Evaporator Design formulas and important design considerations with online calculator

Formulas and design parameters in  Robert Evaporator Design with online calculation sheet

The object of evaporation may be to concentrate a solution containing the desired product or to recover the solvent. Sometimes both may be accomplished. Evaporator design consists of three principal elements: heat transfer, vapor-liquid separation, and efficient utilization of energy.

In our sugar industry the solvent is juice, heat is supplied by condensing steam, and the heat is transferred by indirect heat transfer across metallic surfaces.

Types of Evaporators:

1. Robert type natural circulation evaporator.
2. Long tube vertical raising film evaporator (Kestner)
3. Long tube vertical falling film evaporator (FFE)
4. Plate type evaporator.
5. Thin film evaporators, high viscous liquid
6. Inclined tube evaporators
7. Horizontal tube evaporators.
8. Flash evaporators.
9. Compact evaporators.

The first three type of evaporators are used in Sugar Industries and recently also use the plate type evaporator.

In this article mainly discussed about Robert type evaporator body design criteria.

Steps in calculation of evaporator design:

1. Number of tubes (N ):

• Mean dia of the tube ( Dm) in mtr = Tube OD- Tube Thickness ( In some designers also take ID of the tube in the place of mean dia.
• Effective Length of the tube ( L ) = Tube length – 2(Tube plate thickness)-2(Tube expansion allowance)
• Number of tubes = Heating surface / π x Dm x  L.
• The thickness of the tube generally take for juice heaters and for evaporator 18G and for pans take 16G. ( 18g = 1.22mm , 16g = 1.625mm, 14g = 1.8mm).

2. Tube plate & Downtake dia :

   Area occupied for tubes in tube plate :

• Tube Pitch (P ) = OD of the tube +Legment of the tube + tube clearance+hole clearance
• proportional factor(β) = Generally β value taken for multiple pass(i.e Juice heaters) 0.6 to 0.8 and for single pass(l.e evaporators) 8 to 1.0.
• Take extra dia in percentage on area occupied for tubes in tube plate  for stay roads arrangement, free withdrawal of condensate and noxious gases removal purpose. while providing the  multiple down design than this percentage may go higher side. Its value lies in the range 10 to 20% on area occupied for tubes.
• Tube plate area required for tubes only ( A_T ) = (0.866 x P² x N /β) x %extra
• Tube plate dia required for tubes only = SQRT ( A_T x 4/π )
  

  Dia of the down take :

• The central well or peripheral  downtake is often utilized to collect the concentrated juice in order to remove it from one vessel to the following vessel.
• According to peter rein down take dia consider less than 25% of the tube plate dia.
• According to E. Hugot The diameter of the centre well varies from ¼ to ⅛ of the interior diameter of the vessel.
• Certain manufacturers replace the centre well by a lateral well or by a series of down takes of small diameter distributed over the calendria (Multiple downtakes).
• From the above generally downtake dia take 20% on tube plate.
• Dia of the single downtake = Tube plate dia for tubes x % of downtake on tube plate.
• Dia of the central downtake in multiple down takes design = SQRT [(Area of the single downtake – Total area of peripheral down takes) x 4/π )] .

The final required tube plate diameter.

Final Dia of the tube plate = = SQRT [(Area of the Tube plate for tubes + Downtake area) x 4/π )] .

3. Dia required for vapour inlet & Dia of the Calendria in radial steam/ vapour entry

   Dia of the Vapour Inlet :

   Number of steam/ vapour entries will take according to heating surface of the body, diameter of the body and performance of evaporation.

• Vapour required for calendria = Heating surface x Evaporation rate of the body.
• Area required for the each vapour entry (m²) = Volume of the vapour in each (M³/sec) / Velocity of vapour.
• Dia of the each steam entry = SQRT [ (Area required for the vapour entry / Number of vapour entries) x (4/π ) ]

As Per Hugot given Evaporation Rate of The Several Vessels of a multiple effect working under the condition of temperature drop from 120^oC to 55^oC

Evaporator Bodies Vapour Velocities Recommended by E.Hugot

Calendria dia at the entry of the steam/vapour jocket

• Area for the inlet vapour ( If more than one connection to calendria than take each inlet vapour area) = π/4 * (Dia of the vapour inlet)²
• Height of the steam entry = Take Effective Length of the tube.
• Width of the steam entry = Area for the inlet vapour / Height of the steam entry.
• Dia of calendria at the point of radial steam entry = Final Dia of the tube plate + Width of the steam entry.
• (Note : This dia to be maintained at the vapour entry side later it may reduced in vapour travel direction.).

