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Redesign of an Exhaust Gas Economiser Using Software

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Abstract

Approaches to heat exchanger designs are numerous. Marine heat exchangers are usually single and they do not form part of a large network. Selections are generally based on the duties, area and the heat quantum. Over capacities and un-optimised designs could result. As an exercise to verify the choice, an existing heat exchanger on board of an operational ship was redesigned using computer software with thermodynamic data and standard geometric values. The formulae employed in the software were extracted and verified. The geometric data was used to develop the design drawings using SolidWorks®. Visualising the designs, the physical arrangement was improved. Comparisons and design improvements were made keeping the standard values in view. With the exercises, a method of developing an optimised physical design reducing the number of rating runs has been demonstrated.

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Abbreviations

A :

Total surface area (m2)

A o :

Total outside area, tube (approximated) (mm2)

A b :

Area causing bypass streams (mm2)

A e :

Smallest area for cross flow between baffles (mm2)

A ee :

Area obtained from S e ·L E (mm2)

A f :

Area obtained as A f  = D i S (mm2)

A gsb :

Area of gaps between shell and baffles (mm2)

A gtb :

Total area of gaps between tubes and baffle holes (mm2)

A sg :

Total area given by addition of A gsb and A gtb (mm2)

A t :

Total area of tubes in the window section (mm2)

A wt :

Flow area in window section including tubes in window (mm2)

A w :

Flow area in window cross section (mm2)

C :

Constant obtained from Tables

Cp c :

Specific heat capacity, cold fluid (kJ/kg K)

Cp h :

Specific heat capacity, hot fluid (kJ/kg K)

D i :

Shell inside diameter (mm)

D s :

Shell outside diameter (mm)

D baf :

Baffle diameter (mm)

D bun :

Tube bundle diameter (mm)

D equi :

Equivalent diameter (mm)

D noz in :

Inlet nozzle diameter (mm)

F :

LMTD correction factor

F t :

Fouling factor

G s :

Shell side mass flow velocity (kg/m2 s)

G t :

Tube side mass flow velocity (kg/m2 s)

H :

Baffle cut height (mm)

K e :

Pressure drop factor

K 1 :

Constant based on number of tube passes

LMTD :

Logarithmic mean temperature difference

L :

Length of tubes (mm)

L E :

Sum of the shortest connections (\( 2e_{1} + \mathop \sum \nolimits e \)) (mm)

N b :

Number of baffles

N t :

Total number of tubes

Nu :

Nusselt number

Nu b :

Nusselt number of the tube bundle at operating conditions

Nu cb :

Nusselt number calculated for tube bundle

Nu ib :

Nusselt number of ideal tube bundle

Nu laminar :

Nusselt number for laminar flow in a tube

Nu s :

Nusselt number, shell side

Nu t :

Nusselt number, tube side

Nu turbulent :

Nusselt number for turbulent flow in a tube

NTP :

Number of tube passes

P :

Temperature effectiveness

Pr :

Prandtl number

Pr t :

Prandtl number of tube side fluid

Pr w :

Prandtl number at wall temperature

Q :

Heat transferred (W)

R :

Ratio of fluid capacities

R fi :

Fouling resistance inside (tube) (m2 K/W)

R fo :

Fouling resistance outside (shell) (m2 K/W)

R B :

Ratio A b /A e

R G :

Ratio n w /N t

R L :

Ratio A sg /A e

R M :

Ratio A gsb /A sg

R S :

Ratio n s /n mr

Re :

Reynolds number

Re e :

Reynolds number in end cross flow section

Re s1 :

Reynolds number for obtaining drag coefficients for turbulent and laminar flows

Re t :

Reynolds number for tube side fluid

Re Ψl :

Reynolds number for a tube bundle

S :

Baffle spacing (mm)

S e :

Baffle spacing in end channels (mm)

T 1 :

Shell side fluid inlet temperature (°C)

T 2 :

Shell side fluid outlet temperature (°C)

T h , i :

Hot fluid inlet temperature (°C)

T h , o :

Hot fluid outlet temperature (°C)

T c , i :

Cold fluid inlet temperature (°C)

T c , o :

Cold fluid outlet temperature (°C)

U :

Overall heat transfer coefficient (W/m2 K)

U o :

Overall heat transfer coefficient (approximated) (W/m2 K)

U w :

Wetted perimeter (mm)

\( \dot{V} \) :

Fluid flow rate (m3/s)

a :

Factor obtained from transverse pitch ratio (s 1/d o )

b :

Factor obtained from longitudinal pitch ratio (s 2/d o )

d b :

Hole diameter in baffles (mm)

d g :

Equivalent diameter for window section (mm)

d h :

Hydraulic diameter (mm)

d i :

Tube inside diameter (mm)

d n :

Diameter of nozzle (mm)

d o :

Tube outside diameter (mm)

e :

Shortest connection between adjacent tubes in the same row/adjacent row (mm)

e 1 :

Shortest connection between outermost tube and the shell (measured at the shell diameter parallel to baffle edge) (mm)

f B :

Bypass correction factor, shell side

f Fanning :

Fanning friction factor

f L :

Leakage correction factor, shell side

f Z :

Correction factor for change in physical properties, shell side

f alv :

Factor based on a, b and c

f atv :

Factor based on a and b

f a :

Tube arrangement factor

f b :

Bypass correction factor

f g :

Geometry correction factor

f l :

Leakage correction factor

f n :

