Heat Transfer

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Heat Transfer

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Vessels File Reference: MEX30209

For additional information on this subject, contact M.Y. Naffa’a

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Vessels Heat Transfer

Contents

Pages

INTRODUCTION................................................................................................................ 1 CALCULATING HEAT TRANSFER IN BOILERS AND FURNACES.............................. 2 Radiant Heat Transfer ............................................................................................... 2 Radiant Heat Transfer in Furnaces ................................................................. 3 Maximum Radiant Section Heat Flux ............................................................. 4 Heat Transfer Surface Area............................................................................ 7 Convection Section Heat Transfer ............................................................................. 9 Heat Transfer in Boilers............................................................................................11 CALCULATING INTERNAL HEAT TRANSFER COEFFICIENTS .................................13 Inside Film Coefficients ............................................................................................13 Economizer and Superheater Tubes ..............................................................14 Steam Generation Tubes ...............................................................................15 CALCULATING MAXIMUM TUBE METAL TEMPERATURES....................................17 HEAT TRANSFER PROBLEMS........................................................................................24 External Fouling.......................................................................................................24 Internal Fouling ........................................................................................................24 Deposits .......................................................................................................24 Coking..........................................................................................................25 Flame Impingement on Tubes .......................................................................26 Work Aid 1 - Calculating Heat Transfer in Boilers and Furnaces ...................28 Work Aid 2 - Calculating Heat Transfer in Boilers and Furnaces ...................29 Work Aid 3 - Calculating Internal Heat Transfer Coefficient .........................30 Work Aid 4 - Calculating Maximum Tube Metal Temperatures.....................31 Work Aid 5 - Calculating Maximum Tube Metal Temperature ......................35 Work Aid 6 - Calculating Maximum Tube Metal Temperatures.....................38 REFERENCE......................................................................................................................39 GLOSSARY........................................................................................................................40

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INTRODUCTION This module provides information on the mechanisms by which heat is transferred to fluids flowing through tubes in boilers and furnaces. Information is also provided on the procedures used for calculating rates of heat transfer and the maximum temperatures of boiler and furnace tubes, based on performance data contained in the data sheets. The maximum tube metal temperatures are needed to properly monitor the operation of boilers and furnaces. Several of the equations presented in this module contain variables that are functions of the final calculated temperatures. For example, the maximum rate of heat transfer to a tube, which is a major factor in determining tube metal temperature, is a function of that temperature. In these cases, it is necessary to make a first estimate of the value of the variable, then adjust this estimate based on the resulting calculations. It may take more than one iteration to produce satisfactory results. It is important to use consistent units for the variables in these equations. Many of the common units for the variables must be adjusted for use in these equations. For example, fluid viscosity is often given in centipoise (CF), but must be converted to lb/hr-ft for use in these equations.

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CALCULATING HEAT TRANSFER IN BOILERS AND FURNACES Heat transfer is accomplished by three fundamental processes. All three processes are found in furnaces and boilers. •

Radiation. Energy is transferred, by means of electromagnetic waves, from one body to another body which is not in contact with it. All matter at temperatures above absolute zero emits radiant energy. An example of radiation is the transfer of heat, from burner flames and the resulting high-temperature flue gases, to the surrounding boiler or furnace tubes.

•

Conduction. Heat is transferred from one part of a body to another part (or to another body in contact with it). An example of conduction is the flow of heat through a tube wall, from the outside surface to the inside surface.

•

Convection. Heat is transferred by a combined mechanism of fluid mixing and conduction. An example of convection is the transfer of heat, from hot flue gases flowing over a tube surface, to the tube itself.

Boilers and furnaces have two sections where heat transfer takes place. In the first section, heat is transferred to tubes that surround the combustion zone, mainly by radiation. This is known as the "radiant section" in furnaces and the "furnace section" in boilers. This radiation section is followed by a section in which the flue gas passes through a bank of tubes, and heat is transferred mainly by convection. This is the "convection section" in furnaces. In boilers, this is mainly the "boiler bank" (the tubes directly connecting the steam and mud drums). The "economizer section," which follows the boiler bank, is also a convection section. Radiant Heat Transfer The radiant energy emitted from a black body is proportional to the fourth power of the absolute temperature of the body. Energy emitted can be represented by the Stefan-Boltzman equation: Q = A σ T4

where:

(Eqn. 1)

Q = Total energy emitted, Btu/hr. A = Surface area, ft2. s = Stefan-Boltzman constant. = 0.1712 x 10-8 Btu/hr-ft2-°R4. T = Absolute temperature of the body, °R.

The Stefan-Boltzman equation can be used to calculate the energy transmitted by radiation from a high-temperature body (the flue gases) to a lower-temperature body (the tubes). For non-ideal situations, such as furnaces and boilers, this equation can be represented as the following: Saudi Aramco DeskTop Standards

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Q = φr = F σ ε T g 4 - T m 4 A

where:

(Eqn. 1a)

= Radiant heat density (flux), Btu/hr-ft2. = View factor, which is dependent on furnace geometry and takes into account how well the two bodies "see" each other. e = Emissivity of a body relative to the ideal emissivity of an ideal black body. Tg = Radiating gas temperature, °R. Tm = Tube metal temperature, °R. fr F

Radiant Heat Transfer in Furnaces The calculation of radiant heat transfer in a complex system, such as the radiant section of a furnace, is very difficult. Various furnace designers and manufacturers have developed procedures to simplify these calculations. Many of these use empirical correlations to calculate furnace operating conditions based on test data from similar furnaces. The average rate of radiant heat transfer in a furnace (fr) is determined by the furnace's thermal design. fr equals the radiant section duty (Qr) divided by the effective heat transfer surface area (Ar). fr usually appears on the furnace data sheets. Satisfactory rates of heat transfer in a furnace depend upon the process conditions and the type of furnace being used. For Saudi Aramco furnaces, Table 1.A of AES-F-001 lists allowable values for fr. These are in the range of 6,000-12,000 Btu/hr-ft2, depending upon the type of service. For vertical cylindrical furnaces, fr is further limited to a maximum of 10,000 Btu/hr-ft2 (Par. 2.1.5 of AES-F-001). Heat transfer to the tubes in a furnace radiant section is not uniform. It is necessary to determine the maximum rates of heat transfer (heat flux) in order to calculate the corresponding maximum tube metal temperatures. The front side of the radiant section tubes absorbs heat directly from the radiating mass of flue gas. Some of the radiation emitted by the flue gas passes between the tubes and is absorbed by the refractory wall behind the tubes. This energy is then re-radiated, with a portion of this energy being absorbed by the back side of the tubes. The remaining re-radiated energy is recycled back to the flue gas. The proportion of the total energy that is absorbed by the tubes depends upon the furnace geometry. This is illustrated in Figure 1.

