TP - heat exchanger design.ppt1

Transport ProcessesOverall heat transfer coefficient

From previous studies (CPP module):Q’ = U A ΔTLM

oo

iioi

ii h

1

d

d

2k

)ddln(d

h

1

U

1

Some typical U values (all in W/m2K):steam/water: 6000 to 18000water/water: 850 to 1700steam condenser (water in tubes) 1000 to 6000ammonia condenser (water in tubes) 800 to 1400alcohol condenser (water in tubes) 250 to 700finned tube (air outside, water inside) 25 to 50deduce others from charts

TP - heat exchanger design.ppt2

Transport ProcessesOverall heat transfer coefficient

• Contains many combinations

• May need to transpose top and bottom fluids

• Gives rather conservative estimates

TP - heat exchanger design.ppt3

Transport ProcessesChoosing right shell-and-tube type

Decision as to TEMA code used depends on fluids used

Shell& tube exchangers

Severe thermal exapansion stresses?

Are bellows allowed?

Is chemical cleaning possible?

High shellside fouling > 0.00035

m2K/W?

High tubeside fouling > 0.00035

m2K/W?

Is chemical cleaning possible?

Removable bundle design

Fixed tubesheet

Is interstream leakage allowed?

Are T & P within range for lantern ring?

Is F correction factor < 0.75?

Are there more than 2 passes?

Are F or multi shells allowed?

Frequency of bundle removal

AEL AEM

BEM

No No Yes

Yes No

AEU AFU

AEU AFU

No No Yes

Yes No

AEP

BEP

No No Yes

Yes No

AEW

BEW

No No Yes

Yes No

AET

BET

No No Yes

Yes No

AES

BES

No No Yes

Yes No

Is tubeside fouling > 0.00035 m2K/W?

Do we require tube access without disturbing connections?

Yes No

Yes

Yes

No

Yes

No

No

Yes Yes

No

Yes

No

Yes

No

Yes

No

Yes

No

Low High

No

Yes

TP - heat exchanger design.ppt4

Transport ProcessesLog Mean Temperature Difference

e.g. find TLM for both co-current & counter-current flowFluid A Tin = 120 Tout =90°CFluid B tin = 20 tout = 80°C

temperature

T1

T2

hot fluid

Tin

Tout

cold fluid

tout

tin

21

21LM TlnTln

TTT

TP - heat exchanger design.ppt5

Transport ProcessesLog Mean Temperature Difference

i.e. less driving force with co-current

K deg 53.6ln40ln70

4070TLM

K deg 39.1ln10ln100

10100TLM

120

80 9040

70

20

120

8090

20

100 10

TP - heat exchanger design.ppt6

Transport ProcessesLog Mean Temperature Difference

i.e. more driving force than either of the first twosame value for both co and counter-current

K deg 65.5ln40ln100

40100TLM

120

80

20

100 40

• Now make fluid A condensing steam. What happens?

TP - heat exchanger design.ppt7

Transport ProcessesLog Mean Temperature Difference

• Why a log mean temperature difference rather than any other?

• Consider point along heat exchanger tube:

T T – dT

t + dt t

Area = dA

Heat = dQ’• At this point: T = T – t d(T) = dT – dt

also dQ’ = -m’hCphdT = m’cCpcdt (sensible heat change) / / / /

h h c c h h c c

dQ dQ 1 1d T dQ

m Cp m Cp m Cp m Cp

TP - heat exchanger design.ppt8

Transport ProcessesLog Mean Temperature Difference

• Hence:

/ /h h c c

d T tdQ =

1 1m Cp m Cp

= U(T – t).dA

/ /

h h c c

d T t 1 1U.dA

T t m Cp m Cp

assuming constant Cph & Cpc:

