Tuesday, 11 June 2013

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 (ST )2  (for square pitch)
     =  n x 0.866 (ST )2  (for triangular pitch)
Here ST  is the pitch of tube.
     Also, Area (A) = an

The area occupied by the central downtake is considered to be 40% of the cross sectional area of tubes, Ai
= 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 (Ao )
= 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 Ao / π  )
Where DT  must be in mm.

    2) Calendria Sheet Thickness :-

 ts = (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 ‘ts’ after adding corrosion allowance must be greater than 10mm,if it is less than 10mm then take ts = 10mm.
    
    3)   The Tube Sheet Thickness :-

t = FG ( 0.25P/f )

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

K = ( Es ts (Do - ts ) ) / ( Et N tt (do - 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 Rd factor as,
Rd = ( (V/A)  / 0.0172 x ( ρL - ρV / ρV)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  )

ρL  =  density of liquid

ρV  = density of vapour

Now,
if Rd = 0.5 then no Entrainment Seperator is required & 
if Rd = 1.3 then wire mesh as Entrainment Separator is used &
if Rd = 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.
   Pc = 2.42 E ( t / Do )5/2 /
    ( 1 - µ2 )3/4 ( ( L / Do) – 0.45 ( t / Do )1/2)

Then calculate Pall as,
Pall = Pc / factor of safety

Now, If Pall is less than external pressure of 0.1 N/mm2
then again calculate Pall taking t = 14mm and so on…
    
    6)   Compressive stress is calculated as :-
  
   fc = Pd/2t
t = for which t, Pall is greater than external pressure of 0.1 N/mm2.
If fc 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, Pall is greater than external pressure of 0.1 N/mm2 & L / Do = 1 and calculate Pall as,
       
         Pc = 2.42 E ( t / Do )5/2 /
    ( 1 - µ2 )3/4 ( ( L / Do) – 0.45 ( t / Do )1/2)
     
Then calculate Pall as,
   
 Pall = Pc / factor of safety

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