ABSTRACT

CONTENTS 4.1 Introduction ................................................................................................ 244 4.2 Temperature Control in Food Dehydration and Drying .................... 245

4.2.1 Control Parameters for Heat and Mass Transfer in Drying ........................................................................................ 246

4.2.2 Feedback and Feedforward Control in Dryers ........................ 249 4.2.2.1 Mass and Energy Balance in Dryers ........................... 249

4.2.3 A Single-Input, Single-Output (SISO) Dryer Model ............... 251 4.2.4 A Multiple-Input, Multiple-Output (MIMO)

Dryer Model................................................................................... 252 4.2.5 Feedforward Control in the Food Dryer................................... 255

4.2.5.1 The Control Functions ................................................... 257 4.3 Electronic Controllers................................................................................ 258

4.3.1 On-Off Controller ......................................................................... 260 4.3.1.1 Bimetallic Thermal On-Off Controller........................ 260

4.3.2 Controller Modes .......................................................................... 261 4.3.2.1 Proportional Controller.................................................. 261 4.3.2.2 Integral Controller .......................................................... 263 4.3.2.3 Derivative Controller ..................................................... 264

4.3.3 Fan Direction Control in Food Withering................................. 265 4.3.3.1 The Relay Circuit ............................................................ 268 4.3.3.2 Logic Control Circuit ..................................................... 268 4.3.3.3 The Comparator Circuit ................................................ 269

4.3.4 Cooling Surface Area Control in Chocolate Tempering......... 271 4.4 Flow Ratio Control in Food Pickling Process ....................................... 271

4.4.1 Ratio Controller ............................................................................. 272 4.4.2 Control Valves................................................................................ 273

4.4.2.1 Types of Control Valves................................................. 274 4.4.2.2 Valve Sizing ..................................................................... 275

4.5 Atmosphere Control in Food Preservation ........................................... 277 4.5.1 Control Scheme.............................................................................. 277

4.6 Timers and Indicators in Food Processing............................................ 278 4.6.1 Rolling Program in Tea Manufacturing .................................... 279

4.6.1.1 Roller Timing Sequence ................................................. 281

4.6.1.2 Programmable Timer Circuit ........................................ 282 4.6.2 Temperature Indicator for Tea Dryer......................................... 284

4.7 Food Sorting and Grading Control ........................................................ 287 4.7.1 Objective of Sorting ...................................................................... 287 4.7.2 Various Sorting Techniques ......................................................... 288 4.7.3 Automated Packaging and Bottling........................................... 289

4.8 Discrete Controllers................................................................................... 290 4.8.1 Ladder Diagram ............................................................................ 291 4.8.2 Programmable Logic Controllers ............................................... 292

4.8.2.1 Input and Output Modules........................................... 292 4.8.2.2 Programming................................................................... 293

4.9 Adaptive and Intelligent Controllers ..................................................... 293 4.9.1 Self-Tuning Controllers ................................................................ 294 4.9.2 Model Reference Adaptive Controllers ..................................... 295

4.9.2.1 Some Design Criteria of Adaptive Controllers.......... 295 4.9.3 Intelligent Controllers................................................................... 296

4.9.3.1 Fuzzy Logic Controller .................................................. 297 4.10 Conclusion .................................................................................................. 300 References ............................................................................................................ 301 Further Reading ................................................................................... 302

Due to the advent of modern electronic techniques and automatic control devices supported by instrumentation, power electronics, and digital systems, the control of machines in industries has been revolutionized. Even today, however, many food processing industries are run by either outdated mechanical means or manual methods. As a result maximum quality, productivity, and efficiency are not ensured. Therefore, there are obvious reasons for incorporating scientific and electronic control devices in food industries. For example, most of the tea factories in India use single-phase or three-phase induction motors ranging from 0.5 hp to 25 hp, totaling around 50 to 60 in a tea factory of normal size for 10 to 12 different process operations. However, these machines are still manually operated today. For a production of 5 × 105 kg/year of processed tea, the average power consumption of a tea factory is about 600 MWh and the total energy requirement of the entire tea sector in India is about 7 × 105MWh [1]. This implies that there is ample opportunity to save a substantial amount of electrical energy in the tea industry alone. Automatic control for food processing not only contributes toward saving energy, but substantially improves quality and efficiency as well.