Book - School of Science and Technology

162 Indirect heating systems200800Total heat82800Temperature saturation ºC18016014012010080604020Heat in water kJ/kg700600500400300200100Heat in waterTemperatureLatent heat of steamSpecific volume7654321Specific volume m /kg32700260025002400230022002100Heat in steam kJ/kg0000 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Absolute pressure MPa2000Figure 6.17 Properties of steam. Graphical representationcalcium bicarbonate to form calcium carbonate and CO 2 and the inevitable inwardleakage through valve glands, etc., which follows any seasonal plant shut-down. Withinany heat exchanger, a stagnant air film at the steam-side surface will act as an insulant andprevent condensation and the desired surrender of latent heat. At low pressures this effectis particularly persistent.Flash steam recoveryUse may be made of Table 6.6 to determine the availability of flash steam, as bestillustrated by the examples which follow. If steam at an absolute pressure of 400 kPawere to be used in some item of process equipment, the condensate temperature would be144 C and the heat content would be 605 kJ/kg. If the discharge were to a vessel held atatmospheric pressure, the temperature could not then be more than 100 C, and thus heatmust be abstracted from the condensate until the content reaches the level appropriate tothe reduced pressure, i.e. 417 kJ/kg.The difference of 605 417 ˆ 188 kJ/kg would thus be available to evaporate a part ofthe remaining condensate and, since latent heat at atmospheric pressure is 2257 kJ/kg, thequantity of flash steam so produced would be 188/2257 ˆ 0:083 kg/kg (or 8.3%). It is ofcourse not necessary that the condensate be discharged to atmospheric pressure:flashsteam could be produced at, say, 170 kPa in which case the availability from 400 kPawould be reduced to 0.054 kJ/kg. Flash steam, if not recovered, represents a loss inefficiency and the magnitude of this may be seen from Table 6.7.Pressure reductionSince a steam plant may well supply a number of different needs, the use of pressurereducing valves as described in Chapter 8 will often be a desirable feature. The process of