低品位烟气余热回收换热器热力学分析

低品位烟气余热回收过程存在冷凝现象,烟气的放热过程分为显热、潜热两部分。冷凝时,局部热流率和熵产率明显增大;增加水蒸气质量分数、冷却水质量流量和降低烟气入口温度都会导致烟气提前冷凝;存在最优冷却水质量流量使得热回收过程熵产数最小。另外,提出热回收效率评价烟气热回收程度,该指标受冷凝的影响很大。随着烟气中蒸汽质量分数的增加,冷凝过程的影响明显增强,因此,在低品位烟气的全热回收中必须考虑潜热的影响。

The Thermal Analysis of Low Grade Flue Gas Heat Recovery Heat Exchanger

The heat and mass transfer phenomenon in the heat recovery of low grade flue gas was analyzed and discussed in the present work.The condensation phenomenon may appear in the flue gas side as the temperature of flue temperature decreases,thus the heat released from flue gas could be divided into two parts:the sensible heat and the latent heat.When the condensation occurred,the local heat transfer rate could be increased remarkably,and the entropy generation rate had the similar distribution with the heat transfer rate.The increasing mass fraction of water vapor,the increasing mass flow rate of cooling water and the decreasing inlet temperature of flue gas could lead to the earlier occurrence of condensation in the flue gas.There existed an optimal mass flow rate of cooling water,in which the entropy generation number reached the minimum.A heat recovery efficiency was proposed to evaluate the heat recovery degree of flue gas,the condensation affected the heat recovery efficiency seriously,and the influence enhanced as the mass fraction of vapor increases in flue gas.This indicates that the latent heat has to be considered in the full heat recovery of low-grade flue gas.