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Source: Perry's Chemical Engineers Handbook.

FIGURE 1 Effectiveness Of Radiant Section Tube Geometries Maximum Radiant Section Heat Flux This is a major factor in determining maximum tube metal temperatures. Although the maximum heat flux is usually listed on the data sheets, it should be checked. The maximum flux is often arbitrarily listed as about 1.8 times the average, without a detailed calculation of its actual value. Maximum radiant heat flux can be estimated using the following equation. Work Aid 1 can be used for these calculations. The example in Figure 3 illustrates the calculation of maximum radiant heat flux. φm = F C F L F T φr + φc where:

fm FC FL FT fr fc

(Eqn. 2)

= Maximum radiant heat flux (based on outside surface), Btu/hr-ft2. = Factor accounting for circumferential heat flux variations. = Factor accounting for longitudinal heat flux variations. = Factor accounting for the effect of tube metal temperature on radiant heat flux. = Average radiant heat flux, Btu/hr-ft2. = Average convective heat flux, Btu/hr-ft2.

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The factor FC, which accounts for circumferential heat flux variations, can be determined from Work Aid 2 for common furnace geometries. The factor FL accounts for uneven heat distribution throughout the radiant section. This is the most difficult factor in this equation to determine. Values between 1.0 and 1.5 are most often used. Unless specific data are available, values of FL from Figure 2 are recommended for furnace design. These values are based on the height of the radiant section. These values can be interpolated for intermediate heights. Furnace Radiant Section Height, ft Up to 25 30 35 40 45 50

FL 1.20 1.23 1.28 1.33 1.40 1.48

FIGURE 2 Longitudinal Heat Flux Variation (Fl) The factor FT accounts for the effect of tube metal temperature on radiant heat flux. For areas of maximum tube metal temperatures, FT will be less than 1. FT can be approximated by the following equation, which is an adaptation of the Stefan-Boltzman equation:

FT

where:

=

(T g + 460 )4 - (T m + 460 )4 (Tg + 460 )4 - (Ta + 460 )4

(Eqn. 3)

Tg = Average flue gas temperature in radiant section, °F. Flue gas exit temperature (bridgewall temperature) is often used. Tm = Maximum tube metal temperature, °F. If the heat flux at another point in the coil is to be calculated, the tube metal temperature at that point should be substituted for Tm. Ta = Average radiant section tube metal temperature, °F. This can be calculated using the tube metal temperature calculation procedures covered later in this module. The average radiant section operating conditions listed on the data sheets for heat flux fr, fluid temperature, etc., should be used.

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Since fm is a function of the calculated maximum tube metal temperature Tm, it is necessary to start with an estimation of Tm, calculate fm and the resulting Tm, then recalculate fm. fm is not very sensitive to variations in Tm, so one or two iterations is usually sufficient. If no other data are available, Tm can be estimated to be equal to Tb + (150 to 200) °F. Tb is the temperature of the fluid. The term fc can be used to account for any convective heat transfer. In the radiant section, this term is usually very small compared to the radiant flux, and is therefore usually included in the average radiant flux term, fr. In these cases assume fc = 0. Sample Calculation - Maximum Radiant Heat Density The following example illustrates the use of Work Aid 1 to calculate maximum radiant heat density in furnaces. Reference: Adapted from API RP 530, Section C.5, Sample Calculation. Given: Tube Outside Diameter, Do Average Wall Thickness, ta Tube Spacing, c-c (center-to-center) Flue Gas Temperature, Tg Average Radiant Tube Metal Temperature, Ta Average Heat Flux, fr Height of Radiant Section, Hr Tube Arrangement: Single tube row, against refractory wall

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in. in. in. °F °F Btu/hr-ft2 ft

= 4.50 = 0.25 = 8.0 = 1650 = 582 = 10,000 = 25

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Solution: Tube Spacing: c-c/Do = (8.0)/(4.5) Circumferential Heat Flux Factor, FC (from Work Aid 2) Longitudinal Heat Flux Factor, FL (from Figure 2) Estimated Maximum Tube Metal Temperature, Tm Tube Metal Temperature Factor, FT:

4 4 Tg + 460 ) - (T m + 460 ) ( (1650 + 460 )4 - (690 + 460 )4 = = 4 4 (1650 + 460 )4 - (582 + 460 )4 Tg + 460 ) - (T a + 460 ) ( FT

Convective Heat Flux, fc (Usually = 0) Calculate Maximum Radiant Heat Flux: fm = FC FL FT fr + = (1.91)(1.20)(0.969)(10,000) +

= = = °F =

1.78 1.91 1.20 690

=

0.969

Btu/hr-ft2 = fc (0)

=

0

Btu/hr-ft2 = 22,210

FIGURE 3 Heat Transfer Surface Area This can be calculated from the data supplied on the data sheets. These calculations are illustrated in Figure 4. Ar = N r L e Ao

where:

(Eqn. 4)

= Radiant section tube surface, ft2. = Number of tubes in radiant section. = Effective tube length, ft. = Lr + Lrb Lr = Straight tube length, ft. This is the tube length exposed to heat transfer. Any portions of the tube located in header boxes are not included. Lrb = Effective length of return bend, ft. = (¹/2) c-c/12. c-c = Center-to-center distance between radiant tubes, in. Ao = Tube outside surface area, ft2/ft of tube length. = ¹ Do/12. Do = Tube outside diameter, in.

Ar Nr Le

Sample Calculation - Heat Transfer Surface Area The following example illustrates the calculation of radiant section heat transfer surface areas in furnaces.

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Given: Tube Outside Diameter, Do = 4.5 in. Tube Spacing, c-c (center-to-center) = 8.0 in. Number of Radiant Tubes, Nr = 50 Straight Tube Length, Lr = 40 ft Average Heat Flux, fr (from Example in Figure 3) = 10,000 Btu/hr-ft2 Tube Arrangement: Horizontal tubes in single row against refractory wall. Solution: Heat Transfer Surface Area. This can be calculated from the data supplied on the data sheets. Effective Length of Return Bend:

L rb = π c-c 2 12 = π 8 = 1. 047 ft 2 12

Effective Tube Length: L e = L r + L rb

= 40 + 1. 047 = 41. 047 ft

Tube Outside Surface Area: Ao = π

Do 12

= π 4.5 = 1.118 f t2 /ft of tube length 12

Radiant Section Tube Surface (Eqn. 4): Ar = N r L e Ao = 50 (41. 047) 1. 78 = 2417. 7 ft2

Radiant Duty: Q r = φr A r = 10, 000 B tu/hr-ft2 (2417. 7 ft2 )

= 24.18 MBtu/hr FIGURE 4

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Convection Section Heat Transfer Flue gases leaving the radiant section pass through the convection section before entering the stack. At flue gas temperatures below 1600-1800_F, radiant heat transfer becomes relatively inefficient. If further heat recovery from the flue gas is desired, it is more efficiently accomplished by convection. Typically, 35-40% of total furnace duty is obtained in the convection section. Heat transfer in the convection section is based on the following equation: Q = U A ∆T

where:

Q U A ÆT

(Eqn. 5)

= Heat transferred, Btu/hr. = Overall heat transfer coefficient, Btu/hr-ft2-°F. = Area, ft2. = Temperature difference between the flue gas and the process fluid, °F. Since this varies across the tube bank, the logarithmic mean temperature difference (LMTD) between the flue gas and the process fluid is used.

