Selection Guide · 2026-10-13
Synthetic Heat Transfer Fluids Comparison (300-350°C)
Evaluate high-temperature synthetic aromatic heat transfer fluids for 300°C to 350°C systems. Compare hydrogenated terphenyl, dibenzyl toluene, and alkylbenzenes.
Key takeaways
- For continuous liquid-phase operation at 330°C to 350°C, hydrogenated terphenyl fluids (such as Eurotherm THT) offer high thermal stability with a maximum bulk temperature of 350°C and a maximum film temperature of 380°C.
- For 300°C to 330°C applications requiring winter cold-starts or outdoor resilience, single synthetic aromatic fluids (such as Eurotherm HFT-M) combine a low pour point of -47°C with an open-cup flash point of 205°C.
- Alkylbenzene thermal fluids are typically limited to continuous operating temperatures below 300°C due to side-chain dealkylation at elevated bulk and film temperatures.
- Closed-loop systems operating above 300°C require nitrogen or cold-oil blanketing in expansion vessels to prevent high-temperature oxidation and maintain extended fluid service life.
Thermal and Operational Demands of 300°C to 350°C Systems
In polyester fiber polycondensation, fine chemical syntheses, and petrochemical distillation units, liquid-phase heat transfer loops frequently operate between 300°C and 350°C. At these elevated thermal regimes, fluid bulk temperatures approach the thermodynamic bond dissociation limits of organic molecules. Within the heating furnace tubes, peak boundary layer film temperatures regularly exceed bulk fluid temperatures by 20°C to 40°C.
Improper fluid selection or excessive film temperatures accelerate homolytic carbon-carbon bond cleavage and dehydrogenation polymerization. The resulting low-boiling components lower the operating flash point, generate vapor pockets, and increase cavitation risks in circulation pumps. Simultaneously, high-boiling condensation polymers precipitate onto heat transfer surfaces as hard carbon deposits, lowering thermal conductivity and increasing the risk of tube overheating. Systematic evaluation of chemical structure, thermal stability limits, and low-temperature fluidity is essential for reliable process engineering.
Chemical Structure Comparison: Terphenyls, Benzyl Toluenes, and Alkylbenzenes
Hydrogenated terphenyl consists of partially saturated aromatic rings. This molecular architecture combines resonance-stabilized aromatic rings with naphthenic bonds, offering resistance to free-radical cracking at sustained temperatures up to 350°C. Its thermal degradation products primarily consist of volatile single-ring aromatics that can be vented through the expansion circuit, rather than insoluble polycyclic aromatic sludges, enabling maximum allowable film temperatures up to 380°C.
Dibenzyl toluene and specialized single-ring synthetic aromatics feature isomeric structures that suppress crystallization, achieving pour points down to -47°C alongside open-cup flash points of 205°C. These attributes enable cold pumpability in sub-zero outdoor climates without heavy heat tracing, while supporting liquid-phase stability up to 330°C. In contrast, alkylbenzenes carry aliphatic carbon-hydrogen side chains with lower dissociation energies. Above 300°C, dealkylation accelerates, limiting their thermal durability in continuous 300°C to 350°C duty compared to fully aromatic or partially hydrogenated ring systems.
| Product Model / Base Chemistry | Max Operating Temp (°C) | Max Film Temp (°C) | Pour Point (°C) | Flash Point (°C) | Viscosity at 40°C (mm²/s) | International Benchmark | Recommended Application Scope |
|---|---|---|---|---|---|---|---|
| Eurotherm THT (Hydrogenated Terphenyl) | 350 | 380 | -25 | 185 (Closed Cup) | 28 | Therminol 66 / Dowtherm RP / Diphyl THT | Liquid-phase duty at 330–350°C; polyester fiber spinning and silicone synthesis. |
| Eurotherm 340 (Synthetic Aromatic L-QD340) | 345 | 360 | -30 | — | 25 | Therminol 75 / Dowtherm MX / Marlotherm SH | Liquid-phase duty at 320–345°C; petrochemical and fine chemical heating units. |
| Eurotherm HFT-M (Single Aromatic, High Flash) | 330 | 350 | -47 | 205 (Open Cup) | 16 | Therminol 66 / Dowtherm RP / Marlotherm SH | Outdoor operations in cold environments; balanced 330°C stability and -47°C pour point. |
| Eurotherm XT-1 (Synthetic High-Temp Fluid) | 400 | 420 | -30 | — | 20 | — | High-temperature liquid systems above 350°C requiring film temperature tolerance up to 420°C. |
| Eurotherm 400 (Biphenyl / Diphenyl Oxide Eutectic) | 400 | 410 | 12 | 115 (Closed Cup) | 2.5 | Therminol VP-1 / Dowtherm A / Diphyl | Vapor-phase or pressurized liquid loops up to 400°C; requires trace heating. |
Four-Dimension Technical Evaluation Framework
Criterion 1: Maximum Film Temperature and Thermal Degradation Rates. Eurotherm THT provides a maximum allowable film temperature of 380°C, maintaining low cracking rates during continuous operation between 340°C and 350°C. For specialized thermal loops operating beyond 350°C and up to 400°C, formulations such as Eurotherm XT-1 (film limit 420°C) or the eutectic diphenyl/diphenyl oxide blend Eurotherm 400 (film limit 410°C) provide the required thermal headroom.
