Book - School of Science and Technology

216 Pumps and other auxiliary equipmentmargin but it is almost inevitable that at some time during the life of the system there will be apump failure of some sort: it is usual therefore not to rely upon this added margin.The required volume of the cylinder, if left uninsulated, is a function of the polytropicexpression where PV n is a constant. It has been found by experiment that n ˆ 1:26approximately and thus the movement of the water level may be calculated from:a ˆ h (1 e 0:794 )wherea ˆ movement in the water level (m)h ˆ height of gas space prior to the start of compression (m)e ˆ P 1 /P 2P 1 ˆ initial absolute pressure in gas space (kPa)P 2 ˆ final absolute pressure in gas space (kPa)Control of the pump and spill valve is greatly simplified if the movement of the waterline in the pressure cylinder is at a maximum: this movement is independent of thediameter of the cylinder but that dimension must be considered in conjunction with theduty of the pressure pump which should have a realistic running time of at least twominutes between start and stop when restoring the water level. The pump duty is afunction of the rapid contraction which would take place if the boilers were to be shutdown in emergency while the circulation continued.Application of pressurisationAlthough the equipment and packaged units which have been described under the precedingthree headings were suggested as being appropriate to low, medium and hightemperature heating systems in that sequence, this was merely to note that they were oftenso applied. There is of course no reason why packages for pumped or gas pressurisationshould not be applied to any category of system provided that they have been designed tosuit the pressure and volume conditions obtaining and that their use can be justified ineconomic terms.Non-storage calorifiersA calorifier is a heat exchanger, i.e. an item of equipment used to transmit heat from onefluid, at a higher temperature, to another at a lower temperature. The non-storage pattern,which is the subject of the following paragraphs, is so called to distinguish it from thoserelated to hot-tap supplies which are large by comparison and provide a reservoir of hotwater. While non-storage calorifiers are manufactured to suit either steam or water as thehigher temperature source, the latter are now less commonly used for duties such as thatwhich was shown in Figure 6.5(a), the function they fulfilled now being dealt with byinjection circuits as Figure 6.5(b). They are, however, still required for particular applications,such as the heating of chlorinated swimming pool water, where separation betweenthe higher temperature (primary) and lower temperature (secondary) contents is necessary.Subsequent references here are directed principally to steam-to-water types butapply with little revision to water-to-water equipment.