Shell Tube Heat

Published:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Heat exchanger design:-The most and widely used type of heat transfer equipment for higher pressure applications in oil refineries and large chemical processes is the shell and tube heat exchanger. Heat exchanger is used to achieve heat transfer between two streams. Based on the direction of the flow, these heat exchanges are separated into different types like counter flow, cross flow, parallel flow etc., A shell and tube consists of a shell c large pressure vessel and bundle of tubes enclosed inside. The tube sheets Separates the shell side fluids and tube side fluids by fitting the end of the tubes in a tube sheet.

Two different starting temperatures enters into shell and tube heat exchanger one will goes to the shell side and other will goes into the tube side. The heat transfer takes place inside the shell and tube through tube walls , heat transfer takes place either from shell side to tube side or from tube side to shell side(out side the tube).

This kind of heat transfer equipment can be used single phase heat transfer that means

In this case heat transferring fluid remains in same phase but it goes increase the temperature of the field inside. In case two phase heat exchangers liquid changes to

gas or gas change liquid through the heat transfer. Heat exchangers can be used to increase the temperature of fluid phase from liquid to gas, this type heat exchangers some times called boilers and can be change from vapor phase liquid phase called condensers.

The advantages for this kind of heat exchangers are:

1. It accommodates large surface area in a smaller volume.

2. It can be designed and constructed with different kind and wide range of materials.

3. It allows us good mechanical layout and a good shape for pressure operations.

4. Easily cleanable.

5. Uses well fabricated techniques.

6. Allows well established design procedures.

Heat exchanger tubes: The type of heat exchanger tubes are can also uses in condensers. The out side diameter of the tubes is in inches or mm with in variable phenomenon either in heat exchanger or condensers. The typical heat exchanger tubes are available in range of metals, which are steel, copper, admiral by, mints metal brass, copper nickel, aluminum and the stainless steel. These tubes can be available in a number of different wall thicknesses which can be defined by the Birmingham wire gage. Which is usually denotes by BWG or gage of the tube. The most of tubes in heat exchanger design are of sizes 3/4th outside diameter and 1 inch.

Tube Pitch (Pt):- The tube pitch Pt is the shortest center to center distance between adjacent tubes. The common tube layouts for exchangers are squire pitch, Triangular pitch, square pitch rotated and triangular pitch with cleaning lanes. The widely used tube pitch in process industries is square pitch. It is the advantage of the tube allows easily for external cleaning and causes lower pressure drop when the fluid directs to the square pitch side. The common pitches for square layouts are inch outer diameter on 1(1/4) inch square pitch. For triangular layouts these pitches would be inch diameter on 1inch triangular pitch. The tubes in shell will allow easy cleaning, when the tubes are arranged widely in tube sheet.

SHELLS:

The shells are fabricated for shell and tube heat exchanger from steel pipe with nominal IPS diameter 12 inches. The nominal diameter of 12 and 24 inches as actual outside diameters. The standard wall thickness for shells is inch, when the inside diameters range from 12 inch to 24 inch and the shell side operating pressure can be range of up to 300 psi. for greater operating the shell wall thickness should be greater .same time the shell of above 24 inches in diameter are fabricated by rolling steel plate.

BAFFLES:

The higher transfer coefficient appears when the fluid maintained in a state of turbulence. It is necessary to employ the baffles which can induce turbulence outside the tubes and which allow the liquid to pass right angles to the axis of the tubes. These will cause certain amount of turbulence even the flow rate are small through the shell. The distance between the two centres of baffles called the baffle spacing or baffle pitch.

The mass velocity does not depend entirely up on the diameter of the shell, the baffles may fixed close together or far a part. The baffles are spaced inside at a distance of less than the inside diameter of the shell or near to one fifth the inside diameter of the shell. These are wide range of baffles are available in market but the segment baffles are mostly uses in the oil refineries and large chemical processes.

Shell side coefficient: we call the heat transfer coefficient outside the tube bundles are as shell side coefficients. The higher transfer coefficients appears when the turbulence is higher because when the baffles are spaced in baffle bundle, the flow directed by side to side or top to bottom, the heat transfer coefficient is higher where the undisturbed flow across the axis of the tubes.