The overall heat transfer coefficient, U, takes the following into account: •

Effective outside heat transfer coefficient. This includes the convective heat transfer coefficient and a term to account for radiative heat transfer. Radiative heat transfer can be a large component at the beginning of the convection section, where flue gas temperatures are high.

•

Resistance through the tubewall.

•

Inside heat transfer coefficient.

•

Allowances for fouling.

Extended surface is used on tubes in the convection section, where flue gas temperatures permit. Extended surface increases the effective outside heat transfer coefficient and thus the overall heat transfer from a convection tube. The use of extended surface tubes permits the economic recovery of more heat from the flue gas than would be possible with only bare tubes. The entrance to the convection section faces the radiant section. The first two rows of the convection section (known as the shield, or shock, tubes) receive direct radiation from the radiant section. See Figure 5. Added to this shield radiant duty is the convection duty. The resulting shield heat flux is often the highest heat flux in the furnace. Usually this is not a problem, since the process fluid temperature in the shield tubes is lower than in the radiant section. However, extended surface tubes should not be used in the shield section, because the heat fluxes and tube metal temperatures would be excessive. Saudi Aramco DeskTop Standards

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FIGURE 5 Convection Section Heat Transfer Heat transfer is often expressed as a rate per unit of surface area, as follows: Q = φa = U ∆ T A where :

Q A

= φ a = Average rate of heat transfer

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(or heat flux ), Btu / hr

2

(Eqn. 6)

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Process conditions are usually provided on the data sheets. Average heat flux for convection section tubes is usually not given, but this can be determined by dividing the process heat duty (Q) by the bare tube surface area (A). Some services, such as superheaters, often have only one convection section row, and in these cases this calculation is straightforward. For services with more than one row of convection tubes, the heat flux should be calculated for the first row of the geometry, because this row will have the highest flux (and highest tube temperatures). Heat flux can vary greatly across the convection bank. Convection section heat transfer calculation procedures are beyond the scope of this course. Typical procedures may be found in the article "Fired Heaters," by H. L. Berman, listed in the References. Maximum heat flux (fm) for convection section tubes is approximately 1.5 times the average flux (fa) for that tube row. This maximum heat flux can be used in place of Eqn. 2 to estimate convection section tube metal temperatures. The maximum permissible heat flux for convection section tubes is 1.8 times the allowable flux rate listed in Table 1.A of AES-F-001 (per Par. 3.2.1). This rate is on a bare tube basis. Heat Transfer in Boilers The heat transfer surface in the furnace (radiant) section consists mainly of membrane water walls. By convention, the heat surface area is equal to the projected area of the exposed tubes, not counting the membrane. For example, a 3 in. diameter tube counts as a 3 in. wide projected surface. This is illustrated in Figure 6. Tubes covered with refractory (mainly the floor tubes) are not included. Ar

where:

D

= Nr Le o 12

(Eqn. 7)

Ar = Furnace radiant heating surface, ft2. Nr = Number of tubes. L e = Effective tube length, ft. Do = Tube outside diameter, in.

do FIGURE 6 Boiler Heat Transfer Area Saudi Aramco DeskTop Standards

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The maximum heat transfer rates in the boiler's furnace section are limited by Par. 5.2.2 of 32AMSS-021 in the following two ways: •

Combustion volume: - For heavy liquid fuel (15 °API or heavier), a maximum of 60,000 Btu/hr-ft3. - For gaseous and light liquid fuels (lighter than 15 °API), a maximum of 80,000 Btu/hrft3.

•

Radiant heat flux: a maximum of 150,000 Btu/hr-ft2.

The superheater is located at the exit of the furnace section. Screen tubes are located just ahead of the superheater to shield the superheater from direct radiation from the hot flue gas in the furnace section. Reducing the direct radiation to the superheater helps to produce a more uniform superheater outlet temperature over the boiler's operating range. The screen tubes are part of the steam generation system. Calculating the boiler (convection) section heat transfer surface area is similar to the calculations for process furnaces. The heat transfer area is equal to the total exposed surface of the tubes.

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CALCULATING INTERNAL HEAT TRANSFER COEFFICIENTS Establishing and maintaining adequate process flow through boiler and furnace tubes is essential for satisfactory operation. Because of the high flue gas temperatures in this equipment, rapid overheating of tubes can occur if the flow of the fluid being heated is interrupted. Overheating of some tubes can also occur if the flow through the parallel passes becomes severely unbalanced. This can be caused by inadequate control of the flow to each pass, or by unbalanced firing. Flow blockages or restrictions in the flowpaths can also cause unbalanced flow. Inside Film Coefficients The heat transfer (film) coefficient at the inside wall of the tube (hi), is calculated using the following equations, which are based on Appendix C of API RP 530. Work Aid 3 can be used to calculate film coefficients. The example in Figure 7 illustrates the use of these calculation procedures. Calculated inside heat transfer coefficients are based on uniform flow rates. Variations in flow can seriously affect these coefficients. For single-phase fluids, the following equations are used: For liquid flow: µb 0. 14 = 0. 023 k N R e 0. 8 N P r 0. 33 µw Di

hl

(Eqn. 8)

For vapor flow: hv

T b + 460 0. 5 = 0. 021 k N R e 0. 8 N P r 0. 4 Di T w + 460

N Re =

NPr =

where:

hl hv k Di µb

= = = = =

Di G

µb

C p µb k

(Eqn. 9)

(Reynolds number)

(Eqn. 10)

(Prandtl number)

(Eqn. 11)

Liquid phase film coefficient, Btu/hr-ft2-_F. Vapor phase film coefficient, Btu/hr-ft2-_F. Thermal conductivity of the fluid (at the bulk fluid temperature), Btu-ft/hr-ft2-_F. Inside diameter of the tube, ft. Absolute viscosity of the fluid (at the bulk fluid temperature), Btu/hr-ft.

