Heat Exchangers Complete

HEAT EXCHANGERSHEAT EXCHANGERS

HEAT EXCHANGER HEAT EXCHANGER SO FAR……….. SO FAR………..

Types of Heat Exchanger Application of Heat Exchanger Heat Exchanger Design

Consideration Factors Affecting Design Heat Transfer Co-efficient Temperature Approach

SCOPE OF SCOPE OF PRESENTATIONPRESENTATION

Fouling in Heat Exchanger Pressure Drop Design consideration Fluid Allocation Calculation of Shell and Tube Heat

exchanger

FOULING IN HEAT FOULING IN HEAT EXCHANGEREXCHANGER

“Fouling” is a general term that includes any kind of deposition of extraneous material that appears upon the heat transfer area during the lifetime of the heat exchanger

TYPES OF FOULINGPrecipitation foulingParticulate foulingChemical reaction foulingCorrosion foulingBiological fouling

MATERIALS SELECTION MATERIALS SELECTION FOR FOULING SERVICESFOR FOULING SERVICES

Copper-bearing alloy such as

90-10 copper- nickel

70-30 copper- nickel

83-17-0.4 copper-nickel-chromium

PRESSURE DROPPRESSURE DROP

Pressure drop in Heat Exchanger is made up of three losses

Frictional loss due to flow

Loss due to changes in direction

Losses due to sudden expansion or

contraction

DESIGN CONSIDERATIONDESIGN CONSIDERATION

Number of shells Tube selection

TypeLengthDiameter and Wall Thickness

Type of HeadStationary HeadRear Head

Tube Layout Baffles

Tube Layouts

There are four types of tube layouts

Square (900)

Rotated square (450)

Triangular (300)

Rotated triangular

(600)

PITCH LAYOUTPITCH LAYOUT

30º

PT

90º

60º

45º

PT

TUBE SPACING LAYOUTS TUBE SPACING LAYOUTS FOR TUBE SHEETFOR TUBE SHEET

BAFFLESBAFFLES

Two types of baffles are used in shell and

tube heat exchanger

Transverse Baffles

Longitudinal Baffles

DISC AND DOUGHNUT DISC AND DOUGHNUT BAFFLESBAFFLES

TUBESTUBES

Number of Tubes:

It depends on fluid flow rates and the available pressure drop

No. of tubes selected such that the tube side velocity is 0.9 to 2.4 m/sec and shell side velocity is 0.6 to 1.5 m/sec

Lower velocity limits corresponds to limiting the fouling and upper velocity limits corresponds to erosion

Tube side passes:

Almost all cases we have even number of tube side passes owing to factors of stress etc mechanical design for odd passes is difficult

Large number of tubes increase fluid velocity, heat transfer coefficient and minimize fouling. However this leads to increase in pressure drop and need for more pumping

SHELL AND TUBE HEAT SHELL AND TUBE HEAT EXCHANGEREXCHANGER

FLUID ALLOCATIONFLUID ALLOCATION

Tube side is preferred under these circumstances

Fluid which are prone to foulCorrosive fluidsToxic fluids to increase containmentStream with low flow rates to obtain

increase velocities and turbulenceHigh pressure streams

Material of construction

Material Typical service

Aluminium and austenic chromium-nickel steel

T < -100 0C

Carbon steel -100 < T < 0 0C

Refractory- lined steel

0 < T < 500 0C

Non-Corrosive service

Corrosive ServiceCorrosive Service

Material Typical service

Carbon Steel Mildly corrosive fluids

Ferritic chromium steel

Tubes for moderately corrosive service

Aluminium Mildly corrosive fluids

High nickel-chromium-molybdenum alloys

Resistant to mineral acids

THE CALCULATION OF SHELL AND TUBE HEAT EXCHANGER

Shell Side Film Co-efficient

• ho.De/k = 0.36 (De.G/μ)0.55 (Cp.μ/k)1/3 (μ/μw) 0.14

Shell Side Equivalent Diameter De = Wetted Perimeter 4 x free Area For square pitch De = 4 (PT

2 – πdo2/4)

πdo

For triangular pitch De = 4 (0.43 PT

2 – πdo2/8)

πdo/2

Tube side Heat Transfer Co-efficient Dittus-Bolter equation Sieder-Tate equation

Overall Heat Transfer Co-efficient 1 = 1 + 1 do + xw. do + Ro + Ri diUo ho hi di kw. dm do

True Temperature Difference

Q = U.A. tm = W.C.(T1-T2) = w.c. (t2-t1) tm = True Temperature tm = LMTD x FT

FT = correction factor

Pressure Drop in Shell & Tube Heat Exchanger

Tube Side P = 4.f L u2ρ

2df = 0.079 Re-0.25

f = 0.046 Re-0.2

Shell Side P = f Gs2 Ds L

2 x 106 de B S

f = 1.87 Re-0.2

Where,

Gs = shell side mass velocityDs = shell diameterde = equivalent diameterL = tube lengthS = baffle spacing

Bundle DiameterDb = do (N/k)1/n

Db = bundle diameterN =total no. of tubesdo = Outside diameter of tube

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