Book - School of Science and Technology

98 Survey of heating methodsTable 4.1 Options for indirect heating systemsDistribution mediumItem Water Steam AirTemperature or Warm or low temp. Low pressure Low velocityPressure or Medium temp. Medium pressure Medium velocityVelocity High temp. High pressure High velocityUsageDirect or at reduced levelArrangementVisible in space or concealedReticulationRadial or concentricPipework Single or two-pipe Single run or ring mainReversed returnMotive power Thermosyphon Gravity return Natural`Accelerated' Pumped return MechanicalPumpedCombinedTerminals See text for separation Grilles(primarily radiant or convective)DiffusersLouvresPlantDispersed, semi-dispersed or centralisedFuel or energy sourceSolid, liquid, gaseous or electricalTable 4.2 Moisture released to the enclosure by combustion of fuels in direct heating appliancesRate of fuel consumption or energy use which willrelease 1 litre of water per hourFuel litre/h kg/h Therm/h kWKerosene 1.0 ± ± 10.2Natural gas ± ± 0.23 6.6Butane ± 0.65 ± 8.8Propane ± 0.62 ± 8.5Direct systems ± solid fuelOpen firesThese, man's primitive source of heat, were most economic in mediaeval times when thehearth was open to the heated space and gases escaped through a hole in the roof aftercooling. With this arrangement little of the energy content of the fuel was lost but therewere compensating disadvantages. Later, flues were invented whereby economy wassacrificed in the interests of cleanliness and easier respiration.The modern version of the open fire is usually arranged for continuous burning, withmeans for controlling the air inlet and with a restricted throat to the flue, sometimesadjustable. It is designed to burn smokeless fuel and the efficiency may be of the order of30±40% as compared with 15% or less for the pre-1939 pattern of grate. Some forms ofappliance provide airways for a convection output thus increasing efficiency slightly.