Fired Equipment and Design

Fired equipment transfers heat produced by combustion of

the fuel to the process stream. In gas processing equipment,the fuel is usually natural gas; however, ethane, propane, orlight oils are sometimes used. The process stream varieswidely, e.g., natural gas, heavier hydrocarbons, water, glycol,amine solutions, heat transfer oils, and molten salts.

Fired equipment can be classified as:1. Direct fired heaters where the combustion gases occupy

most of the heater volume and heat the process stream con-tained in pipes arranged in front of refractory walls – the

8

radiant section – and in a bundle in the upper portion –the convection section. Convective heaters are a specialapplication in which there is only a convection section.

2. Firetube heaters where the combustion gases are con-tained in a firetube that is surrounded by a liquid thatfills the heater shell. This liquid may be either the proc-ess stream or a heat transfer medium that surrounds thecoil bundle containing the process stream.

Fig. 8-2 lists the common applications and general charac-teristics of these heaters.

a = constant, Eq 8-21A = area, m2

AO = Dry combustion air mols per dry fuel mols,for stochiometric combustion, mols/mols

B = parameter defined by Eq 8-21c = number of carbon atoms in fuel molecule

C = constant, Eq 8-4 and Eq 8-8Cd = burner spud discharge coefficientCp = specific heat, kJ/(kg • K)

CO = carbon monoxide mols in mols of dry flue gases,mols/mols

CO2 = carbon dioxide mols in mols of dry flue gases,mols/mols

CO2O= carbon dioxide mols in mols of dry flue gases for

stochiometric combustion, mols/molsd = diameter of pipe or cylinder or fin, mmD = diameter of pipe or cylinder, m

EA = volume percentage of excess combustion air, %ff = Fanning friction factorF = view factor, dimensionless

Flo = mols of dry flue gases per mols of dry fuel forstochiometric combustion, mols/mols

g = acceleration due to gravity = 9.8067 m/s2

G = Ratio of air to fuel, kg/kg; or= mass velocity, kg/(s • m2)

GHI = gross heat input, kWGTE = gross thermal efficiency, Eq 8-17a

Gr = Grashof number, dimensionlessh = heat transfer coefficient, W/(m2 • °C)H = heat content or enthalpy, kJ/kg; or

= height of stack, m; height of fin, mmHav = available draft, Pa (gauge)

HHV = higher or gross heating value, kJ/Sm3

hy = number of hydrogen atoms in fuel moleculeI = average tube radiant heat flux intensity

(circumferential), W/m2

k = thermal conductivity, W/(m • °C) L = length of heat transfer surface, m; or

= mean beam length, m; or= characteristic dimension, m

LHV = lower or net heating value, kJ/Sm3

m = constant, Eq 8-4 and Eq 8-8 or= dimension, m

M = mass flow rate, kg/hrn = number of fins per meter, number of tube rows

ni = number of nitrogen atoms in fuel moleculeNu = Nusselt number, dimensionless

NHI = net heat input, kWNPS = nominal pipe size, mmNTE = net thermal efficiency, Eq 8-17b

o = number of oxygen atoms in fuel moleculeO2 = oxygen mols in mols of dry flue gases, mols/mols

O2O= oxygen mols in mols of dry flue gases for

stochiometric combustion, mols/molsP = partial pressure of CO2 + H2O, atm

PB = barometric pressure, kPa∆P = pressure difference, kPaPg = burner fuel gas pressure, kPa (abs)Pr = Prandtl number = Cp µ / k, dimensionlessqgs = gas flow rate at standard conditions, m3/dayql = liquid flow rate, m3/hrQ = heat transfer or heat input or heat content (rates), Wr = ratio of flue gases to heat release, kg/(MW • hr)

rf = fouling heat flow resistance, (m2 • °C)/WR = fraction of total heat liberation absorbed in

radiant section (Fig. 8-20)RD = relative densityRe = Reynolds number, DVρ/µ or LVρ/µ, dimensionless

s = number of sulfur atoms in fuel molecule S = tube spacing, mm

Sm3 = standard cubic meters at 101.325 kPa and 15°C

FIG. 8-1

Nomenc lature

-1

Direct Fired Firetube

Applications

Hot oil heater Indirect fired water bathheaters (line heaters)Regeneration gas heaters

Propane and heavier hydro-carbon vaporizers

Amine and stabilizer reboilers

Hot oil and salt bath heatersGlycol and amine reboilersLow pressure steam generators

Characteristics

More ancillary equipmentand controls

Heat duty usually less than2930 kW

Higher thermal efficiency Easily skid mountedRequires less plot space Forced or natural draft

combustionForced or natural draftcombustion Less likely to have hot spots

or tube rupture

FIG. 8-2

Heater Ap plications and Characteristics

G

HEAT TRANSFER

ConductionFourier’s law of conduction gives the rate of heat transfer through

substances resulting from vibrations and interactions between ad-jacent molecules as opposed to overall motion or mixing of the mole-cules. Conduction always applies to solids and rarely to fluids.

Fundamental equations for steady heat conduction throughsome common solid shapes, ignoring border conditions, are:

For unidimensional perpendicular heat flow through flatwalls, as in heat flow through a square or very large cylindricaltank wall:

8

Q = 1000 • k • A • ∆Twt

Eq 8-1

For heat transfer in cylindrical geometry where the heattransfer is normal to the axis, as in heat flow through a cylin-drical vessel or pipe wall:

Q = 2 • π • L • k • ∆T

ln (Do /Di) =

2 • π • L • k • ∆Tln(do /di)

Eq 8-2a

For radial heat flow through a spherical vessel:

Q = 2 • π • k • ∆T

(1 / Di ) + (1 / Do ) =

π • k • ∆T(500 /di ) + (500 /do )

Eq 8-2b

Fig. 8-3 gives the thermal conductivities and densities ofcommercial refractories and insulation. Similar data for met-als are given in Fig. 8-8 and Fig. 9-8.Example 8-1 — Estimate the loss per linear meter through a25 mm layer of block insulation covering a 200 mm NPS Sch 40steam header. Assume:

Ti = 120°C

To = 10°C

k = 0.0721 W/(m • °C)

Solution Steps

do = 269.9 mm

di = 219.1 mm

L = 1 m

From Eq 8-2a

Q = 2 • π • L • k(Ti − To )

ln (do /di ) =

2 (3.1416)(1)(0.0721)(120−10)ln (269.9 / 219.1)

= 239 W per linear m

T = temperature, K∆T = temperature difference, °C

t = fin thickness, mmU = overall heat transfer coefficient, W/(m2 • K)

UHT = useful heat transfer or heat duty, WV = velocity, m/sw = weight of air, kg

wt = wall thickness, mm∆x = distance in direction of heat transfer, m or mmY = expansion factor, dimensionless

reekβ = volumetric coefficient of thermal expansion,

1/(°C or K)ε1, ε2, = emissivities of combustion gases and wall,

respectivelyµ = viscosity of fluid, mPa • sρ = density of fluid, kg/m3

σ = Stefan-Boltzmann constant,5.67 (10–8) W/(m2 • K4)

π = 3.1416

Subscriptsa = atmospheric air at operating conditions

as = air at standard conditionsB = barometricb = bulkc = convective

cs = cross section, projectionf = fin; fouling; friction; Fanningg = gas

gs = gas at standard conditionsi = inside, internal

LM = log mean base em = middle surfaceo = outside, external, overallp = piper = radiants = stack

w = wall1 = burner operating conditions 22 = burner operating conditions 2

FIG. 8-1 (Cont’d)

Nomenclature

-2

Products

Max Service

Temp, °C(Note 1)

Thermal Conductivity,W/(m • °C)

at Mean Temperature, °CDensity,

kg/m3

ColdCrush

Strength,kPa

Notes

260 540 815 1100 1370FIREBRICKH-W Karundal XD 1815 3.520 2.940 2.827 2.827 2.841 2995 107 588 2H-W UFALA 1650 1.870 1.846 1.889 1.961 2.077 2515 48 263APGreen KX-99 1370 1.400 1.428 1.471 1.543 1.601 2291 73 774APGreen Empire S 1315 1.200 1.269 1.327 1.370 1.428 2114 14 479

INSULATING BRICKAPGreen Greenlite 30 1650 0.375 0.447 0.519 0.577 0.649 977 8101Thermal Ceramics K-25 1370 0.159 0.187 0.216 0.245 609 1517Thermal Ceramics K-23 1260 0.144 0.173 0.216 0.245 497 1000Thermal Ceramics K-20 1090 0.130 0.159 0.202 465 862

HEAVY CASTABLEAPGreen Greencast 94 1870 2.090 1.817 1.630 1.543 1.558 2611 39 990 2,3,4,5APGreen Mizzou 1650 1.120 1.111 1.082 1.067 1.067 2211 20 684 3,11

LIGHT CASTABLEAPGreen Kast-O-Lite 25 1425 0.519 0.505 0.534 0.577 1378 8963 8APGreen 45-L 1370 0.389 0.375 0.361 1137 17 237 3,6,7,8APGreen Castable 22 1200 0.245 0.274 0.317 0.361 849 1896 3,8APGreen Cast Block Mix 870 0.086 0.144 0.216 352 138 8

GUN & RAM MIXESPremier 85 RAM HS 1650 3.130 2.034 1.817 1.716 1.745 2803 34 474 5,9H-W Tuff Shot LI 1425 0.894 0.952 0.966 0.981 0.995 1906 23 787 6,11APGreen Kast-O-Lite 26-LI GR 1425 0.548 0.534 0.562 0.620 1474 13 790

CERAMIC FIBERThermal Ceramics Saffl 1535 0.055 0.101 0.144 0.216 0.332 48 4Fiberfrax Durablanket 2600 1425 0.074 0.166 0.314 0.513 96Fiberfrax Durablanket 2600 1425 0.074 0.141 0.261 0.427 128Fiberfrax 550 Paper 1260 0.061 0.108 0.192 0.306 192Thermal Cer Cerablanket 1175 0.072 0.144 0.245 0.375 96Thermal Cer Cerablanket 1175 0.053 0.115 0.216 0.303 128

BLOCK & BOARDFiberfrax Duraboard LD 1260 0.081 0.123 0.179 0.250 256 345Fiberfrax Duraboard HD 1260 0.081 0.126 0.212 0.232 416 483Thermal Ceramics TR-20 1090 0.092 0.111 0.133 400 1386 12USG K-FAC 19 1035 0.074 0.128 296 228 10,13Schuller Thermo 12 350 0.066 0.092 240 1103 10,14PARTEK Paroc 1212 350 0.061 0.141 192 10,13Schuller 1000 SpinGlas 455 0.072 48 10,15NOTES 1. Maximum Service Temperature listed has no safety factor included. 2. 90-94% Alumina product for extreme temperature or high velocity service. 3. Cast properties listed, gunning product available but properties will be different. 4. Low silica product. 5. For burner blocks and severe service. 6. Can be used as one shot (single layer) lining. 7. High performance medium weight lining. 8. May be used as back-up insulation in 2 layer lining. 9. 85% Alumina ramming plastic for burner blocks, etc.10. For external insulation only.11. May be used as hot face lining in dual layer system.12. Diatomaceous earth base.13. Mineral wool base.14. Calcium silicate base.15. Fiber glass base.

FIG. 8-3

Properties of Commercial Refractories and Insulations

8-3

Configuration D or Y (Y = Gr • Pr) C m

Vertical Platesor Cylinders

Y < 104

104 < Y < 109

109 < Y

1.360.550.13

0.200.250.33

Horizontal Plates:Facing UpFacing UpFacing Down

105 < Y < 2(107)2(107) < Y <3 (1010)3(105) < Y < 3(1010)

0.540.140.27

0.250.330.25

Long HorizontalCylindersL > D

D < 0.10.1 < D < 0.50.5 < D

0.530.470.11

0.250.250.33

Short HorizontalCylindersL = D < 8 in.

Y < 10–5

10-5 < Y < 10–3

10-3 < Y < 11 < Y < 104

104 < Y < 109

109 < Y

0.490.711.091.090.530.13

0.000.040.100.200.250.33

FIG. 8-4

Heat Transfer Constants2 for Eq 8-4(Natural or Free Convection)

ConvectionHeat transfer between a solid and an adjacent fluid occurs

by movement of the fluid molecules. Hot molecules leave thesurface of the solid and are replaced by cold ones. Most of theresistance to this form of heat transfer occurs in a thin film orlayer next to the solid surface. This layer exists even if thebulk fluid flow is violently turbulent.

Newton’s law of cooling applies to convective heat transfer

Q = h • A • ∆T Eq 8-3

Natural or free convection — occurs when the onlyforce promoting the fluid flow results from temperature differ-ences in the fluid. Under these conditions the heat transfercoefficient is obtained from the Nusselt equation.