Tube rows correction factor

f p :

Correction factor due to changes in physical properties near tube surfaces

f w :

Shell side flow correction factor

f zl :

Temperature correction factor (laminar flow)

f zt :

Temperature correction factor (Turbulent flow)

l′:

Characteristic length (mm)

m c :

Mass flow of cold fluid (kg/s)

m h :

Mass flow of hot fluid (kg/s)

m t :

Mass flow in tube side (kg/s)

n :

Exponent for Reynolds number

n pp :

Index based on number of tube passes and pitch arrangement

n mr :

Number of main resistances in cross flow between adjacent baffles (central section)

n mre :

Number of main resistances in end section

n mrw :

Number of effective main resistances in the window section

n s :

Number of pairs of sealing strips

n w :

Total number of tubes in lower and upper windows

r :

Exponent as defined

s 1 :

Transverse tube pitch (mm)

s 2 :

Longitudinal tube pitch (mm)

t 1 :

Tube side fluid inlet temperature (°C)

t 2 :

Tube side fluid outlet temperature (°C)

w :

Characteristic velocity (for Reynolds number) (m/s)

w e :

Velocity in the narrowest section of tube bundle (m/s)

w ee :

Velocity obtained from \( \dot{V}/A_{ee} \) (m/s)

w in noz :

Velocity in the inlet nozzle (m/s)

w n :

Velocity in nozzle (m/s)

w nozzle :

Velocity in the nozzle section (m/s)

w out noz :

Velocity in the outlet nozzle (m/s)

w p :

Velocity in window section as defined (m/s)

w t :

Velocity of fluid flow in tubes (m/s)

w z :

Velocity in window section as defined (m/s)

α i :

Tube side heat transfer coefficient (W/m2 K)

α o :

Shell side heat transfer coefficient (W/m2 K)

η sm :

Dynamic viscosity (mean), shell side fluid (Pa s)

η s :

Dynamic viscosity, shell side fluid (Pa s)

η t :

Dynamic viscosity, tube side fluid (Pa s)

η w :

Dynamic viscosity of fluid at wall temperature (Pa s)

λ s :

Shell side thermal conductivity (W/m K)

λ t :

Tube side thermal conductivity (W/m K)

ξ friction :

Drag coefficient due to friction

ξ lam :

Drag coefficient (laminar flow)

ξ noz :

Drag coefficient for inlet or outlet nozzles

ξ stb :

Drag coefficient for shell side staggered tube bundle

ξ tube in :

Drag coefficient for tube side inlet nozzle

ξ tube :

Drag coefficient obtained after corrections

ξ turb :

Drag coefficient (turbulent flow)

ρ noz :

Density of fluid in the nozzle region (kg/m3)

ρ in noz :

Density of fluid at inlet nozzle (kg/m3)

ρ out noz :

Density of fluid at outlet nozzle (kg/m3)

ρ sm :

Density (mean) of shell side fluid (kg/m3)

ρ s :

Density of shell side fluid (kg/m3)

ρ t :

Density of tube side fluid (kg/m3)

Δp in noz :

Pressure drop in inlet nozzle (Pa)

Δp qe0 :

Pressure drop in end sections without bypass & leakage streams (Pa)

Δp return :

Pressure drop approximated for nozzle entry/exit and flow (Pa)

Δp shell :

Pressure drop in shell side (Pa)

Δp tube friction :

Pressure drop in tubes due to friction (Pa)

Δp tube :

Pressure drop in the tubes (Pa)

Δp wlam :

Pressure drop due to laminar flow regime in window section (Pa)

Δp w turb :

Pressure drop due to turbulent flow regime in window section (Pa)

Δp e in out :

Pressure drop in inlet and outlet nozzles (Pa)

Δp friction :

Pressure drop in tube side due to friction (general equation) (Pa)

Δp n :

Pressure drop in nozzles (Pa)

Δp out noz :

Pressure drop in outlet nozzles (Pa)

Δp qo :

Pressure drop in central section without leakage and bypass streams (Pa)

Δp q :

Pressure drop in central section between adjacent baffles (Pa)

Δp qe :

Pressure drop in end section (Pa)

Δp tube friction :

Pressure drop due to tube friction (Pa)

Δp w :

Pressure drop in window section (Pa)

ΔT lm :

Logarithmic mean temperature difference (°C)

Φ :

Viscosity Correction factor

α :

Heat Transfer coefficient (W/m2 K)

β :

Constant for calculating bypass correction factor

γ :

Central angle due to baffle cut degrees

λ :

Thermal conductivity (W/m K)

ν :

Kinematic viscosity (m2/s)

ξ :

Drag coefficient

ρ :

Density (kg/m3)

Ψ :

Void fraction factor

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Acknowledgments

The author is thankful to MISC Berhad for the information and data on shipboard heat exchangers.

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Authors and Affiliations

  1. Malaysian Maritime Academy (ALAM), Window Delivery 2051, P.O. Masjid Tanah, 78300, Melaka, Malaysia

    R. Balaji

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  1. R. Balaji
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Correspondence to R. Balaji.

Appendix

Appendix

See Tables 6 and 7.

Table 6 Design calculations: heat transfer coefficient
Full size table
Table 7 Design calculations: pressure drops
Full size table

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Balaji, R. Redesign of an Exhaust Gas Economiser Using Software. J. Inst. Eng. India Ser. C 95, 273–289 (2014). https://doi.org/10.1007/s40032-014-0131-3

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