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= 2.42(viscosity in centipoise). µ w = Absolute viscosity of the fluid (at the wall temperature), Btu/hr-ft. Tb = Bulk temperature of the fluid (vapor), _F. Tw = Temperature of the fluid (vapor) at the wall, _F. G = Mass velocity of the fluid in the tube, lb/hr-ft2. Note that G is expressed in hours. This G is 3600 times the mass velocity when expressed in seconds (lb/s-ft2). Cp = Specific heat (or heat capacity) of the fluid, Btu/lb-_F. µb 0. 14 T b + 460 0. 5 The last terms in Equations 8 and 9, µw and T w + 4 60 , account for changes in the viscosity of the fluids due to the temperature difference across the film. These usually amount to less than 10% of the overall film coefficient. The fluid properties (viscosity, thermal conductivity, and specific heat) can be obtained from the furnace data sheets. If these data are not available, they should be obtained from the appropriate process engineers. Eqns. 8 and 9 are valid for turbulent flow conditions and should cover all Saudi Aramco furnace applications. Conditions which have Reynolds numbers (NRe) above 10,000 for liquid flow and 15,000 for vapor flow are in turbulent flow. Lower Reynolds numbers, indicating laminar flow, are very rarely encountered in process furnaces. A possible case with laminar flow could be an extremely viscous liquid flowing at an extremely low velocity. For two-phase flow, the heat transfer coefficient can be approximated using the following equation: h tp = h l W l + h v W v

where:

(Eqn. 12)

htp = Two-phase film coefficient, Btu/hr-ft2-°F. Wl = Weight fraction of liquid flow. Wv = Weight fraction of vapor flow.

The liquid and vapor film coefficients are calculated using the total mass flow rate (G), but using the liquid and vapor material properties, respectively. Boilers Economizer and Superheater Tubes Properties of water and steam are listed in Work Aid 3. These can be used with Eqn. 8 to calculate film coefficients for economizers, and with Eqn. 9 for superheaters.

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Steam Generation Tubes Tubes where steam is being generated have extremely high film coefficients. These are very difficult to calculate, since the coefficient depends on the type and extent of boiling taking place. It is common practice to use a film coefficient of about 1000 Btu/hr-ft2-°F. For economizer and steam generation tubes, the actual film coefficient will have very little impact on the mechanical design of the tube. The ASME Code requires that all tubes in a boiler that are heated shall be designed for a minimum temperature of at least 700°F. Except for superheater tubes, this is well above the calculated tube metal temperature. Sample Calculation - Tube Inside Film Coefficient The following sample problem illustrates the use of Work Aid 3 to calculate the inside film coefficients in a furnace tube. Reference: Adapted from API RP 530, Section C.5, Sample Calculation. Given: Fluid Flow Rate, W Weight Fraction Fluid Temperature, Tb Fluid Pressure, P Viscosity, µ 2.42 (cP) = Thermal Conductivity, k Specific Heat, Cp Tube Outside Diameter, do Average Wall Thickness, ta Coke Thickness, tc Number of Tube Passes, n

Total Flow = 50,000 lb/hr = 520

Liquid 0.62

°F psig cP lb/hr-ft Btu-ft/hr-ft2-°F Btu/lb-°F in. in. in.

= = = = = = = = = =

2 4.84 0.0672 0.68

Vapor 0.38 0.007 0.017 0.020 0.572

4.5 0.25 0 1

Solution: Flow Diameter, dx = do - 2 (ta + tc) = (4.5) - 2 (0.25 + 0) Dx = (dx in.) / 12 Flow Area, Ax = n (p/4) (Dx)2 = (1) (p/4) (0.333)2 Mass Velocity, G = W/Ax = (50,000) / (0.0873) Estimated Temperature Rise Across Film, ÆTf Estimated Wall Temperature, Tw = Tb + ÆTf,

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in. ft ft2 lb/hr-ft2 °F °F

= 4.00 = 0.333 = 0.0873 = 5.73 x 105 = 125 = 645

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Liquid Phase Coefficient: Reynolds Number, NRe = DxG / µ = (0.333) (5.73 x 105) / (4.84) Prandtl Number, NPr = Cpµ / k = (0.68) (4.84) / (0.0672) Viscosity at Wall Temperature, µ w cP 2.42 (cP) = lb/hr-ft Film Coefficient (Eqn. 8): h 1 = 0. 023 h 1 = 0. 023

k Dx

0.33  µ b 

(N Re ) (N Pr ) 0.8

(0. 0672 ) (0.333 )

= 3.95 x 104 = 48.98 = 1.1 = 2.66

0.14

 µw 

 (4. 84 ) 0.14 4 0.8 ( 0.33 3.95 x 10 48 . 98 )  (2. 66 )

(

)

Btu/hr-ft2-°F =

86.7

FIGURE 7 Vapor Phase Coefficient: Reynolds Number, NRe = DxG / µ = (0.333) (5.73 x 105) / (0.017) Prandtl Number, NPr = Cpµ / k = (0.572) (0.017) / (0.020) Film Coefficient (Eqn. 9): h v = 0. 021 h v = 0. 021

k Dx

(N Re )

(0. 020 ) (0. 333 )

0.8

  (N Pr )0.4  Tb + 460   Tw + 460 

= 1.12 x 107 = 0.486

0.5

0.5 7 0.8 ( 0.4  (520 + 460 ) ) 1.12 x 10 0. 486  (645 + 460 )

(

)

Btu/hr-ft2-°F =

388.6

Btu/hr-ft2-°F =

201.4

Two-Phase Coefficient (Eqn. 12): h tp = h l W l + h v W v

= (86.7)(0.62) + (3.886)(0.38)

FIGURE 7 (CONT’D)

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CALCULATING MAXIMUM TUBE METAL TEMPERATURES The method for calculating maximum tube metal temperatures is based on the procedure contained in Appendix C of API RP 530. Work Aid 5 can be used to calculate maximum tube metal temperature. The example in Figure 10 illustrates these calculations. Maximum tube metal temperature is calculated using the following equation: Tm = Tb + ∆ Tf + ∆Tc + ∆T w

(Eqn. 13)

Tb = Bulk fluid temperature, _F. This is the temperature of the fluid inside the tube at the point for which the tube metal temperature is being calculated. Most cases use the maximum process fluid temperature in the section of the boiler or furnace in question. where:

ÆTf = Temperature difference across the fluid film, _F. This is a boundary layer between the bulk process fluid and the inside surface of the tubewall. ÆTc = Temperature difference across a layer of coke or scale (if any), _F. This is an allowance for fouling that may occur on the inside surface of the tubewall during operation. ÆTw = Temperature difference across the tubewall, _F. A temperature profile across the tubewall for the example in Figure 10 is illustrated in Figure 8.

