Book - School of Science and Technology

152 Indirect heating systemsLow temp. circuitLow temp. circuitFlowsafety pipecold feedreturn(a)(b)FlowreturnPumpPumpDiverter valveMixing valveNon-storagecalorifierOr plate heatexchanger( )High/medium temp. flowOver-size injectionheaderHigh/medium temp. returnFigure 6.5 Low temperature circuits served from a high temperature system (isolating and regulating valvesare omitted for clarity)any system will reveal that the various circuits and sub-circuits form a network of parallelpaths for water flow. If the arrangement of these is kept simple and they are arranged in alogical manner, a design has a sound basis from which it may be developed further.Moreover, from such a basis, a system is likely to be produced which can be set to work,commissioned and maintained without undue difficulty.The diagrams included under the subsequent headings do not show circulating pumps,feed and expansion pipes, valves, or connections to pressurising equipment, etc. Theseimportant aspects will be dealt with separately.The single-pipe circuitA piping arrangement of this type, as shown in Figure 6.6, is perhaps the simplestcircuit possible, the various items of heat emitting equipment being connected to thesame single pipe as shown. Each item of equipment is therefore in shunt with the shortsection of piping between the two junctions and, in consequence, the equipment must beof a type which does not offer a significant resistance to water flow. This means, interms of current availability, that a simple radiator having liberally sized waterways isto be preferred.The first radiator (A) will receive water at boiler output temperature and will reject it,somewhat cooler, after having extracted the appropriate heat quantity, back to the singlepipe. Here the water rejected will mix with that which has bypassed the radiator, throughthe single pipe, and a mixture at less than boiler output temperature will reach radiator (B).Thus, the water temperature is progressively cooler as it reaches each following radiatorand the last in the circuit is supplied at little more than boiler return temperature. In orderto compensate for this decay, the size of successive radiators is increased and care is takento select a suitable temperature drop across both them and the piping circuit. Forexample, consider a system having ten radiators, each of which is required to providethe same output. The temperature drop across the circuit might be chosen to be 10 K andthat across each individual radiator to be 15 K. The water temperature in the single pipe