Nu = C (Gr • Pr)m Eq 8-4

Where

Nu = h • Do

k =

h • do

1000 • kEq 8-5a

or Nu = h • L

kEq 8-5b

Configuration Characteristic Length Flat plate parallel to flow Plate length Cylinder axis perpendicu-

lar to flowCylinder diameter

Inside pipes Inside pipe diameter Outside bank of tubes Staggered outside tube diamete

In line

FIG

Heat Transfer Constants for Eq

8

Gr = 106 • Do

3 • ρ2 • g • β • ∆T

µ2 = do

3 • ρ2 • g • β • ∆T

1000 • µ2 Eq 8-6a

or Gr = 106 • L3 • ρ2 • g • β • ∆T

µ2 Eq 8-6b

Pr = Cp • µ

kEq 8-7

The constants C and m depend on the shape and size of thesolid surface, the orientation of the surface to the fluid,whether the solid is hotter than the fluid or vice versa, and themagnitude of (Gr • Pr). A brief summary of C and m for someusual situations is given in Fig. 8-4.

Nu, Gr, and Pr are dimensionless when the units indicatedin Fig. 8-1 are used in equations 8-4 through 8-7. The physicalproperties are those of the fluid at the film temperature, whichis often assumed to be the average of the solid surface and bulkfluid temperatures. Fluid properties may have to be evaluatedat an assumed film temperature, and this assumption thenconfirmed from the results — see Example 8-4.

The coefficient of thermal expansion β for low pressure gas(i.e. ideal gas) equals 1/(T, K). It is left in this form for use inGr and is not converted to 1/°C for dimensional consistency.Example 8-2 — What is the heat transfer coefficient for natu-ral convection around a 75 mm NPS Sch 40 pipe surroundedby water at 88°C? Assume To for the pipe is 45.6°C.

Solution Steps

Tfilm = 88 + 45.6

2

Tfilm = 66.8 °C

From Eq. 8-4, 8-5a, 8-6a and 8-7, and Fig. 8-4:

do = 88.9 mm

Nu = 0.47 (Gr • Pr)0.25

h • do 1000 • k

= 0.47

do3 • ρ2 • g • β • ∆T • Cp

1000 • µ • k

0.25

ρ = 990 kg/m3

g = 9.807 m/s2

β = 0.00063 1/°C∆T = 88 − 45.6 °C

Re Pr C m103 < Re < 105 > 0.6 0.648 0.50

1 < Re < 4 > 0.6 0.99 0.334 < Re < 40 > 0.6 0.91 0.39

40 < Re < 4000 > 0.6 0.68 0.474(10)3 < Re < 4(10)4 > 0.6 0.193 0.62

4(10)4 < Re > 0.6 0.0266 0.81104 < Re 0.7 < Pr < 700 0.023 0.80

r 2(10)3 < Re > 0.6 0.33 0.602(10)3 < Re > 0.6 0.26 0.60

. 8-5

uation 8-82, 3 Forced Convection

-4

Cp = 4.187 kJ /(kg • °C)µ = 0.63 mPa • s k = 0.632 W/(m • °C)

The properties of water (ρ, β, µ, k) are based on the filmtemperature.

ho = 0.47 • 1000 • k

do

do3 • ρ2 • g • β • ∆T • Cp

1000 • µ • k

0.25

ho = 0.47 k

88.91000

(88.9)3(990)2(9.807)(0.00063 )(42.4)(4.187)1000 • (0.63)(0.632)

0.25

= 697.3 W/(m2 • °C)

Forced convection — occurs when the fluid flow adja-cent to a solid is promoted by external force, e.g., pumping,agitation, etc. The result is a substantial increase in the heattransfer rate. The Dittus-Boelter correlation is:

Nu = C • Rem • Pr0.33 Eq 8-8a

Utilization of a viscosity correction term gives the Sieder-Tatecorrelation:

Nu = C • Rem • Pr0.33 •

µb

µw

0.14

Eq 8-8b

Where

Re = 1000 • D • V • ρ

µ =

d • V • ρµ

Eq 8-9a

= 0.3537 • MD • µ

= 353.7 • Md • µ

= 0.3537 • ql • ρ

D • µ =

353.7 • ql • ρd • µ

= qgs • RD

55.4 • D • µ =

18.05 • qgs • RDd • µ

or

Re = 1000 • L • V • ρ

µEq 8-9b

Equation 8-8b need be used only for high viscosity fluids suchas glycol.

As before, Re is dimensionless when the units indicated inFig. 8-1 are used in equations 8-9a and 8-9b. The constants Cand m depend on the configuration and the type of fluid flow —laminar, intermediate, or turbulent — which is characterizedby the magnitude of the Reynolds number. Fig. 8-5 lists valuesfor the more common situations.

Example 8-3 — Find the heat transfer coefficient for280 000 Sm3/day of 0.6 relative density natural gas flowing at13 800 kPa (abs) in a 75 mm NPS Sch 40 pipe when the pipewall and gas temperatures are 40 °C and 22 °C, respectively.

Solution Steps

Tfilm = 40 + 22

2 = 31 °C

From Eq 8-7, 8-8a, and 8-9a and Fig. 8-5:

Nu = 0.023 (Re)0.8 (Pr)0.33

8

h • di

1000 • k = 0.023

18.05 • qgs • RDdi • µ

0.8

Cp µk

0.33

Where:

di = 73.7 mm

µ = 19 (10−3) mPa • s

k = 0.052 W/(m • °C)

Cp = 3.06 kJ/(kg • °C)

qgs = 0.28 106 Sm3 /day

RD = 0.60

hi = 0.023 • (1000 ) • k

di

18.05 • qgs • RDdi • µ

0.8

Cp • µk

0.33

hi = 0.023 (1000 ) 0.052

73.7

18.05 (0.28) (106 ) 0.6

(73.7) (19) (10−3)

0.8

3.06 (19) (10−3)0.052

0.33

hi = 1971 W/(m2 • °C)

Overall Heat Transfer CoefficientSo far, only the individual or local heat transfer coefficients

have been considered. As discussed in Section 9 “Shell andTube Heat Exchangers” the individual heat transfer coeffi-cients are combined into an overall heat transfer coefficient.See Fig. 9-3 for calculation of ∆TLM.

Q = U • A • ∆TLM Eq 8-10

U must be based on some specific area. Considering all theresistances to heat transfer through a hollow cylinder whosewall is made of two different materials (i.e. metal pipe and alayer of insulation), the overall heat transfer based on the in-sulation outside diameter is:

Uo = 1

1ho

+ Do

Di • hi +

Do • ln (Do /Dm)2 • ko

+ Do • ln (Dm /Di)

2 • ki + rfo + rfi •

Do

Di

= 1

1ho

+ do

di • hi +

do • ln (do /dm)2000 • ko

+ do • ln (dm /di)

2000 • ki + rfo + rfi •

do

diEq 8-11

When there is only one solid layer, delete the fourth term inEquation 8-11 and change the subscript m to i and delete thesubscript on k in the third term.Example 8-4 — Find the overall heat transfer coefficient for a75 mm NPS Sch 80 pipe submerged in an 88 °C water bath.280 000 Sm3/day of 13 800 kPa (abs) natural gas is to beheated from 16 °C to 29 °C. Refer to Examples 8-2 and 8-3;note that the heat flow through the pipe is similar to Example8-1.

Solution StepsUse hi, ho, and the conduction through the pipe wall to find

U. Then check the heat fluxes to see that the right film tem-peratures were used. From Fig. 8-8, k = 45.3 W/(m •°C) forthe pipe wall at 48°C. Assume clean pipe.

hi = 1971 and ho = 697.3 W/(m2 • °C)

di = 73.7 and do = 88.9 mm

k = 45.3 W/(m • °C)

-5

FIG. 8-6

Fin Efficiency Chart4

FIG. 8-7

Fin Tip Temperature5

From Eq 8-11

Uo = 1

1ho

+ do

di • hi +

do • ln (do /di)2000 • k

= 1

1697.3

+ 88.973.7 • 1971

+ 88.9 ln (88.9 /73.7)

2000 • 45.3

= 448.4 W/(m2 • °C)

Ao = π • do

1000 = 0.2793 m2

To = 88 Ti1 = 16 Ti2 = 29 °C

From Fig. 9-3

∆TLM = (To − Ti1) − (To − Ti2)

ln

To − Til

To − Ti2

= (88 − 16) − (88 − 29)

ln

88 − 1688 − 29

°C

∆TLM = 65.28 °C

Q = Uo Ao ∆TLM = 448.41(0.2793)(65.28)

1 = 8176 W per linear m

To confirm the film temperatures and the validity of the in-dividual heat transfer coefficients, the heat fluxes outside,through, and inside the pipe must be compared with the over-all heat flux. Consider one linear meter of pipe. At the averagegas temperature of 22.5°C, calculate the fluxes for one linearmeter of pipe.

Qo = ho • Ao • ∆To

8-6

FIG. 8-8

Thermal Conductivity of Ferrous Materials4

8-7

Qo = (697.3) •

π • 88.91000

• (88 − 45.6)

Qo = 8257.3 W per linear m

Qp = 2 • π • L • k • ∆T

ln

do

di

Qp = (2) • (3.14) • (1) • (45.3) • (45.6 − 40)

ln

88.973.7

Qp = 8496.2 W per linear m

Qi = hi • Ai • ∆Ti

Qi = (1971) •

π • 73.71000

• (40 − 22)

Qi = 8214.4 W per linear m

The agreement is close enough for purpose of this example.

8

The above refers to clean pipes. Fouling occurs with contin-ued use. Sometimes, fouling factors are specified; e.g., 0.0002internally and externally. These are correction factors that areadded to 1/Uo. So,

Uof = 11

Uo + 0.0002 + 0.0002 • 88.9

73.7

Uof = 374.34 W/(m2 • °C)

Finned TubesWhen combustion gases flow externally and heat a liquid in

a pipe, there is a significant disparity between hi [usually over850 W/(m2 •K)] and ho [usually less than 60 W/(m2 •K)]. Toovercome this and make better use of a given length of pipe,the external surface is finned. When the gas flow is normal tothe pipe axis, helical fins — typically 1.25 to 3 mm thick, 12.5to 40 mm high, and 80 to 240 fins/m — are used. The result isan increase of up to tenfold in the external area of the pipe.

The total external surface area of a finned pipe and the crosssectional or projected area restricting normal gas flow per lin-ear foot are:

A. Metals and Their Oxides

FIG. 8-9

Normal Total Emissivity of Various Surfaces 3

Surface T, °C* Emissivity*

AluminumHighly polished plate, 98.3% pure 225-575 0.039-0.057Polished plate 23 0.040Rough plate 26 0.055Oxidized at 599°C 200-600 0.11-0.19Aluminum-surfaced roofing 38 0.216Calorized surfaces, heated at 600°C:

Copper 200-600 0.18-0.19Steel 200-600 0.52-0.57

BrassHighly polished:

73.2% Cu, 26.7% Zn 247-357 0.028-0.03162.4% Cu, 36.8% Zn, 0.4% Pb, 0.3% Al 257-377 0.033-0.037

82.9% Cu, 17.0% Zn 277 0.030Chromium; (see Nickel Alloys for Ni-Cr steels) 38-538 0.08-0.26Copper

Commercial, emeried, polished, but pitsremaining 189 0.030

Commercial, scraped shiny but notmirror-like 22 0.072

Polished 117 0.023Plate, heated long time, covered with

thick oxide layer 25 0.78Cuprous oxide 799-1099 0.66-0.54

Iron and steelMetallic surfaces (or very thin oxide layer):

Cast iron, polished 200 0.21Ground sheet steel 938-1099 0.55-0.61Smooth sheet iron 899-1038 0.55-0.60

Oxidized surfaces:Iron plate, pickled, then rusted red 20 0.612Completely rusted 19 0.685

Rolled sheet steel 21 0.657Oxidized iron 100 0.736Cast iron, oxidized at 599°C 199-599 0.64-0.78

-8

Surface T, °C* Emissivity*

Steel, oxidized at 599°C 199-599 0.79Iron oxide 499-1199 0.85-0.89Sheet steel, strong rough oxide layer 24 0.80

Dense shiny oxide layer 24 0.82Cast plate:

Smooth 23 0.80Rough 23 0.82

Cast iron, rough, strongly oxidized 38-249 0.95Wrought iron, dull oxidized 21-360 0.94Steel plate, rough 38-371 0.94-0.97High temperature alloy steels

(see Nickel Alloys)Monel metal, oxidized at 599°C 199-599 0.41-0.46Nickel

Electroplated on pickled iron, not polished 20 0.11Plate, oxidized by heating at 599°C 199-599 0.37-0.48Nickel oxide 649-1254 0.59-0.86

Nickel alloysChromnickel 52-1034 0.64-0.76Nickelin (18-32 Ni; 55-68 Cu; 20 Zn),

gray oxidized 21 0.262KA-2S alloy steel – (8% Ni; 18% Cr),

light silvery, rough, brown,after heating 216-490 0.44-0.36After 42 hr. heating at 980°F 216-527 0.62-0.73

NCT-3 alloy (20% Ni; 25% Cr), brown,splotched, oxidized from service 216-527 0.90-0.97

NCT-6 alloy (60% Ni; 12% Cr),smooth, black, firm adhesive oxidecoat from service 271-563 0.89-0.82

Tin – bright tinned iron sheet 24 0.043 & 0.064Zinc

Commercial, 99.1% pure, polished 227-327 0.045-0.053Oxidized by heating at 750°F 399 0.11Galvanized sheet iron, fairly bright 28 0.228Galvanized sheet iron, gray oxidized 24 0.276

*t

†bA

Ao = π • do

1000 •

1 −

n • t1000

+

n • π2

df2 − do

2

106

Eq 8-12

and

Acs = do

1000 +

n • t • (df − do)106 Eq 8-13

The surface area of the fins is not as efficient as the externalsurface of the pipe because heat absorbed at the fin surfacemust be conducted to the fin base before it can pass throughthe pipe wall. Fin efficiencies are given in Fig. 8-6. These effi-ciencies are applied to the total external area.

