Book - School of Science and Technology

694 Combined heat and power (CHP)Figure 24.5 Sankey diagram of simple CHP systemof input may be recovered by use of a condensing heat exchanger although this may notbe economic in many cases and in any event should never be incorporated when the fuelhas any sulphur content.It will be appreciated that the water jackets of most engine types are not designed towithstand the pressure which would be imposed upon them if they were connected directlyto a building system. It will be necessary therefore to interpose a water-to-water heatexchanger in the circuit and this will,usually,be of the shell-and-tube type mentionedpreviously in Chapter 8. The exhaust gases,similarly,must pass through a heat exchanger,gas-to-water in this case,which may,in some instances,serve a dual role and bearranged to act as a silencer to attenuate exhaust noise from the engine,as Figure 24.6. Asecondary terminal silencer on the exhaust line may also be required.The primary (heat recovery) circuit of the water-to-water heat exchanger will in mostinstances be pumped through the engine jacket and then be boosted in temperature whenpassed,in series,through the gas-to-water unit. The secondary circuit will be that usedwithin the building. Figure 24.7 illustrates a typical CHP installation in combination withconventional boiler plant connected to the building heating system. For times when theheat recovered exceeds that required,the recovery circuit must be arranged to permitdumping of the surplus at a dry cooler or similar blown unit. It is as well,also,that thegas-to-water heat exchanger be provided with a bypass,not shown in the figure,on theexhaust side which should be well protected from accidental human contact.The piping configuration used for connection of the heat recovery system to theutilisation circuits must be selected to suit the pumping and other hydraulic arrangements.