Furnace or Fired Heater 3

8/18/2019 Furnace or Fired Heater 3

1/13

FURNACE / FIRED HEATER

An industrial furnace or direct fired heater is an equipment used to provide heatfor a process or can serve as reactor which provides heats of reaction. Furnacedesigns vary as to its function, heating duty, type of fuel and method of

introducing combustion air.

Several classifications of furnace:

i) Natural draft.ii) nduced draft.iii) Forced ! nduced draft.

"urner

Fuel flows into the burner and is burnt with air provided from an air blower. #herecan be more than one burner in a particular furnace which can be arranged incells which heat a particular set of tubes. "urners can also be floor mounted, wallmounted or roof mounted depending on design.

$amper

#he stac% damper contained within wor%s li%e a butterfly valve and regulatesdraft &pressure difference between air inta%e and air e'it) in the furnace, which iswhat pulls the flue gas through the convection section

1


2/13

(adiant Section

#he flames heat up the tubes, which in turn heat the fluid inside in the first part of the furnace %nown as the radiant section or firebo'. n this chamber wherecombustion ta%es place, the heat is transferred mainly by radiation to tubes

around the fire in the chamber. #he heating fluid passes through the tubes and isthus heated to the desired temperature.

onvection Section

#he gases from the combustion are %nown as flue gas. After the flue gas leavesthe firebo', most furnace designs include a convection section where more heatis recovered before venting to the atmosphere through the flue gas stac%. *eattransfer ta%es place by convection here, and the tubes are finned to increaseheat transfer.

rossover is the term used to describe the tube that connects from theconvection section outlet to the radiant section inlet.

Stac%

#he flue gas stac% is a cylindrical structure at the top of all the heat transferchambers. #he breeching directly below it collects the flue gas and brings it uphigh into the atmosphere where it will not endanger personnel.

2

http://en.wikipedia.org/wiki/Image:Conv_sect.gif


3/13

nsulation

nsulation is an important part of the furnace because it prevents e'cessive heatloss. (efractory materials such as firebric%, castable refractory and ceramic fibre,are used for insulation. #he floor of the furnace are normally castable type

refractories while those on the walls are nailed or glued in place. eramic fibre iscommonly used for the roof and wall of the furnace and is graded by its densityand then its ma'imum temperature rating. For eg: + -,//0F means + lb1ft density with a ma'imum temperature rating of -,//0F

Sootblowers

Sootblowers utili2e flowing media such as water, air or steam to remove depositsfrom the tubes. Sootblowers are found in the convection section. As this sectionis above the radiant section and air movement is slower because of the fins, soot

tends to accumulate here

"urner 3peration

3


4/13

#raditional premi' burners on a process heater premi' the fuel with the primaryair which is inspired to the burner by the fuel gas flow. #he pressure of the fuelgas supply is important since low gas pressure degrades performance. #heprimary air flow should be ma'imi2ed without lifting the flame off the burner. 4ostof the air &as primary air) is delivered to the burner along with the fuel. Secondary

air is introduced and ad5usted with the registers.

#he primary air register supplies primary air, which is the first to be introduced inthe burner. Secondary air is added to supplement primary air. "urners mayinclude a premi'er to mi' the air and fuel for better combustion before introducinginto the burner.

A furnace burner can be lit by a small pilot flame or in some older models,matches... 4ost pilot flames nowadays are lit by an ignition transformer &muchli%e a car6s spar% plugs). #he pilot flame in turn lights up the main flame. #he pilotflame uses natural gas while the main flame can use both diesel and natural gas.

7hen using liquid fuels, an atomi2er is used, otherwise, the liquid fuel will simplypour onto the furnace floor and become a ha2ard. 8sing a pilot flame for lightingthe furnace increases safety and ease. &compared to using a match).

9eneral rocedure to gnite "urner

i) onduct pressure test on the gas pipewor% to ensure that it is gas tight.ii) hec% the movement of the secondary and1or primary air register.

;ubricate the spindle if necessary.iii) hec% that no combustible gas is present inside furnace chamber using

gas meter.iv) 7ith the primary air register closed, insert gas lighting torch or igniter toignite the burner or the pilot burner. For burner that comes with pilotburner, main burner can be ignited by simply open the valve to the mainburner.

gniter or torch can be inserted through secondary air register or throughspecial opening for the torch.

v) Slowly open the primary air register to give short bluish flame.vi) #he secondary air register should be ad5usted to give the required 3 -

content in the furnace e'haust gas.

"urner Shutdown

i) lose the primary air register &to ensure fuel rich mi'ture ! to avoid


5/13

"urner #roubleshooting

4alfunction robable ause ountermeasures

"low>off $raft too high. Ad5ust draft.

logged gas tip. $ismantle the main burner !

clean the gas tip."ac%>fire $raft too low. ncrease the draft.