2 2/ /

1 1 h Ph c Pc

T t 1 1-ln UA

T t m C m C

But/h Ph

1 2

Qm C

T T

/c Pc

2 1

Qm C

t t

TP - heat exchanger design.ppt9

Transport ProcessesLog Mean Temperature Difference

• Giving 2 2 1 2 2 1

1 1

T t T T t t-ln UA

T t Q Q

2 21 1 2 2

1 1

T t UAln T t T t

T t Q

2 2 1 1

2 2

1 1

T t T tQ UA

T tln

T t

= UA.ΔTLM

• Counter-current derivation also true for co-current flow

Co-current flow rarely used in practice

TP - heat exchanger design.ppt10

Transport ProcessesLog Mean Temperature Difference

Shell & tube exchanger NOT in true counter-current flow if there is more than one tube-side pass

TLM < TLM for pure counterflow

In this case, calculate TLM as if for counterflow.Multiply by correction factor F to give true value:

1R

1R

1R1RS2ln1R1RS2ln

SR1lnS1lnF

2

22

tube(in)tube(out)

shell(out)shell(in)

TT

TTR

tube(in)shell(in)

tube(in)tube(out)

TT

TTS

TP - heat exchanger design.ppt11

Transport ProcessesLog Mean Temperature Difference

Alternatively, use charts to evaluate F.

F should be high (0.75 to 1.0) for efficient operation

If F > 0.75 inachievable, use single tube-side pass

F then becomes 1

TP - heat exchanger design.ppt12

Transport ProcessesDuties

For sensible heat (i.e. no boiling or condensing)Q’H = m’H CPh (Tin - Tout) Q’C = m’C CPc (tout - tin)

For latent heat (boiling and/or condensing)Q’ = m’ fg

For perfect balance, Q’H = Q’C

i.e. heat lost by hot fluid = heat gained by cold fluid

In reality, heat losses always occur

TP - heat exchanger design.ppt13

Transport ProcessesFouling

• Standard formula for U assumes clean surfaces

• In reality, surface fouling increases thermal resistance

External fouling layer

Internal fouling layer

TP - heat exchanger design.ppt14

Transport ProcessesFouling

• Occurs for a number of reasons

Slimy film through microbial activity in water

Precipitation of dissolved salts

Reaction of fluid alone (eg. polymerisation of hydrocarbons)

Reaction of surface with fluid (eg. corrosion)

Freezing

Silt

TP - heat exchanger design.ppt15

Transport ProcessesFouling

• Dynamic problem by natureFouling

resistance

Time

• Can be held in check by Regular cleaning High flow velocities Low temperatures Use of special devices and/or chemical additives

TEMA and others usually quote this assymptotic value

TP - heat exchanger design.ppt16

Transport ProcessesFouling

• Fouling resistances incorporated into formula:

• Designers assume static Rfo & Rfi. A few examples:FLUID Rf (m2K/W)Seawater & treated boiler water (<50°C) 1 × 10-4 Seawater & treated boiler water (>50°C) 2 × 10-4 River water (<50°C) 2 to 10 × 10-4

Fuel oil 9 × 10-4 Refrigerating liquids 2 × 10-4 Steam (non-oil bearing) 1 × 10-4

fo

oo

iioifi

iD

Rh

1

d

d

2k

ddlndR

h

1

U

1

TP - heat exchanger design.ppt17

Transport ProcessesMechanical considerations of shell-and-tube heat exchanger

design• Tubes held in place by tube sheets

with drilled holes

• Holes align the tubes in square or triangular arrangement

• Distance between centres of adjacent tubes = “tube pitch”

• Outer diameters:16, 20, 25, 30, 38, 50 mm, 2mm thick

• Lengths:1.83, 2.44, 3.66, 4.88, 6.10, 7.32 metres

TP - heat exchanger design.ppt18

Transport ProcessesMechanical considerations of shell-and-

tube heat exchanger design• Baffle spacing: minimum = Ds ÷ 5 (but > 5 cm)

maximum = 74do0.75 (but < Ds)