Criterion 2: Cold-Start Capability and Pumping Rheology. Industrial units in northern zones require unassisted startup at low ambient temperatures. Eurotherm HFT-M features a pour point of -47°C and a kinematic viscosity of 16 mm²/s at 40°C, while Eurotherm 340 provides a pour point of -30°C. These rheological profiles reduce pipeline freezing risks, minimize electric tracing heat requirements, and prevent cold-start motor overload in primary circulation pumps.
Criterion 3: Vapor Pressure and System Pressurization. Non-eutectic synthetic aromatics such as Eurotherm THT, Eurotherm 340, and Eurotherm HFT-M exhibit low saturation vapor pressures across the 300°C to 350°C range, enabling low-pressure liquid-phase loop designs. Conversely, biphenyl/diphenyl oxide eutectics (Eurotherm 400) boil at approximately 257°C under 1.013 bar, requiring pressurized liquid designs or vapor-phase piping certified for higher mechanical design ratings.
Criterion 4: Fluid Lifecycle Economics. Although high-purity synthetic aromatics require higher initial capital investment than conventional mineral oils or alkylbenzenes, their resistance to carbonaceous fouling lowers coil-cleaning frequency and downtime. Combined with proactive periodic fluid analysis and expansion tank inerting, synthetic aromatics deliver consistent thermal transmission over long service intervals.
Engineering Recommendations for Process Units
Continuous Baseload Units at 330°C to 350°C: For polyester continuous polycondensation (CP) loops, synthetic fiber spinning headers, and silicone monomer reactors, hydrogenated terphenyl fluids such as Eurotherm THT or high-temperature synthetic aromatics like Eurotherm 340 are well suited. Their thermal stability profiles and high film margins (380°C and 360°C, respectively) minimize degradation under continuous heating loads.
Flexible and Batch Units at 300°C to 330°C: For fine chemical batch reactors, jacketed vessels, and outdoor thermal installations subject to ambient cold extremes or periodic shutdowns, Eurotherm HFT-M offers practical operating advantages. Its 205°C open-cup flash point, combined with a -47°C pour point, provides operational flexibility without requiring auxiliary line heaters.
Engineers can utilize the Suzhou Chemie Online Selection Tool (https://calculator.szchemie.com.cn/select) or the AI Selection Portal (https://calculator.szchemie.com.cn/ai) to evaluate fluid hydraulic properties, calculated film temperatures, and flow velocity profiles, or contact our engineering team directly for fluid lifecycle and reclamation consultation.
FAQ
Why are conventional alkylbenzenes restricted in continuous systems above 300°C?
Alkylbenzene molecules incorporate aliphatic alkyl side chains that possess lower carbon-carbon bond dissociation energy than aromatic ring structures. When operated continuously above 300°C, the rate of dealkylation and thermal cracking increases, resulting in light-end volatility, rising fluid viscosity from condensation reactions, and accelerated coking along the heater tube walls.
How should low-boiling thermal degradation products be managed in hydrogenated terphenyl systems?
Over extended operating cycles at elevated temperatures, trace low-boiling components form within the fluid. High-temperature systems should incorporate a continuous thermal degassing loop or a vapor-release circuit linked to the expansion tank. By maintaining correct expansion vessel temperatures and routine venting procedures, light ends can be purged and collected, preserving operating flash point and preventing pump cavitation.
How is the maximum film temperature calculated and monitored in thermal heaters?
The peak boundary film temperature is determined by the fluid bulk temperature, the heat flux density of the furnace tubes, the tube geometry, and the bulk fluid flow velocity. Maintaining turbulent flow inside the furnace coils (typically above 2.0 to 3.0 m/s depending on tube design) minimizes the temperature differential across the boundary layer. The calculated maximum film temperature must always remain below the certified maximum film temperature rating of the selected fluid (e.g., 380°C for Eurotherm THT).
What factors determine whether to select Eurotherm HFT-M or Eurotherm THT?
The primary criteria are operating temperature requirements and ambient cold-start parameters. For continuous duty between 330°C and 350°C, Eurotherm THT is recommended due to its 380°C maximum film temperature margin. For systems running between 300°C and 330°C located in cold climates or requiring unassisted cold startups, Eurotherm HFT-M is advantageous due to its -47°C pour point and 205°C open-cup flash point.
Is nitrogen blanketing mandatory for closed-loop synthetic heat transfer fluid systems?
Yes. Synthetic aromatic heat transfer fluids undergo rapid oxidative degradation when exposed to atmospheric oxygen at bulk or expansion tank temperatures above 60°C. Oxidation generates organic acids, polymer sludges, and corrosive byproducts. Maintaining a positive-pressure dry nitrogen blanket (or a properly designed cold-oil seal) in the expansion vessel isolates the fluid from ambient oxygen, preserving fluid condition.
Sources
Product data are typical laboratory values for reference only; please confirm with a Chemie engineer.