CHAPTER ONE

INTRODUCTION

1.1      Background of the study

Heat exchanger being an indispensable unit in chemical processes is mostly used to transfer heat energy from a fluid of higher temperature to a cooler fluid (Padhee et al., 2011). There are various kinds of heat exchangers available in the process industry such as, plate heat, plate fin, pillow plate heat exchangers, but the most frequently usedis Shell and Tube type Heat Exchanger (STHX) system. The popularity is due to its efficiency and less complexity (Duran et al., 2009). The STHXs are mostly used to control fluid having varying temperatures and pressures. Typically, it consists of some special designed pipes in  a cylindrical shaped shell in which a set of fluid is allowed to flow through the tubes, and while the other fluid is directed to flow  between the tubes and the shell (Sodja & Zupanˇ, 2009). In the operation of STHX, fluid from a storage tank which is to gain heat flows through the tube while the fluid from a steam tank which is to loss heat flows in between the tube and the shell. The main purpose of a heat exchanger (HEx) unit is to ensure that the temperature of an outlet fluid is maintained at a specific value.This is normally achieved by implementing a control law that will regulate the exit temperature of the fluid in the presence of varying  operating conditions such as  noise and disturbances  (Berto & Jr, 2000). Heat Exchangers are extensively used in areas such as petroleum refining, petrochemical processing and food industries as well as nuclear reaction systems, aircraft and space vehicles (Dhakad et al., 2013).

Since the purpose of heat exchanger is mainly to transfer heat energy from the hot fluid in the shell to the fluid in the tube. Therefore, maintaining a specific temperature of the outlet fluid irrespective of disturbances requires the use of controllers(Araki & Taguchi, 2003). Designing controllers for Heat Exchangers have been an area of interest to researchers over time due to its nonlinear dynamics and time delay, which poses challenges in regulating the outlet fluid temperature in the presence of a disturbance(Suthar & Gadit, 2017b). Different controllers ranging from classical PID,  PID with  feedforward as well as internal model based controllers have been implemented and studied with their merits and demerits in terms of transient behavior and computational complexity (Padhee et al., 2011).  In this research, a controller, with three degrees of freedom, capable of achieving a desired setpoint . tracking and disturbance. rejection has been developed. In the developed method, the number of system variables to be controlled will form the degree of freedom. Degree of freedom in control system can be said to be the  number of controlled variables. in the system (Suthar & Gadit, 2017b). The structure of One Degree of Freedom (1DoF) control has a drawback of its inability to optimize two objectives i.e. ensuring setpoint tracking and disturbance rejection concurrently. For this reason, controlling two system objectives at the same time leads to 2DoF control system structure (Araki & Taguchi, 2003;Suthar & Gadit, 2017b). In a typical 2DoF control, the disturbance rejection and the command input response characteristics can be determined separately. However, there exist a trade-off relationship between noise suppression and disturbance rejection characteristic. Hence, there is a need to design a 3DoF control scheme for determining setpoint tracking, disturbance rejection and noise suppression independently (Suzumura & Fujimoto 2015).

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