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FIGURE 8 Temperature Profile Across Tubewall The major variable in determining the total temperature difference between Tb and Tm is the maximum rate of heat transfer to the tube (fm). Since fm is, in turn, dependent upon the maximum tube metal temperature, an iterative calculation procedure is required. The calculation procedures presented in this module start by estimating fm, calculating Tm based on this estimation, and then recalculating fm. Since fm does not change rapidly with changes in Tm, one or two iterations of these calculations is usually sufficient. Temperature differences are determined by the following equations: Film temperature difference: Do 1 ∆ Tf = φm h i D i - 2 tc

(Eqn. 14)

Coke layer temperature difference: D

∆ Tc = φ m F o Di

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(Eqn. 15)

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Do t ∆ Tc = φ m c k c Di - tc

or:

(Eqn. 15a)

Tubewall temperature difference: ∆ Tw = φ m

ta

Do

kw

Di - ta

(Eqn. 16)

where: fm = Maximum rate of heat transfer, Btu/hr-ft2. By convention, this is based on the outside surface area of the tube. In each of these equations, fm is adjusted by a ratio of applicable diameters to reflect the actual surface area over which the term applies. This is illustrated in Figure 9. The average diameters of the tubewall and the coke layer are used. The inside film is assumed to have no thickness. hi

= Fluid heat transfer coefficient at the inside wall of the tube (also called the inside film coefficient), Btu/hr-ft2-°F . This must be calculated, based on the properties of the fluid.

F

= Fouling Factor. This provides an allowance for coking, scale, or other deposits on the inside surface of the tube. This factor is usually in the range of 0.0015-0.005 Btu/hr-ft2-°F. Fouling factors for various services are listed in Table 1.A of AES-F-001.

tc = Thickness of coke (or scale or other deposits) layer, in. This can be used in place of a fouling factor, or to estimate the effect of a given layer of coke. If a fouling factor is used, assume that tc = 0 in Eqn. 14. The following are typical design coke thicknesses: Atmospheric pipestill furnaces: 1/8 in. Vacuum pipestill furnaces: 1/4 in. kc = Thermal conductivity of a coke layer. This can be assumed to be 35 Btu-in./hr-ft2-°F (based on average coke temperature = 850°F). ta

= Average tubewall thickness, in.

kw = Thermal conductivity of the tubewall, Btu-in./hr-ft2-°F. This is a function of the tube material and the tubewall temperature. Thermal conductivities for common tube materials are given in Work Aid 6. Since thermal conductivity is temperature dependent, it is first

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necessary to assume a tube temperature and conductivity, then adjust the calculations. Since thermal conductivity is not extremely sensitive to temperature changes, this step is usually not difficult. It is important to note that units must be consistent in these equations. Many of the variables are often referenced in inconsistent units, and these must be adjusted accordingly.

φ FIGURE 9 Tube Cross Section Sample Calculation - Maximum Tube Metal Temperature The following example illustrates the use of Work Aid 5 to calculate maximum tube metal temperature in a furnace radiant section. Reference: Adapted from API RP 530, Section C.5, Sample Calculation.

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Given: Tube Outside Diameter, Do Average Wall Thickness, ta Bulk Fluid Temperature, Tb Tube Material Maximum Heat Flux, fm (from Example in Figure 3) Film Coefficient, hi (from Example in Figure 7) Service: Reboiler

in. = in. = °F = = Btu/hr-ft2 = Btu/hr-ft2-°F =

4.50 0.25 520 CS 22,210 201

Solution: Inside Diameter, Di = Do - 2ta = (4.50) - 2 (0.25) in. = 4.00 Fouling Factor, F (from Table 1.A, AES-F-001) = 0.0015 Or: Coke Thickness, tc in. = Coke Thermal Conductivity, kc Btu-in./hr-ft2-°F = Approx. TMT = Tb + 1.2 fm [1/hi + F] = (520) + 1.2 (22,210) [1/(201) + (0.0015)] = 692 (Assume TMT = 692 °F) Thermal Conductivity, kw (from Work Aid 6 at TMT): Btu-in./hr-ft2-°F = 302 -------------------------------------------------------------Film Temperature Difference: Do = 22,210 h i D i - 2 tc

∆ T f = φm 1

Coke Layer Temperature Difference: ∆ T c = φm F

Do = 22,210 Di

0.0015

Tubewall Temperature Difference: t

D

4.5 4.0 - 2 0

1 201

4.5 4.0

0.25

°F =

124

°F =

37

°F = °F °F =

19 180 520

°F =

700

4.5

o = 22,210 ∆ T w = φm a 302 4.5 - 0.25 k w D o - ta Total Temperature Difference = ÆTf + ÆTc + ÆTw Bulk Fluid Temperature, Tb Maximum Tube Metal Temperature: Tm = Tb + ÆTf + ÆTc + ÆTw

FIGURE 10

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Check Assumptions Made in Previous Calculations: Check Inside Film Coefficient (example in Figure 7): Liquid Phase Coefficient: Calculated Wall Temperature, Tw = Tb + ÆTf Viscosity at Wall Temperature, µ w h1'

Original µ w  = Original h 1   Revised µw 

0.14

°F = cP = 2.42 (cP) = lb/hr-ft =

644 1.1 2.66

( ) 0.14

 2. 66  = (86 . 7 )   (2. 66 ) Vapor Phase Coefficient: hv'

= Original

 Original T w + 460  hv  Calculated T + 460 

Btu/hr-ft2-°F =

86.7

Btu/hr-ft2-°F =

388.8

Btu/hr-ft2-°F =

201.5

0.5

w

0.5

 (645 + 460 ) = (388 . 6 )  (644 + 460 ) Revised Two-Phase Coefficient: htp' = h1' W1 + hv' Wv = (86.7)(0.62) + (388.8)(0.38) Revised Film Temperature Difference: Original h

201.4

i = 124 ∆ T f '= Original ∆ T f x Revised h i 201.5

Revised Tubewall Temperature Difference: Average Tubewall Temperature, Tma = (Tm =700) (ÆTw/2=10) Thermal Conductivity, kw (at Tma) Original k w 302 = 19 ∆ T w '= Original ∆ T w x Revised k w

°F =

124

°F =

690

Btu-in./hr-ft2-°F =

302

°F =

19

302

FIGURE 10 (CONT’D)

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Revised Maximum Tube Metal Temperature: T m ' = T b + ∆ T f ' + ∆ T c' + ∆ T w '

°F = 700

Recalculate Maximum Radiant Heat Flux (from Figure 3): Recalculate FT (Eqn. 3):

(Tg + 460 )4 - (T m + 460 )4 (1650 + 460 )4 - (700 + 460 )4 = = 4 4 (1650 + 460 )4 - (582 + 460 )4 T + 460 T + 460 ( ) ( ) g a FT Maximum Radiant Heat Flux (Eqn. 2): fm' = FC FL FT fr + fc = (1.91)(1.20)(0.966)(10,000) + (0) % Difference in fm:

