Basic Refinery Equipment- Heat Exchanger (E003/1)

Punnee Kitjareonvong

by

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Outline

Introduction Theory of Heat transfer Basic principles of heat exchangers Types of heat exchangers Functions of each heat exchanger Design Principle parameters and criteria for heat exchanger design

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Why require Heat Exchanger?

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Introduction

Why require?

Heat generate at a high temp ( > 300 deg C) in a furnace (fuel gas , fuel oil, waste gas , off-gas ., etc) by combustion to desired temperature.

After that, heat recovery is required when already satisfy the process requirement for economic optimization .

Down to almost close to air inlet temp (50 degC) or cooling water temp ( 45 deg C) depend on process side requirement.

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Key function

Any devices which allow heat to exchange from one fluid to another, physically separated.

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Heat Transfer Principle

Sensible heat : Temperature change Q = m * Cp * (T2-T1)

where Cp = Specific heat (kg/kj-deg K)

m = Flowrate ( kg/s)

Ti = Temperature (deg K)

Latent heat : Phase change whereas temperature constant Q = m * LHV

where m = Flowrate (kg/s)

LHV = Lower Heating Valve (kj/kg)

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Three Mechanisms of heat transfer : Conduction, : laminar film liquid through the tube wall

in heat exchanger

Convection : in space where gas/liquid flow is turbulent i.e, fluid inside or outside the tube of exchangers.

Radiation : at high temperature ( > 200-300 degC) ie., in furnace.

Heat changer Conduction (tube wall) and

convection ( fluid inside/outside tube )

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Design a shell &tube Heat Exchanger

T1 =250 C T2 = 150 C

t1 = 120 C

t2 = 230 C

Shell : TTube : t

Q = F*U*A*LMTD

Correction factor

Total effective area (m2)

Local overall heat transfer coefficient , W/m2 K (fouled , cleaned)

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GTD

LTD

T1 =250

t2 = 230

T2 = 150

t1 = 120

Greater Temp Difference Lesser

Temp Difference

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LMTD

Long Mean Temperature Difference

= average driving force

LMTD = (GTD- LTD)/ ln(GTD/LTD) , deg K

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Correction Factor (F)

Depend on the heat exchanger internal arrangement and temperature of the respective flow.

Temperature efficiency P = (t2-t1)/(T1-t1)

Capacity Ratio

R = (T1-T2)/(t2-t1)

Use P, R, and geometry to indicate the F valve

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Fouling Resistance/coefficient

Fouling Resistance= ultimate additional resistance to heat transfer cause by deposit and corrosion = f( type of fluid, material, temperature, flow velocity)

Fouling coefficient = the reciprocal of fouling resistance

( an increase fouling = and decrease in fouling coefficient)

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Types of heat exchangers

Shell&tube Heat exchanger feed preheated/ Trim cooler (by cooling water)

Thermosiphon/Kettle-type Reboiler

Air cooler Condensor

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Shell&Tube Heat exchanger

Tube side : 1-pass/2-pass compartment and Floating head for thermal expansion of shell&tube.

Shell Side : baffle to force the cross flow pattern

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Air cooler

Finned tubes, through which the process fluid flows with air being forced passing the tube bundle by fans

There are 2 major types ;Forced draft

Induced draft

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Thermosiphon/Kettle type :Reboiler

a) Kettle type Tubes are kept/submerged in the

boiling liquid by mean of an overflow weir

The heating medium flow through 2-tube passes

To use in cases where large volumes of vapor generated.

Horizontal heat exchanger

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b) Thermosyphon Vertical single pass Heat exchanger liquid will vaporize pass upward

through tube and the remaining

liquid will leave at the top and

leave out from the bottom of

column.

Heating media is on shell side. circulation in tube is driven by

difference of density between the

bottom column liquid and

gas/liquid mixture in tube (gas lift) principle)

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Design Strategy :Heat integrationSimultaneous shutdown of unit involved

acceptable. (inter-unit heat exchange)

Or units with a different shutdown timing in case different in heat sink/source are small and/or shut down duration is short.

