Book - School of Science and Technology

System pressurisation 213Systemconnections(a) (b) (c) (d)DiaphragmChargingconnectionsFigure 8.10 Application of a simple expansion vesselBDACEGFFigure 8.11 Components of a simple packaged expansion unit. A, boiler; B, system circulating pump;C, expansion vessel; D, topping-up control; E, feed cistern; F, pressurising pump; G, non-return valvevalve for water supply and an electrically driven pump all as illustrated in Figure 8.11. Alow pressure switch fitted to or near the boiler will control the operation of the pump toensure that a minimum water pressure exists and a parallel high pressure switch may beincorporated to stop firing of the boiler in the event of over pressure, at a level belowsafety valve operation. The various components of a system of this type may be built upinto a complete packaged unit.Pressurisation by pumpFor medium temperature hot water systems, operating at temperatures up to about110 C and at pressures of about 400 kPa, an alternative approach to pressurisation hasled to the introduction of units of the type shown in Figure 8.12. These rely principallyupon pressure-pump operation in conjunction with a spill valve and a cistern fitted at lowlevel to take up the water of expansion. Some units of this type are factory assembled andare provided with duplicate pumps and controls.Starting a system so pressurised from cold, the spill valve allows the water of expansionto escape into the cistern once a preset pressure is reached. While the system remains atthe design working temperature, and thus at constant pressure, the spill valve remainsclosed and the pump is idle. When the system temperature and pressure fall as a result of