Book - School of Science and Technology

Basic considerations 261The result is that output is likely to decline from the rated level due to irrational attentionto combustion equipment and consequent fouling of heat transfer surface by soot or otherdeposits.To some extent the various corrections cancel out and in the past these refinements incalculation were often ignored, a blanket margin of 25% being added to the total of heatlosses to deal with such adjustments, to make allowance for distribution heat losses and tocover a multitude of other sins. More recent thinking has concluded that much smallerallowances, of the order of 10±15%, are adequate but there is no proper alternative to apainstaking analysis of the various aspects and the addition then of no more than anotional 5% or so to the calculated total. By such means, the designer will know just whatallowance he has made for what eventuality.By way of example, to illustrate the associated problems, if the total heat loss for abuilding, after adjustment for diversity, etc., were 490 kW and a calculated addition of25 kW were made for heat losses from piping outside the occupied space, then a net boilercapacity of 515 kW would be needed. Adding an arbitrary margin of 10% to that figurewould produce a gross requirement of about 570 kW. Boilers, as will be appreciated, arenot purpose made to size and a side issue might be that for physical or other reasons oneparticular make was preferred. The catalogue range then examined would perhaps offer achoice between output ratings of 550 or 600 kW and since these would, respectively,represent margins of either 6.8 or 16.5%, the former would probably be chosen as beingthe more suitable.In days when hand-stoking with coke was the norm, a generously oversized boiler witha large firebox would allow longer periods of running without attention on one charge offuel. With automated firing however, and in particular with fuel oil or gas, it is a positivepenalty on running cost to select a single boiler with too great a margin. The consequentextended periods of firing at low load will lead to frequent cycling of the firing mechanismand a reduced annual average combustion efficiency will result, to say nothing of smokeand other nuisances.Boiler efficiencyHeat input to boilers of all types must not be confused with rated output, both figures areoften quoted by manufacturers. Input is measured in a rational manner best suited to thefuel: coal is weighed and the volume of liquid or gaseous fuel is metered. Detailedprocedures for boiler efficiency tests are set out in various British and European Standards,the duration usually being a 6-hour test period with pre- and post-control periods.From such a test and taking account of an associated analysis of the fuel, a heat balancemay be struck. The complete routine is extended but the final summary may be brief, forexample:(%)Heat content of fuel 100.0Heat loss in flue gases 11.4Loss due to unburnt carbon monoxide 1.9Heat losses (from boiler casing) 4.2 17.5Overall thermal efficiency 82.5The heat content of a fuel is referred to either by its net or gross calorific value. Grosscalorific value includes the latent heat within any water vapour formed as a result of the