable for a steam supplylower than the mains pressure.Butterfly valves are, in effect, two-port control devices, but have poor control characteristicsequivalent to that illustrated as quick opening in Figure 22.11. Applicationis normally limited to two-position (open/closed) but standard equipment cannot beguaranteed to give total shut-off: special valves with liners are available for this purpose.Noise emanating from control valves should not be overlooked in the selection ofcomponents and in the design of the configuration of connecting pipework. The causemay be either unique or a combination of mechanical vibration, turbulence or cavitation.DampersControl of air flow follows the same principles as for water systems, and the function maybe either modulating or two (or more) ± position. There are two forms of control damper,parallel and opposed blade type, and their inherent characteristics are shown in Figure22.14. Dependent upon the damper authority (defined above for valves) the dampercharacteristic will vary when installed in a system (Figure 22.15). The closest to linearityis obtained where, for opposed blade dampers, the authority is 0.05 and, for parallel bladetype, 0.2. To limit the pressure drop in a system, therefore, opposed blade dampers, sizedfor an authority of about 0.05, would normally be selected.When used for controlling outside/exhaust/recirculating air in an air handling plant(Figure 22.16) the outside air damper (A), and the exhaust damper (B), would normally beopposed blade type, dimensioned at the duct size. However, for abnormal applicationswhere, for example, high resistance plant components are to be installed, a check shouldbe made from first principles. The function of the recirculation damper (C) is to impose aresistance in the bypass duct equivalent to the static pressure difference between thesupply and extract ducts and to have a characteristic to complement that of the outsideair damper in maintaining constant the mixed flow to the fan. This damper will ofnecessity have a high authority, up to 100%, and since the characteristic of the parallelblade type is better at higher authority (pressure drop in this case having no influence onrunning costs) the parallel blade type would be preferred. Where the pressure to be