An evaporator tube is a seamless tube through which process fluid is heated to vaporisation — the tube wall transfers heat from a heating medium on the shell side into the boiling fluid inside. Five industrial evaporator configurations are specified: kettle reboilers, thermosiphon reboilers, falling film, rising film, and forced circulation — each with specific tube material and TEMA designation requirements.
For procurement engineers working on Indian refinery, petrochemical, fertiliser, and desalination projects, evaporator tube selection is a material specification decision. The wrong tube material in a kettle reboiler handling amine solution, or undersized wall thickness in a thermosiphon reboiler at high heat flux, cause premature failures. This guide covers industrial evaporator tube types used in Indian process plants, tube materials for each service condition, and the TEMA and IBR code requirements that govern the specification.
What Makes Evaporator Tubes Different from Standard Heat Exchanger Tubes
In a standard shell-and-tube heat exchanger, the tube-side fluid either enters as a liquid and leaves as a liquid at a different temperature, or a condensing vapour on the shell side gives up heat. In an evaporator, the tube-side fluid enters as a subcooled liquid and exits as a vapour-liquid mixture or fully vaporised gas. This phase change introduces operating conditions that do not apply to single-phase heat exchangers:
Higher heat flux: Evaporation requires more energy per unit area than sensible heating — latent heat of vaporisation is typically 100–500 kJ/kg for industrial process fluids. An evaporator operates at much higher heat flux than a sensible-heat exchanger at the same thermal duty. Higher heat flux can cause film boiling (departure from nucleate boiling, DNB), where a vapour film insulates the tube from the liquid and tube metal temperature rises rapidly.
Two-phase flow inside the tubes: In rising film and thermosiphon reboilers, the tube-side contains a two-phase vapour-liquid mixture travelling upward. Two-phase flow creates erosion-corrosion at bends and at tube-to-tubesheet joints from vapour bubble collapse (cavitation erosion), and it requires tube wall thickness calculations that account for slug flow pressure surges rather than steady single-phase flow.
Concentration effects: In evaporators concentrating a solution (sugar evaporators, desalination, amine stripper reboilers), the process fluid becomes progressively more concentrated as vapour is removed. The tube wall is exposed to a more corrosive fluid at the outlet end of the tubes than at the inlet. Tube material selection must be based on the maximum concentration at the outlet, not the average or inlet condition.
Industrial Evaporator Types and Their Tube Specifications
Kettle Reboilers
The kettle reboiler is the most common evaporator configuration in Indian refinery and petrochemical service. A tube bundle is submerged in a pool of boiling liquid inside an enlarged kettle shell. Steam or hot process fluid flows inside the tubes; the process fluid boils on the shell side. The tube bundle is typically a U-tube (single tubesheet) design, accommodating thermal expansion between steam inside the tubes and the boiling liquid outside.
TEMA designation: shell designation K (kettle-type shell), giving common designations AKU or BKU. Tube materials: SA-179 carbon steel for steam reboilers heating clean hydrocarbon bottoms; SA-213 T11 or T22 for high-temperature hot oil heating duty; TP304L or TP316L for amine service, sour water strippers, and corrosive bottoms. Design is per ASME Section VIII Division 1, accounting for both internal steam pressure and external boiling-side pressure.
Thermosiphon Reboilers
Thermosiphon reboilers (natural circulation reboilers) use the density difference between inlet liquid and outlet vapour-liquid mixture to drive circulation without a pump. Process liquid from the bottom of the distillation column enters the tube-side at the bottom header, rises through vertical tubes while absorbing heat from steam or hot fluid on the shell side, and exits at the top as a vapour-liquid mixture that returns to the column.
Thermosiphon reboilers operate at the highest heat flux of all evaporator types in Indian refinery service, making tube material selection critical. SA-179 is used in CDU and VDU service where the process fluid is clean hydrocarbon. SA-213 T11 or T22 is specified above 400°C. TP304L or TP316L stainless steel is standard for amine regenerator reboilers — amine solution at 110–130°C is corrosive to carbon steel at reboiler outlet concentrations.
Falling Film Evaporators
In a falling film evaporator, the process liquid flows as a thin film downward on the inside surface of vertical tubes, evaporating as it falls. Heating medium (steam) is on the shell side. Uniform liquid distribution at the top of each tube is critical — if the film breaks, dry spots cause local overheating and accelerated corrosion. Falling film evaporators are used in Indian sugar industry, seawater desalination (multi-effect distillation), pharmaceutical concentration, and juice processing plants.