Another important consideration is fin tip temperature.This can be obtained from the fin efficiency and Fig. 8-7.Fig. 8-8 gives the thermal conductivities and maximum rec-ommended fin tip temperatures for the more common ferrousconstruction materials.

Example 8-5 — Calculate the external surface and fin effi-ciency for 100 mm NPS Sch 80 pipe with the following finning:118 fins/m, 31.75 mm high, 2.67 mm thick. Assume:

ho = 22.2 W/(m2 • °C)

kf = 43.3 W/(m • °C)

8-

Solution Steps

Abscissa in Fig. 8-6:

Hf = 31.75 mm

t = 2.67 mm

X = 0.045 Hf √ho

kf • t

= 0.045 (31.75) √22.243.3 (2.67)

= 0.626

do = 114 mm

df = do + 2 Hf = 114 + 2 (31.75 )

= 177.5 mm

df

do =

177.5114

= 1.557

From Fig. 8-6, fin efficiency is 87%.

B. Refractories, Building Materials, Paints, and Miscellaneous

FIG. 8-9 (cont’d)

Normal Total Emiss ivity of Various Su rfaces3

Surface T, °C* Emissivity*

AsbestosBoard 23 0.96Paper 38-371 0.93-0.945

BrickRed, rough, but no gross irregularities 21 0.93Silica, unglazed, rough 1000 0.80Silica, glazed, rough 1100 0.85Grog brick, glazed 1100 0.75See Refractory Materials below.

CarbonT-carbon (Gebr. Siemens) 0.9% ash

(this started with emissivity at 125°C of0.72, but on heating changed to valuesgiven)

127-627 0.81-0.79

Enamel, white fused, on iron 19 0.897Glass, smooth 22 0.937Gypsum, 0.02 in. thick on smooth or

blackened plate 21 0.903Marble, light gray, polished 22 0.931

Oak, planed 21 0.895Oil layers on polished nickel (lube oil) 20

Polished surface, alone 0.045+0.001-in. oil 0.27+0.002-in. oil 0.46+0.005-in. oil 0.72Infinitely thick oil layer 0.82

Oil layers on aluminum foil (linseed oil)Al foil 100 0.087†+1 coat oil 100 0.561+2 coats oil 100 0.574

When two temperatures and two emissivities are given, they correspond, firsto first and second to second, and linear interpolation is permissible.

9

Surface T, °C* Emissivity*

Paints, lacquers, varnishesSnowhite enamel varnish on rough iron

plate 23 0.906Black shiny lacquer, sprayed on iron 24 0.875Black shiny shellac on tinned iron sheet 21 0.821Black matte shellac 77-146 0.91Black lacquer 38-93 0.80-0.95Flat black lacquer 38-93 0.96-0.98

White lacquer 38-93 0.80-0.95Oil paints, sixteen different, all colors 100 0.92-0.96Aluminum paints and lacquers

10% Al, 22% lacquer body, on rough orsmooth surface 100 0.52

26% Al, 27% lacquer body, on rough orsmooth surface 100 0.3

Other Al paints, varying age and Al content 100 0.27-0.67Al lacquer, varnish binder, on rough plate 21 0.39Al paint, after heating to 327°C 149-316 0.35

Plaster, rough lime 10-88 0.91Porcelain, glazed 22 0.924Quartz, rough, fused 21 0.932Refractory materials, 40 different

poor radiators

good radiators

599-9990.65-0.75

0.700.80-0.850.85-0.90

Roofing paper 21 0.91Rubber

Hard, glossy plate 23 0.945Soft, gray, rough (reclaimed) 24 0.859

Water 0-100 0.95-0.963

Although this value is probably high, it is given for comparison with the data,y the same investigator, to show the effect of oil layers. See Aluminum, part of this table.

FIG. 8-10

Partial Pressure of CO2 Plus H2O

Dimension Ratio Mean Beam Length, LRectangular Furnaces,

Length-Width-Height,In Any Order

1-1-1 to 1-1-31-2-1 to 1-2-4 (2/3) (Furnace Volume)

1⁄3

1-1-4 to 1-1-∞ (1) (Smallest Dimension)1-2-5 to 1-2-∞ (1.3) (Smallest Dimension)1-3-3 to 1-∞-∞ (1.8) (Smallest Dimension)Cylindrical Furnaces,

Diameter-Height1-1 (2/3) (Diameter)1-2 to 1-∞ (1) (Diameter)

}

FIG. 8-11

Beam Lengths for Gas Radiation8

FIG. 8-12

Gas Emissivity10

From Eq 8-12:

n = 118 per meter

Ao = π • do

1000 1 −

n • t1000

+

n • π2

• (df

2 − do2)

106

= π • 1141000

• 1 − 118 • 2.67

1000 +

118 • π2

• (177.52 − 1142)

106

= 3.676 m2 per linear m

RadiationAn estimate of the radiant heat flux between two surfaces

is:

Q A

= σ • F (T1

4 − T24)

1ε1

+ 1ε2

− 1

Eq 8-14

The geometric or view factor, F, is the fraction of the surfacearea that is exposed to and absorbs radiant heat. The equationfor F must be determined from an analysis of the geometry. In

8

using Eq 8-14, it is recommended6 that F not be less than 0.67.Also, all temperatures are absolute.

Fig. 8-9 gives the emissivities of common construction met-als, oxides, refractories, and insulation materials. The emis-sivity of combustion gases is more complex because it dependson the temperature and the product (P • L). See Figs. 8-10,8-11, and 8-12.Example 8-6 — What is the radiant heat flux to a 0.9 m lengthof a 0.6 m ID firetube when the combustion gases inside thetube are at 1540°C and the firetube wall is at 150°C? Assume20% excess air is used.

Solution Steps

F = curved surface areatotal surface area

D = 0.6 L = 0.9

F = π • D • L

π • D • L + 2 • π • D2

4

= π (0.6) (0.9)

π (0.6) (0.9) + 2 π • 0.62

4

F = 0.75

From Fig. 8-10, P = 0.24 atm

From Fig. 8-11, L = D, L = 0.6 so P • L = 0.144 atm • m or P • L = 14.6 kPa • m

From Fig. 8-12, ε1 = 0.12

From Fig. 8-9, ε2 = 0.79 (steel, oxidized at 600°C)

Equation 8-14,

σ = 5.67 (10−8) W/(m2 • K4)

-10

FIG. 8-13

Combina tion Con vection and Ra diation Fil m Coefficien tsfor Air in Co ntact with Vertical Walls or Su rfaces11

NOTE: Values for rich mixtures depend somewhat on combustion chamberdesign. The average values shown are within 1⁄2% of correct for H2, CO, andCO2, but may be as much as 2% low for other constituents. Some externalheat is usually required for mixtures with less than 70% aeration (dottedlines). Dashed lines show the trends with poor mixing or quenching. Gasconstituent percentages are on a dry basis to allow comparison with gasanalyzer readings, that measure the gas volumes after water vapor hascondensed out of the sample. With the correct amount of air (10.56 m3),each cubic meter of this fuel gas produces 2.14 m3 H2O, 1.15 m3 CO2, 8.39 m3

N2; so %CO2 = 100 × 1.15 m3 CO2 ÷ (1.15 + 8.39) m3 dry flue gas = 12.1%.

FIG. 8-14

Effect of Fuel/Air Ratio o n Flue Gas Analysisfor 41 283 kJ/Sm3 Natural Gas (0.63 Gas Relative Density)

Containing 83 % CH4 and 16% C2H61

8

T1 = 1540 + 273 K

T2 = 150 + 273 K

QA

= σ • F • [ (T1)4 − (T2)4]

1ε1

+ 1ε2

− 1

= (5.67)(10−8) 0.75 [(1813)4 − (423)4]

10.12

+ 1

0.79 − 1

= 53 271 W per square m

Note that T is in K.

Heat LossesHeat losses from equipment surfaces occur primarily by ra-

diation and convection. Fig. 8-13 gives the combined heattransfer coefficient, hc+hr, in terms of the wind velocity andthe temperature difference between the surface and the sur-rounding air.Example 8-7 — How much heat can be saved per linear meterby covering a 200 mm NPS Sch 40 steam header, carrying100 kPa (ga) steam at 120°C, with a 25 mm thick layer of blockinsulation? Assume ambient conditions are -1°C with a24 km/hr wind.

Solution StepsUsing Fig. 8-13 the heat loss from the bare pipe is:

hcr = hc + hr Combined convection and radiation coefficient

L = 1 m

hcr = 33.2

Do = 0.219 m

Tp = 120 °C

Ta = −1 °C

Q = hcr • Ao (Tp − Ta) = 33.2 • π • 0.219 (120 − (−1))

= 2764 W/per linear m

For the insulated pipe, assume the outside surface of theinsulation is at 10 °C. Then from Fig. 8-13:

hcr = 25.5

Q = 25.5 • π (0.219 + 2 • 0.025)[10 − (−1)]

= 237 W/per linear m

This agrees closely enough with the heat flow through theinsulation — Example 8-1.

Heat saved = 2764 − 237 = 2527 W/m

COMBUSTION

Combustion is the rapid chemical reaction between oxygenand a combustible material that releases heat and light. Usu-ally the combustible material is a hydrocarbon and ambientair supplies the oxygen. Complete combustion occurs whenthere is sufficient oxygen to convert all of the carbon to carbondioxide and all of the hydrogen to water. Incomplete combus-tion means that there is either unburned or partially reactedfuel, i.e., carbon monoxide, hydrogen, etc.

Methane is the main constituent of natural gas. It reactswith oxygen to form carbon dioxide and water.

-11

0.5

13

11

10

9

12

0.7 0.80.6

(0.7, 0.2, 0.1)

(1.0, 0, 0)

(0.9, 0.1, 0)

(0.8, 0.2, 0)

(0.7, 0.3, 0)

Relative Density of Fuel Gas(Air = 1.0)

Cub

ic M

eter

s of

Dry

Air

per

Cub

ic M

eter

of

Fue

l Gas

(CH4, C2H6, C3H8)

FIG. 8-15a

Standard Cubic Meters of Dry Air Needed per Standard Cubic Meter of Hydrocarbon for Complete Combustion

FIG.

Mass of Humid Air Per Mass of Dry Air At

8-

CH4 + 2 O2 → CO2 + 2 H2O

This stoichiometry is typical of all hydrocarbons. One atomof carbon requires one molecule of oxygen and four atoms ofhydrogen require one molecule of oxygen. The theoretical airis that needed for complete combustion of the carbon and hy-drogen, i.e., two molecules of oxygen for one molecule of meth-ane. Excess air is that supplied in addition to what is required.For example, 20% excess air means that the air supplied is 1.2times the stoichiometric amount.

The following reaction represents the complete combustionof an arbitrary carbon-based fuel compound.

CcHhyOoSsNni + c + hy

4 − o

2 + s

O2 →

(c) CO2 +

hy2

H2O + (s) SO2 +

ni2

N2

Fig. 8-14 shows how the composition of the flue gases de-pends on the amount of combustion air.

Air RequirementsFig. 8-15a shows the m3 of dry air needed per scf of paraffinic

hydrocarbons for complete combustion in terms of the specificgravity of the fuel. In using this figure any inert componentsin the fuel, e.g., nitrogen, carbon dioxide, etc., must be ex-cluded. Fig. 8-15b shows the mass of humid air required permass of dry air at 760 mm Hg and percent relative humidity.

Air is about 20.9% oxygen on a dry basis, hence 4.77 mols(or m3) of air supply 1.0 mol (or m3) of oxygen. Applying thisto methane, 9.54 mols (or m3) of air are needed for every mol(or m3) of methane.

8-15b

760 mm Hg and Percent Relative Humidity

12

FIG. 8-16

Effect o f Ambient Temperature and Ba rometer Pressureon Air Actually Delivered

FIG. 8-17

Gross Th ermal Efficiency for a Gas withHHV = 37.3 kJ/Sm3

8

The effect of water vapor in the air is relatively small at lowand moderate temperatures. Saturated air at 15°C contains1.75% water. Still this should be considered and 2-3% more airis usually added if exact calculations are not made. The watercontent in saturated air increases rapidly with temperature;e.g., at 38°C saturated air contains about 6.5% water, and at46°C it contains about 10%.

