THERMAL FLUID HYDRAULICS
Thermal Oil Flow Rate & Pipe Size Calculation Guide
Calculate a transparent preliminary circulation flow and pipe internal diameter, then test the result against hot-fluid properties, heater coil requirements, circuit pressure drop and pump operating limits.
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 the normal and peak useful heat duty, heater outlet temperature, system return temperature, thermal-fluid property table, process-user pressure drops, piping route, fittings, valves, elevation, heater coil pressure drop and pump arrangement. Separate useful heat delivered to the process from burner fuel input.
| 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)]
Because 1 kW equals 1 kJ/s, the units cancel directly. When heat capacity changes materially across the operating range, use a suitable average or a more detailed property integration rather than one arbitrary value.
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.
This is a preliminary design flow at the stated property basis. The heater may require a higher minimum circulation rate to control tube or coil film temperature, and parallel users may need balancing or individual control loops.
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
The calculated result is not a nominal pipe size. Select a pipe schedule and material, obtain its actual internal diameter, then recalculate velocity. A thicker wall reduces internal diameter and increases velocity and pressure loss.
| 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 is approximately 2,000,000 kcal/h. In JIELI’s published YY(Q)L gas / fuel-oil-fired thermal oil heater table, the YY(Q)L-200 row lists a preliminary circulation flow of 160 m³/h and a main-pipe value of 200 mm. The heat-balance example above produced approximately 154 m³/h and a calculated internal diameter of 165 mm at 2.0 m/s.
The proximity is a useful arithmetic cross-check, not a final selection certificate. The published 200 mm value is a product-table main-pipe value, while the 165 mm result is a calculated internal diameter under assumed fluid properties. Final sizing still requires the exact pipe schedule, heater minimum-flow requirement, coil and circuit pressure drop, pump curve, NPSH, operating range and thermal-expansion review.
05 · HYDRAULIC CHECK
Total pump head includes more than straight-pipe friction.
Add heater coil pressure drop, process heat exchangers, valves, filters, bends, reducers, instruments and any control-valve requirement. For a closed loop, static elevation does not simply become permanent pump head, but elevation and fill conditions affect system pressure, expansion-tank arrangement, 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.
From the heat-balance formula, increasing ΔT reduces calculated mass flow and may reduce preliminary pipe size and pump power. It can also increase temperature variation at the process, reduce control quality, raise local film temperature or move the return temperature outside the desired operating window.
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.
JIELI THERMAL ENGINEERING
Turn the heat balance into a complete circulation design.
Send the duty, temperatures, fluid data, piping route and process pressure drops. JIELI can review the heater, flow, pump, vessels and controls as one thermal-oil system.
Request a hydraulic review