Tube material in Indian falling film evaporator service: TP304 or TP316L stainless steel is standard for food-grade and pharmaceutical applications. For seawater desalination on the Indian west coast and Andaman projects, titanium Grade 2 tubes are the correct specification — seawater at elevated temperatures is corrosive to both carbon steel and 304/316L stainless steel. Sugar mill evaporators use carbon steel for early effects (lower temperature and concentration), shifting to stainless steel for final effects.
Rising Film (Long Tube Vertical) Evaporators
Rising film evaporators have the process liquid fed from the bottom of long vertical tubes. The liquid at the bottom is heated to boiling, and growing vapour content drives the liquid-vapour mixture upward at increasing velocity. The high tube velocity provides good heat transfer and reduces fouling in solutions with scaling constituents. Used in Indian caustic soda (chlor-alkali) industry, pulp and paper (black liquor evaporation), and concentration of viscous solutions.
Caustic soda concentration requires nickel (Ni 200 or Ni 201) tubes at high concentrations and temperatures — carbon steel and stainless steel suffer stress corrosion cracking (caustic embrittlement) in concentrated NaOH above 50% concentration. For pulp mill black liquor service, TP304L or duplex stainless steel 2205 is specified for the higher-concentration effects. For Indian fertiliser plants concentrating acid or ammonium nitrate solutions, tube material is selected based on chemical resistance data for the specific process fluid.
Forced Circulation Evaporators
Forced circulation evaporators use an external pump to circulate process fluid through a shell-and-tube heat exchanger and back to a flash separator vessel. The process fluid is heated below its boiling point in the heat exchanger, then flashes to vapour in the separator where pressure is lower. Because the fluid in the heat exchanger tubes is subcooled (below boiling point at that pressure), nucleate boiling and scaling on tube surfaces is suppressed. Forced circulation is preferred for heavily scaling solutions: seawater brines, calcium sulphate, and ammonium nitrate in Indian desalination and salt manufacturing plants.
Tube Material Selection for Industrial Evaporator Service
| Evaporator Application | Tube Material | ASTM/ASME Grade | Key Reason |
|---|---|---|---|
| CDU/VDU thermosiphon reboilers — clean hydrocarbon | Carbon steel | SA-179 / SA-192 | Cost-effective; adequate for non-corrosive service below 400°C |
| High-temperature process heaters/reboilers above 400°C | Alloy steel | SA-213 T11 / T22 | Creep resistance and Nelson curve compliance above carbon steel limit |
| Amine regenerator reboilers | Stainless steel | TP304L / TP316L | Amine solution at 110–130°C is corrosive to carbon steel |
| Sour water stripper reboilers (H₂S/NH₃) | Stainless steel | TP304L / TP316L | Sour water at elevated temperature — carbon steel unacceptable |
| Seawater falling film / forced circulation | Titanium | Grade 2 (ASTM B338) | Chloride corrosion resistance — stainless steel fails in hot seawater |
| Sugar mill evaporators (first effects) | Carbon steel | SA-179 | Low concentration, moderate temperature — carbon steel adequate |
| Caustic soda concentration above 50% | Nickel | Ni 200 / Ni 201 | Caustic embrittlement of carbon and stainless steel at high NaOH concentration |
| Urea plant carbamate evaporation | SS Urea Grade | TP316L UG (ASTM A213) | Ammonium carbamate highly corrosive — urea-grade SS with low ferrite required |
TEMA, ASME, and IBR Requirements for Evaporator Tubes
Evaporator tubes in Indian process plant service fall under multiple codes. ASME Section VIII Division 1 governs the pressure vessel design of the evaporator shell and tube bundle. TEMA R (for refinery and chemical process service) or TEMA C (for general commercial service) governs tube material, minimum wall thickness, and tube-to-tubesheet joint specification. For evaporators forming part of an IBR steam circuit, tube material must be SA-192 or SA-179 from an IBR Well Known Maker.
TEMA R minimum tube wall: 2.11 mm (BWG 14) for carbon steel seamless tubes in kettle and thermosiphon reboiler service for OD up to 25.4 mm; 1.65 mm (BWG 16) for non-ferrous and CRA tubes. The specified wall must also satisfy the ASME pressure design calculation for tube-side and shell-side pressures at design temperature — which may require thicker walls than the TEMA minimum.
