Book - School of Science and Technology

600 Piping design for central hot water supply systemsPrimary pipework (indirect system)While the advisability of sizing pipework generously, where circulation is to be by gravity,applies to indirect systems also, in this case no allowance need be made for scale depositionwhen selecting pipe sizes since the water content of the primary circuit is constantlyrecirculated and does not change. The need for a brisk circulation remains, nevertheless,since the temperature of the stored secondary water is raised through some form of heatexchanger whereas, in a direct system, it is heated within a boiler. It is current practice touse a pumped circulation through a primary circuit except for the smallest domesticinstallations.Since the primary circuit will, in all probability, originate from a boiler plant which alsoserves a heating system, it is also probable that the flow temperature therefrom is fixed at80 C. In any event, however, it will be advantageous to provide a primary flow temperatureat a reasonably high level in order to economise in heat exchanger surface. Thetemperature difference, flow to return, may again be about 20 K or, when a pumpedcirculation is used, 10±15 K.For a gravity circulation, the pipe sizes required may again be determined using the samemethods of calculation as those described in Chapter 7. As a first approximation, the data inTable 21.1 may be used, these being based upon a flow temperature of 80 C and otherparticulars as before, plus an allowance equivalent to 3 m of pipe for the coil within thestorage cylinder. The pipework arrangement and effective height are shown in Figure 21.1.In a domestic size system, where a single boiler serves both heating and hot watersupply, the adoption of a mixed pumped/gravity circulation may offer advantages. Figure20.15 illustrates an arrangement where use is made of an injector to accelerate thecirculation in the hot water primary circuit at times when the heating pump is running.Where the primary circuit is to be pumped, it is necessary that it be arranged to be quiteseparate from any space heating circuit served from the same energy source. For a lowriseinstitutional building or group of buildings, it may be more economic to provide anumber of dispersed hot water stores fed from a constant temperature primary distributionsystem, as shown in Figure 21.2, rather than a widespread secondary hot watercircuit. Such an arrangement may offer the facility to serve other equipment requiring aconstant temperature supply such as ventilation heater batteries, etc., and would be quiteseparate from any parallel circuit which, at a temperature varied to suit weather conditions,might serve heating apparatus. A separate cold water cistern will, of course, berequired for each of the dispersed stores.Secondary outflow and return pipeworkThe simplest form of outflow pipework from a secondary system is a series of dead-legs asshown in Figure 21.3. The length of these must however be strictly limited for three verygood reasons:. If the length were too great, an undue time would elapse before hot water reached thetap and, furthermore, cold water would have to be run to waste.. Following draw-off, a long dead-leg would be full of hot water which, being unused,would be an energy wastage.. If draw-off were intermittent, an excessive quantity of water would stagnate in thepipework and might be held on occasions within the critical temperature band for rapiddevelopment of L. pneumophila.