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Heat Exchanger Design

- Anand V P Gurumoorthy
- Associate Professor
- Chemical Engineering Division
- School of Mechanical Building Sciences
- VIT University
- Vellore, India

Heat Exchanger Classification

- Recuperative
- Cold and hot fluid flow through the unit without

mixing with each other. The transfer of heat

occurs through the metal wall. - Regenerative
- Same heating surface is alternately exposed to

hot and cold fluid. Heat from hot fluid is stored

by packings or solids this heat is passed over

to the cold fluid. - Direct contact
- Hot and cold fluids are in direct contact and

mixing occurs among them mass transfer and heat

transfer occur simultaneously.

Heat Exchanger Standards and Codes

- British Standard BS-3274
- TEMA standards are universally used.
- TEMA standards cover following classes of

exchangers - Class R designates severe requirements of

petroleum and other related processing

applications - Class C moderate requirements of commercial and

general process applications - Class B specifies design and fabrication for

chemical process service.

Shell and Tube Heat Exchanger

- Most commonly used type of heat transfer

equipment in the chemical and allied industries. - Advantages
- The configuration gives a large surface area in a

small volume. - Good mechanical layout a good shape for pressure

operation. - Uses well-established fabrication techniques.
- Can be constructed from a wide range of

materials. - Easily cleaned.
- Well established design procedures.

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Types of Shell and Tube Heat Exchangers

- Fixed tube design
- Simplest and cheapest type.
- Tube bundle cannot be removed for cleaning.
- No provision for differential expansion of shell

and tubes. - Use of this type limited to temperature

difference upto 800C. - Floating head design
- More versatile than fixed head exchangers.
- Suitable for higher temperature differentials.
- Bundles can be removed and cleaned (fouling

liquids)

Design of Shell and Tube Heat Exchangers

- Kern method
- Does not take into account bypass and leakage

streams. - Simple to apply and accurate enough for

preliminary design calculations. - Restricted to a fixed baffle cut (25).
- Bell-Delaware method
- Most widely used.
- Takes into account
- Leakage through the gaps between tubes and

baffles and the baffles and shell. - Bypassing of flow around the gap between tube

bundle and shell. - Stream Analysis method (by Tinker)
- More rigorous and generic.
- Best suited for computer calculations basis for

most commercial computer codes.

Construction Details Tube Dimensions

- Tube diameters in the range 5/8 inch (16 mm) to 2

inch (50 mm). - Smaller diameters (5/8 to 1 inch) preferred since

this gives compact and cheap heat exchangers. - Larger tubes for heavily fouling fluids.
- Steel tubes BS 3606 Other tubes BS 3274.
- Preferred tube lengths are 6 ft, 8 ft, 12 ft, 16

ft, 20 ft and 24 ft optimum tube length to shell

diameter ratio 5 10. - ¾ in (19 mm) is a good starting trial tube

diameter.

Construction Details Tube Arrangements

- Tubes usually arranged in equilateral triangular,

square or rotated square patterns. - Tube pitch, Pt, is 1.25 times OD.

Construction Details - Shells

- Shell should be a close fit to the tube bundle to

reduce bypassing. - Shell-bundle clearance will depend on type of

heat exchanger.

Construction Details - Shell-Bundle Clearance

Construction Details Tube Count

- Bundle diameter depends not only on number of

tubes but also number of tube passes. - Nt is the number of tubes
- Db is the bundle diameter (mm)
- D0 is tube outside diameter (mm)
- n1 and K1 are constants

Construction Details - Baffles

- Baffles are used
- To direct the fluid stream across the tubes
- To increase the fluid velocity
- To improve the rate of transfer
- Most commonly used baffle is the single segmental

baffle. - Optimal baffle cut 20-25

Basic Design Procedure

- General equation for heat transfer is
- where Q is the rate of heat transfer (duty),
- U is the overall heat transfer coefficient,
- A is the area for heat transfer
- ?Tm is the mean temperature difference
- We are not doing a mechanical design, only a

thermal design.

