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
INTRODUCTION1.1 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.
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:
- Turbine drive
for electric generating equipment, blowers and pumps
- Process for
direct contact with products, direct contact sterilization and non-contact
for processing temperatures
- 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:
- Instantaneous
tap water: hot tap water is produced only when demanded
- 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.
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
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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