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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