Some situations may result in a higher amount of watervapor coming from the combustion air and fuel gas. Considerthe complete combustion of 1 mol of water saturated methaneat 38°C, 103.4 kPa (ga) with 20% excess air with air also watersaturated at 38°C. This introduces 0.79 mol of water from theair and 0.032 mol of water from the gas. Additionally, 2 molsof water from the methane combustion is added water result-ing in approximately 21% water in the flue gas of which 30%is from the air and gas humidity. Also, steam or water additionfor NOx control introduces more water vapor to the flue gas.These situations increase the wet bulb temperature of the fluegas. Water condensation should be considered in mass andenergy balances and excess air calculations. Errors in consid-eration of water vapor content and air temperature may cancela 10% excess air calculation, resulting in incomplete fuel com-bustion. Designs and operations should consider local weatherconditions and seasonal changes.

The theoretical air requirement of an arbitrary carbon-based fuel compound, in mols of air per mol of fuel, can becalculated with the following equation.

AO = 4.77 • c + hy

4 − o

2 + s

Eq 8-15

Analysis of the flue gases provides useful information aboutthe actual excess air and the efficiency of fired equipment. Thefollowing equations provide the excess air percentage for asulfur and oxygen free, carbon-based fuel combustion withoutsoot formation. Analysis must be molar and on a dry basis.

EA = Flo

AO

CO2o

CO2 + CO • 100

= Flo

AO

O2o

20.9 − O2 + CO • 100 Eq 8-16

Many forced-draft burners supply a fixed volume of air.Fig. 8-16 shows the effect of ambient temperature and baro-metric pressure on the amount of air actually delivered.

Heating Value

The heating value of a fuel is the amount of heat releasedduring complete combustion with the stoichiometric amountof dry air. This involves a heat balance (the usual datum orreference temperature is 15°C).

Hydrogen in the fuel burns to water and when the flue gasesare cooled to 15°C, the physical state — either vapor or liquid— of this water must be assumed. So the latent heat of vapori-zation of the water may or may not be considered to be part ofthe heating value. The result is two definitions for the heatingvalue. The higher or gross heating value, HHV, includes theheat of condensation and the lower or net heating value, LHV,assumes the water remains in the vapor state.

Fig. 23-2 in the “Physical Properties” section gives the netand gross heating values of most pure hydrocarbons. For mix-tures calculate the molar, or volume, average.

-13

FIG. 8-18

Typical Enthalpy of Combustion Gases for a Dry Natural Gas Fuel and 20% Excess Dry Air

8-14

Thermal Efficiency

The two ways to express the heat released during combus-tion result in two definitions for the thermal efficiency, grossand net.

GTE = UHTGHI

Eq 8-17a

NTE = UHTNHI

Eq 8-17b

Therefore NTE exceeds GTE. There is a tendency to use thegross heating value with the net thermal efficiency eventhough the bases are inconsistent because the numerical val-ues are higher than the corresponding net heating value andgross thermal efficiency.

Fig. 8-17 shows how the gross thermal efficiency can be de-termined from the excess air and stack gas temperature. Es-pecially for insulated heaters or furnaces, the combustionefficiency is close to the gross thermal efficiency. The differ-ence is the heat lost through the walls to the surroundings.

The typical enthalpy of the combustion (or stack) gaseswhen natural gas is burned with 20% excess dry air is shownin Fig. 8-18.

Draft

Combustion air is obtained by natural, forced, and induceddraft. Natural draft uses the buoyant effect of the hot fluegases in the stack to draw air into the combustion zone. Forceddraft is the result of an air blower or fan. Induced draft refersto a blower in the stack.

The draft available (Pa) from a stack is the natural (or static)draft less the frictional and exhaust gas velocity losses. Asdefined below the draft is always negative. The available draftshould be sufficient to overcome the head losses in the air inletregisters, convection section tube coils, baffles, damper, andany waste heat recovery devices. Also, it should contain a mar-gin of safety to allow the damper to be in an intermediateposition to deal with site conditions such as atmospheric pres-sure changes, humidity changes, and temperature changes(daily and seasonal variations). Also, the draft should be suf-ficient to obtain a negative pressure along the entire heaterfire side flow path.

In references 35 and 36 information can be found about pres-sure losses in various devices such as coils, duct transitions,etc. Changes in velocity head should be considered in head losscalculations because of the low densities of the air and fluegas. The absolute roughness of the inner stack depends on thematerial, construction, and lining, if any. The absolute rough-ness may range from 0.3 to 9 mm. The equipment manufac-turer should be consulted for more precise values of stackroughness and for other factors which may influence totalhead loss.

Refer to Fig. 17-2 to determine the Fanning friction factor(ff) using the Reynolds number determined from Eq 8-9a andthe relative roughness (dimensionless), which is the quotientof the absolute roughness and the diameter, both in the sameunits.

Hav = Hs • g (ρg − ρa) + V2 • ρg

2 4 • ff •

Hs

Ds + 1

Eq 8-18

8-

Also

Hav = 34.16 • Hs • PB •

RDg

Tg −

RDa

Ta

+

+ 1.7955 • Tg

PB • RDg • Ds4 •

M10 000

2

• 4 • ff •

Hs

Ds + 1

Eq 8-19

Since ρg is less than ρa, the first term in Eq 8-18, which isthe total available driving force, is always negative. This mustbe decreased by adding the losses from friction and resistancein the second term.

BurnersFour types of burners are commonly used in direct fired

heaters:• Inspirating pre-mix burners. The passage of fuel gas

through a venturi pulls in the combustion air. Theseburners have short dense flames that are not affected bywind gusts.

• Raw gas burners. Some of the air required for combus-tion is pulled in by a venturi. The rest of the air is admit-ted through a secondary air register. These burners havelarger turndown ratios, require lower gas pressures, andare also quieter.

• Low NOx burners. The addition of a tertiary air registerreduces the amount of nitrogen oxides in the flue gas.This type also can be operated with less excess air thanthe above types.

• Combination gas and oil burner. An oil burner isadded to the gas spider so that fuel oil can also be used.One-tenth kilogram of steam per kilogram of fuel is usu-ally required to atomize the oil.

Burner air registers are sometimes used to control the ex-cess air. Ten to fifteen percent excess air is a compromise forbest thermal efficiency and lowest NOx levels. Air leakage intothe heater around sight door openings, header box gaskets,terminal penetrations, etc. should be minimized.

Unmuffled burners have 100 to 110 dBA noise levels. Re-quirements for 85 or 90 dBA noise levels, measured one meterfrom the heater, require noise attenuation plenums and orificemufflers.

Most burners have a continuous pilot flame that releases upto 15kW. The pilot is lit by hand or with a spark plug. The pilotshould be left on when:

• The furnace will not overheat during no-flow conditions.• The fuel is ethane or heavier gases.• The furnace is used intermittently, e.g., regeneration gas

heaters.• The pilot heat release does not affect furnace turndown

ratio. • The refractory must remain dry for fast start-up.• Nuisance shutdowns are unavoidable.

Gas Burner PerformanceSeveral factors influence gas burner performance, such as

the gas pressure, temperature, and composition that affectheating value, gas density and combustion characteristics;also, the aeration, partial heat load, local altitude, etc. Sometypical problems that can occur as a consequence of incorrectburner selection, or from changing operating conditions, or us-ing non-interchangeable gas include flashback, yellow tipping,flame lifting, sooting, and inadequate heat input.

15

The change in gross heat input through a gas burner orificecaused by changes in the operating conditions and gas compo-sition can be estimated with the following equation, where thepressure and temperature are absolute. The equation can alsobe used for the lower heating value. The term in parenthesisis called the Wobbe Index, which is a gas interchangeabilityparameter.

GHI2

GHI1 =

Y2 • √Pg2 • ∆P2

Tg2 •

HHV2

√RD2

Y1 • √Pg1 • ∆P1

Tg1 •

HHV1

√RD1

Eq 8-20

In Eq 8-20 the temperature and pressure are absolute, andthe expansion factor (Y) is a function of the burner nozzle char-acteristics and the fuel inlet and outlet pressures. For low inletgas pressure the expansion factor is approximately 1, and forsmall changes in the fuel gas pressure the expansion factorsY1 and Y2 have similar values and may be ignored. In othercircumstances consult the burner manufacturer for data.

Flue Gas CondensationFlue gas water condensation may produce corrosion prob-

lems caused by acid gases such as SO2 when present in theflue because of the sulfur content of some fuels, and thereforetemperature control above the SO2 dewpoint of flue gasesshould be considered. However, highly efficient fired equip-ment or waste heat recovery equipment burning sweet fuelsand/or with acid resistant duct materials may utilize flue gaswater condensation taking advantage of its latent heat.

NOx Control The main factors influencing NOx formation are flame tem-

perature, excess air in the flame, time in which combustiongases are at flame temperature, and fuels containing nitrogencompounds.

In natural draft heaters the most common means of chang-ing NOx emissions is through the use of low NOx burners. Thepremix and raw gas burners produce NOx levels greater than0.056 g NOx /MJ of burner heat release. These are generallyused when no NOx requirement is specified. The upper end ofthe low NOx burner design uses a partially staged raw gasburner to achieve levels of 0.034 to 0.052 g NOx /MJ. The midrange low NOx burner design uses staged gas to achieve levelsof 0.022 to 0.034 g NOx /MJ. For lower NOx requirementsstaged gas tips and internal flue gas recirculation are com-bined to produce NOx emissions at 0.013 g NOx /MJ and below.These numbers are based on clean fuel gas (not oil burning)and are dependent on firebox temperature and excess air.

A staged-air burner is a low NOx burner in which a portionof the combustion air is injected downstream of the burnerblock to mix with the combustion products and unburned fuelfrom the primary combustion zone.

A staged-fuel burner is a low NOx burner in which a portionof the fuel is mixed with all of the combustion air within theburner block while a second portion of the fuel is injecteddownstream of the burner block to provide delayed combus-tion.

The injection of water or steam is also used to reduce theNOx formation by reduction of the peak flame temperature.

8-

DIRECT FIRED HEATERS

Direct fired heaters vary in size from 0.15 MW small pack-age regeneration gas heaters to 300 MW steam hydrocarbonreformer heaters. In the gas processing industry, the usualrange is 0.3 to 6 MW.

TypesThere are two basic configurations: cylindrical and cabin,

see Fig. 8-19. The simplest design is vertical-cylindrical withonly radiant tubes. The NTE is about 60% and the stack gastemperature is 650°C or more. The burner in the floor firesupward. A stainless steel baffle slows the exit flow of the hotgases and reradiates heat back to the top part of the tubes.There is a short stack that usually has no damper. The designis low cost and suited for low cost fuel.

Adding a convection section improves the NTE to about 80%.The radiant section may be either cylindrical or cabin, and thecoil configuration either helical or serpentine. These heaterscost more than the all-radiant type but they use less fuel forany given duty.

By cooling the combustion gases to about 150°C, the NTEcan be increased to over 90%. This requires either a combus-tion air preheater using exhaust gas or an additional convec-tion section. These units have the highest capital cost andlowest fuel requirement for any given duty.

Design modifications are used when the tube material isexpensive. A bridge wall is installed down the center of thecabin. The radiant tubes are placed above the bridge wall sothat they are, in effect, double fired.

Cylindrical or Cabin?Vertical or Horizontal Tubes?

Cylindrical heaters have the following advantages:• They require the smallest plot area for a given duty.• The cost is usually 10% to 15% lower in the larger sizes.• They can accommodate more parallel passes in the proc-

ess coil.• For large duties, a cylindrical heater has a taller firebox

and more natural draft at the burner.• The flue gas velocity is usually higher in the convection

section, hence, the flue gas film coefficient is higher.• Fewer expensive tube supports or guides are required in

the convection section.• The noise plenums or preheated combustion air plenums

are smaller.• Fewer soot blowers are required in the convection sec-

tion. Soot blowers are not needed for gaseous fuel.• If coil drainage is a problem, a helical coil may be used

when there is only one pass.Cabin heaters have the following advantages:• The process coil can always be drained.• Two-phase flow problems are less severe. (Slug flow can

generally be avoided.)• Cabins can accommodate side-firing or end-firing burn-

ers instead of only vertically upward firing. This permitsthe floor of the heater to be closer to the ground. (Someburner manufacturers prefer to fire liquid fuels horizon-tally.)

• A smaller capital investment is required when the dutyis less than 3 MW.

16

FIG. 8-19

Example Cylindrical and Cabin Direct Fired Heaters

Radiant SectionThe radiant section or firebox should:• Obtain complete combustion of the fuel with a reasonable

amount of excess air, i.e., 10% to 15%.• Contain the flame and avoid impingement on the tubes.• Distribute the radiant heat flux.• Cool the combustion gases to 800-1000°C to protect the

convection section.The proportions of the firebox are the key to good perform-

ance. Generally the flame length should be 60% of the fireboxlength and the clearance between the flame and tubes at least0.5 m. For small cylindrical heaters, the tube circle should beequal to the length of the firebox. For small cabin heaters, thewidth, height, and tube length should be equal. For large heat-ers the height of a cylindrical heater is twice the tube circle,

8

and for cabins a good ratio of width to height to length is 1:2:4.The firebox shell is reinforced steel plate. The insulation be-hind the tubes is usually 125 mm of 1:2:6 lumnite (cement),haydite (aggregate), vermiculite (insulation) castable. Thefloor is at least 150 mm of 1:2:6 castable, often with a firebricksurface. The bridge wall is always firebrick.