The velocity of the fluid under goes fluctuation when we use square pitch because of limited area between the adjacent tubes compared to successive rows. Greater turbulence appears if we use the higher fluid velocities shows direct impact on the succeeding row. Which will gives higher shell-side film coefficient and little consequence for the pressure drop and clean ability. So the shell side coefficients are roughly 25 percent greater in triangular pitch than for square pitch.

Shell-side mass velocity (Gs):

The linear and mass velocity of the fluid changes continuously across the bundle because of varying width of shell and tubes, zero at the top and bottom of the shell and maximum in the middle of the shell.

Disgram of7.19mph 139

The tube pitch is the sum of the diameters of the two tubes and clearance c'.

Mass velocity (Gs) = lb/ hr.ft2 or kg/hr.m2

Where

W= fluid flow rate. lb/hr or kg/hr.

The shell side and bundle cross flow area ft2 or m2

Where ID = inner diameter of shell

C' = bundle clearance

B = baffle spacing

PT = tube pitch

Shell side equivalent diameter:

The shell equivalent diameter can be calculated using the flow area between axial direction to the tubes and the wetted perimeter of the tubes.

Fig12.28 pg no 675

Shell side equivalent diameter for square pitch:

De = 4- free area ft or m2

Wetted perimeter

De=

= in or m

Where PT = tube pitch

do = tube outside diameter

de = equivalent diameter in

Mean temperature difference:

For calculating the heat transfer area required for a given duty, mean temperature difference ∆Tm must be needed. This can be calculated from the temperature difference in the fluid inlet and outlet of the exchanger.

∆Tlm =

Where ∆Tlm = lag mean temperature difference

T1 = hot fluid temperature, inlet

T2 = hot fluid temperature, outlet

t1 = cold fluid temperature, inlet

t2 = cold fluid temperature, outlet

True temperature difference ∆Tm can be calculated from the logarithmic mean temperature by applying a temperature correction factor.

∆Tm = Ft ∆Tlm

Where ∆Tm = true temperature difference

Ft = the temperature correction factor

The correction factor is a function of the shell and tube fluid temperatures and the number of tube and shell passes. It is normally as a function of two dimensionless temperatures.

R= shell side fluid flow rate times the fluid mean specific heat, divided by the tube side fluid flow rate times the tube side fluid specific heat

S= measure of the temperature efficiency of the exchanger

Correction factor Ft =

PROCEDURE FOR DESIGN OF HEAT EXCHANGER

The calculation and design of an exchanger:

The outline for design follows:

Given:

Process conditions:

Hot fluid: T1, T2, W, CP,S, h , k, Rd, ∆P

Cold fluid: t1, t2, w, Cp, s, c, k, Rd, ∆P

The tube length, outside diameter, and pitch will be specified by plant practice.

(1) Heat load Q = WCp (T1 - T2)

= WCp (t2 - t1)

(2) True temperature difference are ∆t:

L.M.T.D

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

S = (t2-t1) / (T1-t1)

Therefore ∆t = L.M.T.D x FT

(3) Average temperatures Ta and ta.

For the exchanger:

(a) Assume a tentative value of Ud with the aid of Table 8 and page no 840 from process heat transfer by D Q.kern ,and compute the surface from A = Q / (Ud*∆Tm). It is always better to assume UD too high than too low, as this practice ensures arriving at the minimum surface. Determine the corresponding no.of tubes using Table 9.

(b) Assume a plausible number of tube passes for the pressure drop allowed, and select an exchange for the nearest number of tubes from the tube counts of Table 9.

(c) Correct the tentative UD to the surface corresponding to the actual number of tubes which may be contained in the shell.

The performance calculation for the film coefficient should start with the tube side. If the tube - side film coefficient is relatively greater than UD and the pressure drop allowance is reasonably fulfilled and not exceeded, the calculation can proceed to the shell side. Whenever the number of tube passes is altered, the surface in the shell is also altered, changing the value of A and UD

For the remainder of the calculation it is assumed that the cold fluid flows in the shell side as it does on a majority but not necessarily all cases.

COLD FLUID : shell side

Flow area as = (1DxC'B) / (Ptx 144) ft2

ID = ID of shell

C = Tube clearance

B = Baffle spacing

PT = Pitch.