• Baffle cut (segment opening height Ds) = 0.25 to 0.40

• eg. segmental baffle inside 1.00 m shell

25% means segment 25cm high removed

• Smaller cut leaves smaller holeHigher shell-side film coefficientGreater shell side pressure drop

0.25 m

TP - heat exchanger design.ppt19

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Calculate duty Q’ (add 10% to include losses and errors)

• Find appropriate fouling resistances

• Choose side for each fluid (based on fouling, corrosion and pressure)

• Choose type of exchanger from TEMA tree

• Calculate all temperatures TLM & F

• Keep things simple to start with; assume 4.88m tubes, do = 20 mm, 2 tube side passes (NP=2)

TP - heat exchanger design.ppt20

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Cool 5.0 kg/s of ethylene glycol from 370 to 330K with cooling water from 283 to 323K

• Ethylene glycol at 350K (average) has following properties

k = 0.261 W/m.K Cp = 2637 J/kg.K

μ = 0.00342 Pa.s ρ = 1079.0 kg/m3

Giving Pr = (2637×0.00342)/0.261 = 34.6

• Anticipate fouling resistance of Rf = 0.00018 m2K/W

• Duty is Q’ = 5.0 × 2637 × (370–330) = 527 400 Watts

• Aim to transfer 580 140 W

TP - heat exchanger design.ppt21

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Water at 303K (average) has following properties

k = 0.618 W/m.K Cp = 4179 J/kg.K

μ = 0.000797 Pa.s ρ = 995.6 kg/m3

Giving Pr = (4179×0.000797)/0.618 = 5.39

• Anticipate fouling resistance of Rf = 0.0001 m2K/W

• Water fouls less and is on shell-side

• We need water flowrate

inout ttCp

Qm

15506.3

8322331794

527400

• 3.16 kg/s water on shell-side

TP - heat exchanger design.ppt22

• Problem – we cannot calculate a log mean• Solution – a log mean is just an average after all• What is average of 47 and 47?

?ln47ln47

7474TLM

370

323 33047

47

283

1703303

233832R

4598.0703832

703330S

ΔT = 47, F = 0.87

Transport ProcessesFirst design of a shell-and-tube heat exchanger

TP - heat exchanger design.ppt23

• Do we have severe expansion stresses?

• ie. are the temperatures greatly different to ambient?

• Yes

Shell& tube exchangers

Severe thermal exapansion stresses?

Are bellows allowed?

Is chemical cleaning possible?

High shellside fouling > 0.00035

m2K/W?

High tubeside fouling > 0.00035

m2K/W?

Is chemical cleaning possible?

Removable bundle design

Fixed tubesheet

Is interstream leakage allowed?

Are T & P within range for lantern ring?

Is F correction factor < 0.75?

Are there more than 2 passes?

Are F or multi shells allowed?

Frequency of bundle removal

AEL AEM

BEM

No No Yes

Yes No

AEU AFU

AEU AFU

No No Yes

Yes No

AEP

BEP

No No Yes

Yes No

AEW

BEW

No No Yes

Yes No

AET

BET

No No Yes

Yes No

AES

BES

No No Yes

Yes No

Is tubeside fouling > 0.00035 m2K/W?

Do we require tube access without disturbing connections?

Yes No

Yes

Yes

No

Yes

No

No

Yes Yes

No

Yes

No

Yes

No

Yes

No

Yes

No

Low High

No

Yes

Transport ProcessesFirst design of a shell-and-tube heat exchanger

TP - heat exchanger design.ppt24

• Are bellows allowed?

• No reason why not

• Yes

Shell& tube exchangers

Severe thermal exapansion stresses?

Are bellows allowed?

Is chemical cleaning possible?

High shellside fouling > 0.00035

m2K/W?

High tubeside fouling > 0.00035

m2K/W?

Is chemical cleaning possible?

Removable bundle design

Fixed tubesheet

Is interstream leakage allowed?

