额定热功率 有些制造设备上标有热量输出的信息。这些设计的额定数据有时非常有用,但有时也会产生误导。额 定热功率总是涉及到在特定的蒸汽压力下将一定质量的水或者其它流体加热提高一定的温度。这些公布的 数据考虑了换热器表面结垢因素的影响,具有良好的可信度。 必须清楚任何因素的改变都会改变预定的热量输出,因而改变蒸汽耗量。例如二次侧流体的温度比指 定的要低会增加热负荷,同时蒸汽压力比指定压力低也会降低传热能力。 压力和温度通常能容易地测量,从而能采用相应修正措施。但是,空气、水或其它流体的流率测量要 困难的多。另外无法预计的风机皮带脱落或者泵的叶轮磨损也能导致偏差,同时诸如附加于泵和风机阻力 的减少也会使流率高于设计值。 最常见的错误是假定制造商的额定热功率等于实际负荷。换热器可能具有满足或超过给定负荷的能力, 但与之相连接的负载可能只有该能力的一部分。很显然,了解设备的额定热功率是很有帮助的,但如果将它 和实际的热负荷相联系时必须要小心。 如果负荷用kW表示,蒸汽压力给定,: 负荷kW×3600 蒸汽流率(kg/h) = 工作压力下的hfg 换热器制造公司 系列号 HX12345 型号和口径 AB12345 设计压力测试压力 壳侧 bar g bar g 压力 管侧 bar g barg 正常工作压力 barg 壳体厚度 5mm 测试日期 1985 设计标准—壳侧 BS 853 设计标准—管侧 BS 853 设计功率 250kW 典型的换热器制造商提供的铭牌 蒸汽和冷凝水系统手册 92
Questions 1. What is the result of using a heat exchanger rating to calculate its steam consumption ? a| The true connected heat load may be different from the rated figure b| The rating does not take account of the temperature of the secondary medium c| The rating is based on a steam pressure of bar d| The rating does not allow for condensate forming in the heat exchanger 2. A heat exchanger has a design rating based on a working pressure of 7 bar g. What would be the effect of supplying the exchanger with steam at 3 bar g ? a| The heat output would be greater because the enthalpy of evaporation at 3 bar g is higher than at 7 bar g b| The heat output would be greater because steam at 3 bar g has a greater volume than steam at 7 bar g c| Less weight of steam would be required because steam at 3 bar g has a higher enthalpy of evaporation than at 7 bar g d| The output would be reduced because the difference in temperature between the steam and product is reduced a, 2: d 2: a, Answers 1: 蒸汽和冷凝水系统手册
