Book - School of Science and Technology

584 Hot water supply systemsThe heat exchanger, built up as a `sandwich' from a number of corrugated plates whichprovide passages through which the separated primary and secondary water circuits flow,is mounted to a frame and has cover plates which are pulled together by long end-to-endcompression bolts: flanges on one cover plate provide for pipework connections. Theoutput of the unit is determined by the number of plates makingup the heat exchangerand a range of ratings up to 1115 kW is available.The addition of an accumulator (buffer vessel) interconnected to the plate heat exchangerby continuously pumped pipework circulation circuit, will enhance the output to meetpeak surge demand above the normal demand requirements.Since the efficiency of such a unit depends largely upon the high flow velocity of bothprimary and secondary water, the passages between the plates are necessarily restrictedand any deposition on the drowned surfaces will affect performance. The manufacturersclaim that the heat exchanger is easy to dismantle for maintenance, as a result of thebolted construction, but use of such equipment in a hard water area with an untreatedsupply require some deliberation.Rating of calorifiersThe heatingsurface (tube area) required is a function of the temperature differencebetween the primary water or steam and the secondary water and of the heat transfercoefficient, W/m 2 K. The question now arises as to what is the true temperature differencebearingin mind that the only known conditions are probably those of the enteringprimary flow and the enteringcold water supply.The notional design conditions, taking Figure 20.14 for reference, might be, say, 80 Cflow and 70 C return for the primary circuit, 70 C for the secondary outflow and 10 Cfor the enteringcold water feed. The mean temperatures would thus seem to be 75 C forthe primary and 40 C for the secondary, giving a difference of 35 K. However, this levelof difference is very unlikely to remain stable for any length of time in practice.Followinga period of heavy draw-off, the whole of the hot water content of thecylinder may have been used and the difference then might be 60 K or more. At the otherextreme, when use of hot water is negligible, and storage temperature has reached 70 C,the difference then might be as little as 5 K. As to the heat transfer coefficient, this will bea function of tube size, spacingand of the level of external scaling: it will vary alsoaccordingto the vigour of the convection velocities within the shell which will betemperature dependent.In consequence of all the variables, it is best to consult the calorifier manufacturers andnot to rely upon a theoretical approach alone.Requirements for storage capacity and boiler powerA decision as to the volume of hot water storage and the capacity of the associated boilerplant depends upon considerations which are quite different from those which apply to aheatingsystem. There, as a rule, the quantity of heat which is required under designconditions may be calculated with a fair degree of accuracy and, after adjustments havebeen made to suit particular circumstances, the use of meteorologial data may be appliedto determine the 24 hour pattern of demand.A hot water supply system, except in the case of an industrial process havinga finitetime cycle, has to fill a demand which depends upon a user pattern which can be projected