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对于防止火床炉炉膛 冒正压的若干措施 西安交通大学 徐通模  惠世恩
摘  要 根据流体力学和炉内空气动力学的基本原理,分析了炉内冒正压的机理,提出对大空间、小炉拱的炉膛,只要保证炉膛出口在 0 ~ 20Pa 的负压时不会出现冒正压;对极低、长炉拱小空间炉膛是否出现冒正压现象的关键在于炉膛、炉拱结构和炉内的空气动力参数是否合理。同时提出防止措施。
炉拱的作用及设计 火床炉燃烧存在的问题 煤质恶化,着火困难,燃烧不稳,效率下降 低而长的炉拱可以改善燃烧,但引起冒正压 冒正压时送风不能开足,煤的燃尽率下降,并影响经济性和安全性
炉内冒正压问题 的流体力学分析 ⑴ 物理模型假设和简化 如图是一种典型的工业锅炉炉膛结构示意图,为了简化问题,把烟气在炉内的流动近似作为不可压缩的一元稳定流动处理。
大空间炉拱示意图
基本公式 总流伯努力方程如下 式中, H 3-4 -炉前与炉膛出口的位置高差, [m] ;  ?   h 3-4  -烟气从炉前流到出口的阻力损失, [J/kg] ; P 3 -炉前部的烟气压力, [Pa] ; P 4 -炉膛出口处烟气的压力, [Pa] ; ? -气体密度, [kg/m 3 ] ; w 3 -炉前部烟气的平均速度, [m/s] ; w 4 -炉膛出口截面处的烟气平均速度, [m/s] 。
分析和讨论 大空间炉膛的流动分析 炉膛内烟气流速很低,烟气在炉膛内的流动阻力损失 ?   h 3-4 很小,且 ?   h 3-4  <<  H 3-4 g , 又烟气流速 w 3  ≈  w 4 ,则上式可简化为:
按流体静力学的基本公式,在炉膛外的大气中存在如下静力学关系: 式中, ? a -大气中空气的密度, [kg/m 3 ] ; P 3 a -与炉内压力 P 3 同一标高处的大气压力, [Pa] ; P 4 a -与炉内压力 P 4 同一标高处的大气压力, [Pa] 。
以上两式相减,得下式 一般情况下,炉膛出口的烟气相对压力 ( P 4 - P 4 a ) 为负压。在工业锅炉中, ( P 4 - P 4 a ) 通常控制在 0 ~ -20Pa 范围内。由于上式中的密度差 ( ? -  ? a )<0 ,所以, ( P 3 - P 3 a ) 也一定小于 0 。 以上分析表明,对于大空间炉膛,只要炉膛出口压力在正常的 0 ~ -20Pa 范围内运行,冒正压的现象不会发生。
极低、长的小空间炉膛 的流动分析 极低、长的小空间炉膛的烟气流速高,炉膛形状复杂。从炉膛下部流动到炉膛出口的阻力损失不可忽略,伯努力方程写为如下形式,式中, w 0 为烟气在前后拱喉口的流速, ? 为阻力系数。
公式 (5) 减公式 (3) ,得
重要的启示 由上式可见,炉膛下部压力 ( P 3 - P 3 a ) 是正压还是负压完全取决于右边三项的和。我们可以从中得到如下很重要的启示: ① 炉膛结构越复杂,炉内烟气流速越高,出现炉膛下部冒正压的可能性越大; ② 锅炉在低负荷下运行时,烟气流速下降,则流动阻力减小,冒正压情况改善;反之,锅炉运行负荷越高,冒正压问题越严重。
炉内冒正压问题的防止措施 在炉拱设计时,除应考虑满足燃烧因素外, 还应注重考虑减小炉内阻力的问题,特别应防止片面追求过低过长的不适当的炉拱结构 ,因此,后拱不宜压得过低,倾角不宜过小;一般后拱倾角应大于 10 ? ,后拱末端最小高度与炉排有效长度比应控制在 0.08 ~ 0.10 的范围内; 应合理选择空气动力参数, 喉口流速一般应设计在 5m/s 以下 ; 合理组织配风工况 ,消除风室间窜风、漏风及纵向配风不均匀,对复杂炉膛结构的锅炉,在引风机压头选取上还应进一步放大系数; 对旧锅炉的改造,应对系统阻力及引风机压头进行核算, 对尾部烟道的积灰、堵塞均应予以高度的重视 。
结  论 ⑴ 理论分析及计算表明, 对于大空间、小炉拱的炉膛,只要保证炉膛出口有 0 ~ 20Pa 的负压时不会出现冒正压;对极低、长炉拱结构和小空间炉膛,是否出现冒正压现象的关键在于炉膛及炉拱结构和空气动力参数是否合理。 所以对复杂结构的炉膛及其阻力系数 ? 的研究是很有必要的,在炉膛阻力较大的情况下,引风机的压头按常规方法选取是不适宜的。 ⑵ 防止炉膛冒正压的通常办法应从炉膛、炉拱结构和空气动力参数入手 。

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