Sugar Factory Tubes for Heating, Evaporating and Crystallising
Some desirable characteristics of tubes for juice heaters, evaporators and pans are
- easy to expand into the tube plate
- corrosion resistant
- similar co-efficient of thermal expansion to the shell of the vessel
- have a good heat conductivity
- have a smooth and bright inside surface: a very low surface roughness favours a higher flow of the juices
- have a long life
- have a good cost : benefit ratio
The choice material is between:
- mild steel
- copper (or brass)
- austenitic stainless steel (types AISI 304 and / or 316)
- special alloys (with higher chromium / nickel contents)
- ferritic stainless steel
In practice the choice is between mild steel, 304 stainless steel or 439 stainless steel. 304 for shorter tubes and 439 for longer tubes. Carbon steel is not recommended because in the long run (a period of say 20 years) carbon steel tubes work out more expensive (Watkins and Bartholemew, Development of Low Cost Carbon Steel Tube for Sugar Mill Evaporators in South Africa, SASTA 1998, pg 72)
If it is decided that carbon steel tubes are to be used the recommended specification is BS3605 Gr 320
This grade of stainless steel can be used where the tube length is less than three metres. The coefficient of thermal expansion for 304 is 1.8×10-2 mm/m/°C which is substantial more than that of carbon steel. When the vessel is hot the thermal stresses in the tubes will be high. Tubes of 304 stainless steel should always be annealed after welding.
439 Stainless Steel
ASTM TP439 is a titanium stabilised ferritic grade of stainless steel (17-19% Cr) which is recommended for long evaporator or pan tubes (in excess of 5m long)
Advantages of grade 439
- fully ferritic metallurgical structure (ensured by the titanium stabilisation)
- very good weldability and ductility;
- inter-crystalline corrosion resistance;
- pitting corrosion resistance;
- full immunity to stress corrosion.
- Coefficient of thermal expansion (in the range 0°C - 100°C) is 1.02×10-2 mm/m/°C
Stress corrosion cracking
This type of corrosion occurs when
- A susceptible material is subject to
- mechanical stress in a
- corrosive environment
In an evaporator, pan or juice heater under the above conditions the result will be cracks leading to breakage in the area near the tube plate.
The danger of stress corrosion cracking exists in virtually all evaporators. The risk will be higher if tubes over 7 metres in length (some designs of continuous pans, falling-film evaporators and Kestner evaporators). Ferritic stainless steels are immune to stress corrosion cracking
Thermal conductivity of ferritic stainless material is 40% higher than that of austenitic grades (like 304, 304L, 316 or 316L) i.e.: 26 vs 15 watt/metre/°C.
Recommended Wall Thickness
For evaporators and heaters, with tube length less than five metres a wall thickness of 1.2 mm is acceptable, for tubes longer than five metres a wall-thickness of 1.50 mm is quite sufficient (even on longer lengths up to 11 m) Tubes with 2.0 mm would be harder to swage into the holes and would require a 600°C pre-heating of tube ends.
Wall-thickness 1.6 or 1.75 mm are recommended for those tubes located near steam-entrance and subject to some vibration during the process.
For vacuum pans with 100 mm diameter tubes the recommended wall thickness is 1.5 or 1.6mm.
Allow 0.6 mm clearance between tube and plate.