By far the most common tube material is A-106B carbonsteel. The nominal size range is 50 to 200 mm. with 75 and100 mm the most prevalent. Short radius return bends arestandard and the tubes are usually 1.5 nominal diametersfrom the refractory wall. For these arrangements, the maxi-mum heat flux directly facing the flame is 1.9 times the aver-age flux. (With long radius return bends the maximum heatflux is 1.45 times the average.) The flux to the front 60° of thetube is 1.8 times the average and the front half-tube flux is 1.5times the average. Any flux maldistribution due to tall narrow

-17

FIG. 8-20

Chart to Estimate the Fraction o f Total Heat Libe ration That is Abso rbed in the Radian t Section o f a Direct Fired Heater

fireboxes or short flames, usually less than 15%, must beadded to this. For double firing, the circumferential maldis-tribution is reduced from 1.8 to 1.25.

Equations 8-21 and 8-22, as well as Fig. 8-20, may be usedto obtain an estimate of the absorbed heat in the radiant sec-tion of a fired heater, expressed as a fraction of the total netheat liberation, in terms of the average heat flux to the tubes,the arrangement of the tubes (circumferential), and the air tofuel weight ratio. These equations are solutions of the Wilson-Lobo and Hottel equation.

B = 0.317 • do • n • π • I • G2

S • aEq 8-21

R = 1 − √ (B2 + 70.56 • 106 • B) − B

35.28 • 106 Eq 8-22

where "a" is a constant depending on the arrangement oftubes. The "a" value is:

Tube spacingNo. of rows 2 • NPS 3 • NPS

1 0.88 0.732 0.99 0.91

The total heat liberation consists of the lower heating valueof the fuel and the sensible heat in combustion air, recirculated

8

flue gas, and fuel and atomizing steam, all heat contents re-ferred to a datum of 15°C.

Fig. 8-20 provides an estimate of the fraction of the totalheat liberation that is absorbed in the radiant section in termsof the allowable heat flux to the tubes. The kg air/kg fuel firedis needed and this can be obtained from either Fig. 8-21 if theLHV of the fuel is known, or by stoichiometry.

Fig. 8-20 is for fired heaters with one row of 200 mm NPSpipes, spaced two pipe nominal sizes (NPS). Correction factorsfor other designs, to be multiplied by G prior to graph reading,are shown in the figure.

Example 8-8 — Estimate the radiant tube area for a 3000 kWregeneration gas heater. To avoid overheating the tubes, a ra-diant flux of 30 000 W/m2 is specified. The design calls for100 mm NPS Sch 80 tubes on a 2400 mm tube circle. The fuelis 0.61 relative density gas with LHV of 37 260 kJ/m3. Use 20%excess air.

Fuel gas and combustion air are supplied at 15°C. The heaterNTE is 80%. The tubes are arranged in one row at 200 mmspacing.

Solution Steps

r = 1500 kg flue gas /(MW • hr) (Fig. 8-21).

-18

= 417 kg flue gas/( kJ •106 )

r • LHV = 417 • 37 260 = 15.537 kg flue gas /m3 fuel gas

Mass of 1 m3 fuel gas = (1)(0.61)(29)

23.68 = 0.747 kg

Mass of combustion air = 15.537 – 0.747 = 14.79 kg

G = 14.79/0.747 = 19.8 kg air/kg fuel

Correction factor for 100 mm tubes is 1.02 (Fig. 8-20)

G corrected = 19.8 (1.02) = 20.0 kg air/kg fuel

R = 0.535 (Fig. 8-20)

Eq 8-21

B = 0.317 • do • n • π • I • G2

S • a

B = (0.317)• (114.3) • (1) • (3.14) • (30 000) • (19.82)

(200) • (0.88)

B = 7.603 • 106

Eq 8-22

R = 1 − √ (B2 + 70.56 • 106 • B) − B

35.28 • 106

FIG. 8-21

Flue Gas Rates9

R = 1 − √[(7.603 • 106)2 + 70.56 • 106 • (7.603 • 106)] − (7.603 • 106)

35.28 • 106

R = 0.525 (Note: Fig. 8-20 yields R = 0.535, so use the average of R = 0.530.

Q = UHT • RNTE

= 3000 • 0.5300.80

= 1988 kW

Radiant heat transfer area = Qr

I =

1988 • 100030 000

= 66.3

The surface area of 100 mm NPS pipe is 0.359 m2/m

Total tube length = 66.30.359

= 184.7 m

There are 37 vertical tubes in a cylindrical heater with a2400 mm diameter tube circle when the tubes are 200 mm cen-ter to center.

Tube length = 184.7

37 = 4.992 m

Convection SectionThe purpose of the convection section is to transfer as much

heat as possible from the combustion gases leaving the radiantsection. As always there is the trade-off between capital cost,i.e., adding more tubes, and operating cost, i.e., improved ther-mal efficiency.

FIG. 8-22

Flue Gas Convection-Coefficients for Flow AcrossStaggered Bank s of Bare Tubes 9

8-19

The construction is similar to that for the radiant section, asteel plate shell with internal castable or ceramic fiber insu-lation. The tubes are staggered, and the space between thesidewall and the tube is filled with “corbels” to prevent the fluegases from bypassing the end tubes.

The first two rows of the convection section are called shocktubes and they “see” the firebox flame. The first row receivesthe full radiant heat flux and also some convective heat trans-fer. It has the highest heat transfer flux in the heater and isalways bare tubes. The second shock row receives about one-third of the radiant flux as well as convective heat transferfrom the flue gas. It is also bare tubes. If long radius returnbends are used, the third row will receive radiant heat and ittoo should be bare tubes.

Helical fins, sometimes serrated to increase turbulence, areused as soon as possible, i.e., when the fin tip temperature isnot excessive, e.g., 540°C for carbon steel, see Fig. 8-8. Typi-cally when natural gas is the fuel, the fins are 25 mm high,1.5 mm thick and up to 240 fins per linear meter. For oil firedheaters where soot deposition is possible, the fins are 25 mmhigh, 2.7 mm thick and not more than 120 fins per meter.Often the first finned row has fewer, shorter, and thicker finsto reduce the fin tip temperature. Where ash and soot foulingare expected, a lane is left every four or five rows for soot blow-ers. These are tubes equipped with nozzles that direct steamagainst the tubes. Soot blowing is intermittent and is seldomused more than once every shift.

The fins compensate for the low flue gas heat transfer coef-ficient. Typically, the heat flux in the convection section is 6.3-12.6 kW/m2 of finned surface or 38-76 kW/m2 on a bare tubebasis.

Cast iron tube supports can be used below 425°C and 25%chrome – 12% nickel is good up to 1100°C. With high vana-dium or sodium levels in the fuel oil, 50% chrome – 50% nickelmust be used.

The distance between supports for horizontal tubes shouldbe the lesser of 35 outside tube diameters or 6 m. The distancebetween supports on vertical tubes should not exceed either70 tube diameters or 12 m. Usually the return bends are ex-ternal to the tube sheets. This prevents flue gases from by-passing the tube fins.

Fig. 8-22 shows approximate external heat transfer coeffi-cients for 75, 100, and 150 mm NPS. steel pipe arranged instaggered rows and surrounded by combustion gases.Example 8-9 — Design the convection section for the 3000 kWregeneration gas heater of Example 8-8. The heat loss is as-sumed to be 2% of the heat release. Use six 100 mm NPSSch 80 tubes on 200 mm center-to-center spacing with2400 mm effective length in each row. After two rows of bareshock tubes use finned pipe, 118 fins/m, 32 mm high, 2.7 mmthick. Assume pipe wall temperatures of 90-240°C across thefinned part of the convection section and average values of 250and 260°C for the two shock rows.

Solution StepsFig. 8-23 summarizes the design of both the radiant and con-

vection sections. A trial and error solution for assumed tem-peratures is required. Details follow for the convergedsolution.

Qtotal = duty/GTE = 3000/0.80 = 3750 kW

r = 1500 kg flue gas/(MW • hr) (Fig. 8-21)

Flue gases flow rate = 3.75(1500) = 5625 kg/hr

8-

Assume that the setting loss of 2% or 75 kW occurs in theradiant section.

The heat content rate of the combustion gases leaving radiantsection:

Qradiant exit = 3750 –1988 – 75 = 1687 kW = 6073.2 (103) kJ/hr

The enthalpy of the exit gas from radiant section:

H = 6073.2 (103)/5625 = 1080 kJ/kg

Tg = 918°C (Fig. 8-18, Flue Gas – LHV)

Convection Section:

Area for gas flow = (no. of tubes) (L) (spacing − D)

= (6) (2.4) (0.2 − 0.114) = 1.24 m2

Gg = 5625/(1.24) (3600) = 1.23 kg/(s • m2)

First shock row. Assume the average gas temperature is885°C and tube wall temperature is 260°C.

Tg mean = 260 + 885

2 = 573°C

ho = 21.6 W/(m2 • °C) (Fig. 8-22)

A = 0.359 m2 per linear m (Example 8-8)

Atubes = 14.4 (0.359) = 5.17 m2

Qc = ho A ∆T = (21.6) (5.17) (885 − 260)

= 69.795 kW

Flux = Q/A = 30 000 W/m2 (Example 8-8)

Qr = (Q/A) (A) = 30 000 (5.17) = 155.1 kW

Qc + Qr = (69.795 + 155.1) = 224.9 kW

Qexit gases = (1687 – 224.9) = 1462.1 kW

Hexit gases = (5263.6 (103) kJ/hr)/5625 kg/h = 935.7 kJ/kg

Tg exit = 820°C (Fig. 8-18, Flue Gas – LHV)

Second shock row is analogous except that the radiantheat flux is one third of that for the first row, i.e.,10 000 W/m2

Qr = (10 000) (5.17) = 51.7 kW

With ho = 21 W/(m2 • °C)

Qc = (21) (5.17) (796 – 250) = 59.3 kW

Qc + Qr = 59.3 + 51.7 = 111.0 kW

Qexit gases = 1462.1 – 111 = 1351.1 kW

Hexit gases = 4864 (103) / 5625 = 864.7 kJ/kg

Tg exit gases = 762°C (Fig. 8-18, Flue Gas – LHV)

Finned rows. The combustion gas mass velocity increasesbecause of the increased cross sectional area of finned pipe.From Eq 8-13.

Acs = 1141000

+ 118 (2.7) (178 − 114)

106 = 0.134 m2/linear m

Gg = 5625

[14.4(0.2 – 0.134)(3600)] = 1.64 kg/(s • m2)

20

FIG. 8-24

Natural Draft Profiles

SectionGas Heat Content Rate

kW (LHV) Heat TransferkW

Exit GasEnthalpy

kJ/kg

Exit GasTemperature

°CIn OutRadiant 3750 1687* 1988** 1080 918Shock Row 1 1687 1462.1 224.9 935.7 820Shock Row 2 1462.1 1351.1 111 864.7 762Finned 1351.1 675 676.1 432 395

* Heat losses are 75 kW ** From Example 8-8

FIG. 8-23

3000 kW Regeneration Gas Heater

8-21

Qf = 3000 – 1988 – 224.9 – 111 = 676.1 kW

Q(exit) = 3750 – 3000 – 75 = 675 kW

Hg(exit) = 2.43 (106)/5625 = 432 kJ/kg

Tg(exit) = 395°C (Fig. 8-18, Flue Gas – LHV)

Assuming that HHV is 10% more than LHV, the gross heaterefficiency is 80% / 1.1 = 72.7%. Note that this agrees closelywith Fig. 8-17 for 20% excess air and 390°C.

Pipe and gas temperatures are:

Tp1 = 90°C; Tp2 = 240°C; Tp av = 165°C

Tg1 = 762°C; Tg2 = 395°C; Tg av = 578.5°C

∆TLM = [(762 − 240) − (395 − 90)]/ ln (522/305) = 405°C

Tfilm = (165 + 578.5)/2 = 372°C

ho = 25 W/(m2 • °C) (Fig. 8-22)

Qf = ho Ao ∆TLM (Eq 8-10)

Ao = 676.1 (103 )/[25 (405)] = 66.78 m2

This is the theoretically required surface area. From Exam-ple 8-5, the fin efficiency is 87% and the external surface areaof the finned pipe is 3.676 m2/linear m.

Lpipe = 66.78/[(0.87) (3.676)] = 20.88 m

With 14.4 linear m per row this is 1.45 rows. Use 2 rows.

Stack DraftThe stack draft must overcome the gas friction loss in the

convection section, burner, and stack. The stack diameter isoften sized for 4.6 to 6.1 m/s stack gas velocity. The stack isnormally bare carbon steel but must be lined if the flue gastemperature exceeds 400°C or if the fuel has high sulfur con-tent. All wall temperatures should be above the dew point ofthe flue gas.

