Book - School of Science and Technology

264 Boilers and firing equipmentEssential to the modular concept is the in-built system of control which, via a singlemonitor, will arrange for the optimum number of modules to be fired to meet the imposedthermal load. Facilities are provided in the control arrangements to allow changes in theorder in which the modules are fired to produce even running across all units. Care mustbe taken in the design of the associated water circuits with particular reference to possibleinteraction between the boiler load controller and any diverting or mixing valves fittedthere: some means to maintain constant water flow is required.The advantages claimed for the concept, which is, as will be noted, no more than aworks designed package echoing the ad hoc arrangement for load matching `steps'described under the first heading here, include the low thermal capacity of the boilermodules and the consequent quick response to both firing and control.Thermal storageAn alternative approach, which is in some respects diametrically opposite to that notedabove, takes account of not only the cold weather peak load but also the extent by whichthis will be reduced on even the most severe winter day by lighting, solar radiation,occupancy and other heat sources. As may have been noted from Chapter 5, such aspectsare taken into account as a matter of course when electrical off-peak systems are designed.In essence, the heat storage method noted here consists of a package made up from awell insulated water vessel interposed between the boiler unit and the distribution system.The boiler charges the store and maintains the water temperature in it, quite irrespective ofthe concurrent output of the system. By this means, it is claimed that the frequency of theon/off cycling of the firing arrangements is greatly reduced, particularly under the less thanfull load conditions which persist for most of the year, thus increasing annual efficiency. Itis further claimed that as a result of adoption of such an arrangement, the capacity of theboiler plant may be reduced. Figure 10.2 shows the arrangement of such a system.Certainly, on a domestic scale, this approach has proved effective and further referencewill be found in Chapter 20 to the situation where a so-called combination boiler is usedand both heating and domestic hot water supply systems are served from it.As a word of warning, however, it is worth remembering, when considering this or anyother design hypothesis (however promising in theory) which relies for success upon areduction in plant potential, that building occupiers are inclined to deplore ingenuitywhen full capacity is unavailable on a cold winter morning.Output flowsBoilerThermalstoreThermostat withwide differentialOutput returnsFigure 10.2 Pipework arrangements for heat storage