Design Procedure for Standard Vertical Short Tube Evaporator

Design Procedure for Standard Vertical Short Tube Evaporator

    1)Design of Calendria :-

 Number of tubes(n) =

 Heat transfer area / Tube area

 Here, tube area = π x outside tube dia. x effective tube  length

Area occupied by ‘n’ tubes ( A )

 = n x (S_T )²  (for square pitch)

     =  n x 0.866 (S_T )²  (for triangular pitch)

Here S_T  is the pitch of tube.

     Also, Area (A) = a_n /β

The area occupied by the central downtake is considered to be 40% of the cross sectional area of tubes, A_i

= 0.4 x No. of tubes x cross sectional flow area of a single tube( π/4 x Di ² ) & Di  = DO – 2t  

Here t = tube thickness

The inside diameter, di , is then obtained as :-

di  = ( 4 x A / π )^1/2

Outside diameter of downtake(do ) = di  + 2 x 10

The area of downtake based on outside diameter (A_o )

= π/4 x do ²

Therefore, the total area occupied by downtake and tubes =

area of tubes + area of downtake

 Therefore, Diameter of tubesheet, DT , is calculated as,

                                 DT  = ( 4 x A_o / π  )

Where DT  must be in mm.

    2) Calendria Sheet Thickness :-

 t_s = (PDT / 2fJ – P) + c

If steam pressure or working pressure is given then multiply it with 1.1 , then we get design pressure.

P   = Design Pressure

DT   = Diameter of tubesheet as calculated above

f    = Allowable stress

J    = Joint efficiency = 1

The calculated value of ‘t_s’ after adding corrosion allowance must be greater than 10mm,if it is less than 10mm then take t_s = 10mm.

    

    3)   The Tube Sheet Thickness :-

t = FG ( 0.25P/f )

F = ( k / 2+3k )^1/2

K = ( Es ts (D_o - ts ) ) / ( Et N tt (d_o - tt ) )

G = DT + 25

P   = Design Pressure

DT   = Diameter of tubesheet as calculated above

f    = Allowable stress          

    

    4)   Design of Evaporator or Vaccum Drum  :-

The diameter of the drum is considered to be the same as that calculated for Calendria i.e, DT   = Diameter of tubesheet = Diameter of the Drum

To see whether the diameter is adequate for entrainment separator,we calculate R_d factor as,

R_d = ( (V/A)  / 0.0172 x ( ρ_L - ρ_V / ρ_V)^1/2 )

V = Volumetric flow rate

   = Amount of water evaporated/density of vapour

A = cross sectional area of the drum 

    = π/4 x DT ² ( here DT should be in m  )

ρ_L  =  density of liquid

ρ_V  = density of vapour

Now,

if R_d = 0.5 then no Entrainment Seperator is required & 

if R_d = 1.3 then wire mesh as Entrainment Separator is used &

if R_d = 0.5 then the height of drum considered as the disengaging height is based on Drum Diameter.

   5)   Drum Thickness :- The drum is under vaccum. The outside pressure is atmospheric . Therefore for the design purpose the drum is subjected to external pressure of 0.1 N/mm².Therefore we shall assume thickness t = 12mm and length of shell and calculate allowable stress.

   P_c = 2.42 E ( t / Do )^5/2 /

    ( 1 - µ² )^3/4 ( ( L / Do) – 0.45 ( t / Do )^1/2)

Then calculate P_all as,

P_all = P_c / factor of safety

Now, If P_all is less than external pressure of 0.1 N/mm²

then again calculate P_all taking t = 14mm and so on…

    

    6)   Compressive stress is calculated as :-

  

   f_c = Pd/2t

t = for which t, P_all is greater than external pressure of 0.1 N/mm².

If f_c is less than the given allowable stress then our design is safe.

    

     7)   Conical Heads at Top and Bottom :-

Now take t = for which t, P_all is greater than external pressure of 0.1 N/mm² & L / D_o = 1 and calculate P_all as,

       

         P_c = 2.42 E ( t / Do )^5/2 /

    ( 1 - µ² )^3/4 ( ( L / Do) – 0.45 ( t / Do )^1/2)

     

Then calculate P_all as,

   

 P_all = P_c / factor of safety

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Design Procedure for Flange Joint for a Cylindrical Pressure Vessel

Design Procedure for Flange Joint for a Cylindrical Pressure Vessel

a)  Design of Gasket :-

 i)Gasket inside diameter (G_i)= Do + 10

ii)G_o / G_i = ( y – pm / y – p(m+1) )^1/2

Here p is operating internal pressure

m= gasket factor

 y = gasket seating stress

Now, After calculating the ratio G_o / G_i substitute the value of G_i and calculate

iii) G_o i.e Gasket outside diameter _.

