A LINEAR APPROACH TO SYSTEM ANALYSIS AND DESIGN OF TEMPERATURE CONTROLLER SYSTEM FOR A BOILER PLANT

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
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:
a.       Fire tube boilers-Products of combustion pass through the tubes, which are surrounded by water.
b.      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:

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

b.      Process for direct contact with products, direct contact sterilization and non-contact for processing temperatures
c.       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 demanded. 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:
a.       Fire tube boilers-Products of combustion pass through the tubes, which are surrounded by water.
b.      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:

(a)     review various principles of operation of boiler plant, temperature measurement devices and control systems.
(b)   perform detail design of a prototype solid state temperature control experimental module for domestic boiler plant.
(c)    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.

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Item Type: Postgraduate Material  |  Attribute: 70 pages  |  Chapters: 1-5
Format: MS Word  |  Price: N3,000  |  Delivery: Within 30Mins.
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