Mass velocity Gs = W/as lb / hr. ft2

W = mass flow rate of ethane

Reynolds No Shell Side (Res) = (De Gs) /μ

Obtain the μ values from fig 15

j H from Fig 24

At Tavgobtain Cp from fig 3 and K values from table.5

h0 = j H (k/De) (C/k)l/3Фs

h0/ Фs = j H (k/De) (C/k)l/3

HOT FLUID: tube side

Flow area, at : Flow area per tube at from Table 10.

a t = (N t x a't) / (144xn)ft2

n= no of passes

ID will be obtained from Table 10.

Mass velocity G t = W/at Ib / hr. ft2

` at' = flow area per tube

Reynolds No Tube Side R et = DG t /c

D = ID of tube

Obtain D from Table 10.

Obtain c at ta from figure 15

jh from Fig.24

At tc obtain Cp and K from table 5.

hi = jh(k/D) (c/k)Ф t

h io/ Ф t = (hi/ Ф t) (ID /OD)

(or)

Clean overall co efficient Uc :

Uc =

Assume Dirt Factor Rd :

Rd=

Area A=

heat exchanger design calculations:

T1

3500C T2

t 2 1500C

2000C t 1

240C

Data:

Shell side Tube side

Inner diameter (ID) = 12 in No of tubes = 158

Baffle space = 5 in

Baffle pitch(Pt) = 1.25 tube length = 25 in

No of passes = 1 outer diameter =3/4 in

Birmingham wire gage (BWG) = 13

Tube pitch = 1-in triangular

Passes = 4

Temperature difference of water =350-150

= 1500C

Temperature difference of ethane =200-24

=1750C

Specific heat of water at 1250C = 0.45 Btu/lb. 0F [1]

= 1.88 kj/kg. 0C

Specific heat of ethane (C2H6) at 1750C= 2.147 kj/kg. 0C

Heat load of process stream ethane (Qps) = m Cp ∆T

Where m= mass flow rate of process stream

= 80000/3600= 22.2 kg/sec

Cp= specific heat of process stream

= 2.147 kj/kg. 0C

∆T= temperature difference of process stream

= (T1-T2)

=300-125

=1750C

Therefore Qps =

=30229760 KJ/hr

= 28652282 BTU/hr 1Kj= 0.9485 BTU/hr

According to conservation of energy

Qps = mw*Cpw*∆t

Mass flow rate of water (mw) = Qps / Cpw*∆t

= 28652282/ (4.2*200)

=35988 kg/hr

Area required for heat transfer A= Q/UD*∆T

=30229760/(450*70.8)

= 948 ft2

= 288 m2

The above UD is assumed from table no 8.

HEAT EXCHANGER DESIGN (Calculations)

Note: In all general calculations hot fluid is taken on shell side but here I took opposite because of larger flow rate of ethane stream and other factors like cleaning and scale formation.

Hot fluid cold fluid Difference

350

Higher temperature

200

150

150

Lower temperature

24

126

200

Difference

176

24

(T1-T2) (t2-t1)

(∆t2-∆t1)= 24C

∆Tlm =

=

=

= 1370C or 2780F

R =

=

=

=1.136

S=

=

=

= 0.539

Temperature correction factor (Ft) =

=

=

=

= 0.5168

Therefore true temperature (∆Tm)= ∆Tlm*Ft

= 137*0.5168

= 70.8 0C or 0 F

Tc=T2+Ft*(T1-T2)

=150+0.5*(350-150)

=2500C or 4820F

tc =t1+Ft*(t2-t1)

= 24+0.5*(200-24)

=1120C or 2340F

Shell side calculations for ethane (cold fluid):

Flow area as = (1DxC'B) / (Pt x 144) ft2

=(12*0.25*5) / (1.25 x 144)

=0.0833 ft2

Mass velocity (Gs) = W/as lb / hr. ft2 W=80000 kg/hr

= 176370/0.833 =176370 lb/hr

=2114537.4 lb / hr. ft2

Reynolds No on Shell Side (Res) = (De Gs) /μ

At Tc= 4820F obtained μ is 0.0242 from figure 14.

And De=

= ft or m

=

= 0.9457

(Res) = (Dc Gs) /μ

= (0.9457*2114537.4*)/0.0242

=82636183

JH=60000 from figure 28 using the Reynolds no.