Then calculate,

iv) Mean Gasket Diameter(G)  = G_o + G_i / 2

v) Width of Gasket ( N ) = G_o - G_i / 2

       vi) Basic Gasket seating width b_o = N/2

Now,

If b_o < 6.3 then b = b_o    &

If b_o > 6.3 then b = 2.5(b_o)^1/2

Here, b is Effective gasket seating width

Also, G is calculated as :-  G_o – 2b

b) Design of Bolt :-

i)                 Bolt load under atmospheric condition is calculated as :-

         W_m1 = π b G Y_b

         ii) Bolt load under atmospheric condition is calculated as :-

                 W_m2 = π (2b) G m p + π/4 G² P

Consider the greater value from W_m1 and W_m2 for all the further calculations.

iii)           Area of bolt required at atmospheric condition:-

A _m1 = W_m1 / f_a

_Here  f_{a is allowable stress of bolt material at atmospheric condition.}

iv)           Area of bolt required at operating condition:-

A _m2 = W_m2 / f_b

_{Here  fb is allowable stress of bolt material at operating condition.}

         Consider the greater value from A_m1 and A_m2 for all the further calculations.

v)              No. of Bolts(n) = G/ 25(should be a factor of 4)

vi)           Bolt Diameter ( d_b ) = ( 4 x A_m(max) / π x n )

_vii)        Bolt circle Diameter (B) = G_o + 2 d_b

_viii)     Actual bolt spacing required( b_s ) = π x B / n

c)   Design of Flange :-

    Outside diameter of flange :-

        D_fo = B + 2 d_b

 

            Flange thickness( t_f ) = G ( P/ kf )^1/2

k = 1 / ( 0.3 +( 1.5 W_m(max) h_G/H x G ))

Hydrostatic end force (H) = π/4 G² P

Radial distance from gasket load reaction to bolt circlr (h_G)= B – G / 2

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Design Procedure for Fixed Tubesheet Heat Exchanger

Design Procedure for Fixed Tubesheet Heat Exchanger

SHELL SIDE :-

Note :- For Shell Side J = 0.85

    

   1)Shell Diameter :-

a)Area occupied by each tube (a) = (S_T )²  (for square pitch)

                                                      = 0.866 (S_T )²  (for triangular pitch)

Here S_T  is the pitch of tube.

b) Area occupied by ‘n’ tubes (a_n) = n x (S_T )²  (for square pitch)

                                                       =  n x 0.866 (S_T )²  (for triangular pitch)

      c) Area of shell (A_s) = a_n /β

            Here β is the proportionality factor whose value is,

0.8 - For single pass

0.7 - For double pass

0.6 - For multiple pass

      d) Also, Area of shell (A_s) = π/4 Di ²  

Here Di  is the Shell inside diameter.

       Therefore from c) and d) we can say that,

                                 a_n /β = π/4 Di ²  

From the above equation we can calculate Di ,Which should be a multiple of 50.

2)Shell Thickness :-

t = (PDi / 2fJ – P) = (PDo / 2fJ + P) = (PD / 2fJ)

P   = Design Pressure

Di   = Inner diameter of shell as calculated above

Do  = Outer diameter of shell

D    = Mean diameter of shell = Do + Di /2

f    = Allowable stress of shell material

The calculated value of ‘t’ must be greater than 8mm,if it is less than 8mm than take t = 8mm.

    

      3)Nozzle (Inlet & Outlet) :-

Nozzle Thickness (tn) = (PDi / 2fJ – P)

Here, Di = Nozzle Inlet & Outlet Diameter

After adding corrosion allowance to the above calculated value of tn it must be greater than 4mm,if it is less than 4mm than take tn = 4mm.

  
  4)Head Thickness :- 

     For a Fixed Tubesheet Heat Exchanger the head used is always a Torispherical Head.

Therefore,

Head thickness(th) = (PRcW/2fJ) + c

Here,

Rc = Crown Radius

Rk = Knuckle Radius

W = 1/4 ( 3+ (Rc /Rk)^1/2 )

If Rc is not given then take shell Inside Diameter as Rc .

Also Rk = 6% of Rc

OR

If it is mentioned in question that, U- tube type horizontal heat exchanger head is joined to the shell with the help of flanges which are cold, then, For cold forming calculate

5 x t(shell thickness) and take it as Rk .

