Engineers Guide: Design of Sugar Industry Evaporators

Design Considerations for Evaporator Drums and Calandria

Evaporator drums are critical components in chemical evaporation systems, and their design requires careful consideration of various factors. Most evaporator drums operate under vacuum conditions, with a typical external pressure rating of 0.1 N/m2.

Evaporator Drum Design

The evaporator drum consists of several key components:

Conical Bottom Portion: Designed for a similar pressure rating as the drum, this section provides a smooth transition for the vapor flow.
Top Head Section: This section, which is preferred to be flanged or flared, dished, or conical in shape, facilitates easy access for maintenance and inspection.
Cylindrical Pieces: The vapor drum is constructed from separate cylindrical pieces joined by flanges, allowing for easy assembly and disassembly.
Large Nozzles: Manholes, sight glasses, and other large nozzles are reinforced with compensating rings to ensure structural integrity.
Supports: Brackets welded to the vapor drum or calandria provide additional support and stability.

Calandria Design

The calandria, a tubular heating surface, is designed based on the shell and tube heat exchanger model. Since steam under pressure is typically used as the heating medium, the design takes into account the pressure rating of the system. The calandria and vapor drum are connected by flanged joints or directly welded.

Design Factors and Heat Balance

The design factors, heating surface area, and steam requirement are derived from the overall and individual heat balance of the multiple-effect chemical evaporation system. This involves careful consideration of the heat transfer coefficients, temperature differences, and mass flow rates of the various streams involved.

By carefully considering these design factors and incorporating them into the design of the evaporator drum and calandria, engineers can create efficient and effective chemical evaporation systems that meet the required process conditions.

The nomenclature for quadruple effect calculations for the 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:

The 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	m_f = m_p1 + m_v1 m_fx_f = m_px_p1	m_{s .}λ_s= m_fCp_f(T₁-T_f) + m_v1.λ_v1
Second effect	m_p1 = m_p2 + m_v2 m_p1x_p1 = m_p2x_p2	m_v1.λ_v1= m_p1Cp_f (T₂-T₁) + m_v2.λ_v2
Third effect	m_p2= m_p3 + m_v3 m_p2x_p2 = m_p3x_p3	m_v2.λ_v2 = m_p2Cp_f (T₃-T₂) + m_v3.λ_v3
Fourth effect	m_p3 = m_p4 + m_v4 m_p3x_p3 = m_p4x_p4	m_v3.λ_v3= m_p3Cp_f (T₄-T₃) + m_v4.λ_v4
Fifth effect	m_p4 = m_p5 + m_v5 m_p4x_p4 = m_p5x_p5	m_v4.λ_v4= m_p4Cp_f (T₅-T₄) + m_v5.λ_v5

Assuming the steam temperature is 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	99.819^oC	2257.4 KJ/kg
Second effect	1.19989atm = 480.6 mm Hg	87.616^oC	2289.4 KJ/kg
Third effect	0.81229atm = 302.8mm Hg	75.844^oC	2319.2 KJ/kg
Fourth effect	0.44407atm = 99.6mm Hg	60^oC	2379.1 KJ/kg
Fifth effect	0.09527atm	50^oC

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)	Mass fractions in terms of x_p
1^st effect	66	142.33	0.1804
2^nd effect	64	79.33	0.2313
3^rd effect	17	62.33	0.32
4^th effect	10	52.33	0.6
5^th 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

	m_s λ_s=UAΔT	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
1^st effect	66000	142.33	3564.3	2700	4775
2^nd effect	63000	79.33	7012	2500	4421
3^rd effect	17000	62.33	2530.7	1200	2122
4^th effect	9320	52.33	2658.86	800	1415
5^th 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

The actual area of downtake is 3.1m²
The total area of the tube sheet is = 24.03m²
The 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:

The 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
The diameter of drum is 8m

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Design of Sugar Industry Evaporators