Furnaces Video Lecture | Chemical Technology - Chemical Engineering

hydrocarbon processing involves a series
of chemical and heat processes to
convert raw materials into an array of
essential products such as gasoline
kerosene olefins rubber and plastics at
the heart of these processes are fired
heaters commonly called furnaces these
furnaces are used with many processes
including distillation reforming olefins
manufacturing and hydro cracking as the
operator it's your responsibility to
operate your furnaces as efficiently and
safely as possible the objective of this
program is to provide the basic concepts
of safe and efficient furnace operations
efficient furnace operation has become a
major concern with the rising cost of
fuels today fuel for processing is about
two-thirds of the total utility cost
most of this fuel is used in fired
heaters
environmental regulations have also made
it necessary to closely monitor
emissions from furnaces and other
processing equipment in this program
will cover the principles of heat
transfer and combustion furnace
equipment performance monitoring startup
and shutdown and troubleshooting
fired heaters are used to transfer heat
in the furnace heat is produced by the
combustion of fuel at the burners this
heat is then transferred to the process
fluid that circulates through the tubes
of the furnace there are three methods
of heat transfer radiation convection
and conduction radiation is the means by
which heat is transferred from the Sun
to earth
radiant heat travels in waves as light
does for this type of heat transfer to
occur the object being heated must be in
direct line of sight with the source of
heat as with visible light these radiant
heat waves can be reflected in a furnace
the tubes that are exposed to direct
heat from the burners are called the
radiant tubes heat is transferred to
these tubes primarily by radiation a
second method of heat transfer is
convection convection involves a flow of
gas or liquid to carry heat a central
heating system uses this type of heat
transfer to distribute heat throughout a
room with a flow of hot gases in a
furnace the tubes that are located above
and out of sine of the burners are
called the convection tubes as hot gases
rise to go out of the stack they
transfer heat to these tubes this design
increases furnace efficiency by allowing
more heat to be used before it escapes
out of the stack the third method of
heat transfer is conduction conduction
is the way that heat is transferred
through solids such as metal or glass
this is how heat is transferred through
the metal tubes and the furnace to the
process flow inside when heat reaches
the process flow inside the tube a
convection current is set up that
transfers the heat throughout the liquid
in the tube so there are three different
ways that heat is transferred in any
furnace radiation convection and
conduction but what determines how fast
or how much heat is transferred
while there are several different
factors that affect the rate of heat
transfer the most important one is
temperature difference in order for heat
transfer to occur there must be a
temperature difference between two
substances the greater this difference
is the greater the rate of heat transfer
many of the factors that affect the heat
transfer rate are due to the design of
the furnace and are beyond your control
however later in the program we'll look
at some of the things you can do to
improve the efficiency and utilization
of heat transfer in your furnace right
now let's consider how heat is produced
in a furnace this is accomplished by a
chemical reaction called combustion for
combustion to occur three things must be
present at the same time and in the
proper proportions fuel air and a source
of ignition the fuel used in a fired
heater may be natural gas fuel oil or a
combination of gas and oil natural gas
is preferred in many installations
because it is cleaner and burns more
consistently than oil air is needed for
combustion because it provides oxygen
that reacts with a fuel the final thing
needed for combustion is a source of
ignition the ignition source provides
the heat energy that is required to
start the combustion reaction in the
operation of a fired heater it's
important to maintain the right
proportion of fuel to air when this is
done properly combustion will be
complete and the maximum amount of heat
will be generated without any fuel waste
incomplete combustion occurs when
there's not enough oxygen available to
burn all the fuel when this happens
unburned fuel can accumulate in the
furnace which is a potentially dangerous
situation
in addition incomplete combustion
drastically reduces the production of
heat
one of the challenges to the operator is
to provide enough oxygen to the furnace
to allow complete combustion while at
the same time minimizing excess oxygen
to assure efficient use of fuel the key
to safe and efficient furnace operations
is controlling the combustion process in
an emergency situation it might be
necessary to quickly shut down the
furnace the best way to do this is to
remove the source of fuel we have seen
that by controlling fuel oxygen and heat
the operator can control combustion in
the furnace now stop the video tape and
turn to section 1 in your workbook for a
review of heat transfer and combustion
make sure that you work through these
exercises as they contain information
not covered in the video that you'll
need to know as the program progresses
virtus is a custom designed to control
the flow of heat and handle the load
needed for the process they service the
one shown here is one of the more common
furnace designs this type of furnace is
shaped like a house and is called an