22 ,140 - 22 , 210 22 , 210

x 100

∆ T = 180 Þ x Resulting Difference in Tm:

=

0.999

Btu/hr-ft2 =

22,140

= - 0. 3%

22 .140 22 , 210

= 179 . 4ÞF

FIGURE 10 (CONT'D)

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HEAT TRANSFER PROBLEMS For normal operations, most heat transfer problems are caused by fouling of the heat transfer surfaces. This results in reduced heat transfer and increased tube metal temperatures. Fouling can occur on both the outside and inside surfaces of the tubes. External Fouling The most common external fouling occurs in the convection section, where the tubes and extended surfaces can be covered with a layer of dirt or ash. This insulating layer reduces heat transfer and thermal efficiency. This reduced heat transfer can be overcome by increased firing and higher temperatures. However, high firing rates needed to overcome severe fouling may lead to further problems, such as direct flame impingement on tubes. External fouling can be satisfactorily removed by periodic use of sootblowers or by cleaning during turnarounds. Therefore, provisions for extensive external fouling are not normally included in the thermal design of most boilers and furnaces. In some cases, an allowance for external fouling is included in convection section heat transfer calculations. Internal Fouling The main effect of internal fouling of tubes is to raise the tube metal temperature. A term to account for potential internal fouling is included in the tube metal temperature calculation procedure (Eqn. 13). Two types of internal fouling can be encountered: deposits and coking. Deposits These are most commonly found in boilers. Deposits consist of materials that originate somewhere in the system and are conveyed to the boiler where they are deposited. The following types of deposits are found in boilers: • • • •

Waterborne minerals that have not been removed during water treating. Excess water treatment chemicals. Corrosion products. Contaminants.

Deposits in boiler tubes can form in locations with excessive boiling rates. These can be caused by high rates of heat input, which may be due to flame impingement, as discussed below.

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Superheater deposits are often caused by the carryover of boiler water. This water evaporates in the superheater, usually near the inlet, leaving behind any minerals carried over with the water. Water carryover can be the result of poor operation of the steam drum, possibly due to foaming or high water levels. Superheater deposits can also be caused by contaminated attemperator water. It is very difficult to provide for internal deposits in the design of a boiler or furnace. Consequently, an internal fouling factor is usually not included in the design of boiler tubes. With proper design and operation, significant deposits may never occur. An internal fouling factor may be used to provide an allowance for moderate fouling. However, severe fouling can be so great that it would be impractical to provide an adequate allowance. In extreme cases, severe internal fouling can completely block the tube, causing excessive tube metal temperatures. Coking Coke deposits are generated in furnaces due to the decomposition of hydrocarbons in the process feed. The amount of coke formed is related to the composition of the process stream and the process temperatures in the furnace, particularly the film temperatures. Furnaces are designed and operated to minimize coke formation, but some coke formation is expected in some services, even with good operating practices. In hydrocarbon vaporizing services, such as atmospheric and vacuum pipestill preheaters, coking usually begins at film temperatures above about 660-700°F. Due to this situation, an allowance for coking is included in furnace tube design. The fouling factors are specified in AES-F-001. Even in noncoking services, a fouling factor is used to provide an allowance for any other fouling that may occur. Furnaces in coking services must be periodically decoked during turnarounds. Permanent steamair decoking facilities are required for Saudi Aramco furnaces with fouling factors of 0.003 Btu/hr-ft2-°F (Par. 2.2.5 of AES-F-001). This includes atmospheric and vacuum pipestill furnaces. Coke deposits are usually greatest on the side of the tube with the maximum rate of heat input, where film temperatures are highest. This is illustrated in Figure 11. The temperature effects of internal fouling (either deposits or coke) may be estimated using Eqn. 15a. The example in Figure 12 illustrates the effect of an internal coke layer on maximum tube metal temperature. This is a continuation of the example started in Figure 10. In this example, the effects of coking are severe, because the furnace is in a service that would normally be considered noncoking. This coking would most likely be caused by an upset in operating conditions, such as a loss of process flow or severe flame impingement.

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Direct Radiation

Refractory Wall

1"

1"

FIGURE 11 Typical Coke Formation Flame Impingement on Tubes High tube metal temperatures can be caused by direct flame impingement on the tubes. Furnaces and boilers are designed to provide adequate room for complete combustion of the fuel without the flames coming in contact with the heat transfer surfaces. However, when operations are upset, flame patterns may be distorted, resulting in impingement. This could be caused by overfiring, or by dirty or poorly adjusted burners. Excessive rates of heat input can increase tube metal temperatures in the following ways: •

By directly increasing the tube metal temperature, due to the high heat flux.

•

By causing high coking or fouling rates, due to high film temperatures. Tube metal temperatures are then further increased due to the layer of coke.

Sample Calculation - Effects of Internal Fouling The following example illustrates the effects of internal fouling on tube metal temperatures. For the example in Figure 10, the increase in tube metal temperature for a 1/4 in. coke layer is calculated below. Tube Outside Diameter, Do Tube Inside Diameter, Di Average Wall Thickness, ta Bulk Fluid Temperature, Tb Tube Material Average Heat Flux, fa Maximum Heat Flux, fm (from Example in Figure 3) Film Coefficient, hi (from Example in Figure 7) Coke Thickness, tc Coke Thermal Conductivity, kc

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in. in. in. °F Btu/hr-ft2 Btu/hr-ft2 Btu/hr-ft2-°F in. Btu-in./hr-ft2-°F

= 4.50 = 4.00 = 0.25 = 520 = CS = 10,000 = 22,210 = 201 = 0.25 = 35

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Solution: Approx. TMT = Tb + 1.2 fm [1/hi + tc/kc] = (520) + 1.2 (22,210) [1/(201) + (0.25/35)] = 843 °F (Estimated Maximum = 850 °F) TMT Thermal Conductivity, kw (Work Aid 6 at TMT): Btu-in./hr-ft2-°F = 290 –––––––––––––––––––––––––––––––––––– Tube Metal Temperature Factor, FT (from Example in Figure 3): 4 4 Tg + 460 ) - (T m + 460 ) ( (1650 + 460 )4 - (850 + 460 )4 = = 4 4 (1650 + 460 )4 - (582 + 460 )4 Tg + 460 ) - (T a + 460 ) ( FT

Estimated Maximum Radiant Heat Flux: fm = (FC) (FL) (FT) fa = (1.91) (1.20) (0.905) (10,000) Film Temperature Difference: Do = 20,700 h i D i - 2 tc

∆ T f = φm 1

Coke Layer Temperature Difference: t ∆ T c = φm F c

Do = 20,700 k c D i - tc

Tubewall Temperature Difference: t

D

Btu/hr-ft2 =

4.5 4.0 - 2 0.25

1 201 0.25 35

4.5 4.0 - 2 0.25

0.25

4.5 4.0 - 0.25

o = 20,700 ∆ T w = φm a 290 k w D o - ta Bulk Fluid Temperature, Tb Maximum Tube Metal Temperature: Tm = Tb + ÆTf + ÆTc + ÆTw