4. Vapour outlet pipe dia :

• Vapour volume outlet vapour in M³/sec = Heating surface X Evap. Rate x Specific volume of outlet vapour/3600.
• Vapour outlet pipe dia in mtrs = SQRT [vapour volume /(0.785 x velocity of vapour)]

5. Dia of the condensate line :

• Number of condensate withdrawal points = Consider minimum two numbers of withdrawal points and it can be increase according to the diameter of the body.
• Volume of the condensate in M³/sec = [Heating surface X Evap. Rate ] / [ Density of water x 3600].
• Dia of the each condensate line = SQRT (Volume of the condensate each./(0.785 x velocity of condensate)).

6. Noxious gases connections :

• Generally 10 m²   heating surface area required 1cm² area for removal of noncondensable gases.
• Cross section area of non condensable gases in cm² = Heating surface in m² /10
• Dia of the each non condensable gases line = SQRT( Total area of non condensable gases /0.785*no. of points)
  

  7. Vapour space height :

• Generally for Robert type bodies will take for lost effect 2.5 times on calendria tube height and for remaining bodies will take 2 times on Calendria tube height.
• Tromp quotes an American view that the height of the cylindrical body, above the calandria, should be 1.5 times to twice the length of the tubes. It is wise to specify at least twice; moreover, Tromp later recommended 2 In Europe, a minimum of 3.6 – 4.0m is adopted.

8. Velocity in vapour space of body (Cross checking of the system ):

• Generally in evaporator design vapour space dia may be take same as to calendria dia meter. But we have to check,  how much vapour velocity maintained in body. It is helpful to avoid the entertainment of the system. The velocity of vapour leaving the liquid surface would then be approximately 10 cm/sec.
• In present designs, The vapour velocities in the vessels to be maintained below 3.6m/sec and for lost body it can go upto 4.6 m/sec . Hence the necessity of providing entrainment separators or save all at the vapour outlets from the evaporator vessels.
• In any case, it is considered that entrainment rapidly becomes excessive from the moment when the vapour velocity in the vessel reaches more than 6m/sec.

9. Calendria shell thickness :

• P = Maximum allowable pressure in kg/cm²
• Di = ID of the Calendria in mm
• F = Allowable stress in kg/cm²
  
• J = Welding Joint efficiency in mm
• C= corrosion allowance in mm
• Calendria shell thickness in mm = (P*  Di / (2*F*J – P) ) + C
  

  10. Vapour shell thickness :

• P = Maximum allowable pressure in kg/cm²
• F = Allowable stress in kg/cm²
• J = Welding Joint efficiency in mm
• C= corrosion allowance in mm
• Di = ID of the Calendria in mm
• Vapour shell thickness = (P* Di / (2*F*J – P) ) + C

11. Tube plate thickness :

• C= corrosion allowance in mm
• F = Allowable stress in kg/cm²
• P = Maximum allowable pressure in kg/cm²
• Es = Modulus factor for MS sheet in kg/cm²
• Et = Modulus factor for SS sheet in kg/cm²
• G = ID of the shell inmm
• ts = Thickness of the shell in mm
• tt = Thickness of tube in mm
• do= OD of the tube in mm
• Do = OD of the calendria sheet in mm
• Nt = Number of tubes
• K =( Es x ts x (Do -ts)) /(Nt x Et x tt x(do -tt))
• f = SQRT ( K / (2 + 3K))
• Tube plate thickness in mm = f x G x SQRT((0.25 x P)/F) + C

Generally shell thickness will be taken as follows as (in mm).

[supsystic-tables id=3]

12. Vapour doom dia :

• Generally for Robert type bodies vapour doom dia taken 2 to 2.5 times for vapour outlet pipe area.
• Cross sectional area of the vapour doom = 2 x vapour outlet pipe area.
• Vapour doom dia = SQRT (area of the vapour doom x 4/π ).

13. Top cone Height :

• Top cone angle (φ) = generally take 30 to 35 deg.
• Top cone height = Tan φ( (ID of body – ID of doom)/2)

14. Center Umbrella area :

• In Center Umbrella area calculation follows two types of methods. They are
• Area of Umbrella = Cross sectional Area of the body – cross sectional area of the doom
• Area of Umbrella = consider 60 to 65% on body dia
• Gap between Umbrella to top cone at the place of vapour inlet (  Hi ) = Area of vapour doom / π * Dia of Umbrella
• Gap between Umbrella to top cone at the place of vapour outlet (  Ho ) = Area of vapour doom / π * ID of the vapour doom*

Online Calculation Sheet for Robert Evaporator Body Design

( Note :In this calculator provided with two types of sheets. First sheet provided with formulas for better understanding and another one having simple calculation sheet)

Click Here

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