In this session, we will discuss the design criteria for the vacuum pan used in the crystallization process in a sugar factory.
Vacuum Pan Design Aspects in Sugar Factory | Sugarprocesstech
The following factors are playing an important role in the design aspect of a vacuum pan.
- a) Incoming
- b) Outgoing
- c) Internal
- d) External
a) Incoming
- Heating medium (Vapour/Steam) – Steady flow and uniform quality of vapor
- Footing / Seed material – Uniform grain size and predetermined ratio grain with liquor.
- Liquor ( Syrup / molasses ) – Uniform composition and flow rate of feed
- Moment Water – Constant Temperature and flow rate.
- Vacuum of the Pan – Steady and uniform.
b) Outgoing
1. Massecuite – Uniform consistency, Purity, and grain size.
2. Condensate flow – Complete withdrawal of condensate for Smooth flow
3. Non-condensable gases – Complete removal of NCG without stagnancy
c) Internal
1. Massecuite head (Hydrostatic head) – As minimum as possible for better circulation and boiling point rise of massecuite
2. Boiling point rise – Minimum fluctuation
3. Circulation of massecuite – Velocity should be as high as possible
4. Temperature of the massecuite – Steady and uniform.
e) External
1. Head loss – Constant and uniform pan temperature
2. Injection water – Flow rate and temperature condition should be uniform
Now Discuss one by one the major design considerations of vacuum pan
Graining Volume of the batch pan ( Go through the below link)
The Concept of Graining Volume
Heating surface to ratio of the pan ( S/V ratio)
S/V ratio is important factor in the design and performance of vacuum pan. The heating surface is expressed in square meter and working volume expressed in cubic meter.
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So the unit of S/V ratio becomes m2/m3.
This ratio is mainly depends on the heating medium and type of the pan.
According to present scenario, 2nd bleed or 3nd bleed or 4th bleed vapours is used as a heating medium for pan boiling.
For batch pan the S/V ratio will be in the range of 6.5 to 6.8 m2/m3
For continuous pan the S/V ratio will be in the range of 9.5 to 10.5 m2/m3
Hydrostatic head or Hydrostatic pressure
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The height of the strike
The massecuite circulation will be slow down while the massecuite reaching level 1000 mm above the top tube plate.
Generally, the highest massecuite level above the top tube plate is kept around 1000 mm to 1200 mm in batch pans and 300mm to 500 mm in continuous pans without using any mechanical circulator. While using the mechanical circulators, it will be increased by 200 to 300 mm height
Bottom Cone
The bottom of the pan generally takes the form of a truncated cone, but a segment of a sphere, or even a torus may be followed.
This part of the pan is situated at the bottom of the pan below the calendria making an angle of 170 to 250 with the extended portion of the vertical side wall of the calendria.
The extended portion is about 150mm to 200mm.
This is to facilitate the repairs/replacement of the end tubes of the calendria. It is advantageous for graining volume that these angles should be small but, for rapid flow of the massecuite at the moment of discharge, an angle of less than 17° will not generally be used.
Generally, the cone angle is 180 to 200 is the value while considering all advantages and disadvantages
Downtake Diameter
The downtake diameter is generally not less than 0.4 times the pan diameter unless a stirrer is fitted. A smaller diameter has been shown to restrict circulation.
Circulation ratio is one of the important criteria for estimating the downtake diameter.
Ideally, this should be less than 2.5 to obtain a pan with good circulation, although several pans that have circulation ratio values up to 2.8 have given reasonable results.
Tubes
Generally, the tubes are made up of Stainless steel. The diameter of the tube is 100 mm and the length of the tubes varies from 700-1500 mm.
Stainless steel for tubes is generally AISI 304 grade (18% Cr + 8% Ni).
The tubes are generally, 100 mm in diameter and installed on a triangular pitch. The legment having 16 mm.
Tube diameters may be for low-grade and smaller for high-grade massecuites.
Boiling Point Rise
The boiling point rise or ‘boiling point elevation’ is defined as the temperature difference between the boiling point of the boiling massecuite at a given absolute pressure and the boiling point of water at the same absolute pressure.
i.e Difference between the temperature of a boiling sugar solution and the temperature of boiling pure water, both measured at the same pressure.
The boiling point of the massecuite depended on
1. Brix of the material (Density or Dissolved solids of material
2. Height of the massecuite above the calendria of a pan. ( Strike height)
3. Circulation of massecuite. (Mechanical and Natural circulation)
Various heights and dia of the batch vacuum pan for design aspects
( Note: These measurements will be given a approximate values in design of vacuum pan)
Dia of Vacuum pan
D1 – Calendria shell dia ( 1.21 P √N – P = Pitch of the tube & Number of tubes)
D2- Down take dia (0.4D1)
D3 – Vapour body dia = D1
D4 – Vapour dome dia (1.8 to 2 Dvo)
D5 – Discharge dia
Dvo – Vapour O/L dia
Dvi – Vapour I/L dia
Height of the vacuum pan
H1 – Calendria shell height ( Depends on tube height)
H2 – Strik level height from the top tube plate ( Depends on graining volume of pan)
H3 – Vapour space above the strik level (1200mm to 1500 mm )
H4 – (D3-D4) x Tan ∅ /2 ( ∅ = 180 to 200)
H5 – Height of the vapour dome (1.5 to 1.75 Dvo)
H6 – Bottom ring ( 150mm to 200 mm)
H7 = (D1-D5) x Tan ∝ /2 ( ∝ = 180 to 200)
H8 – 150mm to 200mm
Steam Entry
For pans with central downtake, it is preferable to distribute the steam through several entries placed around the calandira. The quantity of steam surging against the outer tubes immediately in front of the steam entry produces a very rapid circulation at that point at the beginning of the strike at the expenses of other zones of the Calandria. At the end of the strike, this zone near the steam entry may produce false grain.
From this point of view, it is preferable to provide each steam entry with a conical baffle to distribute the steam and avoid direct overheating of massecuite. For large capacities pans shall be provided two steam entry points.
Condensate Removal
Condensate water is removed by either an inverted siphon or a sealing mond. The sizing of condensate shall be calculated on the basis of velocity of condensate having 1 m/sec.
The number of condensate points will be chosen on the basis of calendria dia meter
Non Condenssible Gases (NCG) Removal
Adequate arrangements for the removal of condensate and incondensable gases must be made.
Generally, non-condensable gasses are released by number of perforated pipes inside calandria and connected through main line and vented outside the pan. The total cross section are of the all NCG connection shall be required 1 cm2 area for a 10 m2 heating surface of vacuum pan.
Sight Glasses
The batch vacuum pans are equipped with 4 – 6 sight glasses at different height from front side and one sight glass from backside at the top level so as to facilitate easy supervision. For continuous pan equipped with sight glasses for each compartment.
Design of 80 MT capacity Batch Pan
Please go through the link Click Here
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