=

0.905

20,750

°F = 133

°F

= 178

°F = 18 °F = 520 °F = 849

FIGURE 12 Sample Calculation - Effects of Internal Fouling Summary: Clean Tm (from Figure 10) (Approx.) Tm with Fouling Allowance (from Figure 10) Tm with 1/4 in. Coke Layer

°F = 663 °F = 700 °F = 849

FIGURE 12 (CONT’D)

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Work Aid 1 - Calculating Heat Transfer in Boilers and Furnaces MAXIMUM RADIANT HEAT DENSITY CALCULATION SHEET The following procedure can be used to calculate maximum radiant heat density in furnaces. Plant Location Service Given:

Furnace Coil

Tube Outside Diameter, Do Average Wall Thickness, ta Tube Spacing, c-c (center-to-center) Flue Gas Temperature, Tg Average Radiant Tube Metal Temperature, Ta Average Heat Flux, fr Height of Radiant Section, Hr Tube Arrangement:

in. in. in. °F °F Btu/hr-ft2 ft

= = = = = = =

Solution: Tube Spacing: c-c/Do = ( )/( ) Circumferential Heat Flux Factor, FC (from Work Aid 2) Longitudinal Heat Flux Factor, FL (from Figure 2) Estimated Maximum Tube Metal Temperature, Tm Tube Metal Temperature Factor, FT: 4 4 Tg + 460 ) - (T m + 460 ) ( ( FT = = (Tg + 460 )4 - (T a + 460 )4 (

+ 460 )4 4 + 460 ) -

Convective Heat Flux, fc (Usually = 0) Calculate Maximum Radiant Heat Flux: fm = FC FL FT fr + fc =( )( )( )( ) + ( )

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= = = °F =

( (

+ 460 )4 = _________ 4 + 460 )

Btu/hr-ft2 = Btu/hr-ft2 =

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Work Aid 2 - Calculating Heat Transfer in Boilers and Furnaces

Note 1: Curve 1 = Double row against wall, triangular spacing. Curve 2 = Double row with equal radiation from both sides and two diameters between rows, equilateral spacing. Curve 3 = Single row against wall. Curve 4 = Single row with equal radiation from both sides. Note 2: These curves are valid when used with a tube-center-to-refractory-wall spacing of 1-1/2 times the nominal tube diameter. Any appreciable variation from this spacing must be given special consideration. Source: API RP 530.

FIGURE 13 Ratio Of Maximum Local-To-Average Heat Flux

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Work Aid 3 - Calculating Internal Heat Transfer Coefficient TUBE INSIDE FILM COEFFICIENT CALCULATION SHEET The following procedure can be used to calculate inside film coefficients for boiler or furnace tubes. Plant Location Service

Boiler/Furnace Coil

Given: Total Flow Fluid Flow Rate, W Weight Fraction Fluid Temperature, Tb Fluid Pressure, P Viscosity, µ 2.42 (cP) = Thermal Conductivity, k Specific Heat, Cp Tube Outside Diameter, do Average Wall Thickness, ta Coke Thickness, tc Number of Tube Passes, n

Liquid

Vapor

= lb/hr = °F psig cP lb/hr-ft Btu-ft/hr-ft2-°F Btu/lb-°F in. in. in.

= = = = = = = = = =

Solution: Flow Diameter, dx = do - 2 (ta + tc) = ( )-2( Dx = (dx in.) / 12 Flow Area, Ax = n (p/4) (Dx)2 = ( ) (p/4) ( Mass Velocity, G = W/Ax = ( )/( ) Estimated Temperature Rise Across Film, ÆTf Estimated Wall Temperature, Tw = Tb + ÆTf, Liquid Phase Coefficient: Reynolds Number, NRe = DxG / µ = ( Prandtl Number, NPr = Cpµ / k = ( Viscosity at Wall Temperature, µ w

)(

+

)

)2

)/( )

in. ft ft2 lb/hr-ft2 °F °F

= = = = = =

= = cP = 2.42 (cP) = lb/hr-ft =

)/(

) (

)

Film Coefficient (Eqn. 8):

µb 0. 14 h l = 0. 023 k N R e 0. 8 N P r 0. 33 µw Dx = 0. 023

(

)

(

)

(

)0.8 (

)

0.33

  

( (

)0.14 ) Btu/hr-ft2-°F =

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Vapor Phase Coefficient: Reynolds Number, NRe = DxG / µ = ( Prandtl Number, NPr = Cpµ / k = ( Film Coefficient (Eqn. 9):

)( ) (

)/( )/(

T b + 460 0. 5 h v = 0. 021 k N R e 0. 8 N P r 0. 4 Dx T w + 460

= 0. 021

(

)

(

)

) 0.8

(

) 0.4

(

  

( (

)

) = =

) 0.5  460 )

+ 460  +

Btu/hr-ft2-°F = Two-Phase Coefficient (Eqn. 12): h tp = h l W l + h v W v

= (

)(

)+(

)(

) Btu/hr-ft2-°F =

Work Aid 4 - Calculating Maximum Tube Metal Temperatures Specific Heats of the Vapor for Constant Pressure

Source: Thermodynamic Properties of Steam Including Data for the Liquid and Solid Phases by John H. Keenan and Frederick G. Keyes, 33rd Printing, © 1936. Reprinted by Permission of John Wiley & Sons, Inc.

FIGURE 14 Thermal Properties Of Steam Absolute Viscosity of Saturated and Superheated Steam

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With Permission from Babcock & Wilcox.

FIGURE 15

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Viscosity of Liquid Water

FIGURE 16

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Thermal Conductivity of Liquid Water

FIGURE 17 THERMAL PROPERTIES OF STEAM (CONT’D)

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Thermal Conductivity of Steam

FIGURE 18 THERMAL PROPERTIES OF STEAM (CONTÍD) Work Aid 5 - Calculating Maximum Tube Metal Temperature The following procedure can be used to calculate maximum tube metal temperatures. Plant Location Service

Boiler/Furnace Coil

Given: Tube Outside Diameter, Do Average Wall Thickness, ta Bulk Fluid Temperature, Tb Tube Material Maximum Heat Flux, fm (from Work Aid 1) Film Coefficient, hi (from Work Aid 4)

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in. = in. = °F = = Btu/hr-ft2 = Btu/hr-ft2-°F =

