Design of Sugar Industry Evaporators

Most of the evaporator drums operate under vacuum condition and designed for an external pressure of 0.1 N/m2. Conical bottom portion designed for similar pressure rating and top head section preferred to be flanged or flared, dished or conical shape. Calandria, having tubular heating surface designed as per shell and tube heat exchanger model. Since steam under pressure in usually accepted as the heating medium, the design is based on the pressure so the calandria and vapor drum are connected by flanged joints or directly welded. The vapor drum made up of separate cylindrical pieces and joined by flanges. Large nozzles like manholes, slight glasses are reinforced with compensating rings. Supports placed below the brackets are welded to the vapor drum or to the calandria. External calandria is also designed as shell and tube heat exchanger.The design factors, heating surface area and steam requirement are derived by the overall and individual heat balance of multiple-effect chemical evaporation system.
The nomenclature for quadruple effect calculations for design of sugar industry evaporators :
  • CF = Specific heat of feed (KJ/KgoC) 
  • TF =Temperature of feed,o
  • WF = Feed, Kg/hr 
  • Ts = Saturation temperature of steam to first effect, o
  • Ws = Steam to first effect, Kg/hr 
  • W1-4 = Total water removed by evaporation 
  • m = mass flowrate
  • p = product
  • v = vapor 
  • f = feed 
  • U = heat transfer coefficient

Design of five effect evaporators system:

Overall balance of the evaporator would be: m·f = m·p4 +m·v
Overall vapor balance: m·v = m·v1 + m·v2 + m·v3 + m·v4
Solute balance Energy balance equations  
First effect mf = mp1 + mv1
mfxf  = mpxp1
ms .λs = mf Cpf (T1-Tf) + mv1 v1
Second effect   mp1 = mp2 + mv2
mp1xp1 = mp2xp2 
mv1 v1= mp1Cpf (T2-T1) + mv2 v2
Third effect   mp2 = mp3 + mv3
mp2xp2 = mp3xp3
mv2 v2 = mp2Cpf (T3-T2) + mv3 v3
Fourth effect   mp3 = mp4 + mv4
mp3xp3 = mp4xp4
mv3 v3= mp3Cpf (T4-T3) + mv4 v4
Fifth effect   mp4 = mp5 + mv5
mp4xp4 = mp5xp5
mv4 v4= mp4Cpf (T5-T4) + mv5 v5

Assuming the steam temperature 120oC
The energy value of steam = 2201.6 KJ/kg from steam tables
Basis: For the mass flow rate of the entering juice ‘m’= 208.33tonnes/hr.

Pressure maintained Temperature from steam tables Latent heat of vaporization
First effect 1.60687atm= 755 mm Hg 99.819oC 2257.4 KJ/kg
Second effect 1.19989atm = 480.6 mm Hg 87.616oC 2289.4 KJ/kg
Third effect 0.81229atm = 302.8mm Hg 75.844oC 2319.2 KJ/kg
Fourth effect 0.44407atm = 99.6mm Hg 60oC 2379.1 KJ/kg
Fifth effect 0.09527atm 50oC

Assuming the value of  xf =0.15

By substituting the values in the balance equations and solving the simultaneous equations we get the following values:
Type Vapor flow rates(tonnes/hr) Product flow rate(tonnes/hr) Massfractions in terms of xp
1st effect 66 142.33 0.1804
2nd effect 64 79.33 0.2313
3rd effect 17 62.33 0.32
4th effect 10 52.33 0.6
5th effect 1 51.33 0.6

Assuming the mass flow rate of steam, ms = 66tonnes/hr
Steam balance:
  • Steam consumption ms is = 66tonnes/hr 
  • Capacity mv = 41532.3499 Kg/hr 
  • Steam economy = 4.619 
The formula for design of evaporators Q=Q1=Q2=Q3=Q4
Where Q=ms λs
  • ms λs = Q1 = U1A1ΔT1 
  • ms λs = Q2 =U2A2 ΔT2 
  • ms λs = Q3 =U3A3 ΔT3 
  • ms λs = Q4 =U4A4 ΔT4
ms λs=UA ΔT U, KJ/(m2.hr. oC)
First effect 66000×2187.18 = U1×2700×(125-110) 3564. 3
Second effect 63000×2201.6   = U2×2500×(110-102) 7012
Third effect 17000×227.168 = U3×1200×(102-90) 2530.6
Fourth effect 9310×2281.6     = U4×800×(90-80) 2658.861
Fifth effect 9320×2353        = U5×300×(80-60) 3663.41

Calculation for number of tubes:  

Formula for number of tubes= n × π × D × L = A

Type Vapor  flow rates(kg/hr) Product flow rate(kg/hr) U(KJ/m2.hr oC) Area, A=m2 n, no.of Tubes
1st effect 66000 142.33 3564.3 2700 4775
2nd effect 63000 79.33 7012 2500 4421
3rd effect 17000 62.33 2530.7 1200 2122
4th effect 9320 52.33 2658.86 800 1415
5th effect 1000 51.33 3663.41 300 531

Model mechanical design of sugar industry evaporators:   

  • Temperature in evaporator = 99.819 o
  • Diameter of tubes: 42 ID × 45 OD 
  • No. of tubes: 4775 
  • Length of the tube = 4m 

Calandria: 

  • Pressure = 755 mm 
  • Heating surface area = 2700 m2 
  • Material of construction: Stainless Steel 
  • Permissible stress for low carbon steel—98 N/mm2 
  • Modulus of elasticity for low carbon steel = 19.0 X 104 N/mm2 
  • Modulus of elasticity for SS 304= 9.5 X 104N/mm2
Calandria with vertical tubes:
  • No. of tubes Nt = 4775
  • Pitch of the tube = Pt/Do = 67.5mm
  • Area occupied by tubes As = n X 0.866 X ST2 / β      = 18.4 m2
  • Required area for central down take = 40% X Cross sectional area of the tube = 0.4 X 2110 X π X0.0452 / 4    = 3.04m2
  • Actual area of down take is 3.1m2
  • Total area of tube sheet is  = 24.03m2
  • Diameter of the tube sheet is = 5.96m
  • Calandria sheet thickness: ts = pD / 2fJ – p  = 5.48mm

Tube sheet thickness:
F = √(K/[2+3K])
Where K = Ests(D0-ts) / EtN Ti(d0-tt)
  • Es = modulus of elasticity of shell material
  • t= Sheet thickness
  • tt   = Tube thickness
  • k = 0.15527
  • F = 0.250936
  • t = FG√(0.25p)/f  = 5.48mm

Bottom flange of calandria:
  • Thickness of the flange = 40mm
  • Number of bolts = 112
  • Outside diameter = 3894mm
  • Pitch circle diameter = 3825mm
  • Size of bolts = 20M

Evaporator drum:
  • Rd =     V/A   0.0172 X (ρL  – ρV / ρV)1/2
  • Let Rd = 0.8
  • Here V = volumetric flow rate of vapor in m3/sec = 66
  • A = 31.32m2
  • Diameter of drum is 8m