Miscellaneous Design Guide
Hot Oil Systems
Chapter from the Miscellaneous Design Guide.
HOT OIL (HEAT TRANSFER FLUID) SYSTEMS
Common Heat Transfer Fluids
| Fluid | Brand Names | Max Temp | Min Temp | Flash Point | Key Properties | Applications |
|---|---|---|---|---|---|---|
| Mineral oil | Mobiltherm 605, Therminol XP | 600°F | 0°F | 380-450°F | Low cost, moderate range | General heat transfer, moderate T |
| Synthetic (diphenyl oxide/biphenyl) | Dowtherm A, Therminol VP-1 | 750°F | 60°F | 255°F | Highest temperature capability | High-T reactors, distillation reboilers |
| Synthetic (alkylated aromatics) | Therminol 66, Dowtherm Q | 650°F | -15°F | 350°F | Good low-T pumpability | Wide temperature range applications |
| Silicone | Syltherm 800, Duratherm S | 750°F | -40°F | 450°F | Non-toxic, wide range | Food/pharma, extreme cold startup |
| Glycol/water | Dowtherm SR-1, Dowfrost | 350°F | -60°F (with glycol) | N/A | Freeze protection, low T | Cooling/heating below 350°F |
System Design Considerations
Key Design Elements:
1. Expansion Tank: sized for thermal expansion from ambient to max operating T
• Liquid volume change: 30-50% increase from ambient to 600°F (fluid-specific)
• Blanketed with N2 (2-5 psig) to prevent oxidation and moisture ingress
• Located at highest point in system
• Level indication: LI at expansion tank, LAH/LAL alarms
2. Circulation Pumps: centrifugal, API 610 OH2 for hot service
• Minimum 2 pumps (1 operating + 1 spare)
• Seal type: Plan 23 (cooled flush) or Plan 53 for high temperature
• NPSH consideration: fluid VP at operating temperature can be significant
3. Fired Heater: primary heat source
• Film temperature limit: do not exceed fluid's max film temperature
• Tube velocity: 6-10 ft/s minimum to prevent coking
• Low-flow trip: protect against tube overheating
4. System Controls:
• Heater outlet temperature control (primary)
• User temperature control (secondary, via 3-way valve or bypass)
• Low flow alarm and trip on heater
• High temperature alarm on expansion tank (indicates fluid degradation)
Source: Miscellaneous_Design_Guide_v1.xlsx · sheet “Hot Oil Systems”
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