ENERGY CONSERVATION HANDBOOK – Fired...

ENERGY CONSERVATION HANDBOOK – Fired Heaters

HINDUSTAN PETROLEUM CORPORATION LTD

Mumbai Refinery

ENCON SECTION

TECHNICAL DEPARTMENT

2

Energy Conservation Hand Book-2010

INDEXSr. No. Subject Page No.

1 Introduction 3

2 Classification of Furnaces 6

3 Furnaces Efficiency Calculations 9

3 Furnace Design Data 11

4 Furnace Tuning 12

5 Factors affecting Furnace Parameters 14

6 Burner Management 16

3


Industrial Fired Heaters

An industrial furnace or direct fired heater is equipment used to provide heat for a process. Furnace designs vary as to its function, heating duty, type of fuel and method of introducing combustion air. However, most process furnaces have some common features.

1. Introduction: Fuel flows into the burner and is

burnt with air provided from an air blower. There can be more than one burner in a particular furnace which can be arranged in cells which heat a particular set of tubes. The flames heat up the tubes, which in turn heat the fluid inside in the first part of the furnace known as the radiant section or firebox (Fig 1.1).

In this chamber where combustion takes place, the heat is transferred mainly by radiation to tubes around the fire in the chamber. The heating fluid passes through the tubes and is thus heated to the desired temperature. The gases from the combustion are known as flue gas. After the flue gas leaves the firebox, most furnace designs include a convection section where more heat is recovered before venting to the atmosphere through the flue gas stack.

Radiant Section:The radiant section is where the tubes receive almost all its heat by

radiation from the flame. In a vertical, cylindrical furnace, the tubes are vertical. Tubes can be vertical or horizontal, placed along the refractory wall, in the middle, etc., or arranged in cells. Studs are used to hold the insulation together and on the wall of the furnace. They are placed about 1 ft (300 mm) apart in this picture of the inside of a furnace. The tubes, shown below, which are reddish brown from corrosion, are carbon steel tubes and run the height of the radiant section; The tubes are a distance away from the insulation so radiation can be reflected to the back of the tubes to maintain a uniform tube wall temperature. Tube guides at the top, middle and bottom hold the tubes in place.

Convection section:

The convection section is located above the radiant section where it is cooler to recover additional heat. Heat transfer takes place by convection here,

Fig 1.1

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and the tubes are finned to increase heat transfer. The first two tube rows in the bottom of the convection section and at the top of the radiant section is an area of bare tubes (without fins) and are known as the shield section, so named because they are still exposed to plenty of radiation from the firebox and they also act to shield the convection section tubes, which are normally of less resistant material from the high temperatures in the firebox. The area of the radiant section just before flue gas enters the shield section and into the convection section called the bridge zone. Crossover is the term used to describe the tube that connects from the convection section outlet to the radiant section inlet. The crossover piping is normally located outside so that the temperature can be monitored and the efficiency of the convection section can be calculated. The sight glass at the top allows personnel to see the flame shape and pattern from above and visually inspect if flame impingement is occurring. Flame impingement happens when the flame touches the tubes and causes small isolated spots of very high temperature.

BurnerThe burner in the vertical,

cylindrical furnace as above is located in the floor and fires upward (Fig 1.2). Some furnaces have side fired burners, such as in train locomotives. The burner tile is made of high temperature refractory and is where the flame is contained in. Air registers located below the burner and at the outlet of the air blower are devices with movable flaps or vanes that control the shape and pattern of the flame, whether it spreads out or even swirls around. Flames should not spread out too much, as this will cause flame impingement. Air registers can be classified as primary, secondary and if applicable, tertiary, depending on when their air is introduced. The primary air register supplies primary air, which is the first to be introduced in the burner. Secondary air is added to supplement primary air. Burners may include a pre-mixer to mix the air and fuel for better combustion before introducing into the burner. Some burners even use steam as premix to preheat the air and create better mixing of the fuel and heated air. The floor of the furnace is mostly made of a different material from that of the wall, typically hard cast-able refractory to allow technicians to walk on its floor during maintenance.

A furnace can be lit by a small pilot flame or in some older models, by hand. Most pilot flames nowadays are lit by an ignition transformer (much like a car's spark plugs). The pilot flame in turn lights up the main flame. The pilot flame uses natural gas while the main flame can use both diesel and natural gas. When using

Fig 1.2

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liquid fuels, an atomizer is used; otherwise, the liquid fuel will simply pour onto the furnace floor and become a hazard. Using a pilot flame for lighting the furnace increases safety and ease compared to using a manual ignition method (like a match). Soot blower:

Soot blowers are found in the convection section. As this section is above the radiant section and air movement is slower because of the fins, soot tends to accumulate here. Soot blowing is normally done when the efficiency of the convection section is decreased. This can be calculated by looking at the temperature change from the crossover piping and at the convection section exit. Soot blowers utilize flowing media such as water, air or steam to remove deposits from the tubes. This is typically done during maintenance with the air blower turned on. There are several different types of soot blowers used. Wall blowers of the rotary type are mounted on furnace walls protruding between the convection tubes. The lances are connected to a steam source with holes drilled into it at intervals along its length. When it is turned on, it rotates and blows the soot off the tubes and out through the stack.