Overall Heat Transfer Coefficient

- Overall coefficient given by
- h0 (hi) is outside (inside) film coefficient
- hod (hid) is outside (inside) dirt coefficient
- kw is the tube wall conductivity
- do (di) is outside (inside) tube diameters

Individual Film Coefficients

- Magnitude of individual coefficients will depend

on - Nature of transfer processes (conduction,

convection, radiation, etc.) - Physical properties of fluids
- Fluid flow rates
- Physical layout of heat transfer surface
- Physical layout cannot be determined until area

is known hence design is a trial-and-error

procedure.

Typical Overall Coefficients

Typical Overall Coefficients

Fouling Factors (Dirt Coeffs)

- Difficult to predict and usually based on past

experience

Mean Temperature Difference (Temperature Driving

Force)

- To determine A, ?Tm must be estimated
- True counter-current flow logarithmic

temperature difference (LMTD)

LMTD

- LMTD is given by
- where T1 is the hot fluid temperature, inlet
- T2 is the hot fluid temperature, outlet
- t1 is the cold fluid temperature, inlet
- t2 is the cold fluid temperature, outlet

Counter-current Flow Temperature Proflies

12 Heat Exchanger Temperature Profiles

True Temperature Difference

- Obtained from LMTD using a correction factor
- ?Tm is the true temperature difference
- Ft is the correction factor
- Ft is related to two dimensionless ratios

Temp Correction Factor Ft

- Temperature correction factor, one shell pass,

two or more even tube passes

Fluid Allocation Shell or Tubes?

- Corrosion
- Fouling
- Fluid temperatures
- Operating pressures
- Pressure drop
- Viscosity
- Stream flow rates

Shell and Tube Fluid Velocities

- High velocities give high heat-transfer

coefficients but also high pressure drop. - Velocity must be high enough to prevent settling

of solids, but not so high as to cause erosion. - High velocities will reduce fouling
- For liquids, the velocities should be as follows
- Tube side Process liquid 1-2m/s
- Maximum 4m/s if required to reduce fouling
- Water 1.5 2.5 m/s
- Shell side 0.3 1 m/s

Pressure Drop

- As the process fluids move through the heat

exchanger there is associated pressure drop. - For liquids viscosity lt 1mNs/m2 35kN/m2
- Viscosity 1 10 mNs/m2 50-70kN/m2

Tube-side Heat Transfer Coefficient

- For turbulent flow inside conduits of uniform

cross-section, Sieder-Tate equation is

applicable - C0.021 for gases
- 0.023 for low viscosity liquids
- 0.027 for viscous liquids
- µ fluid viscosity at bulk fluid temperature
- µwfluid viscosity at the wall

Tube-side Heat Transfer Coefficient

- Butterworth equation
- For laminar flow (Relt2000)
- If Nu given by above equation is less than 3.5,

it should be taken as 3.5

Heat Transfer Factor, jh

- j factor similar to friction factor used for

pressure drop - This equation is valid for both laminar and

turbulent flows.

Tube Side Heat Transfer Factor

Heat Transfer Coefficients for Water

- Many equations for hi have developed specifically

for water. One such equation is - where hi is the inside coefficient (W/m2 0C)
- t is the water temperature (0C)
- ut is water velocity (m/s)
- dt is tube inside diameter (mm)

Tube-side Pressure Drop

- where ?P is tube-side pressure drop (N/m2)
- Np is number of tube-side passes
- ut is tube-side velocity (m/s)
- L is the length of one tube
- m is 0.25 for laminar and 0.14 for turbulent
- jf is dimensionless friction factor for heat

exchanger tubes

Tube Side Friction Factor

Shell-side Heat Transfer and Pressure Drop

- Kerns method
- Bells method

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Procedure for Kerns Method

- Calculate area for cross-flow As for the

hypothetical row of tubes in the shell equator. - pt is the tube pitch
- d0 is the tube outside diameter
- Ds is the shell inside diameter
- lB is the baffle spacing, m.
- Calculate shell-side mass velocity Gs and linear

velocity, us. - where Ws is the fluid mass flow rate in the

shell in kg/s

Procedure for Kerns Method

- Calculate the shell side equivalent diameter

(hydraulic diameter). - For a square pitch arrangement
- For a triangular pitch arrangement

Shell-side Reynolds Number

- The shell-side Reynolds number is given by
- The coefficient hs is given by
- where jh is given by the following chart

Shell Side Heat Transfer Factor

Shell-side Pressure Drop

- The shell-side pressure drop is given by
- where jf is the friction factor given by

following chart.

