ABSTRACT

Excess heat in a boiler plant or any other industrial process can have detrimental effect on the performance and this will result to a waste of energy resources. Given the importance of temperature measurement and control, this project work aimed to review various temperature measurement devices and control systems, review principle of operation and types of boiler plant, perform detail design of a prototype solid state temperature control experimental module for domestic boiler plant and construct an electrically heated boiler plant. Module was designed and implemented for each critical function of the controller; the temperature signal voltage amplifier, reference voltage source, voltage comparator, priority time delay, relay switching and power supply sections. The temperature display module was implemented using a three figure decimal counting digital thermometer. Various temperature measurement devices and process control techniques were reviewed. Detail design of temperature control experimental module for an electrically heated domestic boiler plant was developed. The controller was implemented, tested and calibrated for controlling the constructed domestic boiler plant temperature in the range of 70 to 100 degree centigrade.

CHAPTER ONE: INTRODUCTION

• General Introduction 1
• Justification of Study 4-6
• Aim and Objectives of Study 7
• Methodology 7-9

CHAPTER TWO: LITERATURE SURVEY

• Temperature Control Systems 10
  • ON/OFF Action Temperature Control 10
  • Proportional Action Temperature Control 11
  • Integral Action Temperature Control 12
  • Derivative Action Temperature Control 12
  • PID Action Temperature Control 12
• Temperature Instrumentation and Measurement 13
  • Thermocouple 14
  • Resistance Temperature Detectors 14
  • Thermistors 15-16
• Home Heating Systems 17
  • Heat Exchanger 18-19
  • Jacket Reactor 20
  • Cascade Jacket Reactor 21
  • Electric Heating 22-23
• Industrial Boilers and their Classification 24
  • Fire Tube Boilers 25
  • The Wagon Boiler 25
  • Multitubular Boiler 26

• Marine Boilers 27
• Scotch Marine Boilers 27
• Tube Plates 28
• Package Boilers 28
• Reverse Flame Boiler 29
• Water-tube Boilers 29
• Composite Boilers 30

• Superheaters and Classification 30
  • Convective Superheaters 30
  • Radiant Superheaters 31
  • Platen Superheaters 31
• Operation of Superheaters 31

CHAPTER THREE: DESIGN OF TEMPERATURE CONTROLLER

• Introduction to Temperature Controller Design 33
• Voltage Amplifier 33
• Reference Voltage Circuit 37
• Voltage Comparator 38
• Priority Delay 40
• Relay Drive 43
• Heater 45
• Digital Thermometer 45
• Power Supply Module 46
• Operational Principle of temperature controller for boiler plant 46

CHAPTE FOUR: CONSTRUCTION, TESTING AND RESULTS

• Construction 48
• Testing 52
• Results 53

CHAPTER FIVE: CONCLUSION AND RECOMMENDATIONS

• Conclusions 57
• Contributions to knowledge 57
• Recommendations 57

REFERENCES                                                                                                          58

LIST OF FIGURES

LIST OF ABBREVIATIONS

PID                                         Proportional-Integral-Differential

Pb                                            Proportional Band

SV                                           Set Value

PV                                           Process Value

RTD                                        Resistance Temperature Detector

Th                                            Thermocouple

PTC                                         Positive Temperature Coefficient

NTC                                        Negative Temperature Coefficient

0C                                            Degree Centigrade

Hz                                           Hertz

ID                                            Internal Diameter

Sec                                          Seconds

RL                                           Relay

AC                                          Alternating Current

DC                                          Direct Current

IC                                            Integrated Circuit

TDM                                       Temperature Display Module

emf                                          Electromotive Force

Q                                             Transistor

CHAPTER ONE

INTRODUCTION

1.1      Background to the Study

Steam temperature is one of the most challenging control loops in a power plant boiler because it is highly nonlinear and has a long dead time and time lag. Adding to the challenge, steam temperature is affected by boiler load, rate of change of boiler load, air flow rate, the combination of burners in service, and the amount of soot on the boiler tubes (http://blog.opticontrols.com/archives/182)

After separation from the boiler water in the drum, the steam is superheated to improve the thermal efficiency of the boiler-turbine unit. Modern boilers raise the steam temperature to around 1000F (538C), which approaches the creep (slow deformation) point of the steel making up the superheater tubing. Steam temperatures above this level, even for brief periods of time, can shorten the usable life of the boiler. Keeping steam temperature constant is also important for minimizing thermal stresses on the boiler and turbine (ashrae.org. June 2006)

Steam temperature is normally controlled by spraying water into the steam between the first and second-stage superheater to cool it down. Water injection is done in a device called an attemperator or desuperheater. The spray water comes from either an intermediate stage of the boiler feedwater pump (for reheater spray) or from the pump discharge (for superheater spray). Other methods of steam temperature control include flue gas recirculation, flue gas bypass, and tilting the angle at which the burners fire into the furnace. This discussion will focus on steam temperature control through attemperation. The designs discussed here will apply to the reheater and superheater, but only the superheater will be mentioned for simplicity (WIREs Energy Environ 2015)

