换热器壳程流路分析及折流与逆流的换热偏差

Analyses of Flow Path in Shell Side of Heat Exchangers and Heat Transfer Deviation of Baffling Flow from Counterflow

热流体与冷流体的出口温度比α对换热器的有效传热温差有重要影响,不同的α代表不同的换热深度.为探讨管壳式换热器换热深度与长径比的关系,文中采用流路分析法对换热器壳程折流区域的传热性能进行数学分析,并与纯逆流情况作对比.结果表明:在深度换热的临界点(α=1),折流区域的换热性能远低于逆流换热,应避免折流区域靠近临界点操作;换热器折流与逆流区域传热温差的偏离量随α变化,为避免偏移量过大,应控制折流区域面积占总传热面积的比例;α<1时,为使传热温差偏移小于5%,应使折流区域面积占总传热面积的比例小于0.6/R1a,c(R1a,c为临界点逆流冷流体出口、进口温差与算术平均温差之比).文中揭示了现有换热器结构大型化之后难以实现α<1的原因,并给出了一种可以增加换热深度受限的有效结构——壳程多通道结构.

The ratio of outlet temperature of hot fluid to that of cold one,a, which reflects the heat transfer depth, greatly influences the effective heat-transfer temperature difference. In order to investigate the relationship between the length-to-diameter ratio and the heat transfer depth of shell-and-tube heat exchangers, the heat transfer performance in the baffling flow region in the shell side is numerically analyzed via a flow path approach and is compared with that in the counterflow region. The results show that （ 1 ） at the critical point （ a = 1 ） of heat transfer, the heat transfer performance in the baffling flow region is far poorer than that in the eounterflow region, so that the baffling flow region should be kept far from the critical point; （2） as the deviation of heat-transfer temperature difference between the baffling flow region and the counterflow region Changes with a, the ratio of baffling flow area to the whole area should be controlled to prevent the deviation from being too large ; and （3） when a 〈 1, the ratio should be smaller than 0.6/R1a,c to keep the deviation within 5%. The reasons for the difficulty m achieving an a vatue less than 1 for large-scale heat exehangers are also revealed, and a multi-parallel ehannel strueture in the shell side is finally presented to inerease the heat transfer depth.