a-frame or cabin furnace the inner walls
of the furnace are lined with a material
that reduces heat losses and reflects
heat back onto the tubes this material
is called a refractory lining the
burners in this design are located on
the floor they can also be placed on the
walls or the arch of the furnace the air
required for combustion enters the
furnace through air registers located
directly under the burners the section
directly above the burners is called the
firebox the firebox contains the flames
in the furnace and does not let any
unwanted air into the combustion zone
the radiant tubes are located in the
firebox of the furnace they're in direct
line with the burners so they receive
most of their heat by radiation the
section of tubes above the burners is
called the shock bank these tubes
receive both convection and radiant heat
above the shock bank is the convection
section sometimes call the economizer
section because it extracts heat that
would be otherwise wasted the tubes in
this area are heated by the hot flue
gases before they leave the furnace
stack the process flow is sent through
the tubes and is quickly heated it then
flows out of the furnace to other
process equipment the tubes which carry
the process flow are generally divided
into passes or coils if the passes are
balanced properly each pass receives the
same amount of process flow and absorbs
the same amount of heat after the
process flow is circulated through the
passes it is rejoined at the outlet in
the stack of the furnace is the damper
the damper adjusts to control the flow
of hot gases out of the furnace this
upward flow of hot gases is known as the
furnace draft
now let's see what happens in our simple
model of a furnace the air registers are
open to provide the correct amount of
air for combustion the damper is
adjusted for the correct amount of
draught the feedstock is pumped into the
furnace and divided between the passes
after circulating through the tubes of
the furnace the feedstock is rejoined at
the outlet of the furnace there is a
target outlet temperature for the
process flow and this is used to
determine the amount of fuel going to
the burners in an automatic control
scheme the outlet temperature is
monitored by a temperature controller
which controls the flow of fuel to the
burners in order to increase the outlet
temperature more fuel is burned to
decrease outlet temperature less fuel is
burned the furnace draft is important
because it moves hot flue gases out of
the stack and draws fresh air through
the registers to the burners the type of
furnace we've discussed so far is a
natural draft furnace it's given this
name because the draft occurs naturally
due to the fact that hot air rises the
rising flue gases cause the pressure
inside the furnace to be slightly lower
than the pressure of the atmosphere this
lower or negative pressure is what
maintains the draft through the furnace
other furnaces use fans to maintain a
draft in an induced draft furnace a fan
is located in the flue gas ductwork to
the stack this fan pulls the flue gasses
out of the furnace to maintain an
adequate draft a forced draft fired
heater uses a fan of supply air for
combustion even though the combustion
air is under positive pressure a
negative pressure is maintained in the
rest of the heater by the natural draft
of the hot flue gas fired heaters which
use both forced and induced draft fans
are called balanced draft furnaces in
this system a forced draft fan supplies
air for combustion
while the induced draft fan pulls the
flue gasses from the fireball
to maintain a negative pressure in the
furnace pre heaters are used on some
fired heaters to reduce fuel consumption
these systems use the heat carried by
the flue gasses which otherwise would be
exhausted to the atmosphere to preheat
the air for combustion this conserves
energy because the combustion air enters
the furnace at a higher temperature and
therefore requires less heat for the
combustion reaction an essential piece
of equipment in any fired heater is the
burner the function of the burner is to
mix fuel with air in a manner that will
maintain stable and continuous
combustion the design and operation of a
burner will vary depending on the type
of fuel that is being used
let's take a closer look at how burners
work and review the material on furnace
equipment stop the tape and turn to
workbook section to
in order to obtain the best results were
fired heater key variables from both the
process and combustion side of the
furnace must be closely monitored and
the furnace itself must be physically
inspected to make sure it's operating
properly this segment of the program
will focus on these key operating
variables and the related furnace
inspections an important factor in
operating a furnace is controlling
excess oxygen there must always be
enough oxygen in the firebox to ensure
complete combustion too much oxygen
however waste heat because extra fuel is
used to heat the surplus air in the
furnace excess oxygen is the most
important variable affecting furnace
operating efficiency the amount of
oxygen in the furnace is monitored by an
oxygen analyzer most furnaces operate
between one and five percent excess
oxygen depending on the design of the
furnace the air registers are the main
control of air to the furnace when the
furnace is lit the air registers should
be adjusted to allow the proper amount
of air to each burner
the air registers on out of service
burners should be closed to prevent
excess air from entering the furnace
people should also be kept closed to
prevent excess air from entering the
furnace another important variable that
must be monitored and controlled is the