Stack The flue gas stack is a

cylindrical structure at the top of all the heat transfer chambers. The breeching directly below it collects the flue gas and brings it up high into the atmosphere where it will not endanger personnel (Fig 1.3).

The stack damper contained within works like a butterfly valve and regulates draft (pressure difference between air intake and air exit) in the furnace, which is what pulls the flue gas through the convection section. The stack damper also regulates the heat lost through the stack. As the damper closes, the amount of heat escaping the furnace through the stack decreases, but the pressure or draft in the furnace increases which poses risks to those working around it if there are air leakages in the furnace, the flames can then escape out of the firebox or even explode if the pressure is too great.

Fig 1.3

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2. Classification of Furnace

Classification based on furnace draft

Natural Draft:In this the buoyancy of the hot flue in the stack & furnace provides the suction

to pull combustion air into the furnace (Fig. 2.2 C)

MR Natural Draft Furnaces

Sr. No.

Furnace Service

1 LR VPS(F101)

RCO

2 SEU I(F201/02)

Extract/Raffinate

3 SEU II(F3201/02)

Extract/Raffinate

4 SEU III(F4201/02)

Extract/Raffinate

5 IOH(F401)

Hydrofiner

6 DHDS(71F1)

Raw Diesel & Hydrogen

7 NHT CCR Charge Heater(102F1001)

Naphtha & Hydrogen

8 CCR Charge Heater(102F2001)

Naphtha & Hydrogen

9 CCR No. 1 Inter Heater Naphtha & Hydrogen

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(102F2002)

10 CCR No. 2 Inter Heater(102F2003)

Naphtha & Hydrogen

11 CCR No. 3 Inter Heater(102F2004)

Naphtha & Hydrogen

12 NHT ISOM (103F1001)

Naphtha & Hydrogen

13 Prime G+(105F1001)

Naphtha & Hydrogen

Forced Draft:Use of a fan to supply combustion air to the burners and to overcome the

pressure drop through the burners; This is in contrast to natural draft, where the buoyancies of the column of hot flue in the stack and furnace provide the “suction” to pull combustion air into the furnace. (Fig. 2.2 B)

MR Forced Draft Furnaces

Sr. No.

Furnace Service

1 NSU(101F1001)

Naphtha

Balanced/Induced Draft:Use of a fan on the flue gas side of the furnace, to provide the additional draft

required over that supplied by the stack to draw the flue gas through the convection section. (Fig. 2.2 A)

MR Balance Draft Furnaces

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Sr. No.

Furnace Service

1 FR APS(11F1/11F2)

Crude

2 FR VPS(12F1)

RCO

3 FRE APS(31F1)

Crude

4 FRE VPS(32F1)

RCO

5 FCCU(14F1X)

CAT Feed

6 PDA(F4101)

Asphalt Heater

Classification based on furnace size Vertical-Cylindrical furnace (Fig. 2.1 A)These furnaces are probably most commonly used furnaces with heat duties up

to about 150 MM BTU/hr. In the radiant section, tubes stand or hang vertically in a circle around the floor mounted burners. Thus firing is parallel to the radiant section tubes. These furnaces are designed either with or without a convection section. Most of vertical cylindrical furnace is being provided with a horizontal convection section located above radiant section. In small sizes under about 120 MM BTU/hr, these furnace are more economical& in sizes larger than about 150 MMBTU/hr the vertical tube box furnaces is more economical . Also these furnaces require less plot area.

Horizontal tube cabin furnace (Fig. 2.1 B)These furnaces have been built with the heat duties up to about 500 MM

BTU/hr. The radiant section includes horizontal tube surface on the side walls &the slopping roof of the cabin. The convection section extends over the entire length of the radiant section. Burners are floor mounted in a row down the center of the cabin & are fired vertically. This firing is normal to both radiant &convection tubes.

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Horizontal tube box furnace (Fig. 2.1 C)

The radiant & convection section are separated by a wall called bridge wall. Larger box furnaces have two radiant sections with a common convection section located between them. Burners are usually located in the end walls & fired towards the bridge wall. The furnace tubes all lie horizontally &firing is normal to the tubes.

