Book - School of Science and Technology

Central systems 573NightelementDayelementDayelementNightelementDayelementNightelement(a) (b) (c)Figure 20.11 Arrangement for electric immersion heatersTable 20.1 Annual energy consumption (%) for electric hot-water supplySingle heaterStandard cylinder sizeDual heaterTall cylinderDomestic water useDay and night a Day Night Day Night Day NightVery small 100 50 50 14 86 1 99Small 100 55 45 21 79 4 96Average 100 57 43 28 72 7 93Large 100 60 40 33 67 11 89Overall average 100 55 45 24 76 6 94Cost ratio b 1.00 0.78 0.62 0.53a Thermostatically controlled, no time control, normal tariff.b Based upon night rate charged at half day rate.determined pattern of usage. For instance, in a hospital there may be a continuousdemand for hot water all day and in this case a small storage capacity with a rapidrecovery period (large boiler power) is probably appropriate. Conversely, for a sportspavilion where there may be a single sudden demand following a game, a large storagecapacity and a longrecovery period (small boiler power) may be adequate.Direct systemsIn the recent past, many systems were direct followinga traditional pattern in which thewater drawn off at the various taps etc. was the same water as that which circulatedthrough the boiler, as shown in Figure 20.12. If the water were hard then a scale depositwould occur on the heatingsurfaces and thus a special type of boiler was necessary, of asimple type with large waterways and bolt-on access to ample cleaning `mud-holes'.On the other hand, if the water were soft, then an ordinary boiler of cast iron ormild steel would cause discolouration as a result of rustingof internal surfaces. Specialboilers were thus made of cast iron subjected to an anti-corrosion treatment such as