Shell Side Friction Factor

(Figure 8 in notes)

(Figure 4 in notes)

(Figure 2)

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(Figure 9 in notes)

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(Table 3 in notes)

(Figure 10 in notes)

(Figure 12 in notes)

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Bells Method

- In Bells method, the heat transfer coefficient

and pressure drop are estimated from correlations

for flow over ideal tube banks. - The effects of leakage, by-passing, and flow in

the window zone are allowed for by applying

correction factors.

Bells Method Shell-side Heat Transfer

Coefficient

- where hoc is heat transfer coeff for cross flow

over ideal tube banks - Fn is correction factor to allow for no. of

vertical tube rows - Fw is window effect correction factor
- Fb is bypass stream correction factor
- FL is leakage correction factor

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Bells Method Ideal Cross Flow Coefficient

- The Re for cross-flow through the tube bank is

given by - Gs is the mass flow rate per unit area
- d0 is tube OD
- Heat transfer coefficient is given by

Bells Method Tube Row Correction Factor

- For Regt2100, Fn is obtained as a function of Ncv

(no. of tubes between baffle tips) from the chart

below - For Re 100ltRelt2100, Fn1.0
- For Relt100,

Bells Method Window Correction Factor

- Fw, the window correction factor is obtained from

the following chart - where Rw is the ratio of bundle cross-sectional

area in the window zone to the tube bundle

cross-sectional area (obtained from simple

formulae).

Bells Method Bypass Correction Factor

- Clearance area between the bundle and the shell
- For the case of no sealing strips, Fb as a

function of Ab/As can be obtained from the

following chart

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Bells Method Bypass Correction Factor

- For sealing strips, for NsltNcv/2 (Ns is the

number of baffle strips) - where a1.5 for Relt100 and a1.35 for Regt100.

Bells Method Leakage Correction Factor

- Tube-baffle clearance area Atb is given by
- Shell-baffle clearance area Asb is given by
- where Cs is baffle to shell clearance and ?b is

the angle subtended by baffle chord - ALAtbAsb
- where ßL is a factor obtained from following

chart

Coefficient for FL, Heat Transfer

Shell-side Pressure Drop

- Involves three components
- Pressure drop in cross-flow zone
- Pressure drop in window zone
- Pressure drop in end zone

Pressure Drop in Cross Flow Zone

- where ?Pi pressure drop calculated for an

equivalent ideal tube bank - Fb is bypass correction factor
- FL is leakage correction factor
- where jf is given by the following chart
- Ncv is number of tube rows crossed
- us is shell-side velocity

Friction Factor for Cross Flow Banks

Bells Method Bypass Correction Factor for

Pressure Drop

- a is 5.0 for laminar flow, Relt100
- 4.0 for transitional and turbulent flow, Regt100
- Ab is the clearance area between the bundle and

shell - Ns is the number of sealing strips encountered

by bypass stream - Ncv is the number of tube rows encountered in

the cross- flow section

Bells Method Leakage Factor for Pressure Drop

- where Atb is the tube to baffle clearance area
- Asb is the shell to baffle clearance area
- AL is total leakage area AtbAsb
- ßL is factor obtained from following

chart

Coefficient for FL

Pressure Drop in Window Zones

- where us is the geometric mean velocity
- uw is the velocity in the window zone
- Ws is the shell-side fluid mass flow
- Nwv is number of restrictions for cross-flow in

window zone, approximately equal to the number of

tube rows.

Pressure Drop in End Zones

- Ncv is the number of tube rows encountered in the

cross-flow section - Nwv is number of restrictions for cross-flow in

window zone, approximately equal to the number of

tube rows.