Are T & P within range for lantern ring?

Is F correction factor < 0.75?

Are there more than 2 passes?

Are F or multi shells allowed?

Frequency of bundle removal

AEL AEM

BEM

No No Yes

Yes No

AEU AFU

AEU AFU

No No Yes

Yes No

AEP

BEP

No No Yes

Yes No

AEW

BEW

No No Yes

Yes No

AET

BET

No No Yes

Yes No

AES

BES

No No Yes

Yes No

Is tubeside fouling > 0.00035 m2K/W?

Do we require tube access without disturbing connections?

Yes No

Yes

Yes

No

Yes

No

No

Yes Yes

No

Yes

No

Yes

No

Yes

No

Yes

No

Low High

No

Yes

Transport ProcessesFirst design of a shell-and-tube heat exchanger

TP - heat exchanger design.ppt25

• High shellside fouling?

• 0.0001 < 0.00035 m2K/W

• No

Shell& tube exchangers

Severe thermal exapansion stresses?

Are bellows allowed?

Is chemical cleaning possible?

High shellside fouling > 0.00035

m2K/W?

High tubeside fouling > 0.00035

m2K/W?

Is chemical cleaning possible?

Removable bundle design

Fixed tubesheet

Is interstream leakage allowed?

Are T & P within range for lantern ring?

Is F correction factor < 0.75?

Are there more than 2 passes?

Are F or multi shells allowed?

Frequency of bundle removal

AEL AEM

BEM

No No Yes

Yes No

AEU AFU

AEU AFU

No No Yes

Yes No

AEP

BEP

No No Yes

Yes No

AEW

BEW

No No Yes

Yes No

AET

BET

No No Yes

Yes No

AES

BES

No No Yes

Yes No

Is tubeside fouling > 0.00035 m2K/W?

Do we require tube access without disturbing connections?

Yes No

Yes

Yes

No

Yes

No

No

Yes Yes

No

Yes

No

Yes

No

Yes

No

Yes

No

Low High

No

Yes

Transport ProcessesFirst design of a shell-and-tube heat exchanger

TP - heat exchanger design.ppt26

• High tubeside fouling?

• 0.00018 < 0.00035 m2K/W

• No

Shell& tube exchangers

Severe thermal exapansion stresses?

Are bellows allowed?

Is chemical cleaning possible?

High shellside fouling > 0.00035

m2K/W?

High tubeside fouling > 0.00035

m2K/W?

Is chemical cleaning possible?

Removable bundle design

Fixed tubesheet

Is interstream leakage allowed?

Are T & P within range for lantern ring?

Is F correction factor < 0.75?

Are there more than 2 passes?

Are F or multi shells allowed?

Frequency of bundle removal

AEL AEM

BEM

No No Yes

Yes No

AEU AFU

AEU AFU

No No Yes

Yes No

AEP

BEP

No No Yes

Yes No

AEW

BEW

No No Yes

Yes No

AET

BET

No No Yes

Yes No

AES

BES

No No Yes

Yes No

Is tubeside fouling > 0.00035 m2K/W?

Do we require tube access without disturbing connections?

Yes No

Yes

Yes

No

Yes

No

No

Yes Yes

No

Yes

No

Yes

No

Yes

No

Yes

No

Low High

No

Yes

Transport ProcessesFirst design of a shell-and-tube heat exchanger

TP - heat exchanger design.ppt27

• Is tube access required without dismantling?

• Unlikely unless we had solids or other things that may block

• No

Shell& tube exchangers

Severe thermal exapansion stresses?

Are bellows allowed?

Is chemical cleaning possible?

High shellside fouling > 0.00035

m2K/W?

High tubeside fouling > 0.00035

m2K/W?

Is chemical cleaning possible?

Removable bundle design

Fixed tubesheet

Is interstream leakage allowed?

Are T & P within range for lantern ring?

Is F correction factor < 0.75?

Are there more than 2 passes?

