Thermo-economic Optimization of STHE

P M V SubbaraoProfessor

Mechanical Engineering Department

I I T Delhi

Minimizing capital and operating costs of shell

and tube Heat Exchangers…..

Shell and Tube as a Thermal System

Two major cost components of a heat exchange system are :1. Capital, operating and maintenance costs of the pump/compressors.2. Capital and maintenance costs of the heat exchanger.

Case Study :Total Cost Minimization

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Thermodynamic Strategy for Lowering Cost

Selection of Shell Side Gas Pressure

Cost Analysis of Heat Exchanger Operation

• The cost of owning and operating a heat exchanger is the sum of:

• The cost of providing the heat transfer surface, CHT : This cost is taken as proportional to heat transfer area.

• The costs of maintaining the flow through it, Cpump : This one is assumed as proportional to the pumping power.

• The cost of heating or cooling effect supplied to the exchanger Cmaint: This one is represented by an amount in proportion to the heat duty of the exchanger.

• An optimum design would be defined as an exchanger which has the maximum ratio of the heat duty to the cost

Total Cost of SHTE

intmapumpHTtotal CCCC

HaPaAaCtotal 311

The optimum design is defined as an exchanger which has the maximum ratio of the heat duty to total cost.

HaPaAa

H

C

HJ

total 311

HaPaAa

HJ

321

321

1

aH

PaAaJ

Define C

H

PaAaC 21

Normalizing C using a reference cost function:

H

aaCr

21

Objective function

21

21

aa

PaAa

C

CF

r

21

2

21

1

aa

Pa

aa

AaF

PWAWF 21

Independent Variables for Objective Function

• The area and pumping power vary considerably with respect to baffle space.

• As the baffle spacing is reduced, the pressure drop increases at a much faster rate than does the heat transfer coefficient.

• Thus, due to the considerable pressure drop, the pumping power cost increases as the area cost decreases.

• This means that there will be an optimum value of baffle spacing corresponding to minimum dimensionless F.

Heat Transfer Area Vs Baffle Spacing

B/Ds

Required Pumping Power Vs Baffle Spacing

B/Ds

P/Pmax

Heat Transfer Area Vs Baffle Spacing

B/Ds

P/Pmax

Normalized required heat transfer area vs dimensionless baffle spacing for four different pairs of sealing strips.

B/Ds

Normalized pumps power consumption vs dimensionless baffle spacing for four different pairs of sealing strips.

P/Pmax

Scope for Parametric Study

• A parametric study for obtaining the optimum design should be made for all types of single phase shell and tube heat exchangers (fixed tube sheet, floating head and U tube) in a wide range of normal operational conditions.

• Tube and shell diameter, tube pitch, number of tube passes, tube arrangement, baffle spacing and number of sealing strips should be varied for all recommended values .

• The heat duty and viscosity coefficient of both streams should also be varied extensively.

• A collection of optimal designs is a treasure of data base for a designer.

How to convert Data Base into Knowledge Base

• Is it possible to identify the technical characteristic of the optimized design.

• A correlation of non-dimensional variables.

Physics of Optimal Solution : E type SHTE

W1

Thermo-economics of Optimal Designs

E type SHTE

Floating Head SHTE

U tube SHTE

Generality of Optimal Design

W1

Experience Vs Optimization : E Shell

Baffle spacing in optimal designs

Baf

fle

spac

ing

in e

xper

ienc

e b

ased

Des

ign

Experience Vs Optimization

Baffle spacing in optimal designs

Baf

fle

spac

ing

in e

xper

ienc

e ba

sed

Des

ign