Book - School of Science and Technology

670 Running costsaddition, consideration must be given to the utilisation efficiency of the heating or othersystem (as distinct from that of the plant) which will depend upon the facilities offered forcontrol, the disposition of equipment and other kindred aspects. Suggested values, basedupon data included in the Guide Section B18, are given also in the two parts of Table 23.9.It will be appreciated that these efficiency values are necessarily figures for guidance: theyshould be varied in circumstances where it is known that either plant or system performanceis significantly better or worse than the values in the table; pending Part L of theBuilding Regulations will increase the stated efficiencies.Taking, as a single example, a boiler plant fired by light grade fuel oil (class E), whichoperates to serve the system considered above, the relevant facts may be marshalled asfollows:Calorific value of fuel (Table 11.4)ˆ 41 MJ/kgSpecific gravity of fuel (Table 11.4) ˆ 0:93Annual load (previously calculated)ˆ 3474 GJRoutine of system operationˆ intermittentSystem typeˆ automaticcentralradiatorHeat conversion efficiency (Table 23.9(a)) ˆ 0:65Utilisation efficiency (Table 23.9(a)) ˆ 0:97ThusConsumption ˆ (3474 1000)/(41 0:93 0:65 0:97)ˆ 144 500 litre/annumRunning of auxiliariesIn addition to the fuel consumption, allowance must be made for usage of electrical powerby a variety of auxiliary equipment such as circulating pumps and pressurising equipment,etc. In particular, boiler ancillaries must be considered; oil preheaters, transfer pumps,burners and fans; gas boosters and burners; solid fuel stokers, transporters and ashhandling equipment. In making estimates of the running times of such equipment ininstances where the heat generating plant is operated intermittently, an inclusion must bemade for the preheating period which will average about two hours where optimum startcontrols are used and three hours or more in the case of simple time switching.As a rule, for the lower range of boiler sizes up to say 1 MW, the cost of the electriccurrent for burner operation is not very significant and is often ignored. For a boiler ofthis size, when burning oil requiring preheating:Hourly fuel consumption ˆ about 0.03 kg/sTemperature rise (say 10±80 C) ˆ 70 KSpecific heat capacity ˆ about 2 kJ/kgThus, approximatelyHeat required ˆ 0:03 70 2 ˆ 4.2 kWBurner fan and pump, etc. ˆ say 2.0 kWInduced draught fan (if any) ˆ say 3.0 kWTransfer pumps and sundries ˆ say 1:0kWTotal10.2 kW