furnace draft the draft is controlled by
adjusting the stack damper to maintain a
slight negative pressure inside the
furnace measurements are usually taken
at three points in the furnace to
monitor the draft the indication of
draft is a negative pressure reading at
each of these points process flow rates
are another key operating variable the
flow rates through each furnace pass for
example must be kept balanced to assure
even heating of the process fluid and to
avoid tube damage due to overheating the
pass in lids and outlets are usually
equipped with temperature pressure or
flow instruments that allow an operator
to monitor the situation and keep the
passes balanced
product outlet temperatures the
temperature of the process flow as it
leaves the furnace normally there is a
set temperature a range of temperatures
that must be maintained to change the
outlet temperature you either increase
or decrease the amount of fuel going to
the burners when you adjust the fuel
rate however you must also change the
amount of air going to the burners to
ensure safe and complete combustion some
furnaces are equipped with automatic
controls to perform these functions if
the furnace is not equipped with these
automatic controls the operator is
responsible for controlling the air and
fuel flow for combustion another
variable that tells you a great deal
about the operation of your furnace is
the stack temperature the stack
temperature indicates the amount of heat
that is lost from the furnace while it's
desirable to keep this temperature low
it shouldn't be lower than the
condensation point of the flue gasses
this is because flue-gas condensation
can cause corrosion in the stack a high
stack temperature or rapid increase in
temperature often indicates a problem in
the furnace such as a leaking tube
problems in the furnace can sometimes be
detected by analyzing the flue gas for
oxygen if a flue gas from the radiant
section of the furnace contains excess
oxygen this indicates that too much air
is entering through the air registers
wasting heat too much excess oxygen at
the outlet of the convection section may
indicate that air is leaking into this
part of the furnace while it's possible
to monitor much of the furnace activity
from the control room it's also
necessary to physically inspect the
furnace during operation
the header box houses the tube end
fittings outside the main case of the
furnace smoke coming from the header box
indicates a tube leak another indication
of this problem would be smoke rising
out of the stack never open the doors of
a leaking header box while the furnace
is in operation because a flash fire
could result pilots should be lit at all
times while the burners are firing and
they should be checked on a routine
basis the pilot goes out it should be
repaired and relived
flame pattern should also be inspected
on a regular basis the flame should be
bright and clean with no trace of smoke
a properly burning gas flame is blue and
compact while an oil flame tends to be
bright yellow flame should not touch the
tubes this is called flame impingement
flame impingement can quickly damage a
tube and cause it to rupture the fire
should always be kept as uniform in
length and size as possible let's look
at some other undesirable flame patterns
and consider the causes a long smoky
flame is caused by insufficient
combustion air or insufficient atomizing
steam the flame may be shortened by
increasing primary or secondary air or
by increasing atomizing steam a thin
fluttery oil flame indicates too much
atomizing steam is being used a reddish
dusty flame with smoke over the bright
part of the flame indicates a lack of
oxygen for complete combustion sparks in
the flame could be caused by a dirty
burner tip wet steam solids in the fuel
water in the fuel or improper oil
atomization a dazzling white flame
indicates too much excess oxygen or too
much atomizing steam check your fuel
pressure especially when lighting the
burners too much pressure will cause the
flame to lift off the burner
too little pressure on the other hand
will cause the flame to blow back into
the burner erratic flame pattern and a
puffing sound from the furnace indicates
loss of draught and a serious shortage
of oxygen the flames may start burning
out of the heater and cracks were air
enters this is a hazardous situation
which can damage the furnace puffing and
wolfing can also be caused by flame
instability due to burner damage or by
too high of fuel pressure it's time for
a review of the material on monitoring
furnace performance stop the tape
and turn to workbook section 3
every plane has its own set of
procedures for furnace startup the
general procedures outlined here are
designed to provide an overview over to
startup however each operator should
carefully follow company procedures for
his unit there are five major steps to
furnace startup pre startup inspection
purging the furnace lighting the pilots
lighting the burners and placing the
furnace in normal operating mode
the first step in the pre startup
inspection is to check that the pilot
and main fuel systems are blinded and
that the fuel supply for each burner is
closed then inspect the furnace to
assure that it is ready for start-up
this involves inspecting the furnace to
make sure it's clear of debris checking
the burners and making sure all the
furnace doors are closed the damper and
burner air registers should be fully
open during startup so that the furnace
can be adequately purged with air or
steam
purging clears the furnace of any
combustibles and starts Draft purging
will usually last a minimum of 15
minutes or longer depending on the size
and type of furnace you're dealing with
while the furnace is being purged you