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Solution: Inside Diameter, Di = Do - 2ta = ( )-2( ) in. = Fouling Factor, F (from Table 1.A, AES-F-001) = Or: Coke Thickness, tc in. = Coke Thermal Conductivity, kc Btu-in./hr-ft2-°F = Approx. TMT = Tb + 1.2 fm [1/hi + F] = ( ) + 1.2 ( ) [1/( )+( )] =_______°F (Assume TMT =_______°F) Thermal Conductivity, kw (from Work Aid 6 at TMT): Btu-in./hr-ft2-°F = –––––––––––––––––––––––––––––––––––– Film Temperature Difference: ∆T f = φm

Do

1

h i Di - 2 tc

=

(

)

1

(

)[

Coke Layer Temperature Difference: ∆T c = φm F

Do Di

=(

)(

)

(

( (

) )

ta

Do

k w D o - ta

=(

)

( (

)]

°F =

 tc Do  or  φ m km D i - tc 

Tubewall Temperature Difference: ∆ T w = φm

- 2(

)

) )(

(

) -

Total Temperature Difference = ÆTf+ÆTc+ÆTw

)

°F =

°F = °F =

Bulk Fluid Temperature, Tb

°F =

Maximum Tube Metal Temperature: Tm = Tb + ÆTf + ÆTc + ÆTw

°F =

Check Assumptions Made in Previous Calculations: Check Inside Film Coefficient (from Work Aid 3): Liquid Phase Coefficient: Calculated Wall Temperature, Tw = Tb + ÆTf Viscosity at Wall Temperature, µ w h1'

Original µ w  = Original h 1   Revised µw  =(

 ) 

( (

Saudi Aramco DeskTop Standards

)0.14 )

°F = cP = 2.42 (cP) = lb/hr-ft =

0.14

Btu/hr-ft2-°F =

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Vapor Phase Coefficient: hv'

= Original

 Original T w + 460  hv  Calculated T + 460 

=(

( )  (

0.5

w

+ 460 ) 

0.5

+ 460 ) 

Btu/hr-ft2-°F =

Revised Two-Phase Coefficient: htp' = h1' W1 + hv' Wv = ( )( )+( )( Revised Film Temperature Difference:

Btu/hr-ft2-°F =

)

Original h

i= ∆ T f ' = Original ∆ T f x Revised h i Revised Tubewall Temperature Difference: Average Tubewall Temperature, Tma = (Tm = ) Thermal Conductivity, kw (at Tma)

∆ T w ' = Original ∆ T w x

°F = ) - (ÆTw/2=

Btu-in./hr-ft2-°F =

Original k w = Revised k w

°F =

Revised Maximum Tube Metal Temperature: T m ' = T b + ∆ T f ' + ∆ T c' + ∆ T w ' Recalculate Maximum Radiant Heat Flux (from Work Aid 1): Recalculate FT (Eqn. 3): 4 4 Tg + 460 ) - (T m + 460 ) ( ( = = 4 4 ( Tg + 460 ) - (T a + 460 ) ( FT

Maximum Radiant Heat Flux (Eqn. 2): fm' = FC FL FT fr + =( )( )( )( )+ % Difference in fm: Resulting Difference in Tm:

Saudi Aramco DeskTop Standards

°F =

)4 - ( 4 + 460 ) - ( + 460

(

fc

)

°F =

)4 4 + 460 ) + 460

=

Btu/hr-ft2 =

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Work Aid 6 - Calculating Maximum Tube Metal Temperatures

k for 9Cr: 177 + 0.0207 (T) (Btu - in.)/(hr - ft2 - °F) Figure 19 Thermal Conductivity Of Common Tube Materials

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REFERENCE Saudi Aramco Standards AES-F-001 Process Fired Heaters. 32-AMSS-021 Water-Tube Boilers.

API Publications Recommended Practice 530 Calculation of Heater-Tube Thickness in Petroleum Refineries.

ASME Publications 1967 ASME Steam Tables.

Other Publications Berman, H.L., "Fired Heaters," Chemical Engineering Magazine, June-September 1978 issues. Port, R.D., and Herro, H.M., "The Nalco Guide to Boiler Failure and Analysis," McGraw-Hill, Inc., New York (1991).

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GLOSSARY bulk temperature

The average temperature of the process fluid at any tube cross section.

coil

A series of straight tube lengths connected by 180° return bends, forming a continuous path through which the process fluid passes and is heated.

convection section

The portion of a heater, consisting of a bank of tubes, which receives heat from the hot flue gases, mainly by convection.

economizer

A tube bank for transferring heat from the flue gas to the boiler feedwater before the BFW enters the steam drum.

extended surface

Surface added to the outside of bare tubes in the convection section to provide more heat transfer area. This may consist of cylindrical studs butt-welded to the tube, or fins continuously wound around and welded to the tube.

film

A thin fluid layer adjacent to a pipewall which remains in laminar flow, even when the bulk flow is turbulent. The velocity profile in the film is approximately linear, with zero velocity existing at the wall.

film coefficient

The convective heat transfer coefficient of the film.

film temperature

The maximum temperature in the film, at the tubewall.

flue gas

A mixture of gaseous products resulting from combustion of the fuel.

fouling

The building up of a film of dirt, ash, soot, or coke on heat transfer surfaces, resulting in increased resistance to heat flow.

header

The fitting which connects two tubes in a coil. In common usage, "header" refers to cast or forged 180° "U-bends" ("return bends").

heat density

The rate of heat transfer per unit area to a tube, usually based on total outside surface area. Typical units are Btu/hr-ft2. Also called "heat flux."

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heat duty

The total heat absorbed by the process fluid, usually expressed in MBtu/hr (million Btu per hour). Total fired heater duty is the sum of heat transferred to all process streams, including auxiliary services such as steam superheaters and drier coils.

mass velocity

The mass flow rate per unit of flow area through the coil. Typical units are lb/s-ft2.

one-side fired tubes

Radiant section tubes located adjacent to a heater wall have only one side directly exposed to a burner flame. Radiation to the back side of the tubes is by reflection/re-radiation from the refractory wall.

pass

A coil which transports the process fluid from fired heater inlet to outlet. The total process fluid can be transported through the heater by one or more parallel passes.

radiant section

The section of the fired heater in which heat is transferred to the heater tubes primarily by radiation from high-temperature flue gas.

shield section

The first two tube rows of the convection section. These tubes are exposed to direct radiation from the radiant section and usually receive about half of their heat in this manner. They are usually made of more resistant material than the rest of the tubes in the convection section. Extended surfaces are not used in this section.

sootblower

A steam lance (usually movable) in the convection section for blowing soot and ash from the tubes, using high-pressure steam.

superheater

Heat transfer surface downstream of the steam drum, which is designed to raise the steam temperature above the saturation temperature. The superheater is arranged within the boiler to absorb heat by radiation, convection, or both.

two-side fired tubes

Radiant section tubes which are exposed on both sides to direct radiation from the burners.

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