THERMAL FLUID HYDRAULICS
Thermal Oil Flow Rate & Pipe Size Calculation Guide
Calculate thermal oil flow from heat duty, then estimate pipe internal diameter. Check the result against minimum heater flow, circuit losses and the pump curve.
QUICK ANSWER
Start with heat balance, then convert mass flow into pipe diameter.
Mass flow
ṁ = Q ÷ (cp × ΔT). Use useful heat duty and the thermal fluid’s heat capacity at the representative operating temperature.
Volume flow
V̇ = ṁ ÷ ρ. Use density at operating temperature, not a room-temperature brochure value.
Pipe diameter
D = √(4V̇ ÷ πv). Select an actual pipe internal diameter, then recalculate velocity and total pressure drop.
Preliminary only: final design must satisfy the heater manufacturer’s minimum circulation requirement, fluid supplier limits, applicable piping code and pump NPSH.
01 · INPUT DATA
Use hot-fluid properties and useful process duty.
Collect useful heat duty, supply and return temperatures, fluid properties, pipe dimensions and all circuit pressure losses. Keep burner input separate from useful heat delivered.
| Symbol | Meaning | Use in the example |
|---|---|---|
| Q | Useful heat duty | 2,400 kW |
| cp | Fluid specific heat at representative temperature | 2.50 kJ/kg·K |
| ΔT | Supply minus return temperature | 30 K |
| ρ | Fluid density at representative temperature | 750 kg/m³ |
| v | Preliminary average pipe velocity | 2.0 m/s |
The values above illustrate the method; obtain project properties from the selected fluid supplier.
02 · FLOW FORMULA
Calculate mass flow from the heat carried across the temperature difference.
ṁ (kg/s) = Q (kW) ÷ [cp (kJ/kg·K) × ΔT (K)]
With kW and kJ/kg·K, the result is kg/s. Use a representative average heat capacity; if it varies substantially, calculate the fluid enthalpy change over the temperature range.
Convert mass flow to volume flow:
V̇ (m³/s) = ṁ (kg/s) ÷ ρ (kg/m³)
03 · WORKED EXAMPLE
A 2.4 MW loop with a 30 K design temperature difference.
- Mass flow: 2,400 ÷ (2.50 × 30) = 32.0 kg/s.
- Volume flow: 32.0 ÷ 750 = 0.0427 m³/s.
- Hourly volume: 0.0427 × 3,600 = approximately 154 m³/h.
The heat-balance flow is approximately 154 m³/h. Increase it if the heater’s minimum-flow requirement is higher. Balance parallel branches to maintain each user’s required flow.
04 · PIPE INTERNAL DIAMETER
Calculate internal diameter from volume flow and selected velocity.
D (m) = √[4 × V̇ (m³/s) ÷ (π × v (m/s))]
For 0.0427 m³/s and a preliminary velocity of 2.0 m/s:
D = √[4 × 0.0427 ÷ (π × 2.0)] = approximately 0.165 m
Choose a standard pipe using its actual internal diameter, then recalculate velocity and pressure loss. Nominal diameter alone is insufficient.
| Check after selecting pipe | Why it matters |
|---|---|
| Actual internal diameter | Nominal diameter does not equal flow diameter |
| Velocity at minimum and maximum flow | Confirms the operating range rather than one point |
| Reynolds number and friction factor | Hot-fluid viscosity changes hydraulic behavior |
| Pressure loss per length | Affects pump head and operating cost |
| Thermal expansion and supports | High-temperature piping requires flexibility and controlled loads |
PUBLISHED-DATA CROSS-CHECK
The worked result can be compared with a published JIELI model row—but it cannot be selected from that row alone.
The 2.4 MW example equals approximately 2.06 million kcal/h. The YY(Q)L gas / fuel-oil-fired thermal oil heater table lists 160 m³/h circulation and a 200 mm main-pipe value for YY(Q)L-200. The example gives 154 m³/h and a calculated internal diameter of 165 mm at 2.0 m/s.
These values use different sizing assumptions. The table’s main-pipe value is not an internal diameter. Confirm the actual pipe schedule, minimum heater flow and circuit losses before selecting the pump and pipe.
05 · HYDRAULIC CHECK
Total pump head includes more than straight-pipe friction.
Include coil, heat-exchanger, valve, filter and fitting losses. In a filled closed loop, rising and falling static heads balance. Elevation still affects local pressure, venting and NPSH.
Plot the system resistance curve against the pump curve using hot-fluid properties. Confirm the normal operating point, minimum flow, parallel-pump behavior, motor margin, seal and bearing temperature suitability, and NPSH available. Continue with the thermal oil circulation pump selection guide.
06 · DESIGN ΔT
A larger temperature difference reduces flow, but creates other consequences.
At fixed duty, a larger ΔT reduces calculated flow. If this reduces coil velocity, film temperature may rise. Check process temperature uniformity, minimum heater flow and the return-temperature limit.
Select ΔT with the heater designer, process-equipment supplier and thermal-fluid supplier. Check normal, startup and upset conditions rather than optimizing one steady-state calculation.
Thermal oil hydraulic design checklist
- Useful heat duty at normal, peak and startup conditions;
- Supply and return temperature at the heater and each user;
- Fluid density, heat capacity and viscosity across the operating range;
- Heater minimum flow and coil pressure-drop curve;
- Actual pipe internal diameters, equivalent lengths and valve coefficients;
- Process heat-exchanger and control-valve pressure drops;
- Pump curve, hot-service construction, NPSH and parallel operation;
- Expansion, venting, low-point drainage, filtration and commissioning plan.
RELATED ENGINEERING
Complete the hydraulic and heater selection together.
FREQUENTLY ASKED QUESTIONS
Thermal oil flow and pipe sizing FAQ
How do I calculate thermal oil flow rate?
For preliminary sizing, mass flow equals useful heat duty divided by the product of fluid specific heat and supply-return temperature difference. Use fluid properties at the representative operating temperature and keep units consistent.
How do I estimate thermal oil pipe diameter?
Convert mass flow to volumetric flow using hot-fluid density, select a preliminary design velocity, and calculate internal diameter from D equals the square root of four times volumetric flow divided by pi times velocity. Then check the actual pipe internal diameter, pressure drop and heater requirements.
Can I increase temperature difference to reduce flow and pipe size?
A larger temperature difference reduces calculated flow, but it may raise process temperature variation, fluid film temperature and control challenges. The heater, process users and fluid supplier limits must be reviewed together.
Is velocity alone enough to select thermal oil piping?
No. Final sizing also requires hot-fluid viscosity and density, straight-pipe and fitting losses, control valves, heater coil pressure drop, elevation, pump operating point, NPSH, thermal expansion and applicable piping code.
Preparing for first startup? JIELI’s original thermal oil dehydration notes connect tank level, suction conditions, pressure changes and staged heat-up, with a downloadable observation log.
JIELI THERMAL ENGINEERING
Turn the heat balance into a complete circulation design.
Compare thermal oil heater models and system scope against your calculated flow, then send the duty, temperatures, fluid data and piping route. State whether you need a new heater package or a review of an existing loop; a preliminary flow result alone is not a final pipe or pump selection.