Are F or multi shells allowed?

Frequency of bundle removal

AEL AEM

BEM

No No Yes

Yes No

AEU AFU

AEU AFU

No No Yes

Yes No

AEP

BEP

No No Yes

Yes No

AEW

BEW

No No Yes

Yes No

AET

BET

No No Yes

Yes No

AES

BES

No No Yes

Yes No

Is tubeside fouling > 0.00035 m2K/W?

Do we require tube access without disturbing connections?

Yes No

Yes

Yes

No

Yes

No

No

Yes Yes

No

Yes

No

Yes

No

Yes

No

Yes

No

Low High

No

Yes

Transport ProcessesFirst design of a shell-and-tube heat exchanger

TP - heat exchanger design.ppt28

• BEM exchanger

• A fixed tubesheet design

Shell& tube exchangers

Severe thermal exapansion stresses?

Are bellows allowed?

Is chemical cleaning possible?

High shellside fouling > 0.00035

m2K/W?

High tubeside fouling > 0.00035

m2K/W?

Is chemical cleaning possible?

Removable bundle design

Fixed tubesheet

Is interstream leakage allowed?

Are T & P within range for lantern ring?

Is F correction factor < 0.75?

Are there more than 2 passes?

Are F or multi shells allowed?

Frequency of bundle removal

AEL AEM

BEM

No No Yes

Yes No

AEU AFU

AEU AFU

No No Yes

Yes No

AEP

BEP

No No Yes

Yes No

AEW

BEW

No No Yes

Yes No

AET

BET

No No Yes

Yes No

AES

BES

No No Yes

Yes No

Is tubeside fouling > 0.00035 m2K/W?

Do we require tube access without disturbing connections?

Yes No

Yes

Yes

No

Yes

No

No

Yes Yes

No

Yes

No

Yes

No

Yes

No

Yes

No

Low High

No

Yes

Transport ProcessesFirst design of a shell-and-tube heat exchanger

TP - heat exchanger design.ppt29

Transport ProcessesFirst design of a shell-and-tube heat exchanger

TP - heat exchanger design.ppt30

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Choose best case for each

Usuggested=500 W/m2K

2m .38824787.0500

580140A

TP - heat exchanger design.ppt31

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Use NT to fix estimated coefficient as Uestimate

• L = 4.88 m, do = 20 mm:

• Area of one tube = π ×4.88 × 0.020 = 0.3066m2

• Number of tubes needed = 28.38 ÷ 0.3066 = 92.54

• Obviously, should be an integer

• Round up here, as 92 tubes means U>500

K W/m6.4974787.03066.093

580140U 2

estimate

• Aim to build exchanger with U = 497.6 W/m2K

TP - heat exchanger design.ppt32

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Calculate tube side velocity

T2it

P/t

t Nd

N4mu

Suggested ranges

Tubeside process liquids 1 to 2 m/s(up to 4 m/s if fouling risk)

Tubeside water 1.5 to 2.5 m/s

Vacuum gases/vapours 50 to 70 m/sAtmospheric pressure gases/vapours 10 to 30 m/sHigh pressure gases/vapours 5 to 10 m/s

• Note: di = 0.020 – 2(0.002) = 0.016 m

m/s 4956.0

39016.01079

25.042

TP - heat exchanger design.ppt33

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Lower than the suggested 1 to 2 m/s

• If tube side passes tripled to NP = 6, ut = 1.487 m/s

750600342.0

016.0487.11079Re t

• Use Nusselt turbulent correlation for forced convection in tubes:

• Nu = 0.036 (Re)0.8 Pr0.33 (di ÷ L)0.055

• Nu = 0.036 (7506)0.8 (34.6)0.33 (0.016 ÷

4.88)0.055

• Nu = 106.6 = hidi÷k

• hi = 106.6×0.261 ÷ 0.016 = 1739 W/m2K

TP - heat exchanger design.ppt34

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Find tube bundle diameter DB thus:assume tube pitch (pt)= 1.25 do

