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 :

• C_F = Specific heat of feed (KJ/Kg^oC) 
• T_F =Temperature of feed,^oC 
• W_F = Feed, Kg/hr 
• Ts = Saturation temperature of steam to first effect, ^oF 
• Ws = Steam to first effect, Kg/hr 
• W_1-4 = 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 .λv1
Second effect	mp1 = mp2 + mv2 mp1xp1 = mp2xp2	mv1 .λv1= mp1Cpf (T2-T1) + mv2 .λv2
Third effect	mp2 = mp3 + mv3 mp2xp2 = mp3xp3	mv2 .λv2 = mp2Cpf (T3-T2) + mv3 .λv3
Fourth effect	mp3 = mp4 + mv4 mp3xp3 = mp4xp4	mv3 .λv3= mp3Cpf (T4-T3) + mv4 .λv4
Fifth effect	mp4 = mp5 + mv5 mp4xp4 = mp5xp5	mv4 .λv4= mp4Cpf (T5-T4) + mv5 .λv5

Assuming the steam temperature 120^oC
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  x_f =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, m_s = 66tonnes/hr
Steam balance:

• Steam consumption m_s is = 66tonnes/hr 
• Capacity m_v = 41532.3499 Kg/hr 
• Steam economy = 4.619 

The formula for design of evaporators Q=Q₁=Q₂=Q₃=Q₄
Where Q=m_s λ_s

• m_s λ_s = Q₁ = U₁A₁ΔT₁ 
• m_s λ_s = Q₂ =U₂A₂ ΔT₂ 
• m_s λ_s = Q₃ =U₃A₃ ΔT₃ 
• m_s λ_s = 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 ^oC 
• 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 N_t = 4775
• Pitch of the tube = Pt/D_o = 67.5mm
• Area occupied by tubes As = n X 0.866 X S_T² / 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: t_s = pD / 2fJ – p  = 5.48mm

Tube sheet thickness:
F = √(K/[2+3K])
Where K = E_st_s(D₀-t_s) / E_tN T_i(d₀-t_t)

• E_s = modulus of elasticity of shell material
• t_s  = Sheet thickness
• t_t   = 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 (ρ_L  – ρ_V / ρ_V)^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