Book - School of Science and Technology

590 Hot water supply systemsStorage temperatureIt should be noted that the temperatures listed in Table 20.3 are for hot water use and donot represent those which are required at the storage vessel. Taking no account of otherrelevant factors, it is obviously reasonable to store hot water at a temperature higher thanthat required at the point of use since a smaller volume in storage is then necessary.It was for many years considered to be good practice to design for, and control, hotwater storage temperatures at 65 C (150 F) for all but special applications. Dishwashers,for instance, required an elevated temperature of about 85 C, whereas service to primaryschools, old people's homes and prisons was provided at some 15±20 K lower than thenormal level. Propositions were made duringthe early 1970s, with energy conservation inmind, that the traditional temperature levels were not actually needed and figures of theorder of 45 C were proposed, with seemingly little thought given to the increased storagevolume ± and increased heat loss from larger vessels ± which would have been required.These unfortunate suggestions have however been overtaken by the results of investigationsinto the incidence of Legionnaires' disease and the conclusion that it is related toaerosol contamination, some of which has originated in hot water systems. Without goinginto great detail, it appears that the bacterium can be present in most mains water suppliesbut is dormant at temperatures below about 20 C. It multiplies rapidly between 25 and45 C and is killed instantly at a temperature of 70 C.Interim recommendations are, in consequence, that hot water should not be stored at atemperature less than 55 C and, where it has been held at a lower and more criticaltemperature for any length of time, i.e. overnight, it should be reheated to 55±60 C for anhour prior to exposure and use. Thus, if complex and fallible control cycles are to beavoided, a practical solution is to maintain a storage temperature of 60±65 C wheneverthe plant is in operation, particularly where night shut-down or any other intermittentheatingroutine is adopted.In instances where higher temperatures are necessary, as in the case of dishwashers asmentioned previously, practice in recent years has been to fit such equipment with localelectrical or other booster heaters supplied with make-up from the normal hot watersystem. Any requirements for water at temperatures lower than 55 C may be met bymixingof hot and cold water, which process should be arranged as near to the point of useas is practicable.Temperature controlSince the volume of hot water stored acts as a cushion, close control of secondary watertemperature is neither possible nor necessary. In common with the familiar rod-typethermostat switchingarrangements associated with electrical immersion heaters, thesimplest forms of control for water temperature are direct-acting, which is to say thatexpansion of the sensingelement provides the motive power for the controller.In Figure 20.19, a simple and robust type of direct-acting valve is shown, the primaryoutlet pipework commonly beingworks mounted: this pattern may be used whether theprimary supply is water or steam. The next stage in complexity is a separated direct-actingvalve which may be connected by a capillary to the sensingelement mounted in thestorage vessel. Apart from these simple types, thermostats and control valves may beelectrical, electronic or pneumatic and be either two- or three-way pattern.A disadvantage inherent to both types of direct acting valve, and of other types whenmisapplied, is that they provide proportional control. Thus, with the sensingelementimmersed in the water store, the valve will be wide open to the primary medium when