;ac% of air. ncrease the air flow.

9as tip burnt. (eplace the gas tip.

Firing rate is not o%. Ad5ust the firing rate.

5

http://en.wikipedia.org/wiki/Image:Burner1.jpg


6/13

ombustion Fundamental

ombustion is a chemical reaction between combustible material or fuel !o'ygen releasing heat. #he basic reactions of combustion are:

#ypes of ombustion

i) erfect combustion:7hen the amount of 3- supplied is in stoichiometric:

ii) omplete combustion:7hen the amount of 3- is bigger than stoichiometric:

iii) ncomplete combustion:7hen the amount of 3- supplied is not enough:

*eating ?alue:

$efinition **?: when a perfect mi'ture air>fuel at -@o is burned and the productcomposition is cooled to -@o, the total liberated heat is called the high heatingvalue.

st reaction : the liberated heat is the **?.-nd reaction: the liberated heat is the ;*?.

#he heating value of gaseous mi'ture can calculated using following formula:

;*? mi' B Σ;*?i yi

6


7/13

7


8/13

Air> fuel (atio

#he percentage of e'cess air can be e'pressed as follows:

t can also be e'pressed as:

#he amount of o'ygen in the stac% is obtained by analysis of the flue gas,

therefore the sampling ! analysis of such gas is very important.

8


9/13

9


10/13

10


11/13

S#A(# 8 3F A F8(NAC 1 F(C$ *CA#C(

) Cnsure there is flow inside heater tube.-) Start $F.) Start F$F.

D) 7ait for / minutes to purge any combustible gas inside thefurnace.@) ontrol draft at >D to >@ mm *g.E) ;ea%test of the gas line. f no lea%age, no pressure drop is

observed.) f lea%test pass, reset to open shut off valves &double bloc% !

bleed).+) 3pen valve to flare and use ? to control pressure at /.@ %g1cm-.G) gnite burner according to schedule & or - at one time). *eating rate is

@/o1hr./) After all burner ignited, firing is increased by increasing the burner

pressure. Stic% to the rate @/

o

1hr to G-/

o

..) ncrease flow, temperature etc as scheduled.

11


12/13

S*8# $37N

Shut down is the reverse of start up.

) (educe the flow as scheduled.

-) At the minimum gas flow rate reduce the firing at the rate of @/o

1hr byreducing the bac% pressure of the burner followed by burner e'tinguishing.3pen the valve to flare if required.

) 7hen the temperature is at +//o, shut down the furnace by pressing theshut down button without vent &$F ! F$F will shutdown, double bloc% !bleed valves to the burners will close).

D) lose the individual burner valve.@) lose the bloc%ing valves for N9 to double bloc% ! bleed shut off valves !

N9 valve to flare.E) (estart the $F ! F$F to cool the coil.

lose the dampers ! burner air register to avoid rapid cooling inside the

furnace. Stic% to the cooling rate of @/

o

1hr of the radiant chamber &byad5usting the dampers.) f available, let N- flowing through the furnace tube to avoid local

overheating.

$ry 3ut

$ry out is procedure to dry the refractory bric%s during commissioning at a veryslow heating rate &/o1hr H /o1hr) to avoid crac% from occurring duevapori2ation of humidity inside the bric%s.

$ry out need to be carried out for any new installed bric%s during initial start up or after changing to new bric%s.

Normal 3peration

3perator needs to monitor the followings:

i) #he temperature at every parts of the heater.ii) #he draft.iii) ?isual chec% of the tubes by opening the peep holes.iv) ?isual inspection of heater outer shell.v) hec% the burner firing, ad5ust the air register if necessary. #he flame

should not touch the furnace tube1coil, no bac% fire ! not e'tinguished .vi) 4easure the s%in temperature of the tube. S%in temperature should not

e'ceed the design s%in tube temperature. Ad5ust the burner firing ifnecessary.

vii) $F ! F$F operations: chec% any abnormal sound, chec% the lubricationoil, bearing and motor temperature.

12


13/13

Furnace nterloc%

Some of the important interloc%s for furnace:

i) Furnace will shut down if the temperatures are e'ceeding certain limit

&depends on the location of the temperature detector).ii) Furnace will shut down if $F or F$F is not operating.iii) Furnace will shutdown if the pressure of fuel is too high or too low.iv) Furnace will shutdown if the draft is too high or too low.v) Furnace will shutdown if no flow is detected inside the tubes.

ontrol ! nstrumentation

As in simplified F$. n general there are important control loop:

i) $raft control.ii) #emperature control.

nstrumentation:Some of the important instrumentation in a furnace:

i) $raft.ii) #emperatures of the radiant, convention ! stac%.iii) #emperature of the gas inlet, cross over ! the outlet.

13

Furnace or Fired Heater 3

Documents

Transcript of Furnace or Fired Heater 3