Temperature measurement and control is a major requirement in boiler plant and other process industries. Chemical reactions, material separation, distillation, drying, evaporation, absorption, crystallization, baking, extrusion, and thermal therapies are processes that normally occur at controlled temperatures, (Wei,  and Claridge, 2001). Many domestic and commercial applications such as air-conditioning, space heating, grilling, roasting, ironing, baking and water heating also require temperature control. Some of the applications require temperature to be regulated at a constant value or to follow a prescribed temperature profile, (Ian and Kamel, 2003). Temperature control action may be classified into three types; ON/OFF action temperature control, Proportional action temperature control and Proportional+Integral+Differential temperature control actions. Each action has its own advantages and disadvantages, and it cannot be said which action is the best. The particular temperature control requirements will dictate the best control action for the application. The development of an ON-OFF temperature control system meant for automatic control of domestic water boiler is the subject of this study.

An industrial boiler is a closed vessel in which water under pressure is transformed into steam by the application of heat. In the boiler furnace, chemical energy in the fuel is converted to heat energy and it is the function of the boiler to transfer this heat to the contained water in the most efficient manner, (Wei, and Claridge, 2001). The boiler should be designed to absorb the maximum amount of heat released in the process of combustion and generate high quality steam for plant use. Heat is transferred to the boiler water through radiation, conduction and convection. The relative percentage of each is dependent upon the type of boiler, the designed heat transfer surface and the fuels. Two principal types of boilers used for industrial applications are:

1. Fire tube boilers-Products of combustion pass through the tubes, which are surrounded by water.
2. Water tube boilers- Products of combustion pass around the tubes containing water.

The tubes are interconnected to common channels or headers and eventually to a steam outlet for distribution to the plant system, (Payne, 1984).

The increase in the temperature of water to a level that is well above the saturation temperature is simply referred to as “Steam Generation”. The “Saturation Temperature” is the temperature at which the water in a boiler starts to evaporate. There are three reasons why water is employed as a vehicle for the transmission of heat in the industries, (Idsinga, et al, 1977); (Cranfield and Wilkinson, 1981) it is cheap and plentiful, able to carry large quantity of heat in the form of steam and at a temperature at which it may be used conveniently. It is the most widely used medium for the distribution of the heat required for manufacturing and industrial processes. Chemically, water and steam are identical and the one may be transformed into the other without any basic chemical property change taking place and steam is therefore simply gaseous water and when dry, that is devoid of any liquid, behaves similarly to any other typical gas. At atmospheric pressure, water changes into steam at a temperature of 212°F (100°C). However, the boiling point of water is subject to the value of pressure acting on it, (Obinabo, 2008).

Steam is generated for the following plant uses:

1. Turbine drive for electric generating equipment, blowers and pumps

1. Process for direct contact with products, direct contact sterilization and non-contact for processing temperatures
2. Heating and air conditioning for comfort and equipment, (Yunusa,2004):

Steam superheat temperature control is critical to the efficient operation of a boiler plant. Steam temperature must be stable to achieve peak turbine efficiency and reduced fatigue in the turbine blades. Adjusting the amount of water that is sprayed into the steam header after the steam has passed through the super heater controls the steam temperature. The control is difficult because of time delay between the additions of spray water and when the steam temperature is measured. The gain, delay, and time constant of the system response also change significantly with the load on the steam turbine due to changes in steam flow rates, (Bill,2002). Many people use hot tap water from boilers daily for showering, bathing, washing clothes and dishwashing. When a tap is opened, hot water is supplied within a few seconds, usually at a temperature of about 40-65°C. However, hot water is often required by several users at the same time of day especially during the bathing period. There is therefore a problem of hot water availability and the risk of the water temperature getting too high in a tap water system, which is the case most of the times when the hot water is not properly manually mixed with the tap water which is at ambient temperature.

Direct boiler produces hot tap water in two ways:

1. Instantaneous tap water: hot tap water is produced only when demanded

2. Semi-instantaneous tap water: hot water is also produced when there is no demand, and not necessarily when demande The hot water is accumulated in a tank.

Indirect boiler heats water in a primary circuit, heat from this hot water is transferred to tap water or space heating water before the water returns to the boiler and is reheated.

1.2: Justification of the Study

Excess heat in the undesirable range in a boiler plant was hitherto gotten rid of manually by periodically letting off steam from the industrial boiler system and dousing the hot tap water with water at ambient temperature in the domestic boiler system. Given the importance and widespread use of temperature control systems in boiler plant and other process industries the need for an experimental module that exposes students to principles of temperature control for thermal processes cannot be over emphasized. The developed module will be an important contribution to experimental work in automatic control laboratories in higher institutions. An industrial boiler is a closed vessel in which water under pressure is transformed into steam by the application of heat. In the boiler furnace, the chemical energy in the fuel is converted into heat, and it is the function of the boiler to transfer this heat to the contained water in the most efficient manner (Woodrufff and Lammers, 1985). The boiler should also be designed to generate high quality steam for plant use. A boiler must be designed to absorb the maximum amount of heat released in the process of combustion. This heat is transferred to the boiler water through radiation, conduction and convection. The relative percentage of each is dependent upon the type of boiler, the designed heat transfer surface and the fuels.

