Analysis on the reconfiguration of the control system of the heat exchanger in response to the slow and time-varying fouling

doi:10.11949/0438-1157.20210574

Abstract

Abstract:

Heat exchanger as an important heat transfer equipment in the chemical process, generally needs to continue to run for a long time, during which due to the continuous accumulation of fouling resistance, heat transfer efficiency of the heat exchanger will gradually decline with time, until it cannot meet the process requirements. In the actual industrial heat exchange process, the process personnel usually carry out margin design on the heat exchanger. However, the heat exchanger needs to run longer or the allowance design is insufficient, the regulating effect of the commonly used flow control strategy gradually deteriorates at the end of the heat exchanger operation while the bypass control scheme cannot realize the continuous control of the whole cycle in the face of the inevitable fouling growth. Therefore, an integral asymptotic model of fouling accumulation was established based on the asymptotic grouth model, which considered the influence of process parameters on the fouling rate. Secondly, taking a small area heat exchanger used for water circulation as an example, the regulating effect of flow control and bypass control under the condition of considering fouling growth is analyzed. Finally, according to the characteristics of slow time-varying and continuous fouling process, a reconstruction control strategy based on heat carrier flow and bypass opening was designed. The operation results of the example show that for the same heat exchanger with limited area margin, this control scheme can prolong its service life and achieve its goal of continuous control throughout the whole cycle.

Key words: process systems, process control, heat exchanger, fouling resistance, full cycle control, control system reconfiguration

摘要：

换热器作为化工过程中重要的热量传递设备，一般需要持续运行较长时间，期间由于结垢热阻的不断累积，换热器的换热效率将随时间逐渐下降，直至无法满足工艺要求。在实际的工业换热过程中，工艺人员通常会对换热器进行裕量设计，然而，当换热器需要运行的时间增长或者裕量设计不足时，面对不可避免的结垢增长，常用的流量控制策略在换热器运行末期的调节效果逐渐变差，旁路控制的方案无法实现全周期的持续控制。因此，本文首先在渐近增长模型的基础上建立了换热器结垢累积的积分渐近模型，该模型考虑了过程参数对结垢速率的影响；其次，以一个用于水循环的小面积换热器为例，分析了考虑结垢增长的条件下流量控制和旁路控制的调节效果；最后，针对结垢过程的慢时变和持续性等特点，设计了基于流量与旁路开度的控制系统重构方案。示例的运行结果表明，对于同样面积裕量有限的换热器，这一控制方案能延长其使用寿命，实现其全周期持续可控的目标。

关键词: 过程系统, 过程控制, 换热器, 结垢热阻, 全周期控制, 控制系统重构

CLC Number:

TQ 021.8

Chao REN,Lin SUN,Xionglin LUO. Analysis on the reconfiguration of the control system of the heat exchanger in response to the slow and time-varying fouling[J]. CIESC Journal, 2021, 72(10): 5273-5283.

任超,孙琳,罗雄麟. 换热器因应结垢慢时变的控制系统重构分析[J]. 化工学报, 2021, 72(10): 5273-5283.

Figures/Tables 12

Fig.1 Curves of cold end outlet temperature changing with heat carrier flow rate

Fig.2 Analysis of bypass design when considering fouling

Fig.3 Schematic diagram of heat carrier flow and bypass opening reconstruction control process

Table 1 Flow data of heat exchanger

项目	壳程（热流）	管程（冷流）
物流名称	水	水
流量/(kg·s^-¹)	90	90
进口温度/K	366.49	283.15
出口温度/K	338.2	310.5
进口压力/kPa	551.59	551.59
密度/(kg·m^-³)	965.3	999.7
比热容/(J·kg^-¹·K^-¹)	4177	4187
黏度/(mPa·s)	0.355	0.9

Fig.4 Selection and influence of fouling parameters under different calculation models in flow control

Table 2 Results of overdesign area of heat exchanger

项目	计算值
壳程传热系数/(W·(m²·K)^-¹)	7363.6
管程传热系数/(W·(m²·K)^-¹)	5089.8
不考虑结垢热阻时的换热器传热系数/(W·(m²·K)^-¹)	2288.6
不考虑结垢热阻和旁路设计时所需的换热面积/ m²	80.48
旁路设计中不考虑结垢热阻时所需的换热面积/ m²	96.15
旁路设计中考虑结垢热阻时所需的换热面积/ m²	144.4
本文选取的换热面积/ m²	125
旁路设计中结垢裕量计算值/%	50.16
本文中结垢裕量取值/%	30

Fig.5 Curves of cold end outlet temperature changing with flow rate in different time period

Fig.6 Output response curves of flow and bypass control

Fig.7 Reconstruct control system structure diagram

Fig.8 Reconstruction design of the full cycle control system of heat exchanger

Fig.9 Curves of parameter change in three reconstruction schemes of control system

Table 3 The residual value of the bypass opening in the three control schemes

方案	旁路开度剩余量/%
控制方案 1	10.8
控制方案 2	0.0
控制方案 3	17.0

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