Bells Method Total Shell-side Pressure Drop

Effect of Fouling

- Above calculation assumes clean tubes
- Effect of fouling on pressure drop is given by

table above

Condensers

- Construction of a condenser is similar to other

shell and tube heat exchangers, but with a wider

baffle spacing - Four condenser configurations
- Horizontal, with condensation in the shell
- Horizontal, with condensation in the tubes
- Vertical, with condensation in the shell
- Vertical, with condensation in the tubes
- Horizontal shell-side and vertical tube-side are

the most commonly used types of condenser.

Heat Transfer Mechanisms

- Filmwise condensation
- Normal mechanism for heat transfer in commercial

condensers - Dropwise condensation
- Will give higher heat transfer coefficients but

is unpredictable - Not yet considered a practical proposition for

the design of condensers - In the Nusselt model of condensation laminar flow

is assumed in the film, and heat transfer is

assumed to take place entirely by conduction

through the film. - Nusselt model strictly applied only at low liquid

and vapor rates when the film is undisturbed. - At higher rates, turbulence is induced in the

liquid film increasing the rate of heat transfer

over that predicted by Nusselt model.

Condensation Outside Horizontal Tubes

- where (hc)1 is the mean condensation film

coefficient, for a single tube - kL is the condensate thermal conductivity
- ?L is the condensate density
- ?v is the vapour density
- µL is the condensate viscosity
- g is the gravitational acceleration
- G is the tube loading, the condensate flow per

unit length of tube. - If there are Nr tubes in a vertical row and the

condensate is assumed to flow smoothly from row

to row, and if the flow is laminar, the top tube

film coefficient is given by

Condensation Outside Horizontal Tubes

- In practice, condensate will not flow smoothly

from tube to tube. - Kerns estimate of mean coefficient for a tube

bundle is given by - L is the tube length
- Wc is the total condensate flow
- Nt is the total number of tubes in the bundle
- Nr is the average number of tubes in a vertical

tube row - For low-viscosity condensates the correction for

the number of tube rows is generally ignored.

Condensation Inside and Outside Vertical Tubes

- For condensation inside and outside vertical

tubes the Nusselt model gives - where (hc)v is the mean condensation coefficient
- Gv is the vertical tube loading, condensate per

unit tube perimeter - Above equation applicable for Relt30
- For higher Re the above equation gives a

conservative (safe) estimate. - For Regt2000, turbulent flow situation analyzed

by Colburn and results in following chart.

Colburns Results

Boyko-Kruzhilin Correlation

- A correlation for shear-controlled condensation

in tubes simple to use. - The correlation gives mean coefficient between

two points at which vapor quality, x, (mass

fraction of vapour) is known. - 1,2 refer to inlet and outlet conditions

respectively - In a condenser, the inlet stream will normally be

saturated vapour and vapour will be totally

condensed. For these conditions

Flooding in Vertical Tubes

- When the vapor flows up the tube, tubes should

not flood. - Flooding should not occur if the following

condition is satisfied - where uv and uL are velocities of vapor and

liquid and di is in metres. - The critical condition will occur at the bottom

of the tube, so vapor and liquid velocities

should be evaluated at this point.

Condensation Inside Horizontal Tubes

- When condensation occurs, the heat transfer

coefficient at any point along the tube will

depend on the flow pattern at that point. - No general satisfactory method exists that will

give accurate predictions over a wide flow range.

Two Flow Models

- Two flow models
- Stratified flow
- Limiting condition at low condensate and vapor

rates - Annular flow
- Limiting condition at high vapor and low

condensate rates - For stratified flow, the condensate film

coefficient can be estimated as

- Condensation of steam
- For air-free steam a coefficient of 8000 W/m2-0C

should be used. - Mean Temperature Difference
- A pure, saturated, vapor will condense at a

constant temperature, at constant pressure. - For an isothermal process such as this, the LMTD

is given by - where Tsat is saturation temperature of vapor
- t1 (t2) is the inlet (outlet) coolant

temperature - No correction factor for multiple passes is

needed.