Typical draft profiles for direct fired natural draft heatersare shown in Fig. 8-24. There are two ways to control the flowof combustion air: stack dampers or combustion air registers.There should be a slight vacuum in a natural draft heater toprevent leakage of the flue gases. There is usually an increasein pressure of 8 Pa per meter of firebox height and several mmpressure drop across the convection section.Example 8-10 — Find the available draft in a 0.6 m ID by 6 mlong stack attached to the top of the convection section for the3 x 106 W regeneration gas heater of examples 8-8 and 8-9.Assume dry air at 15°C and 101.4 kPa.

Solution Steps

Assume µg = µa from Fig. 23-21

From Eq 8-9M = 5625 kg/hr

D = 0.6 m

µ = 0.024 mPa • s

Re = 0.3537 • M

D • µ

Re = 0.3537 • (5625)

0.6 • (0.024)

8

Re = 138 164For an absolute roughness of 0.6 mm, the relative roughnessis 0.001 then

f = 0.005 (From Fig. 17-2)From Eq 8-18

Hav = Hs • g (ρg − ρa) + V2 • ρg

2 4 • ff •

Hs

Ds + 1

Calculate ρg and ρa from the ideal gas law.

ρg = 0.746 ρa = 1.227

V = M

π • D2

4 • ρg

V =

56253600

π • 0.62

4 • 0.746

V = 7.408 m/s

Hav = 6 • 9.8067 • (0.746 − 1.227)

+ 7.4082 • 0.7462

• 4 • 0.005 • 6

0.6 + 1

Hav = −3.739 Pa

InsulationInsulation protects the heater shell from the hot combustion

gases and usually reduces the heat losses to less than 2% ofthe heat release. Three common types of insulation are:

LHV Castable Refractory — This is a mixture of lum-nite (cement), haydite (aggregate), and vermiculite (insula-tion) in 1:2:6 proportions. This low cost, concrete-typeinsulation has a density of 880 kg/m3 a low coefficient ofexpansion, and negligible shrinkage. It is held to the verticalwalls with bullhorns — V-shaped steel wire anchors welded tothe outer casing. If a high sulfur (more than 1%) fuel is burned,to prevent the sulfur trioxide in the flue gas from attackingthe iron in the lumnite, use a low-iron (1%) cement. LHV cas-table withstands high (45-60 m/s) gas face velocities. Rain orsnow entering through the stack will not hurt the concrete.Before start-up, cure and dry out the refractory to avoid “spall-ing” or flaking caused by unequal thermal expansion.

Ceramic Fiber — A sandwich construction of this mate-rial in two densities is sometimes used. Because the ceramicfiber is porous, a protective coating is normally applied firstto the steel casing, for protection against sulfur oxides in theflue gas. A 50 or 75 mm layer of 64 kg/m3 ceramic fiber is im-paled over thin stainless steel studs welded to the casing. Overthis is placed a 25 mm layer of 128 kg/m3 material. Stainlesssteel washers twisted onto the studs hold the ceramic fiber inplace. This light weight material is much lighter than LHVand is suitable for convection sections. Advantages includeease of application in freezing weather, when water in refrac-tory is a problem; and no need to dry out or cure the ceramicfiber when it is first applied. For cyclic operations, less heat isstored in the fiber which reduces warm-up time. Rain or snowentering the heater during shutdown may drench the fiber,causing it to tear away from the studs on a vertical wall. Itcannot withstand gas face velocities above 15 m/s. If soot blow-

-22

FIG. 8-25

Example Direct Fired Reboiler

ing or steam lancing of finned tubes is required, LHV castablerefractory must be used or the fiber must be covered with a3 mm stainless steel erosion shield. The erosion shield is alsoneeded for gas turbine exhaust heat recovery units.

Insulating Firebricks (Ifb) — These are used forbridgewalls and floors. Ifb are quite dense, ca. 2400 kg/m3, areshipped loose, and field installed. They must be dried outslowly. After proper installation, firebricks are sturdy and re-sist deterioration from weather and high gas velocities.

External Insulation — In sulfur plants and in cold cli-mates, external insulation is frequently used to maintain aminimum stack temperature to prevent condensation withinthe stack.

Other Design ConsiderationsFilm temperature — Temperatures above 260°C will

cause many hydrocarbons to decompose and coke layers to de-posit in the tube. This increases the film and metal tempera-tures and leads to tube failures. Actual decompositiontemperature is highly dependent upon the fluid charac-teristics.

Snuffing — If a tube ruptures, fire could break out in thefirebox. Connections are needed to admit CO2 or a steam orwater spray to snuff out the fire. The velocity of this vaporshould be kept below 24 m/s to avoid erosion of the refractory.

Purging — If the pilot flame or electric spark fails, thefirebox may contain an explosive mixture of gases. Before at-tempting to relight the heater, this mixture must be purged.

Sampling — To control the excess air, samples of flue gasare needed from various points of the heater. The amount of

Note: Alarms and shutdowns as shown are not to be considered asentative of types used for control systems.

Basic Criterion – The failure of any one device will not allow the hea

SchematicLabel

Alarm/ShutdownDescription

TSH-1 High Stack TemperatureTSH-2 High Outlet TemperatureFSL Low Mass Flow Through TubesBSL Flame Failure DetectionPSL Low Fuel PressurePSH-1 High Fuel PressurePSH-2 High Cabin Pressure

Notes:1. A direct immersion jacketed thermocouple is preferred because the respo

system is a poor third choice. The high stack gas temperature shutdown2. An orifice plate signal should be backed up by a low pressure shutdown 3. The measurement should be on the heater inlet to avoid errors due to tw4. Differential pressure switches mounted directly across an orifice plate a

transmitter with a pressure switch on the output is recommended. The be readily compared with the shutdown point.

5. The flame scanner should be aimed at the pilot so that a flameout signal6. If the heater design precludes flame scanners, a low fuel gas pressure sh

should detect gas pressure at the burner.7. Either burner pressure or fuel control valve diaphragm pressure may be

expected. It prevents the heater from overfiring when the temperature insufficient air.

8. This shutdown should block in all lines to the heater because, when actithe heater.

FIG.

Heater Alarm/Shutd

8-

excess air at the burner may differ from that in the stack dueto leakage of ambient air through terminal holes, box gaskets,cracks in the casing, etc.

Flue gas temperature — To determine the thermal effi-ciency of the heater, the stack temperature must be monitored.

s meeting any minimum safety requirement but are shown as repre-

ter to be damaged.

RegenerationGas Heater

Hot Oil Heater and DirectFired Reboiler

See Note 1 See Note 1See Note 1See Notes 2 & 4 See Notes 3 & 4See Note 5 See Notes 5 & 6

See Note 6See Note 7 See Note 7See Note 8 Not applicable, if natural draft

nse is ten times faster than a grounded thermocouple in a well. A filled bulb should be set approximately 110°C above normal operation.to ensure adequate process stream flow under falling pressure conditions.o-phase flow.

re not satisfactory due to switch hysteresis. An analog differential pressureanalog signal should be brought to the shutdown panel so that the flow level can

will be generated if the pilot is not large enough to ignite the main burner.utdown should be installed to prevent unintentional flameout. This shutdown

used. This shutdown should be used whenever large load changes arecontroller drives the fuel valve wide open to increase heat output with

vated, a tube has probably ruptured. Gas is probably burning vigorously outside

8-26

own Description

23

Process coil thermowells — The firing rate is control-led to maintain the correct process stream outlet temperature.

Draft gauges — These are needed to set the stackdamper or burner air registers.

Soot blowers — In liquid fuel systems these can be usedintermittently to reduce fouling of the finned tubes.

ControlsFig.8-25 shows an example control system for a fired heater.

Fig.8-26 lists the shutdown/alarms as shown with some com-ments about the proper installation and use of these particulardevices. The control system as depicted by these figures shouldnot be considered complete but only representative of the con-ditions to be carefully considered in designing a control systemfor fired equipment.

The following are some indications of possible trouble:• The burner flame is not symmetrical, pulsates or breathes,

is unusually long or lazy, lifts off the burner, etc.• The burner is not aligned and/or the flame is too close to

the tubes.• Lack of negative pressure at the top of the firebox.• The stack gas is smoky.• The gas in the firebox appears hazy.• There are unequal temperatures, more than 6°C differ-

ence, on the process pass outlets.• The stack temperature increases steadily with no change

in the process heat duty.

FIG.

Convecti

8-

• The fuel gas control valve is wide open.• The fuel gas composition or pressure varies widely.• The tubes in the heater are not straight.

Options to Improve the Thermal Efficiency

Option I. Add Convection Surface

Effects:1. Stack temperature is reduced.2. Furnace efficiency is increased.3. Heat release is decreased.4. Flue gas pressure drop in the convection section is in-

creased.5. Draft is decreased.6. Tube side pressure drop is increased.7. NOx is reduced.

Things to consider:1. Increasing stack height.2. More weight from added convection tubes.3. Check structure and foundation to see if added weight

can be supported.4. If not, design adjacent structure to house convection

tubes and support stack.5. Consider increased pumping cost for process stream.6. If fuel is to be changed, some existing convection tubes

may have to be removed to accommodate soot blowers.

8-27

on Heater

24

FIG. 8-28

Water Bath Indi rect Heater

Option II. Add Economizer for Waste Heat Recovery

Waste heat options:Steam generationSteam superheatingBoiler feedwater heater

Effects:1. Stack temperature is reduced.2. Furnace efficiency is increased.3. Flue gas pressure drop in the convection section is increased.4. Draft is decreased.5. No change in process stream operation.6. No change in NOx.

Things to consider:1. Study increased load on structure and foundation as in

Option I.2. Will added boiler capacity lower efficiency of existing

boilers?3. Check possibility of a temperature cross.

Option III. Install Air Preheat SystemEffects:

1. Stack temperature is reduced.2. Natural draft is decreased.3. Furnace efficiency is increased.4. Firebox temperature is increased.5. Heat flux rates are increased.

8

6. NOx increases unless burners are changed.

Things to consider:1. Induced draft and forced draft blowers must be installed.2. Burners must be replaced.3. Check if tube supports and refractory will withstand

higher temperatures.4. Plot space near furnace must be available.5. System is self-contained.6. Considerably more instrumentation must be installed.

Convection HeatersHeaters in which all the heat transfer is convective (there is

no radiant section) are unique modifications of direct firedheaters. Because all aspects of the operation, e.g., fuel com-bustion, combustion gas temperature, tube wall temperature,etc., are controlled, these heaters are ideally suited for offshoreplatforms and other applications that demand a high degreeof safety.

Fig. 8-27 is a sketch of a convection heater, with recycleregulating the heat flux. The cylindrical steel shell is inter-nally insulated with light castable or ceramic fiber, and thecombustion and heat exchange sections are separated by aninsulated wall. The fuel gas and all of the combustion air arefed to the short-flame, pre-mix burners. The combustion gasesmix rapidly with recycled stack gases to produce the inlet gasto the heat exchange section. This gas at 650-815°C flows un-der the dividing wall and then upward across the finned tubesof the process coil. Part of the cooled gas at 175-260°C leavesas stack gas and the remainder is recycled. The flow in theprocess coil is downward and counter-current to the hot gases.

-25

FIG. 8-29

Methane Pressure-Enthalpy Diagram

Recycling some of the stack gas controls the inlet gas tem-perature to the heat exchange section, and in turn the maxi-mum film temperature of the process stream. The combinedvolume of the combustion and recycled gases is much greaterthan that in most direct fired heaters. This results in highergas velocities, higher external heat transfer coefficients, anda smaller process coil surface area. The thermal efficiency ishigh because pre-mix burners operate satisfactorily at 10%excess air and the stack gas temperature can be as low as 38°Cabove the inlet process stream temperature. The price paid forthis is the electricity to run the recirculation fan; this is usu-ally more than offset by the fuel saving.

Heater Bath Temp.°C

Outside CoBundle hoW/(m2 • K)

Water Bath 82–91 910 50 % Ethylene Glycol 91–96 650 Low Pressure Steam 118–121 5680 Hot Oil 149–288 230 Molten Salt 204–427 1135 TEG Reboiler 177–204 – Amine Reboiler 118–132 –

FIG.

Typical Bath Propertie

8

FIRETUBE HEATERS

Firetube heaters range in duty from 17.6 kW glycol reboilersto 3500 kW oil or gas pipeline heaters. The design, controls,and operation of firetube heaters vary widely from those usedin simple, unattended “wellhead” equipment to those used incomplex, well-instrumented, gas plants.

Water Bath HeatersFig. 8-28 is a sketch of an indirect fired water bath heater.

This design is typical of all indirect fired vessels. The firetubeis in the bath in the lower half of the vessel and the processcoil is in the upper half of the vessel. The heat transfer me-dium, in this case water, fills the vessel. A fill hatch, drain,wells for thermostats, and a coil to preheat the fuel are stand-ard. Sizes range from 600 mm OD by 1.5 m long to 3600 mmOD by 9 m long. Only one firetube is needed below 1465 kW.

