process design of heat exchangers.pdf

7/25/2019 Process Design of Heat Exchangers.pdf

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(Shell & Tube HEs)

Unit III

Niteen R Yeole



Most widely used – chemical industry

Parts- standardized in size

Design – TEMA standards

IS: 4503 – Equivalent Indian Std



Std sizes of shell, tubes, tierods etc

Max allowable baffle spacing, minimum tubesheet thickness, baffle thickness, no. of tierods

etc. For mechanical design and fabrication, in

addition to TEMA std, ASME Code Section VIIIDiv. I is used.



Class R- severe duties in hydrocarbon industry

Class B – chemical process industries notinvolving severe duties

Class C – commercial and in less importantprocess applications



HTRI: Heat Transfer Research Institute, USA

HTFS : Heat Transfer Fluid Flow Services, UK

BJAC, USA

HEI: Heat Exchange Institute, USA

For ht without phase change-design methodsused by these softwares are based on Tinker’s

flow model.



Costliest part

Size range (TEMA) – 6 in to 60 in

Std pipes available upto 24 in

Above 24 in – to be fabricated by rolling a plate

Fixed tube sheet STHE – gap between shell &bundle is minimum (10 to 20 mm)

Floating head HE – gap is max (90-100 mm)



1. Based on actual tube sheet layout drawing

6 Parameters to be fixed

OD of tube

No of tube side pases

Tube pitch

Tube arrangement

No of tubes

Type of HE – fixed tube, U tube or floatinghead



2 Use of standard tables

Max no of tubes – shell (6 in – 120 in), tube sidepasses, tube pitch, arrangement of tubes.



3 Use of approximate equations

Shell dia or tube bundle dia can be found out Db = d0 (Nt/k1)

1/n1

where Db = Tube bundle dia, mm

d0 = Tube OD, mm Nt = Total no. of tubes

k1 & n1 are constants

Above equation is used for fixed tube sheettype and floating head shell and tube heatexchangers.



Single pass shell is used in the most of thecases.



Direct the fluid stream across the tubes

Increase the fluid velocity

Increase the turbulence

Increase the rate of ht

Indirectly support the tubes and therebyreduce the vibration in tubes

Most commonly used type-segmental baffle



Size range – 0.25” -2.5”

For std tubes size ie equal to tube OD

Tube thickness – Birmingham Wire Gauge

(BWG) As BWG increases thickness decreases.

For no phase change HEs & condensers, ¾ in

(19.05 mm) OD tube is widely used. For Reboiler, 1 in (25.4 mm) OD is common.

Std lengths-6 ft, 8 ft, 12 ft, 16 ft & 6m.



Tube side passes are provided to decrease thetube side flow area and to increase tube sidefluid velocity thereby to improve the tube side

htc at the expense of pressure drop.



1. Calculation of heat duty

2. Selection of Cooling medium or heatingmedium

3. Fluid Allocation4. Establishment of energy balance or heat duty

balance/ finding of the mass flow rate of

heating medium or cooling medium required5. Calculation of Mean temperature difference

6. Estimation of overall heat transfer coefficient



7. Finding shell diameter

8. Calculation of tube side htc

9. Calculation of tube side pressure drop

10. Calculation of shell side htc

11. Calculation of shell side pressure drop

12. Calculation of the overall htc13. Calculation of heat transfer area



More versatile than fixed tube and U-tubeexchangers.

Suitable for high temperature differentials

Easier to clean and can be used for foulingliquids.



Triangular and rotated square patterns givehigher heat transfer rates but at the expense ofa higher pressure drop than the square pattern.

A square or rotated square arrangement , isused for heavily fouling fluids where it isnecessary to mechanically clean the outside ofthe tubes.

Recommended tube pitch (distance betweentube centres) is 1.25 times the tube OD



No of passes is selected to give the requiredtube side design velocity.

Exchangers are built from 1-16 tube passes.



m/n

m-no of shell passes

n-no of tube passes



Baffle cut specifies the dimensions.

It is expressed as a percentage of the baffle discdiameter.

Baffle cuts from 15 to 45 % are used. Optimum cut- 20 to 25 %

Spacing- 0.2 to 1.0 times shell diameter

Optimum spacing- 0.3 to 0.5 times shelldiameter



Gas oil at 2000C is to be cooled to 400C. The oil

flow rate is 50000 kg/hr. The cooling water is

available at 250C and the temperature rise is

limited to 300C . The pressure drop allowance

on shell side is 100 kN/m2. Calculate the shell

side heat transfer coefficient as well as thepressure drop. Neglect the viscosity correction

on shell side.



Data:

1] Properties of Gas Oil at 1200C

Specific heat capacity (Cp) = 2.28 KJ/(Kg 0C)Thermal conductivity (k) = 0.125 W/(m0C)

Dynamic Viscocity (µ) = 0.17 mN/(m2 s)

Density = 850 kg/m3

2] Tube Dimensions & Thermal conductivity



2] Tube Dimensions & Thermal conductivity

Outside diameter (d0) = 20 mm

Inside diameter (di) = 16 mmLength = 4 m

Thermal conductivity = 50 W/(m0C)

Pitch = Triangular (Pt = 1.25 d0 )



3] Baffle details

Type = Segmental

Cut = 25%

Spacing = 0.5* Shell inside diameter

4] Exchanger Type

1-4, Split-ring floating head

process design of heat exchangers.pdf

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