Thermal rating of Shell & Tube Heat Exchanger

THERMAL RATING OF SHELL & TUBE HEAT EXCHANGER:

SINGLE-PHASE HEAT TRANSFER

Author: Vikram SharmaDate: 20th February 2017

What is a S&T HEX? Fluid flow in HEX – counter vs. co-current Fluid allocation in S&T Heat Exchanger Thermal design principles

◦ Overall duty determination◦ Initial heat transfer area (Ao)◦ Tube pitch, tube size, tube length & shell diam.◦ Calc. tube-side heat transfer coeff. (hi)◦ Calc. shell-side heat transfer coeff. (hs)◦ Calc. overall heat transfer coeff. (Uo)◦ Calc. tube-side pressure drop (ΔPT)◦ Calc. shell-side pressure drop (ΔPS)

Summary

Table of Contents

As per Wikipedia, it consist of a shell (a large pressure vessel) with a bundle of tubes inside it.

One fluid flows through the tube and the second fluid flows through the shell.

Heat transfer occurs when the fluid in the shell flows over the tubes.

What is S&T heat exchanger?

Counter-current flow: ◦ Fluids are flowing in the opposite direction

Co-current flow: ◦ Fluids are flowing in the parallel direction

Why counter-current is preferred over co-current?◦ Thermal stresses are minimized due to more uniform

ΔT between two fluids;◦ Cold fluid temp. can approach the inlet temp. of the

hot fluid; and◦ More uniform of HEX can be achieved – uniform ΔT

throughout the HEX

Fluid flow in HEX: counter vs. co-current

Fouling fluids: ◦ Should be placed in tube-side;

Corrosive fluids: ◦ Should be placed in tube-side as to minimize the purchase

of expensive alloys & cladding material; High temperature fluid:

◦ Use of expensive alloys High pressure fluid

◦ Minimize the cost of construction of mechanically strong shell.

High viscosity fluids:◦ Shell-side provided it is at turbulent flow (Re>200). Viscous

fluids in tube-side results to high ΔP

Fluid Allocation Criteria in S&T HEX

i) Overall duty determination Begins with the determination of duty of the

heat exchanger.

Thermal design principles

In calculating t2, the Cp of the other fluid taken at t1.

Once t2 is calculated, a mean temp. of t1 & t2 is computed.

This mean temp. is used as ref. to obtain the Cp of the other fluid.

An iterative procedure is carried out to determine if the Cp of the other fluid is insignificant, the Cp is taken as the mean temp.

Thermal design principles (cont’d)

ii) Initial heat transfer area (Ao) Calculate the Log Mean Temp. Different (LMTD). Assumption underlying LMTD are:

◦ No change in specific heats;◦ Constant Uo◦ No heat losses

The corrected log mean temp. difference (ΔTm) is a f(FT, LMTD).

Thermal design principles (cont’d)

ii) Initial heat transfer area (Ao) (cont’d) Correction factor (FT) shall not be < 0.75 due to:

◦ Inefficient use of heat transfer area;◦ Violating the simplifying assumptions used in this

approach◦ Uncertainties in design data have more significant effect

when the slopes are steep The initial heat transfer area is calculated by:

Thermal design principles (cont’d)

ii) Initial heat transfer area (Ao) (cont’d) Uo is selected based on the service of the HEX

Thermal design principles (cont’d)

ii) Initial heat transfer area (Ao) (cont’d) Calculate the corrected log mean temp.

difference (ΔTm). 1st, LMTD is calculated using inlet and out temp. of HEX

The above is for counter-current HEX For co-current, the terminal temp. difference shall be (T1-t1) and (T2-t2)

Thermal design principles (cont’d)

ii) Initial heat transfer area (Ao) (cont’d) The LMTD equation is based on the following

assumptions:◦ No change in specific heats;◦ Constant heat transfer coefficients; and◦ No heat losses

Once the ΔTm, Uo and Q are determined, calculate the Ao (refer to Slide #9)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter Four (4) tube pitch layout:

◦ Triangular (30°)◦ Rotated Triangular (60°)◦ Square (90°)◦ Rotated Square (45°)

Adv. & Disadv. of triangular pitch layout?◦ Accommodate more tubes than other patterns◦ Produce high turbulence → better heat transfer◦ Typical pt = 1.25do → restricts mech. cleaning of tubes to

restricted access lanes◦ Preferred when the diff. in OP between 2 fluids are significant

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) Adv. & Disadv. of Triangular pitch layout? (cont’d)

◦ Limited to clean shell-side services◦ Can be used in dirty shell-side services if a suitable &

effective chem. cleaning is available. Adv. & Disadv. Of Square pitch layout?

◦ Typicallly used for dirty shell-side services & when mech. Cleaning is required

◦ Not used in the fixed head design as cleaning is unfeasible◦ Used when the shell-side Re < 2,000 to induce higher

turbulence

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) pt = 1.25do → smallest shell dia. for a given

number of tubes Min. tube pitch for triangular pattern shall be:

◦ pt = 1.25do

TEMA also recommends an additional min. 6mm of cleaning lane between adjacent tubes for square pitch

Min. tube pitch for square pitch shall be:◦ Max (pt = 1.25do ; do + 6mm)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) Tube sizes ranging from ¼” (6.350mm) to 2”

(50mm) Smaller tube size → more compact & economical

size HEX Larger tube size → heavy fouling & ease via

mech. Cleaning Preferred length of HEX tubes → 6ft (1.83m), 8ft

(2.44m), 12ft (3.66m), 16ft (4.88m), 20ft (6.10m) & 24ft (7.32m)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) Once the tube size is selected, calculate the area of 1

tube (A1,tube)

Calc. the tube-side velocity. Ensure the fluid velocity conforms to the requirement (refer next Slide #18)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) Calc. the tube-side velocity. Ensure the fluid

velocity conforms to the requirement (cont’d)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) If Ut is within the limits but at the lower side,

select smaller tube size & repeat the calc. above. Adequacy is determined frm. the tube-side

pressure drop! Next, calc. tube bundle dia. (Db) (mm) BS 3274: HEX dia. Frm 6” (150mm) → 42”

(1,067mm) TEMA: shell dia. → 60” (1,520mm)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) TEMA: shell dia. → 60” (1,520mm) (cont’d):

Parameter K1 & n1 tube pitch & no. of tube passes

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) K1 & n1 → tube pitch type & no. of tube passes

(cont’d)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) Shell inner dia. (Ds) → find out the shell-bundle

clearance Shell bundle clearance → type of HEX rear head

◦ Pull through floating heads (Type T)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) Shell bundle clearance → type of HEX rear head

(cont’d)◦ Split-Ring floating heads (Type S)

◦ Outside packed floating heads (Type P)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) Shell bundle clearance → type of HEX rear head

(cont’d)◦ Fixed tube sheet (Type L, M & N)

◦ U-tube (Type U)

Thermal design principles (cont’d)

iii) Tube pitch, tube size, tube length & shell diameter (cont’d) Shell bundle clearance → type of HEX rear head

(cont’d)◦ Externally sealed tube sheets (Type W)

Ds = Db + Shell-bundle clearance◦ Convert Db & Shell-bundle clearance frm. mm → m

Thermal design principles (cont’d)

iv) Calc. tube-side heat transfer coeff. (hi) First, calc. the tube-side Reynolds number Re < 2,100 Laminar Re > 10,000 Turbulent

Thermal design principles (cont’d)

iv) Calc. tube-side heat transfer coeff. (hi) If 100 < Re < 2,100, use Sieder-Tate’s eq.

◦ Nu ≥ 3.5, if Nu < 3.5 → Nu = 3.5

Thermal design principles (cont’d)

iv) Calc. tube-side heat transfer coeff. (hi) If Re > 10,000, use Sieder-Tate’s eq.