    5) Baffles :-  

       The baffles used will be of the Transverse type with spacing = Di / 5

Thickness of baffles will be = 6mm

The baffles are 25% cut

Therefore, Baffle length = 0.75 x Di

  

     6) Tie Rods :-

No. of Tie rods = 6

Dia. Of tie rod = 10mm

  

     7) Flange Joint Between Shell & Tubesheet :-

Note:- Some times G is taken same as calculated for Tube Side.

a)  Design of Gasket :-

i)Gasket inside diameter (G_i)= Do + 10

ii)G_o / G_i = ( y – pm / y – p(m+1) )^1/2

Now, After calculating the ratio G_o / G_i substitute the value of G_i and calculate

iii) G_o i.e Gasket outside diameter _.

Then calculate,

iv) Mean Gasket Diameter(G)  = G_o + G_i / 2

v) Width of Gasket ( N ) = G_o - G_i / 2

       

       vi) Basic Gasket seating width b_o = N/2

Now,

If b_o < 6.3 then b = b_o    &

If b_o > 6.3 then b = 2.5(b_o)^1/2

Here, b is Effective gasket seating width

b) Design of Bolt :-

i) W_m1 = π b G Y_b

          

       ii) W_m2 = π (2b) G m p + π/4 G² P

Consider the greater value from W_m1 and W_m2 for all the further calculations.

iii) Bolt Area (A) = W_m(max) / f

iv) No. of Bolts(n) = G / 25 (should be a factor of 4)

v) Bolt circle Diameter (B) = G_o + 2 d_b

c)   Design of Flange :-

             

        Flange thickness( t_f ) = G ( P/ kf )^1/2

k = 1 / ( 0.3 +( 1.5 W_m(max) h_G/H x G ))

H = π/4 G² P

h_G = B – G / 2

TUBE SIDE :-

Note :- For Tube Side J = 1

   1)Tube Thickness :-
 

         t = (PDi / 2fJ – P) = (PDo / 2fJ + P)

If t is less than 2mm then take t = 2mm.

  

    2)Tubesheet Thickness :-

       t = FG ( 0.25P/f )

For Fixed Tubesheet Heat Exchanger 

F = ( 2+k / 2+3k )^1/2  &

F = ( k / 2+3k )^1/2

K = ( Es ts (D_o - ts ) ) / ( Et N tt (d_o - tt ) )

Take that F whose value is greater & take G = 380 if not given.

    3)Channel & Channel Cover :-

It is proposed to make the channel & channel cover out of a single plate.

Therefore,

Thickness of Channel Plate( t ) = G_c ( kp/f )^1/2

For Ring type gasket or gasket bolted with narrow faced k = 0.3

For cover bolted with full faced gasket k = 0.25

     4) Flange joint between Tubesheet and Channel :-

a)  Design of Gasket :-

If G is assumed 380 then take Ring Gasket width(w) = 

22mm  & bo = w/8

Y_a = 126.6 N/mm²

m = Gasket Factor = 5.5

And if G is to calculated then calculate G by the method as given below.

i)Gasket inside diameter (G_i)= Do + 10

ii)G_o / G_i = ( y – pm / y – p(m+1) )^1/2

Now, After calculating the ratio G_o / G_i substitute the value of G_i and calculate

iii) G_o i.e Gasket outside diameter _.

Then calculate,

iv) Mean Gasket Diameter(G)  = G_o + G_i / 2

v) Width of Gasket ( N ) = G_o - G_i / 2

        

       vi) Basic Gasket seating width b_o = N/2

Now,

If b_o < 6.3 then b = b_o    &

If b_o > 6.3 then b = 2.5(b_o)^1/2

Here, b is Effective gasket seating width

Design of Bolt & Flange is same whether G is calculated or assumed.

b) Design of Bolt :-

i) W_m1 = π b G Y_b

   

        ii) W_m2 = π (2b) G m p + π/4 G² P

Consider the greater value from W_m1 and W_m2 for all the further calculations.

iii) Bolt Area (A) = W_m(max) / f

iv) No. of Bolts(n) = G / 25 (should be a factor of 4)

v) Bolt circle Diameter (B) = G_o + 2 d_b

b)Design of Flange :-

Flange thickness( t_f ) = G ( P/ kf )^1/2

k = 1 / ( 0.3 +( 1.5 W_m(max) h_G/H x G ))

H = π/4 G² P

h_G = B – G / 2

     5)Nozzle (Inlet & Outlet) :-

Nozzle Thickness (tn) = (PDi / 2fJ – P)

Here, Di = Nozzle Inlet & Outlet Diameter

After adding corrosion allowance to the above calculated value of tn it must be greater than 4mm,if it is less than 4mm than take tn = 4mm.

Also,

Area compensated for each nozzle is (A) =

Nozzle inlet & outlet diameter x cover plate thickness

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