3. Furnace Efficiency calculation

There are two methods to calculate furnace efficiency.Direct method

Heat absorbed by Process Fluid = m * cp * (COT-CIT)

Heat Fired by Fuel = m FO/FG * Calorific value of FO/FG

Efficiency = ( Heat AbsorbedHeat Fired )∗100

Indirect Method Heat Release by FG mmkcal/hr = m FG * LHV FG

Heat Release by FO mmkcal/hr = m FO * LHV FO

Fig 2.1

Fig 2.2

10


Total Heat Release mmkcal/hr = Heat Release by FG + Heat Release by FO

Total CO2 kg moles =

C / H FO∗m FOC / H FO+1 + C /H FG∗m FG❑

C /H FG+1

Theoretical air FO kgmoles =(C /H FO∗3)∗1000

(C / H FO+1 )∗12∗0.21

Theoretical air FG kgmoles = (C /H FG∗3)∗1000

(C / H FG+1 )∗12∗0.21

Total theoretical air Kgmoles =(Theoretical air FO kgmoles * m FO )

+ (Theoretical air FG kgmoles * m FG)

Total H2O kgmoles =

( m FOC /H FO+1 +

m FG❑

C / H FG+1 )∗1000/12

Total N2 kgmole = (Total theoretical air ) * 0.79

Total flue gas O2 kgmole = O2%*

( TotalCO2+Total H 2 O+Total N 2100−O 2% /0.21 )

Actual air kgmole = Total theoretical air + (Total O2 kgmole

/0.21)

Total Air Nm3/hr = Actual air kgmole *(22.4/24)

Total air T/D = Actual air kgmole * 29/1000

Total air m3/hr = Total Air Nm3/hr *((273+35)/273)

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Excess air % =

( Actual air kgmole−Theoretical air kgmoleTheoretical air kgmole )

H2O kgmole due to humidity = Actual air kgmole * 0.022 *29/18

Total flue gas kgmoles =(Total CO2 + Total H2O + Total N2

+ Total O2/0.21 + Atm steam*1000/18

+ H2O kgmole due to humidity)

Flue Gas avg molecular weight =

((Total CO2∗44+(Total H 2O+H 2 O kgmoledue¿humidity)∗18+¿Total N 2∗28+TotalO 2/0.21∗28.84)+ Atm steam∗1000¿

ual air kgmole−Theoretical air kgmole¿¿Total flue gas kgmoles ) Total Flue Gas Nm3/hr = Total flue gas kgmoles * 22.4/24

Total Flue Gas T/D =

(Total flue gas kgmoles∗FlueGas avg molwt1000 )

Total Flue Gas m3/hr = Total Flue Gas Nm3/hr *((273+ T)/273)

Flue Gas Heat loss mmkcal/hr =

(Total FlueGasT /D∗1000∗0.245∗(¿ – Ambient air )24∗106 )

% Heat loss to flue gas =

( FlueGas Heat loss MMKcal /hrTotal Heat Release MMKcal /hr ) *100

Setting loss mmkcal/hr = (Radiation loss * Total heat release)/100

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Actual furnace efficiency = 100 – (% Heat loss to flue gas) – (%Setting

loss) – (% loss due to APH) – (%Setting loss) – (% loss due to APH)

4. Furnace Design Data FR/FRE Block

Temperature profile O2 Profile & Efficiency

Furnace

CIT °C

COT °C

BWT °C

HAT °C

Stack °C

O2 %

η %

11F1 262 361 830 333 160 2.5-3.0 90.711F2 262 361 830 333 160 2.5-3.0 90.712F1 340 416 800 388 221 2.5-3.0 87.731F1 270 355 940 354 160 2.5-3.0 90.732F1 318 410 951 230 160 2.5-3.0 90.714F1X 270 390 694 280 165 2.5-3.0 90.0

LR/LRE Block


Furnace CIT °C

COT °C

BWT °C

HAT °C

Stack °C

O2 %

η %

13


F101 238 410 993 NA 329 4.5-5.0 83.5

F201/02 225 327 858NA 330

4.5-5.084.0

210 357 782 4.5-5.0F3201/02

225 327 841NA 330

4.5-5.084.0

210 357 782 4.5-5.0F4201/02

225 327 858NA 330

4.5-5.084.0

210 357 782 4.5-5.0F4101 81 260 550 204 180 4.5-5.0 90.0

GFEC Block


Furnace CIT °C COT °C BWT °C

HAT °C

Stack °C

O2 %

η %

101F1001 147.1 151.1 766 154 200 2.5-3.0

89.8

102F1001 279 341 765 NA 685 4.5-5.0

64.3

102F2001/02

464/437

549/549

758 NA 206 4.5-5.0

89.3

102F2003/04

481/498

549/549

760 NA 215 4.5-5.0

92.1

103F1001 258 310 630 NA 325 4.5-5.0

82.5

105F1001 285 345 730 NA 335 4.5-5.0

82.4

5. Furnaces Tuning

Natural Draft Heater Tuning Guide

Check flame characteristic Open all peepholes then check flame pattern. If there is bad flame then increase air flow. Then see whether flame is OK if it is No then shut down burner, clean burner

and then light up. Recheck flame pattern. If it is Good flame a)Check arch draft b)Check Excess O2 Close all peep holes.