NP 1 2 4 6 8 K1 0.215 0.156 0.158 0.0402 0.0331 n1 2.207 2.291 2.263 2.617 2.643

1n1

1

ToB K

NdD

• NT = 93, NP = 6, pt = 1.25 0.020 = 0.025 m

m 386.00402.0

93020.0

2.6171

• So tube bundle is 0.386 m in diameter, but shell needs to be wider still

TP - heat exchanger design.ppt35

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Use graph to find clearance between bundle and shell diameter DS

• 12mm added so

• DS = 0.386 + 0.012 = 0.398 m

• Number of tubes at “equator” n = DB ÷ pt

tubes4.15025.0

0.386n

TP - heat exchanger design.ppt36

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Find minimum baffle spacing

• 0.398 ÷ 5 = 0.0796 m

• Divide tube length by bmin to find number of chambers created by baffles

• 4.88 ÷ 0.0796 = 61.3

• Number of chambers should be integer i.e. round down

• Actual baffle spacing b = tube length number of chambers

• b = 4.88 ÷ 61 = 0.08 m

TP - heat exchanger design.ppt37

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Calculate equivalent diameter of shell-side fluid (De)

perimeter wetted

area flow4De

do

pt

m 0198.0020.0

020.0

0.0254D

2

e

• So for do = 0.020 and pt = 0.025

oo

2t

o

2o

2t

dd

4p

d

d4

p4

circle 1 of ncecircumfere

area circlearea square4

TP - heat exchanger design.ppt38

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Calculate cross-section for flow (S) for hypothetical tube row mid-shell of “n” tubes

DS

b

pt do

S = b(DS – ndo) = 0.08 [0.398 – 15(0.02)] = 7.84×10–3 m2

• Choose tube materialif stainless steel, k = 16 W/m.K

TP - heat exchanger design.ppt39

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Calculate shell side velocities

s

/s

s S

mu

Suggested ranges

Atmospheric pressure gases/vapours 10 to 30 m/sVacuum gases/vapours 50 to 70 m/sHigh pressure gases/vapours 5 to 10 m/sShell-side liquids 0.3 to 1.0 m/s

• Falls within accepted range

m/s 4048.06.99500784.0

3.16

10013000797.0

0198.04048.06.959Res

TP - heat exchanger design.ppt40

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Using Grimison correlation where C = 0.348 and n = 0.592

• Nu = 1.13×0.348 (Re)0.592 Pr0.33

• Nu = 0.39324 (10013)0.592 (5.39)0.33

• Nu = 160.11 = hoDe ÷ k

• ho = 160.11×0.618 ÷ 0.0198 = 4997 W/m2K

• Now have all information needed for U-value

fo

oo

iioifi

iD

Rh

1

d

d

2k

ddlndR

h

1

U

1

TP - heat exchanger design.ppt41

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Inside resistance

K/Wm10550.700018.01739

1 24

K/Wm10116.1

162

0.0160.02ln0.016 24

K/Wm10401.20001.04997

1

0.02

0.016 24

• Wall resistance

• Outside resistance

• Overall resistance(7.550 + 1.116 + 2.401)×10–4 = 1.1067×10– 3 m2K/W

• Overall heat transfer coefficient1 ÷ (1.1067×10–3) = 903.6 W/m2K

TP - heat exchanger design.ppt42

Transport ProcessesFirst design of a shell-and-tube heat exchanger

• Here, 903.6 ≠ 497.6 W/m2K, over 81% out

• Main resistance is tubeside, so ponder options

• If Uactual ≠ Uestimate (±30%) then do any of the following:

A by reducing tube length ( Uestimate) A by increasing tube length/diameter (Uestimate) number of tube-side passes ( Uactual) number of shell-side baffles ( Uactual)

• If possible, alter the side where the MAIN resistance lies