Book - School of Science and Technology

212 Pumps and other auxiliary equipmentSome designers provide a small bore (say 25 mm) cross-connection between the suctionand delivery pipework at the pump and fit this with isolating valves and a strainer. Thepump capacity is increased by a small percentage and the arrangement ensures that aproportion of the system water content is being filtered continuously.System pressurisationLow temperature hot water heating systems, where the operating temperature is heldbelow atmospheric boiling point, derive an adequate working pressure as a result ofconnection to an open feed and expansion cistern fitted above the highest point. Nevertheless,a cistern so sited suffers, particularly in domestic situations, from being out ofsight and out of mind with the result that odd noises may be the first sign that it is empty,following seizure of the ball valve. Furthermore, the very requirement that the cistern befitted high may well lead to it being positioned where frost is a potential hazard inexceptionally severe weather.It has been explained previously (Chapter 6, p. 150), that the early high temperature hotwater systems were pressurised by means of a steam space which was either, in the case ofshell boilers, within the shell or, in the case of water tube boilers, within a separate steamdrum. The disadvantage of this method, as explained, was the inherent instability resultingfrom the close association between the pressure applied and the temperature of thewater circulated, with the result that skilled operators were required.At either end of the temperature spectrum, therefore, the adoption of some alternativemethod of pressurisation needed consideration.*Pressurisation by expansionThis, at the simplest domestic level, involves little more than the addition of an unventedexpansion vessel to a heating system which is then charged with water and sealed. Thefunction of the vessel is to take up the increased volume of the water content of the systemas it is heated and, by so doing, apply additional pressure. In practice, proprietary typevessels are used which incorporate a flexible rubber diaphragm separating the watercontent of the system on one side, from a factory applied charge of nitrogen on the other.The size of vessel required is a function of the initial and final pressures and the watercapacity of the system: suppliers rate a standard range accordingly.The way in which such a vessel performs is shown in Figure 8.10, the sequence being(a) before connection, with the diaphragm held to the vessel wall by the nitrogen charge;(b) connected to the system which has been filled; (c) during heating, as the water expands;(d) at system working temperature, the water content now fully expanded. In addition tothe vessel, other fittings necessary are a safety valve fitted to the boiler (of a rather betterquality than is normally provided for domestic systems), a fill/non-return valve which willaccept a temporary hose connection from a water supply and an automatic air releasevalve, possibly associated with a centrifugal air separator.For larger systems operating at low temperature, the principles of operation remain thesame. The expansion vessel will increase in size and may even be duplicated. A smallfilling unit is usually provided for `topping up' purposes, consisting of a cistern with a ball* Kell, J. R., `A survey of methods of pressurization', JIHVE, 1958, 261, 1.