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Check arch draft Read arch draft If ok then Arch draft = 0.1’wc If high then close stack damper check flame characteristic & check Excess O2. If low then open stack damper, then check flame characteristic & check excess

O2.

Excess O2

Read Excess O2. If Ok then Excess O2=3% & Heater is tuned. If low then open burner registers. Check flame characteristic. Then check Arch Draft & then Excess O2 =3%. If excess O2 is high close burner air registers. Check flame characteristic & then Check Arch draft then Excess O2 = 3%. Heater is tuned

Balanced Draft Heater Tuning Guide

Check flame characteristic Open all peepholes then check flame pattern. If there is bad flame then increase air flow. Then see whether flame is OK if it is No then shut down burner, clean burner

and then light up. Recheck flame pattern. If it is Good flame a)Check arch draft b)Check Excess O2

Close all peep holes.

Check arch draft Read arch draft If ok then Arch draft = 0.1’wc

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If high then close ID fan damper & check flame characteristic & check Excess O2. If low then open stack open ID fan damper & then check flame characteristic &

check excess O2.

Excess O2 Read Excess O2

If Ok then Excess O2=3% & Heater is tuned If low then open FD fan damper. Check flame characteristic Then check Arch Draft & then Excess O2 =3%. If excess O2 is high close the FD fan damper. Check flame characteristic & then Check Arch draft then Excess O2 = 3%. Heater is tuned

6. Factors affecting Furnace Parameters

Excess Air (high/low):Effect: Excess air in the heater will result in poor heater efficiency. With a constant stack temp of 530oC the heat loss @20% excess air is 3 times of that at 10% excess air.Measures taken: Adjust dampers, Adjust air registers maintain optimum air/fuel ratio.Results: 10% drop in excess air amounts to 1% saving on fuel fired.

Deposits on convection tubes:Effect: Less Heat transferMeasures taken: Regular soot blowingResults: 3 mm spot deposit increase fuel consumption by 2.5%

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Open peep hole doors:Measures taken: Close peep hole doors when not in use.Results: Unwanted air entry reduces excess air

Low oil temp to burners:-Measures taken: Maintain correct oil temperature.Results: Normally in the range of 110-120 C ensures proper atomisation and complete combustion of oil.

Oil Dripping from burners:-Measures taken: attend leaks at joints and clean burners.Results: Ensures good housekeeping and saves oil.

Wet steam and condensate to burners:-Measures taken: Check steam superheat.Results: Results in steady flame and proper fuel atomization.

Damage to burner tips & plugsMeasure taken: Use proper wire brush & maintain correct alignment during installation.Results: Well maintained tips will ensure good combustion and flame pattern.

Liquid Condensate To Gas Burner:Measure taken: Proper draining of condensate in fuel gas line.Results: Avoids backfires, ensures steady pattern and safe conditions in furnace.

Incomplete Combustion In Furnace BoxWays to be ensured: Maintain proper flame pattern, Maintain correct excess air, Maintain proper atomisation for oil Burners.Results: Results in high efficiency & reduced fuel consumption.

Air Preheater Bypass OpenWays to be ensured: Keep always closed.Results: Reduces heat loss. Entry 22C drop in air preheat temperature amounts to 1% loss in efficiency.

Non Uniform Firing In FurnaceWays to be ensured: Adjust burners for uniform firing.Results: Avoids localized heating resulting in faster coking in tubes and / or failure of tubes.

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7. Burner Management

The following practice is being followed in our refinery with regard to burner management system

Daily Weekly Monthly Semi annually

Annually

Check combustion visually

Check fuel and air linkage

Inspect burner

Check oil preheater

Clean fireside surface

Check Check indicating Analyze Inspect Clean

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general burner operation

lights ad alarms combustion refractory breaching

Check operating and limit controls

Inspect for fuel gas leaks

Clean oil pump strainer & filter

Check oil storage tanks.

Check safety and interlock controls

Inspect for hot spots

Reset combustion

Check fluid level and hydraulic valves

Check for leaks, noise,vibration unusual conditions Check operations of all motors.

Check combustion air supply

Remove and recondition safety valves.

Check general burner operation

Check all filter elements

Check flame scanner assembly.

Check fuel system.

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