Some small heaters are uninsulated. More frequently 25 to50 mm of weatherproof ceramic fiber or light castable insulationis placed around the cylindrical shell. The ends are left bare soas not to impede access to the coil, burner, or stack. As shown inExample 8-7, insulation can reduce heat losses up to 75%.

A long lazy yellow flame increases the fire tube life and in-creases the radiant flame area.

Almost every coil bundle requires a unique design to meetthe requirements of heat duty, working pressure, corrosion al-lowance, sour gas service, NACE MR-01-75, and governingcodes, usually ASME Section VII or ANSI B31.3. Use of a P-Hdiagram will simplify calculations when both sensible and la-tent heat changes occur in the process stream.

Example 8-11 — What heat duty is required to vaporize10 m3/hr of liquid propane at 15°C and 1600 kPa (ga) and su-perheat the vapor by 10°C?

Solution Steps

Refer to Section 24 “Thermodynamics” and the P-H diagramfor propane. At 15°C and 1701 kPa (abs) the enthalpy of pro-pane is 547 kJ/kg. The exit enthalpy at 61°C (10°C above thedew point) and 1701 kPa (abs) is 950 kJ/kg. The liquid densityis 510.2 kg/m3.

ql = 10m3/hr

M = (10) (510.2) = 5102 kg /hr

UHT = (M / 3600) (H2 − H1)

il Firetube FluxQ/A

kW/m2

Stack Temp.°C

FiretubeEfficiency

NTE %

32–41 400–480 76–82

25–32 425–480 76–80

47–57 425–480 76–80

19–25 480–590 71–76

47–57 535–650 68–74

19–25 425 75–80

21–32 480 75–80

8-30

s for Firetube Heaters

-26

FIG. 8-31

103 kPa (ga) Steam Bath Heater

= (5102 / 3600) (950 − 547) = 571.1 kW

Usually long radius return bends connect the passes in theserpentine coil; however, short radius return bends sometimes“fit” the coil bundle into the shell. The heat duty and pressuredrop determine the pipe diameter and the number of parallelflow paths and passes in the process coil. Often the internalprocess stream heat transfer coefficient is much larger thanthe external water bath coefficient, e.g., Example 8-4. Twoparallel flow paths of four passes may be an alternative heattransfer design to one flow path of eight passes. The effect onthe process stream pressure drop is significant. Because thepressure drop for turbulent flow is proportional to the 1.83power of the velocity and the pipe length per pass has beenhalved, the two flow path pressure drop approaches one sev-enth of that for one pass.

In many wellhead applications the coil contains a choke thatthrottles the gas from well pressure to processing or pipelinepressure. This divides the coil into preheat and postheatpasses. The rapid expansion is isenthalpic and, if the gas tem-perature falls too low, hydrates form.

Example 8-12 — Find the optimum distribution between pre-heat and postheat duty for expanding methane from 20 000kPa (abs) and 25°C (see point C in Fig. 8-29) to 7000 kPa (abs)and 35°C.

Solution Steps

Refer to Fig. 8-29 which is a P-H diagram for CH4 on whichthe line AGB for hydrate formation is superimposed. (This isa combination of Figures 20-15 and 24-23.) Consider the fol-lowing three alternative paths.

8

Path CDE expands the gas immediately and then heats it.This results in the smallest coil area because the largest logmean temperature difference between the water bath and themethane is available. But the expansion crosses the hydrateline and the gas will freeze.

Path CHE supplies all the heat needed and then expandsthe gas. This is feasible, but not desirable because the lowesttemperature difference requires the largest coil area.

Path CFGE first heats the methane so that the expansionjust touches the hydrate line, expands it, and heats to the exittemperature. This is the minimum coil area that correspondsto an operable path. So:

Preheat duty: (970 – 900) = 70 kJ/kg

Postheat duty: (1040 – 970) = 70 kJ/kgIn practice, some penetration of the hydrate line is possible;

and the balance between “preheat” and “postheat” passes is suchthat the lowest temperature is 5 to 8°C below the hydrate line.

Freezing of the water bath is a potential problem. If theheater is insulated, a continuous pilot suffices. Several differ-ent antifreeze additives have been tried and all have short-comings:

• Methyl alcohol is volatile and has to be replenished. It isalso a fire hazard.

• Calcium chloride and rock salt in concentrations that areeffective are very corrosive.

• Glycols are generally accepted as the safest and mosttrouble-free additive. The decomposition products areacidic; it is recommended that corrosion and rust inhibi-tors be used concurrently.

-27

FIG. 8-32

Typical Physical Properties of Hot Oil

Insulation Drain

Section "B" - "B"Tubesheet Detail

Stack

ExpansionJoint

ThermostatConn.

BurnerFront

Fill Conn.

Inlet

TubeBundle

Outlet

Section "A" -"A"

FIG. 8-33

Salt Bath Heater

8-28

LLC

LeanAmine Outlet

Weir

VaporOutlet

FillConn.

ReliefValve Conn.

Press.Gauge Conn. Press.

Conn.

LLSD

Stack

BurnerFront

RemovableFiretube

Rich AmineInlet

Drain

Firetube

Gauge Glass Conn .Water Level

FIG. 8-34

Amine Reb oiler

Glycols reduce the heat transfer coefficient of the bath signifi-cantly. For example, a 50% by weight solution of ethylene glycolreduces the firetube flux by 20% and the external bath heat trans-fer coefficient for the process coil by 40%. (See Fig. 8-30)

Fig. 8-30 compares the bath properties of fire tube heaters.The advantage of using water as the heat transfer medium isapparent. The relatively low bath temperature results in thelowest stack temperature and the highest firetube efficiency.Note that the firetube efficiency does not account for heatlosses. For large well insulated heaters the overall processNTE may exceed 80%. But for small uninsulated heaters withintermittent operation the process NTE may be as low as 60%.

Low Pressure Steam HeatersWhen a process stream temperature of 70 to 100°C is

needed, a 104 kPa (ga) steam heater can be used to reduce therequired size of the tube bundle. Construction, shown inFig. 8-31, is under ASME Section IV code. Steam outlet andcondensate return connections are standard so that the steammay be used in external exchangers if desired. The condensingsteam has an external process coil heat transfer coefficient of4540-6800 W/(m2 • K). It is important to vent all air at start-up.With insulation and controls, the NTE can be as much as 80%,which is close to the efficiency for a firetube.

Hot Oil HeatersThese heaters furnish a heating bath to 315°C or higher,

which is hot enough for dry desiccant or hydrocarbon recoveryregeneration gas. Another less severe application is heavierhydrocarbon vaporization prior to injection into a gas pipelineto raise the heating value.

8-

Manufactured heat transfer oils are “blended” for about a110°C operating range. For example, Fig. 8-32 gives typicalheat transfer properties for a 150 to 290°C polyphenyl ether.

The advantages of hot oils are:

• Low vapor pressure at ambient temperature• Always liquid and easy to handle• Blended for a specific temperature range• Higher specific heat than normally occurring hydrocar-

bons

The disadvantages include:

• Escaping vapors are environmentally undesirable• Low heat transfer properties, see Fig. 8-30. (The firetube

flux is half that of a water bath heater and the externalprocess coil heat transfer coefficient is about one quarter.)

• Usually an ANSI Class 300 flange design is required• When overheated, the oils will oxidize and coke on the

firetube. Also, they can be ignited• Ethers are expensive• The ethers are hygroscopic and must be kept dry

Molten Salt HeatersMolten salt baths operate at 200 to 425°C. They are mix-

tures of sodium nitrate and the nitrites of sodium and potas-sium. The advantages include:

• Good heat transfer properties, see Fig. 8-30. The firetubeflux is as high as that for a low pressure steam heater.Note the process coil heat transfer coefficient of

29

1135 W/(m2 • K) is partially due to the small diametertubes that usually comprise the coil bundle.

• Thermally stable to 540°C.• The salt will not ignite.

The disadvantages are:• More difficult to handle, large lumps must be broken up.• The salt is hygroscopic. If wet, it decrepitates on melting

and is hazardous.Because of the higher operating temperature there are no

flange joints on the shell except for the salt loading hatch, seeFig. 8-33. On large heaters the expansion of the firetube maybe sufficient to warrant an expansion joint for the stack. Theshell is insulated to protect personnel.

One of the requirements for heating regeneration gas is avery low pressure drop across the coil bundle. To realize thisand still obtain good heat transfer, the coil bundle consists ofmany, small diameter, U-tubes in parallel. These are weldedinto a tube sheet that is attached to a channel header. To re-duce thermal stresses caused by the cold inlet gas, the inlethalf of the tube sheet is insulated.

Direct Fired ReboilersGas treating and dehydration frequently employ direct fired

reboilers. More detailed process descriptions are given in Sec-tions 20 and 21. Fig. 8-34 is a sketch of a typical direct firedreboiler. The rich fluid containing the sour gas or water isboiled in the reboiler to remove the sour gas or the water. Thelean fluid then is used to treat or dehydrate the process gasstream again. Surge tanks for the lean fluid may be integralwith the reboiler as shown, or may be mounted as a separatevessel beneath the reboiler.

Firetubes, Burners, StacksFiretubes typically range from 150 to 750 mm ID and from

1525 to 9140 mm long. Normally the burner flame extendshalfway down the first leg. A mitered joint return bend is usedto reduce the resistance to flow of the combustion gases.Example 8-13 — What is the firetube flux when the combus-tion gases are at 1540°C and the firetube wall is at 150°C.Assume that the fuel is natural gas and the heat release is1.2 MW with 10% excess air in a 0.6 m ID pipe.

Solution StepsRefer to example 8-6. The radiant heat flux is 53.27 kW/m2.

The convective heat flux must be added.r = 1400 kg / MW heat release (Fig. 8-21)

Combustion gas flow is (1.2)(1400) = 1680 kg/hr

From Eq 8-7, 8-8b and 8-9a and Fig. 8-5

Nu = 0.023 • Re0.8 • Pr0.33 •

µb

µw

0.14

(Fig. 8−5)

h • Dk

= 0.023 • Re0.8 • Pr0.33 •

µb

µw

0.14

k = 0.066 W/(m • °C) at 950°C

Cp = 1.04 kJ/(kg • °C)

µb = 0.045 mPa • s

µW = 0.023 mPa • s

8-

h = 0.023 • Re0.8 • Pr0.33 •

µb

µw

0.14

• kD

= 0.023

(0.6)(1680)

π 0.62

4 0.0451000

3600

0.8

•

(1.04)(0.045)0.066

0.33

0.0450.023

0.14

0.0665

0.6

= 7.45 W/(m2 • °C)

Q/Ac = 7.45 (1540 - 150) = 10 355 W/m2

Total heat flux = 53 271 + 10 355 = 63 626 W/m2

This is a maximum firetube flux and is typical only for waterbath or low pressure steam heaters.

Even though firetube failure is rare, it is advisable to pre-vent movement or flexing with restraining bars. This preventsweakening of the weld joints at the end plate. In addition,when the fuel is oil, include a drain with plug in the bottom ofthe firetube leg between the end plate and the burner flange.Then the firetube can be drained if any oil accidentally getspast the burner.

Inspirating partial pre-mix burners are used in the vast ma-jority of firetube heaters. The gas is preheated before expan-sion, flow control, and flow through the burner. While theburner draws the primary air into the firetube, it is the stackdraft, usually less than 25 mm H2O, that overcomes the pres-sure drop of the combustion gas flow and admits the secondaryair. The stack height is 3 to 6 m.

An alternative to the burner front shown in the equipmentsketches is a flame arrestor, the element of which providesmany small tortuous paths between rolled sheets of thin cor-rugated aluminum. Sucking the combustion air through theelement is an additional pressure drop for the stack draft toovercome. The term arrestor is somewhat of a misnomer be-cause any fire in the firetube or around the burner is containedrather than extinguished. The passage through the arrestorcools the gas so that external combustion does not occur.

ControlsFig. 8-35 shows an example control system for an indirect

fired heater. Fig. 8-36 lists the shutdown/alarms as shownwith some comments about the proper installation and use ofthese particular devices. The control system as depicted bythese figures should not be considered complete but onlyrepresentative of the conditions to be carefully considered indesigning a control system for fired equipment. The controlsprobably vary more than the design of the heater. For example,a wellhead line heater or glycol dehydrator may have no morethan an on-off thermostat for the main burner and a smallcontinuous pilot. A line heater, hydrocarbon vaporizer, oramine reboiler may have all of the controls listed in Fig. 8-36.

TroubleshootingThe following problems can occur with firetube heaters.• Bath level loss can be the result of too high a bath tem-

perature. This is often caused by the temperature con-troller on the process stream. Fouling of the process coil,internal and/or external, means a hotter bath is neededto accomplish the same heat transfer. The coil should beremoved and cleaned.

30

Note: Alarms and shutdowns as shown are not to be considered as meeting any minimum safety requirement but are shown as repre-sentative of types used for control systems.