can start the process flow through the
coils and balance the passes this
circulation must be established before
you light any of the burners
before starting the lighting procedure
you may be required to put on special
protection light off gear check your
safety regulations to see if this
equipment is needed the next step in the
lighting procedure is to check that all
fuel gas valves are shut tight and then
remove the fuel gas blinds each pilot
must be lit separately light a pilot at
each end and then one in the middle to
provide a source of ignition throughout
the furnace
once all the pilots are led and burning
steadily you're ready to light the
burners light the burners in the same
order as you lift the pilots slowly open
the fuel gas valve to ignite the burner
if the first burner does not light close
the main fuel and pilot valves and
repurchase the furnace then restart the
lighting procedure
as the burners are being lit you adjust
each air register for the proper amount
of oxygen and the proper flame pattern
then you adjust the damper for the
proper draught during the light-up
procedure it's also important to monitor
the fuel pressure and keep it stable
since the fuel flow is on manual at this
time there will be a tendency for the
pressure to drop as you light additional
burners after lighting the burners
gradually increase the fuel so that the
furnace is slowly heated up to the
desired outlet temperature it'll be
necessary to make further adjustments
for draft and excess oxygen until the
desired conditions are achieved
when the target outlet temperature is
reached and the passes are balanced
place the controls on automatic the
furnace is now in normal operation
normal furnace shutdown procedures
involve gradually reducing the
temperature of the furnace to avoid
damaging the equipment again every
company has its own detailed procedures
the general procedures are outlined here
to provide an overview of the process
during normal shut down operations the
process flow rate and firing rate are
gradually reduced together when minimum
firing has been reached you shut off
fires and open the peep doors process
circulation is continued to cool down
the tube slowly next you open the stack
damper and air registers to purge the
furnace then block the fuel systems
including the burner and regulator
valves
and when the furnace is cool enough stop
the process flow and block the process
systems next you purge the process tubes
with steam each individual pass must be
purged after purging make sure all
burners are isolated and that all main
and pilot fuel headers are blinded this
completes the normal furnace shutdown
operation some abnormal situations may
call for an emergency furnace shutdown
under these circumstances the burner
fuel is stopped immediately and the
process flow may have to be dumped you
should be familiar with your company's
emergency procedures so that you can
quickly shut down the furnace in the
event of trouble let's review the
material and furnace startup and
shutdown procedures stop the tape and
turn to workbook section for
when an emergency situation occurs you
should be prepared to respond quickly
and correctly furnaces have unique
problems some of which may be detected
only in the field correct diagnosis of
the problem as well as quick response is
important in this segment of the program
we'll consider both the symptoms and
solutions to these common operating
problems let's start with flame
impingement flame impingement occurs
when the flames are actually touching
the metal surfaces inside the furnace
usually the tubes this contact weakens
the metal and can quickly plug up a tube
with coke deposits flame impingement can
sometimes be corrected by adjusting the
position of a burner or by pinching back
the fuel supply to the problem burner if
these adjustments don't correct the
situation it may be necessary to remove
the burner for cleaning or repair a hot
spot in a furnace is an area that has
become overheated due to an improper
distribution of heat or flame
impingement hot spots may occur on the
tubes the refractory or even on the
outside skin of the furnace they're a
problem because they compromise the
strength of a metal and cause coke
deposits some hot spots can be detected
with the human eye as a glowing red spot
on the metal in other instances special
heat sensing instruments such as a
pyrometer or an infrared scanner are
used to detect hot spots sometimes hot
spots on the furnace tubes can be
corrected by redistributing the process
flow so that the hotter passive tubes
receives more charge the increased flow
of charge absorbs more heat and carries
it away from the hot tubes in other
situations it may be necessary to
reposition a burner or remove it from
service
another common furnace problem is the
buildup of coke in the tubes a certain
amount of coking occurs naturally due to
the presence of carbon particles in the
process flow high temperatures however
aggravate the problem by increasing the
amount of carbon deposits the coke acts
as an insulator so less heat is
transferred to the process flow and more
heat accumulates in the metal tube if
the coking is severe the tube may
eventually rupture due to the excessive
temperatures a furnace can be decoded by
injecting superheated steam and air
through the tubes this operation burns
off the coke and cleans out the tubes a
disadvantage of this operation is that
it interferes with the ongoing
processing because the process flow must
be removed from the tubes during
cleaning in furnaces which burn oil and
ash deposit or soot gradually
accumulates on the outside of the
convection tubes these deposits must
periodically be removed because they