For IBR-registered steam reboilers and steam-heated evaporators in Indian process plants, Anand Seamless holds IBR Well Known Maker status — allowing direct supply of SA-179 and SA-192 evaporator tubes for IBR-registered service without per-lot third-party inspection. This significantly reduces procurement lead time for BHEL, HPCL, IOCL, and EPC-contracted evaporator bundle projects where IBR documentation is part of the purchase order requirement.
Evaporator Tubes from Anand Seamless
Anand Seamless manufactures cold drawn carbon steel seamless tubes in SA-179 and SA-192 and alloy steel seamless tubes in SA-213 T11 and T22 for kettle reboiler, thermosiphon reboiler, and steam-heated evaporator applications across Indian refinery, petrochemical, and fertiliser plant projects. We hold IBR Well Known Maker status, ISO 9001:2015 certification, and EIL vendor approval, with EN 10204 Type 3.1 or 3.2 MTRs as standard for all evaporator tube supply.
Contact our technical team at +91 90999 96853 or +91 99099 68550, or email inquiry@anandseamless.com with your evaporator type, tube grade, OD, wall, TEMA class, and IBR documentation requirements.
Frequently Asked Questions
Q: What is the difference between a kettle reboiler and a thermosiphon reboiler in terms of tube specification?
A: A kettle reboiler has a U-tube bundle submerged in a pool of boiling liquid on the shell side, with no circulation pump. A thermosiphon reboiler has straight vertical tubes with the process fluid rising through the tube-side driven by the density difference between inlet liquid and outlet vapour-liquid mixture. Thermosiphon reboilers operate at higher heat flux and require closer attention to tube wall thickness and material at the outlet end, where the highest vapour fraction and most corrosive conditions occur.
Q: Why is TP316L stainless steel specified for amine regenerator reboilers instead of carbon steel?
A: Amine solution at regenerator reboiler temperatures of 110–130°C is corrosive to carbon steel — corrosion rates typically above 0.5 mm/year, consuming a standard 2.11 mm wall tube in under 4 years. TP316L stainless steel reduces corrosion rates in amine service to below 0.05 mm/year, giving tube bundles a 20-year service life. TP304L may be used where chloride content is very low; TP316L is preferred where trace chlorides may be present.
Q: What tube material is correct for seawater falling film evaporators in Indian coastal plants?
A: Titanium Grade 2 (ASTM B338) is the standard. Seawater at elevated temperatures (50–80°C in multi-effect distillation) causes rapid chloride-induced corrosion in carbon steel and stress corrosion cracking in 304/316L stainless steel. Titanium is virtually immune to seawater corrosion. India’s coastal desalination plants — Jamnagar, Mundra, Chennai — consistently specify titanium Grade 2 tubes for seawater evaporator bundles.
Q: When does IBR apply to evaporator tubes in Indian process plants?
A: IBR applies when the evaporator forms part of a steam circuit — when steam at pressure above 1 kg/cm² flows inside the tubes. This covers steam-heated reboilers, steam-heated falling film evaporators, and process steam generators. When IBR applies, tube material must be SA-192 or SA-179 from an IBR Well Known Maker. Anand Seamless holds IBR Well Known Maker status for SA-179 and SA-192 carbon steel seamless tubes.
Q: Does TEMA R apply to evaporator tubes in Indian refinery service?
A: Yes. TEMA R applies to evaporator tube bundles in Indian refinery applications including kettle reboilers, thermosiphon reboilers, and steam-heated evaporators. TEMA R specifies minimum tube wall thickness (2.11 mm for carbon steel, 1.65 mm for non-ferrous and stainless), tube-to-tubesheet joint type, bundle-to-shell clearance, and baffle requirements. EIL and HPCL project specifications default to TEMA R for all evaporator bundles in refinery process service.
Q: Does Anand Seamless supply SA-179 and alloy steel tubes for evaporator bundles?
A: Yes. Anand Seamless manufactures SA-179, SA-192, and SA-213 T11/T22 cold drawn seamless tubes from Gujarat facilities at Kadi, Mehsana and Changodar, Sanand, covering the carbon steel and alloy steel tube range for kettle reboiler, thermosiphon reboiler, and steam-heated evaporator applications. We hold EIL vendor approval, IBR Well Known Maker status, and ISO 9001:2015 certification, supplying with EN 10204 Type 3.1 or 3.2 MTRs.