Achieve the best targets (duties, target temperature etc.,)

Green field project

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Design Strategy :Heat integration(cont)

Sensitivity cases should be considered to absorb the disturbances in actual operation 5 deg C lower (hot stream) or higher (hot stream)

supply temperature

10%duty to be process adjustment ,related to cut-point ie., reflux duty

10%duty lower upstream of desalter (not too low)

OHTC (U) from the existing testrun is very useful for revamped case.

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Process Engineer

New /Revamped Heat Exchanger1. Known parameter from process datasheet.

a) The required duty (MW)

b) Hot & cold fluid temperature inlet/outlet (deg C)

c) Flowrate (TPD)

2. Heating curve at each pressure and varied temperature. do either the HTRI (Heat Transfer Research Inc. Xistprogram) or the HTFS (Heat Transfer and Fluid Flow Service TASC Program.

3. Unknow parameter U fouled/ U cleaned and the required area (m2)

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Process Engineer (cont.)4. The case which result the largest theoretical area should be

selected as the design heat exchanger(base case) when min fuel consumption is the main objective.

5. The case with the largest hot utility requirement should be taken as base case when min investment is the objective and no constraint of utility availability.

6. In case furnace duty in base case is lower than other modes, to consider reduce the heat exchanger area in base case should be reviewed utilize stable furnace capacity

7. To check the pressure drop , velocity ,likelihood of mechanical and/or acoustic vibration and nozzle momentum (rV2)

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Process Engineer (cont.)

7. To cater the increased dP on shell&tube side from fouling , rule of thumb can be applied

Typical dP,shell =0.5 bar/tube side = 0.5-1.0 bar

8. Tube side erosion :

rV2 < 7000 kg/m s2 for gas/vapor

rV2 using the max. water velocity

Fouling resistance(m2K/w)

Multiplication factor

0.00009 to 0.00033 1.1

0.00034 to 0.00085 1.2

0.00086 and higher 1.3

Heat Exchanger Datasheet22

Process Engineer (cont.)

9. Cost concern: To reduce dP (pressure drop) parallel heat exchanger train lower shut-off head and design pressure system.

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Engineering Requirement

For Shell and Tube heat exchanger

1. The heated fluid : flow upward/ the cooled fluid : flow downward.

2. More fouling and corrosive fluid is on tube side for easy to maintenance (No need to pull tube bundle for cleaning) including the cooling water service

3. Better heat transfer, lower pressure drop, smaller size viscous fluid should be on shell side against the fouling aspect.

4. For economic, the higher design pressure is on tube side. (on mechanical construction)

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Engineering Requirement (cont.)

5. For economic, more expansive construction material due to fluid properties, it should be on the tube side.

6. Heat exchanger should be horizontal except thermosiphon reboiler.

7. For longer cycle , the velocity on shell and tube side shall be > 0.5 and 1 m/s respectively during design phase.

8. Isolation valves and by-pass line should be provided for maintenance with the unit still in operation.

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9. Flange connection in hydrogen service (>200 C) which are not readily accessible can have smothering steam ring. for fire extinguish from hydrogen leakage. Or steam lance (with good access)

10. Max. surface area of a heat exchanger shell is approx. 600 m2 for floating head and 600-700 m2 for U-tube

11. Min F-factor for shell and tube is 0.8 more shell in series should be considered.

12. Piping and heat exchanger in high viscosity service shall not provided with capacity relief valves due to blockage design system for shut-off head pump

Engineering Requirement (cont.)

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For air cooler

13. The process fluid usually flow downward.

14. Forced draft : process oultet air inlet > 15 deg C / ease of fan, motor maintenance

15. Induced draft : Process outlet air inlet < 15 deg C

16. If process temperature is required, variable speed (VRS)drives to adjust automatic pitch fan.

17. When water is injected , good distribution shall be critical and be noted in the datasheet for symmetrical lay out of parallel tube bundle.

Engineering Requirement (cont.)

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