Two principal types of boilers used for industrial applications are:

1. Fire tube boilers-Products of combustion pass through the tubes, which are surrounded by water.
2. Water tube boilers- Products of combustion pass around the tubes containing water. The tubes are interconnected to common channels or headers and eventually to a steam outlet for distribution to the plant system (Payne, 1984).

The increase in the temperature of water to a level that is well above the saturation temperature is simply referred to as “Steam Generation”. The “Saturation Temperature” is the temperature at which the water in a boiler starts to evaporate. There are three reasons why water is employed as a vehicle for the transmission of heat in the industries (Idsinga, et al, 1977); (Cranfield and Wilkinson, 1981) it is cheap and plentiful, able to carry large quantity of heat in the form of steam and at a temperature at which it may be used conveniently. It is the most widely used medium for the distribution of the heat required for manufacturing and industrial processes. Chemically, water and steam are identical and the one may be transformed into the other without any basic change taking place and steam is therefore simply gaseous water and when dry, that is devoid of any liquid, behaves similarly to any other typical gas.  At atmospheric pressure, water changes into steam at a temperature of 212°F (100°C). However, the boiling point of water is subject to the value of pressure acting on it. (Obinabo and Chijoke, 1991).

Steam is generated for the following plant uses:

1. Turbine drive for electric generating equipment, blowers and pumps
2. Process for direct contact with products, direct contact sterilization and non-contact for processing temperatures
3. Heating and air conditioning for comfort and equipment (Aschner, 1977).

Steam superheat temperature control is critical to the efficient operation of a boiler plant. Steam temperature must be stable to achieve peak turbine efficiency and reduced fatigue in the turbine blades. Adjusting the amount of water that is sprayed into the steam header after the steam has passed through the super heater controls the steam temperature. The control is difficult because there is a time delay between the additions of spray water and when the steam temperature is measured. The gain, delay, and time constant of the system response also change significantly with the load on the steam turbine due to changes in steam flow rates (Bill, 2002).

Many people use hot tap water from boilers daily for showering, bathing, washing clothes and dishwashing. When a tap is opened, hot water is supplied within a few seconds, usually at a temperature of about 40-65°C. However, hot water is often required by several users at the same time of day especially during the bathing period. There is therefore a problem of hot water availability.

Hot tap water can be produced in two ways:

1. Instantaneous tap water: hot tap water is produced only when demanded
2. Semi-instantaneous tap water: hot water is also produced when there is no demand, and not necessarily when demanded. The hot water is accumulated in a tank.

There is a risk of the water temperature getting too high in a tap water system, which is the case most of the times when the hot water is not properly manually mixed with the cold water.

1.3: OBJECTIVES OF THE STUDY

The overall aim of this study is a linear approach to system analysis and design of                                                                               temperature control for a boiler plant (i.e to develop a prototype temperature control experimental module suitable for temperature control of electric powered domestic water boiler plant in the range of 70 to 100 degree centigrade).

The Specific Objectives are to:

• review various principles of operation of boiler plant, temperature measurement devices and control systems.
• perform detail design of a prototype solid state temperature control experimental module for domestic boiler plant.
• construct and evaluate the performance of the electrically heated boiler plant

1.4: RESEARCH METHODS

Design Layout and Procedure: A modular approach will be used in implementing the domestic water boiler temperature controller, a module will be designed and implemented for each critical function of the controller; the voltage converter and amplification, reference voltage source, voltage comparator, priority time delay, relay switching and power supply sections. The temperature display module will be implemented using a three figure decimal counting digital thermometer. First, the detailed design analysis and calculations will be carried out to choose the appropriate components for the design; this will be followed by construction and testing to validate the design work.

The domestic boiler plant will consist of a water container (reservoir), heating element which will be switched ON and OFF automatically by the temperature controller about a preset desired temperature value between 70 and 100 0C.  The domestic water boiler temperature controller will be designed and constructed.

In this study a prototype temperature control experimental module suitable for temperature control of electric powered domestic water boiler plant is developed and the block diagram is shown in Fig. 1.

Boiler plant                               Voltage amplifier                        Voltage Comparator                        Priority Delay

Heating Element                                    Temp Display                          Variable Volt Source                         Relay Switch

Module

220 AC Main

Power source

Fig.1 Block diagram of a Domestic Boiler Temperature Controller

The changes in the temperature of the boiler plant are detected by an NTC thermistor used as the sensor. Increase in the process temperature causes decrease in the thermistor resistance. The output voltage generated from the thermistor resistance changes is suitably amplified using transistors circuitry. The amplified voltage with a reference voltage of preset desired process temperature are fed into the voltage comparator which is used to detect whether or not the temperature which was detected with the thermistor is equal or different from preset temperature voltage, by the condition of the difference an ON/OFF of the relay drive circuit is activated. A 555 timer configured in a mono-stable mode is used to introduce priority delay to ensure process stability by preventing hysteresis effect or noise from switching the heater rapidly and unnecessarily when the temperature is near the set-point. About three degree drop in temperature should cause the relay to toggle back on and remain on until the temperature again rises to the preset level.

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