Thermal oil system basics
Circulation rate from heat duty and temperature difference, expansion tank sizing, film temperature check and pipe velocity for common thermal oils.
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How this calculator works
A thermal oil (hot oil) system circulates a heat transfer fluid in a closed loop between a heater (fired or waste-heat) and one or more users. The circulation rate follows directly from the energy balance: ṁ = Q / (Cp × ΔT), where Q is the total heat duty, Cp is the oil's heat capacity and ΔT is the difference between supply and return temperatures. A typical ΔT for mineral oils is 20–60 °C.
The expansion tank (or expansion vessel) must hold the volume increase when the oil heats from cold-fill temperature to operating temperature: V_exp = V_system × β × (T_avg − T_cold). For mineral oils β is about 0.0007 /°C; a common rule of thumb is to size the expansion tank at 25–30 % of the total system volume.
The film temperature at the heater tube wall is the most critical limit in a hot oil system. If the film temperature exceeds the oil's maximum, the oil cracks, cokes the tubes and degrades rapidly. The film temperature is estimated as T_film = T_bulk + q″ / h_film, where q″ is the heat flux (kW/m²) and h_film is the oil-side film coefficient. Keeping the velocity above 1 m/s and the heat flux below the oil maker's limit are the main safeguards.
Example: 500 kW duty, Therminol 66 at 300/250 °C (ΔT = 50 °C): circulation rate 4.76 kg/s = 19.7 m³/h. With 5 m³ system volume the expansion from 15 °C to 275 °C average is 0.91 m³ (18 % of system volume). At 25 kW/m² heat flux and 500 W/(m²·K) film coefficient: T_film = 300 + 50 = 350 °C, below the 380 °C limit with a 30 °C margin. In a 150 mm pipe the velocity is 0.31 m/s — too low; a smaller pipe or higher flow is needed.
Limits: the oil properties built in are typical values at the upper operating range. Actual Cp and density vary with temperature; use the oil maker's data sheet for detailed design. The film temperature estimate assumes a uniform heat flux; real heaters have a distribution — the peak flux sets the limit. This calculator does not size the heater, piping or pump; it gives the basic parameters for screening.