SchematicLabel

Alarm/ShutdownDescription

LineHeater

HydrocarbonReboiler

Low PressureSteam Heater

Hot Oil orSalt Heater

GlycolReboiler

AmineReboiler

TSDH-2 High Bath Temperature NoNote 1, 2

Yes No Yes Yes Yes

LSL Low Bath Level NoNote 2

No Yes YesNote 3

NoNote 2, 3

YesNote 2, 3

PSL Low Fuel Pressure YesNote 4

Yes Yes YesNote 4

Yes Yes

PSH High Fuel Pressure YesNote 4

Yes Yes YesNote 4

Yes Yes

BSL Flame Failure Detection NoNote 2, 5

YesNote 5

NoNote 2

YesNote 5

NoNote 2

NoNote 2

PSH High Vessel Pressure No No YesNote 6

No No YesNote 6

Notes:1. When the process stream is oil, a high bath temperature shut down precludes the danger of coking.2. This instrumentation is for heaters located in gas processing sections. Wellhead units have a minimum of controls.3. Low bath level protects both the firetube and bath when it will coke (hot oils, glycol, amine) or decompose (molten salt).4. Low/high fuel pressure is always used when the fuel gas is taken from the exit process stream.5. Optical UV scanners or flame rods should be used because of the speed of response.6. Code requirements, ASME Section IV or VIII.

FIG. 8-36

Bath Heater Alarm/Shutdown Description

FIG. 8-35

Indirect Fired Heater

8-31

End View ofHeat Exchanger

Firetube Economizer

TC

Flame Arrestor

FIG. 8-37

Methods to Increase Firetube Heat Transfer

Firetube Turbulator

If fouling of the coils is not the problem, water losses can bereduced with a vapor recovery exchanger mounted on top ofthe heater shell. It consists of thin tubes that condense thewater vapor. Vapor losses can also be reduced by altering thecomposition of the heat medium or, in drastic cases, by chang-ing the heat medium.

• Shell side corrosion is caused by decomposition of thebath. (The decomposition products of amines and glycolsare corrosive.) Some decomposition and corrosion is in-evitable; however, excessive decomposition is usuallydue to overheating near the firetube. Corrosion inhibi-tors are commonly added. There are numerous reasonsfor overheating the bath: localized ineffective heattransfer caused by fouling, excessive flame impinge-ment, etc. An improper flame can sometimes be modifiedwithout system shutdown. Fouling, however, requires re-moval of the firetube.

8-3

• Inadequate heat transfer may result from improperflame, under-firing, firetube fouling, coil fouling, poorshell fluid dynamics, too small a firetube or coil, etc. If itis not improper design, then it is most likely fouling oran improper flame. The solution may be a simple burneradjustment to correct the air to fuel mixture.

• High stack temperature can be the result of an im-proper air to fuel mixture. A leak of combustible materialfrom the process side to the firetube is also a cause. Itcan also be the result of excessive soot deposition in thefiretube.

• Firetube failure is most commonly caused by localizedoverheating and subsequent metallurgical failure. These“hot spots” are caused by hydrocarbon coking and deposi-tion on the bath side. Firetube corrosion is caused by burn-ing acid gases for fuel. The most damaging corrosion occursin the burner assembly. There is little that can be done ex-

2

FIG. 8-38

Example Hot Oil System

cept to change the fuel and this may be impractical.Proper metallurgy is essential when burning acid gases.

• High or low fuel gas pressure can have a dramaticeffect on the operation of a firetube heater. Burners aretypically rated as heat output at a specified fuel pressure.A significantly lower pressure means inadequate heatrelease. Significantly higher pressure causes over-firingand overheating. The most common causes of a fuel gaspressure problem are the failure of a pressure regulatoror an unacceptably low supply pressure.

Improved Thermal EfficiencyEconomic incentives have promoted the development of de-

vices to improve the thermal efficiency, i.e., reduce the excessair and the stack temperature.

Control of the flow of air into the firetube or the gas flow in thestack with dampers is sensitive because the relatively weak stackdraft is easily influenced by an additional pressure drop. Severaldesigns are available: axially rotating vanes around the burner,a pivoting horseshoe around the burner, a hinged plate over theair inlet duct, a rotating plate in the stack, etc.

Methods to increase firetube heat transfer are shown inFig. 8-37. An economizer (end view) consisting of longitudi-nally finned tubes inserted into the return leg of the firetubeadds heat transfer surface; a turbulator increases the heattransfer coefficient; and internal fins both add area and in-crease turbulence. Often additional equipment, e.g., a pump

8

to circulate the bath through the economizer or an actuator toposition the damper, is needed.

With good control of the excess air, i.e., 5% to 10% and a stacktemperature of 200°C, the NTE approaches 90%. However, thepressure drop across the firetube increases and the stack draftdecreases. This means that a forced draft burner may be re-quired. The economics are usually favorable and short payoutperiods for these modifications are common.

HOT OIL SYSTEM

A simplified schematic of the major components of a hot oilsystem is given in Fig. 8-38. The heat transfer medium ispumped through a fired heater to the heat exchangers andreturns to the pump suction surge vessel. In some cases a firedheater is replaced by a waste heat source, such as the exhauststack of a fired turbine. The slip-stream (typically less than5%) filter is optional but it will help to retain the performancecharacteristics of the heat medium.

Proper design of the heater is critical for satisfactory opera-tion. The heat transfer fluid must have sufficient velocity, gen-erally 1.2 to 3 m/s, to avoid excessive film temperatures on theheater tubes. Hot spots can lead to tube failure and fluid deg-radation. Design and capacity of a heater should be limited sothat the maximum film temperature does not exceed the maxi-mum recommended operating temperature of the fluid.

-33

The surge vessel is provided with blanket gas and vent con-nections. Expansion room for the hot oil from ambient to op-erating conditions must be provided. On small systems thesurge tank may be sized to hold all of the heat medium. Twopiping arrangements are used for the surge vessel; flowthrough the vessel or the vessel as a surge riding on the pumpsuction line.

Pump head requirements are usually 275 to 550 kPa. Fre-quently the pump is spared and provided with isolating valvesfor servicing. A slip-stream should pass through the off-linepump to maintain the off-line pump at operating temperature.Selection of pumps should consider the high operating tem-perature and its effect on seals, packing rings, and gaskets.Slight leakage may occur at startup; the pump gland shouldnot be tightened until the system reaches operating tempera-ture. Suction strainers should be used during startup.

Piping design and installation must include considerationto minimize vapor traps and to relieve expansion and contrac-tion stresses. The number of flanges in a hot oil system shouldbe minimized; ANSI Class 300 lb flanges will aid in minimiz-ing leakage.

During initial startup, the system may contain water, whichmust be slowly vaporized and removed. The surge vessel issometimes the high point in the system to aid in this operation.On shutdown, fluid should be kept circulating to dissipate re-sidual refractory heat. Planned maintenance should includeanalysis of the heat transfer medium, inspection of insulation,and inspection of heater and mechanical equipment.

Consult suppliers of hot oils to obtain design informationsuch as Fig. 8-32 or suggestions such as reference 12.

WASTE HEAT RECOVERY

Economical and environmental considerations may lead tothe use of waste heat recovery systems. Flue gases from firedequipment, combustion engines and gas turbines are commonheat sources for consideration to use waste heat recovery. Therecovered heat may be used in the same equipment to increaseits thermal efficiency and to supply heat to other equipment.Fired equipment also may combine the heating of several proc-ess streams at different temperatures. Typical applications in-clude combustion air and/or fuel preheating in the sameequipment. Burners can be self-recuperative and self-regen-erative. Some applications include additional heat input byrefiring.

Heat transfer systems may have:• Streams contact, as in bubbling flue gases in water heat-

ers.• No contact of streams, as in shell and tube heat exchang-

ers.• An intermediate heat transfer fluid by forced circulation

as in pumped oil systems, or by natural circulation as inthermosyphons. The intermediate fluid may have aphase change as in heat pipes.

• A solid heat transfer media, such as in rotary or alternat-ing regenerative devices.

In some special devices, such as combustion engine and turbineexhaust gas silencers, a fluid is circulated for heat recovery.

Applications of waste heat recovery from fired equipment,combustion engine and gas turbine flue gases include processstream heating, water heating, steam generation, liquids and

8

solids regeneration, heat supply to absorption chillers, com-bined heat and power systems, and mechanical power as inturbochargers. Heat recovery equipment introduces addi-tional head losses in the flue gas path, with the consequenceof possible changes in required stack height, fan power, anddamper position. In the case of combustion engines and gasturbines, there is a loss of power.

Exhaust gases from equipment with high excess air, such asgas turbines which contain 13 to 17% molar volume of oxygen,temperatures between 455 and 565°C and pressures up to2.5 kPa (ga) may be used to supply the oxygen for the combus-tion of fuels in fired heaters.

REFERENCES

1. North American Manufacturing Corp., Combustion Handbook,Second Edition, 1978.

2. Lauer, B. E., Oil & Gas Journal, Series 8-18-52 to 11-2-53.

3. Perry, R. H. and Chilton, C. H., Editors, Chemical EngineeringHandbook, Fifth Edition, 1973.

4. Escoa Fintube Corp. Engineering Manual, 1979.

5. Kentube Company, Design Brochure, 1973.

6. Buthod, A. P. and Manning, W. P., University of Tulsa, PersonalCommunication.

7. McAdams, W. H., Heat Transmission, Third Edition, 1954.

8. Kern, D. Q., Process Heat Transfer, 1950.

9. Wimpress, P. H., Hydrocarbon Process & Petroleum Ref., Vol-ume 42, No. 10, 117, 1963.

10. Lobo, W. E. and Evans, J. F., AIChE Trans., Volume 35, 743, 1939.

11. Nelson, W. L., Oil & Gas Journal, January 4, 1947, p. 77.

12. Dow Corning Corp., “Heat Transfer System Design Checklist,”Version 1.2, Form 24-250-85.

13. API RP 11T, “Installation and Operation of Wet Steam Gener-ators,” USA.

14. API SPEC 12K, “Indirect-Type Oil Field Heaters,” USA.

15. API RP 12N, “Operations, Maintenance and Testing of FireboxFlame Arrestors,” USA.

16. API RP 530, "Calculation of Heater Tube Thickness in PetroleumRefineries," USA.

17. API RP 531M, “Measurement of Noise from Fired Process Heat-ers,”USA.

18. API RP 532, "Measurement of Thermal Efficiency of Fired Proc-ess Heaters," USA.

19. API RP 533, “Air Preheat Systems for Process Heaters,” USA.

20. API STD 534, “Heat Recovery Steam Generators,” USA.

21. API PUBL 535, “Burners for Fired Heaters in General RefineryServices,” USA

22. API RP 550, "Manual on Installations of Refinery Instrumentsand Control Systems, Part III, Fired Heaters and Inert Gas Gen-erators."

23. API STD 560," Fired Heaters for General Refinery Services,"USA.

24. API RP 573, “Inspection of Fired Boilers and Heaters,” USA.

25. API PUBL 4365, “Characterization of Particulate Emissionsfrom Refinery Process Heaters and Boilers,” USA.

26. AIChE, “Fired Heaters, A Guide to Performance Evaluations,”USA.

-34

27. AGA, “Gas Engineers Handbook, 1st Edition,” Industrial PressInc., N.Y., USA.

28. IGE/TM/2, “International Index of Safety Standards and CodesRelating to Gas Utilisation in Industry and Commerce,” The In-stitution of Gas Engineers, England.

29. IGE COMMUNICATION 1507, “The Development of Groundwa-ter Heating at Pressure Reduction Stations,” The Institution ofGas Engineers, England.

30. SALFORD UNIVERSITY, Centre for Natural Gas Engineering,Gas Utilisation Courses Notes, Dr. Robert Pritchard, England.

31. NFPA 85A, "Prevention of Furnace Explosions in Fuel Oil- andNatural Gas-Fired Single Burner Boiler-Furnaces," USA.

32. NFPA 85B, "Prevention of Furnace Explosions in Natural Gas-Fired Multiple Burner Boiler-Furnaces," USA.

33. NFPA 85D, "Prevention of Furnace Explosions in Fuel Oil-FiredMultiple Burner Boiler-Furnaces," USA.

8

34. NFPA 85G, "Prevention of Furnace Implosions in MultipleBurner Boiler-Furnaces," USA.

35. Idelchick, I.E., Handbook of Hydraulic Resistance, 1960, Trans-lated from Russian by Israel Program for Scientific Translations1966, Reproduced by U.S. Department of Commerce. Russia.

36. Miller, D.S., Internal Flow Systems, Vol. 5 in the BHRA FluidEngineering Series, 1978, England.

37. Nelson, W.L., Petroleum Refinery Engineering, 4th Edition, USA.

38. Pitts, D.R. and Sissom, L.E., Heat Transfer Theory and Problems,1977, USA.

39. Potter, J.P., Power Plant Theory and Design, 2nd Edition, 1959,USA.

40. Pritchard, R., Guy, J.J., and Connor, N.E., Industrial Gas Utili-sation, 1977, England.

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