Book - School of Science and Technology

214 Pumps and other auxiliary equipmentBACDFEFigure 8.12 Components of pump-pressurising packaged unit. A, boiler; B, system circulating pump;C, water pressure spill valve; D, pressure pumps; E, expansion water spill cistern; F, non-return valvedecreasing demand or boiler cycling, the pump will start and the preset pressure will berestored. Since the rate of fall in pressure may vary considerably, the pump does not runcontinuously on demand but is arranged to cycle `on/off ' by means of timed interruptionof the electrical supply. This arrangement provides, with admirable simplicity, the equivalentof a variable pump output. Arrangements such as this are sometimes criticised asbeing too reliant upon the availability of an electrical supply to maintain the necessaryworking pressure. Since automatic boiler firing depends also upon an electrical powersource, avoidance of an unbalanced condition may be overcome by prudent design andthe provision of self-actuating override controls.Pressurisation by gasThis was, in chronological sequence, probably the first independent method to be introduced.It was conceived as a result of the difficulties experienced in application of steam asa pressurising agent for high temperature hot water systems, as noted previously. Earlyversions were bespoke to suit the characteristics of particular installations and were builtup on site from standard components to the requirements of the system designer. A rangeof packaged units, rather more, sophisticated as to detail, soon followed but theseretained the original principles of operation.The logic of the design sequence was that a pressure cylinder would be connected to thepipework arrangements in a chosen position, generally in the main return near the boilerwhere the water is coolest. This cylinder would be filled in part by water and in part by airor an inert gas, the initial supply of which derived from a small air compressor or from agas bottle. By these means, an initial pressure could be applied at a level suited to the limitsimposed by the system construction but in any event well above the boiling point of thewater content. The water of expansion would be discharged from the system through a spillvalve to a cistern open to atmosphere and, as the system cooled and contracted in turn, apressure pump would draw water from the spill cistern and return it to the system.The basic elements of the arrangement are shown in Figure 8.13, one of the moreimportant being the pressure controller which regulates the admission of water from thepump or its expulsion through the spill valve. The use of an inert gas such as nitrogen asthe cushion, in place of air, offers the advantage that it is less soluble in water: at a laterstage in the development of packaged units, the spill cistern also was provided with a