At Tc= 4820F,

C= 0.55 BTU/lb. 0F

K=0.0151 BTU/(hr)(ft2)( 0F/ft)

=0.9588

h0 = j H (k/De) (C/k)l/3Фs

h0/ Фs = j H (k/De) (C/k)l/3

= 60000*(0.0151/.9457)* (0.55*0.242/0.0151)l/3

=918.52

Tube wall temperature (tw):

=

=346.270F

At tw =346.70F, μw=0.03146 lb/ft.hr

Фs= (μ/μw) 0.14

=(0.242/0.03146)0.14

= 0.9639

Corrected coefficient,

=918.52*0.9639

=885.4BTU/hr.ft2.0F

Hot fluid: tube side, steam:

Flow area at1 = 0.334 in2

at=(Nt*at) / (n x 144) ft2

=(158*0.334) / (4 x 144)

=0.091618 ft2

Mass velocity (Gt) = W/at lb / hr. ft2

= 35987.9/0.0916

=865203.9 lb / hr. ft2

Reynolds No on Shell Side (Res) = (D Gt) /μ

At tc= 2340F obtained μ is 0.02904 from figure 14.

D=0.0652 ft

(Res) = (D Gt) /μ

= (0.0652*865203.9)/0.02904

=19425378

JH=50000 from figure 28 using the Reynolds no.

At tc= 2340F,

C= 0.23 BTU/lb. 0F

K=0.0241 BTU/(hr)(ft2)( 0F/ft)

=0.6519

hi = j H (k/D) (C/k)l/3Фt

hi/ Фt = j H (k/D) (C/k)l/3

= 50000*(0.0241/0.0652)* (0.23*0.02904/0.0241)l/3

=1204.96

hio/Фt= (hi/ Фt)*(ID/OD)

=1204.96*(0.652/0.75)

=1047.518

At tw =346.70F, μw=0.0363 lb/ft.hr

Фt= (μ/μw) 0.14

=(0.02904/0.0363)0.14

= 0.9692

Corrected coefficient,

=1047.5*0.9639

=1167.906 BTU/hr.ft2.0F

Now the clean overall coefficient Uc=

=

=503.6BTU/hr.ft2.0F

Dirt factor, Rd= 0.0005 is assumed.

=402.306

So the assumed UD for calculating the heat transfer area required (A) and obtained UD from calculation is almost near . So the area we calculated for heat exchanger is right value.

CONDENSORS:

We can use the design of shell and tube exchangers for condensers. The construction and design of the shell and tube exchanger for condenser is similar but the baffle spacing is lengthier i.e; typically baffle spacing lB = inside diameter of the shell.

Using the design of shell and tube exchanger four kinds of condenser configurations can design.

1. Horizontal cooling medium in tubes and condensation in shell side

2. Horizontal cooling medium in shell side and condensation in tubes

3. Vertical with condensation in the shell

4. Vertical with condensation in tubes

Mostly in process industries for condensation prefers horizontal shell side and vertical tube side. We can find mean condensation film coefficient for a single tube is from

where

= mean condensation film coefficient for single tube w/m2 ...c

(KL)= condensate thermal conductivity w/m ...c

L = condensate density kg/m3

V = vapor density kg/m3

μL = condensate viscosity Ns/m2

g = gravitational acceleration 9.81 m/s2

- = the tube loading the condensate flow per unit length of tube kg/m.s

The above equation is used for finding mean condensation film coefficient for single tube when condensation outside horizontal tubes.

Using the kerns methods we can find mean coefficient for a tube bundle

Where -n =

L = length of tube

Wc = total condense flow

Nt = total number of tubes in the bundle

Nr = average number of tubes in a vertical tube row.

Flooding in tubes:

When designing the condensers care should be taken for tubes, to ensure the tube should not flood. If the vapour comes up the tube, reflux is the usual alternative method.

Flooding could not occur if it satisfies the following condition.

Where

uv and uL= vapor and liquid velocities.

di= inner diameter of the tube

Condensation of mixtures:

The designing of condenser for a mixture of vapor component is difficult because it covers the three related situation.

  1. Total condensation of mixtures such as methane, H2, H2O from methanator and multi component mixtures from distillation column.

  2. In multi component mixtures, only part of component vapour mixtures condense, where the dew point of component is the above coolant temperature. There is chance of uncondensable components soluble in condensable components.

  3. The gas from the non condensable gas is not soluble to any extent in the condensed liquid; these exchangers are called cooler-condensers.