◦ With 0.7 < Pr < 700 & L/Ds > 60

If 40,000 < Re < 100,000, use ESDU eq. ◦ With 0.7 < Pr < 160 & L/Ds > 60

Thermal design principles (cont’d)

iv) Calc. tube-side heat transfer coeff. (hi) Transitional regime shall be avoided for

design, if cannot:◦ Min. (Nu from Slide #28, Nu from Slide #29)◦ Nu from Slide #28 & #29 are Sieder-Tate’s eq.

v) Calc. shell-side heat transfer coeff. (hs) Calc. baffle spacing (B). Why have baffles?

◦ Tube support◦ Maintain suitable shell-side fluid velocity◦ Prevent tube failure due to flow induced vibration

Thermal design principles (cont’d)

v) Calc. shell-side heat transfer coeff. (hs) (cont’d) Baffle spacing (B): Max. (Ds/5; 2 in.) → ensure

same units Max baffle spacing (B) is:

Max baffle spacing is expressed in inches Baffle cut of 25% is used, can vary from 15%

→ 45% Why? → Kern’s shell-side ΔP is based on 25%

Thermal design principles (cont’d)

v) Calc. shell-side heat transfer coeff. (hs) (cont’d) Calc. shell-side cross flow area (As): Calc. linear velocity (Us) (0.3m/s<Us<1.0m/s)

Thermal design principles (cont’d)

v) Calc. shell-side heat transfer coeff. (hs) (cont’d) Calc. shell-side equiv. dia. (de) → based on

type of tube pitch Calc. shell-side Re → to obtain the Shell-side

heat transfer factor (jh) (Refer Slide #34)

Thermal design principles (cont’d)

v) Calc. shell-side heat transfer coeff. (hs) (cont’d) Calc. hs: (units same as tube-side)

Jh obtained from the graph below (refer to Slide #35)

Thermal design principles (cont’d)

v) Calc. shell-side heat transfer coeff. (hs) (cont’d) Jh obtained from the graph below (refer to

Slide #35) (cont’d)

Thermal design principles (cont’d)

vi) Calc. overall heat transfer coeff. (Uo) Uo → reciprocal of the overall resistance to

heat transfer & it’s a sum of several heat transfer resistances

Each resistance depend on several factors:◦ Physical properties of fluids◦ Heat transfer process (conduction, convection,

condensation, boiling or radiation)◦ Physical arrangement of the heat transfer surface

Thermal design principles (cont’d)

vi) Calc. overall heat transfer coeff. (Uo) Each resistance depend on several factors:

◦ Physical arrangement of the heat transfer surface (cont’d)

Thermal design principles (cont’d)

vi) Calc. overall heat transfer coeff. (Uo) The Uo calc. shall not be taken as the final

answer Compare it with the assumed Uo frm. Slide

#10 Uo from Slide #37 should be 30% of Uo,ass

from Slide #10

If not, repeat calc. starting from Slide #9

Thermal design principles (cont’d)

vii) Calc. tube-side pressure drop (ΔPT) ΔPT calc. from:

Index m is a f(fluid flow regime)◦ Laminar flow (Re < 2,100), m = 0.25◦ Turbulent flow (Re > 2,100) m = 0.14

Tube-side friction factor is dependent on tube-side Re (refer Slide #40)

Thermal design principles (cont’d)

vii) Calc. tube-side pressure drop (ΔPT) Tube-side friction factor is dependent on

tube-side Re (refer Slide #40) (cont’d)

Thermal design principles (cont’d)

vii) Calc. tube-side pressure drop (ΔPT) ΔPT shall be within the specifications If lower than specs, select diff. tube dimensions &

layout, repeat the calcs. frm. Slide #18.viii) Calc. shell-side pressure drop (ΔPS) ΔPS shall be within the specifications Similar approach as (vi), obtain jf from Slide #42.

Thermal design principles (cont’d)

viii) Calc. shell-side pressure drop (ΔPS)

Thermal design principles (cont’d)

Thank you!