interfere with heat transfer some
furnaces are equipped with stationary
soot blowers which are long tubes that
run through the convection section steam
is circulated through holes in the soot
blower to blow off any accumulations of
soot a second type of soot blower is the
retractable type it's a steam Lance that
is taken out of the furnace when not in
use this type of soot blower can be
programmed to automatically blow
selected tubes in any given sequence
soot blowing can be done while the
furnace is in operation however it may
be necessary to completely open the
damper to prevent the steam and falling
suit from snuffing out the flames it's
important to closely monitor and
maintain the excess oxygen and draft
during soot glowing furnace equipment
like pumps fans valves and burners can
and does occasionally fail when this
happens an operator must be prepared to
respond quickly to prevent damage to the
furnace or a possible accident the feed
pump circulates the process flow through
the furnace if this pump fails the
circulation will stop and the furnace
will immediately start to overheat this
can damage the tubes and refractory and
cause severe coking your first reaction
should be to restart the pump if
possible or go to a spare if you can't
get the flow started immediately you'll
need to stop the input of heat by
cutting off the fuel to the burners the
damper should be open fully to allow the
maximum amount of heat to leave the
furnace and steam should be added which
will have a cooling effect on the
furnace by cutting off the fuel and
removing as much heat as possible you
minimize the chances of furnace damage
once the situation stabilizes you would
want to call in maintenance and have the
defective pump repaired a second type of
equipment failure is a fan failure if
one of the fans in a balanced or induced
draught furnace fails this will result
in a loss of draft if the furnace is
equipped with natural draft air doors
the flame safety system will
automatically open the doors and it may
be possible to keep the furnace
operating at a reduced load some
furnaces are equipped with steam rings
that discharge steam into the furnace in
the event of a fan failure the hot steam
rises upward and creates enough of a
draft to allow continued operation but
if air doors or steam rings are not
available the safety system will
automatically cut off fuel to the
burners and the furnace will have to be
shut down
a valve failure is a third type of
equipment problem if the valve is
controlling the process flow it will
usually fail open or become stuck in one
position in either case you won't be
able to adjust the rate of flow if a
bypass valve is available you may be
able to avoid a shutdown by routing the
flow around the failed valve when a
valve in the fuel system goes down it
usually fails in the closed position
this stops combustion in the furnace and
means a shut down the furnace burners
can also break down burner failures are
often caused by plugging or fouling in
the burner tip if our burner fails
you'll need to remove it from service
for cleaning or repair instrument
failure is not always easy to detect the
operator must understand the interaction
of variables in the furnace to judge
whether a particular reading is accurate
if the furnace has automatic controls
the controls may try to correct a
situation which doesn't exist if this
happens the operator must take the
controls off automatic and manually
adjust the operating variables correct
diagnosis and quick reaction can often
prevent a furnace shutdown let's
consider an example in an automatic
system the controller adjusts the fuel
flow to the burners based on the outlet
temperature of the charge
now if the measuring or transmitting
instrument sends out a faulty low
temperature signal to the controller the
controller will automatically increase
the flow of fuel to the burners trying
to compensate for the apparent decrease
in outlet temperature this in turn will
cause the furnace to overheat and could
cause coking and hot spots if the
operator suspects this is occurring he
can check the individual outlet
temperatures for each passive tubes if
these are above normal this would
indicate that the measuring or
transmitting device has probably
malfunctioned the operator would then
manually control the amount of fuel to
the burners to prevent the furnace from
over firing
another furnace problem is flame out
this is a situation where the burner
flames go out but the fuel is still
entering the burners when it happens
there is a dangerous buildup of unburned
fuel in the furnace which poses a threat
of explosion so the furnace must be
immediately shut down and purged a flame
out can be caused by several things such
as a loss of draught too much fuel or
not enough oxygen entering the air
registers in the situation where oxygen
is decreasing and the amount of
combustibles is rising it may be
possible to correct the problem before a
flameout occurs this can be done by
decreasing the flow of fuel to the
burners opening the damper to increase
draught or by opening the air registers
to increase oxygen but if you're unsure
how much uncombusted fuel is in the
furnace the safe thing to do is proceed
with a furnace shut down
in this program we've covered the basic
principles of heat transfer and
combustion we introduced you to furnace
equipment and discuss its operation and
provided some basic guidelines for
furnace startup and shutdown we also
looked at what you the operator can do
in terms of performance monitoring and
troubleshooting to make the operation
safer and more efficient we hope that
this program will help you in your
day-to-day operation of fired heaters
thank you for watching
you