Some features are same to all the situations, when developing the design of vapour condenser such as heavy components dew point changes out the composition of the vapour, so the condensation would not be isothermal because transfer of sensible heat from vapour to cool the gas to its dew point; that's why the condensation is not isothermal. The physical property of vapour and liquid varies entire the condenser.

Design calculations for condenser:

Data:

Shell side Tube side

Inner diameter (ID) = 25 in No of tubes = 370

Baffle space = 12 in tube length = 16 in

No of passes = 1 outer diameter =3/4 in

Birmingham wire gage (BWG) = 16

Tube pitch = 1-in square

T1

5000C T2

t 2 3250C

1500C t 1

250C

Temperature difference of water =150-25

= 1250C

Temperature difference of mixed gas =500-325

=1750C

Specific heat of water at 1250C = 0.45 Btu/lb. 0F [1]

= 1.88 kj/kg. 0C

Specific heat of methane (CH4) at 1750C= 0.65 Btu/lb. 0F

Specific heat of hydrogen (H2) at 1750C= 3.5 Btu/lb. 0F

Specific heat of carbon monoxide (CO2) = 0.24 Btu/lb. 0F

Process fluid specific heat (Average of three) = (0.65+3.5+.24)/3

= 1.466 Btu/lb. 0F

=6.137kj/kg. 0C

Heat load of process stream (Qps) = m Cp ∆T

Where m= mass flow rate of process stream

= 80000/3600= 22.2 kg/sec

Cp= specific heat of process stream

=6.13 kj/kg. 0C

∆T= temperature difference of process stream

= (T1-T2)

=500-325

=1750C

Therefore Qps= 22.2*6.137*175

= 23294.33 KW

According to the conservation of energy

Heat load on tube side = heat load on shell side

Therefore Qps= Qw (water)

= mw*Cpw*∆t

Mass flow rate of water (mw) = Qps / Cpw*∆t

= 23294.33/ (1.88*125)

=99 kg/sec

So the amount of mass flow rate of water needed is 99 kg/sec.

Log mean temperature (∆Tlm) =

=

=

=

= 3240C

R= shell side fluid flow rate times the fluid mean specific heat, divided by the tube side fluid flow rate times the tube side fluid specific heat.

R=

=

And S=

S= measure of the temperature efficiency of the exchanger

And S=

S=

=0.2631

And Correction factor Ft =

=

=

=

=

=0.869

Therefore ∆Tm= Ft*∆Tlm

= 0.896*324

= 1650C

Area of the condenser=

=

=207 m2

Outer diameter of tube (OD)= in

=0.0195 m

And length of the tube L= 16 ft

= 4.88 m

Surface area of one tube=

=3.147*0.0195*4.88

=0.29 m2

Therefore

No of tubes=

=1267

The pitch used for tube (Pt) =1-square pitch

=25.4mm

Bundle diameter (Db) =

Where Nt = no of tubes

= 1267

Db= bundle diameter

do= tube outside diameter

=19.05mm

n1 = 2.263 if the no of pass is one in shell

K1 = 0.158 if the no of pass is one in shell

Db=

=19*53.11

= 1009 mm

No of tubes in centre row (Nr) =

=

= 40

Writing Services

Essay Writing
Service

Find out how the very best essay writing service can help you accomplish more and achieve higher marks today.

Assignment Writing Service

From complicated assignments to tricky tasks, our experts can tackle virtually any question thrown at them.

Dissertation Writing Service

A dissertation (also known as a thesis or research project) is probably the most important piece of work for any student! From full dissertations to individual chapters, we’re on hand to support you.

Coursework Writing Service

Our expert qualified writers can help you get your coursework right first time, every time.

Dissertation Proposal Service

The first step to completing a dissertation is to create a proposal that talks about what you wish to do. Our experts can design suitable methodologies - perfect to help you get started with a dissertation.

Report Writing
Service

Reports for any audience. Perfectly structured, professionally written, and tailored to suit your exact requirements.

Essay Skeleton Answer Service

If you’re just looking for some help to get started on an essay, our outline service provides you with a perfect essay plan.

Marking & Proofreading Service

Not sure if your work is hitting the mark? Struggling to get feedback from your lecturer? Our premium marking service was created just for you - get the feedback you deserve now.

Exam Revision
Service

Exams can be one of the most stressful experiences you’ll ever have! Revision is key, and we’re here to help. With custom created revision notes and exam answers, you’ll never feel underprepared again.