Double Pipe Heat Exchanger Design

Guidelines to do design sizing for Double Pipe Heat Exchanger and estimate length of double pipe required.

Step 1

Determine Heat Load Obtain flowrate

(W ),

inlet, outlet temperatures and fouling factor for both hot and cold stream. stream. Calculate

physical properties like density

(ρ ) ,

viscosity

(μ ) ,

specific heat

(C p )  and

thermal conductivity

(k )  at

mean

temperature. temperature. Determine Determine heat load by energy balances on two two streams Q

= mH * CpH * ( THot In - THot Out) = mC * CpC * ( tCold Out - tCold In)

where, mH , mC : Mass Mass flow flow rate of Hot Hot and and Cold Cold Stre Stream am CpH , Cp CpC : Spec Specif ific ic Heat Heat of of Hot Hot and and Cold Cold Stream Stream T Hot In , T Hot Out:  Inlet and outlet temperature of Hot Stream tCol d In , tCol d Out:  Inlet and outlet temperature of Cold Stream

Step 2

Calculate Calculate Logarthmic Logarthmic Me an Temper Temper ature Differ en ce (LMTD) LMTD LMTD

= (Δ (ΔT1 - ΔT2 ΔT2)) / ln( ΔT1 / ΔT2 ΔT2))

For Counter-current flow  ΔT1

= THot In - t Cold Out

 ΔT2

= THot Out - t Cold In

For Co-current flow  ΔT1

= THot In In - t Cold In

 ΔT2

= THot Ou Out - t Cold Ou Out

Step 3

Calculate Calculate Film Coe Coe fficient  Allocate hot a nd cold str eams either in inner tube or o r annular ann ular space. spa ce. General Gene ral criter ia for fluid placement place ment in inner tube is corrosive fluid, cooling water, fouling fluid, hotter fluid and higher pressure stream. Calculate Calculate equivalent diameter Inner Tube De

= Di

 Af

= π Di²/4

(De ) and

flow area

(Af  )  for

both b oth streams.

Annular Space De

= D1 - Do

 Af

= π (D 1² - D o²)/4

where, Di  : Inside Pipe Inner Diameter  Do : Inside Pipe Outer Diameter  D1 : Outside Pipe Inner Diameter  Calculate velocity

(V ),

Reynolds

V

= W / ( ρ * Af  )

Re

= De * V * ρ / μ

Pr

= Cp * μ / k

(Re )  and

Prandtl

(Pr )  number

for each stream.

For first iteration a Length of double pipe exchanger is assumed and heat transfer coefficient is calculated. Viscosity correction factor (μ / μw ) 0.14 due to wall temperature is considered 1. For Laminar Flow (Re <= 2300), Seider Tate e quation is used. Nu

= 1.86 * (Re.Pr.De / L )1/3 * (μ / μw )0.14

For Transient & Turbulent Flow (Re > 2300), Petukhov and Kirillov equation modified by Gnielinski can be used. Nu

= (f/8).(Re - 1000).Pr.(1 + De/L)2/3/ [1 + 12.7.(f/8)0.5.(Pr2/3 - 1)] * (μ / μw ) 0.14

f

= ( 0.782* ln(Re) - 1.51 )-2

where, L : Length of Double Pipe Exchanger   μw : Viscosity of fluid at wall temperature Nu : Nusselts Number (h.De / k)

Step 4

Estimate Wall Temperature Wall temperature is calculated as following. TW

= ( hi .tAve + ho .TAve .Do/ Di ) / ( hi + ho .Do/ Di )

where, hi  : Film coefficient Inner pipe ho : Film coefficient for Annulus t Ave : Mean temperature for Inner pipe fluid stream T Ave : Mean temperature for Annulus fluid stream

Viscosity is calculated for both streams at wall temperature and heat transfer coefficient is multiplied by viscostiy correction factor.

Step 5

Overall Heat Transfer Coefficient Overall heat transfer coefficient 1/ U

(U ) is

calclated as following.

= Do/hi.Di + Do.ln(Do / Di )/ 2kt + 1/ho + Ri . Do / Di + Ro

where, Ri  : Fouling factor Inner pipe Ro : Fouling factor for Annulus k t  : Thermal conductivity of tube material  Calculate Area and length of double pipe exchanger as following.  Area

= Q / (U * LMTD )

L

= Area / π * Do

Compare this length with the assumed length in step 3, if considerable difference is there use this length and repeat from step 3, till there is no change in length calculated. Number of hair pin required is estimated as following. N Hairpin

= L / ( 2 * Length Hairpin )

Step 6

Calculate Pressure Drop Pressure drop in straight section of pipe is calculated as following.  ΔPS

= f.L.G² / (7.5x1012.De.SG. (μ / μ w )0.14 )

where,  ΔP : Pressure Drop in PSI  SG : Specific Gravity of fluid  G : Mass Flux ( W / Af  ) in lb/h.ft²  For Laminar flow in inner pipe, friction factor can be computed as following. f

= 64 / Re

For Laminar flow in the annulus. f

= (64 / Re) * [ (1 - κ²) / ( 1 + κ² + (1 - κ²) / ln κ) ]

κ

= Do / D1

For tubulent flow in both pipe and annulus

= 0.3673 * Re -0.2314

f

Pressure Drop due to Direction Changes For Laminar Flow.  ΔPR

= 2.0x10-13. (2NHairpin - 1 ).G²/SG

For Turbulent Flow.  ΔPR

= 1.6x10-13. (2NHairpin - 1 ).G²/SG

Total Pressure Drop Total Pressure drop in both inner pipe and annulus is calculated by adding pressure drop due to straight section and direction change.  ΔPTotal

= ΔPS + ΔPR

References 1. Process Heat Transfer: Principles and Applications 2. VDI Heat Atlas (VDI-Buch)

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