Why can two thermal oil heaters with the same heat output have very different circulation rates? The main starting point is the design temperature rise across the heater. Coil geometry and the fluid’s temperature limits determine whether that flow is suitable.
1. Chinese and European design practice
In JIELI’s engineering experience, Chinese thermal oil heaters commonly use a design inlet-to-outlet temperature rise of about 20°C, with some designs around 30°C. This produces relatively high circulation flow for a given heat duty. JIELI has observed a similar approach in Japanese and Korean equipment; this is an engineering observation, not a rule covering every manufacturer.
There is published Japanese support for this approach: Maeda’s HC-series catalogue states that its circulation pumps are selected for a heater inlet/outlet oil-temperature difference of 20–30°C. Maeda HC catalogue, printed pages 4–5.
European designs also vary. Pirobloc describes both 20°C and 40°C arrangements. The useful comparison is therefore a smaller-ΔT, higher-flow design versus a larger-ΔT, lower-flow design—not a universal distinction between countries. Pirobloc, “Heat differential. Passes in the coils”.
2. Calculate flow from temperature difference
For steady liquid-phase heating, the initial heat balance is:
Mass flow ṁ = Q ÷ (cₚ × ΔT)
Volume flow V̇ = 3,600 × Q ÷ (ρ × cₚ × ΔT)
Use Q in kW, cₚ in kJ/(kg·K), density ρ in kg/m³ and ΔT in K to obtain V̇ in m³/h. A temperature difference of 20°C equals 20 K. Use fluid properties at the operating conditions; use enthalpy difference if specific heat varies significantly.
Example: useful heater output 2,400 kW, average cₚ = 2.5 kJ/(kg·K), density = 750 kg/m³. These are illustrative fluid properties.
| Design ΔT | Mass flow | Volume flow |
|---|---|---|
| 20°C | 48 kg/s | 230.4 m³/h |
| 30°C | 32 kg/s | 153.6 m³/h |
| 40°C | 24 kg/s | 115.2 m³/h |
At the same duty and fluid properties, the 20°C design needs twice the flow of the 40°C design. Actual ΔT falls at part load if flow stays constant. For the full method, see thermal oil flow-rate and pipe-size calculation.
3. Higher flow can provide thermal margin—but check coil velocity
The oil next to the heated tube wall can be hotter than the measured bulk oil. For the oil-side boundary layer, a useful local relationship is Twall − Tbulk ≈ q″/h, where q″ is local heat flux and h is the oil-side heat-transfer coefficient.
With the same coil geometry, fluid and heat flux, higher flow generally raises velocity and improves convective heat transfer. This can reduce the wall-to-bulk temperature difference and provide more margin against local overheating. It explains the practical value JIELI places on adequate circulation.
Total flow alone is insufficient. Larger tubes or more parallel paths can reduce velocity at the same total flow. A properly designed lower-flow heater can still maintain adequate velocity and film-temperature margin. Check each coil path, flow distribution, local heat flux and the fluid supplier’s limits. Eastman Therminol design resources.
High circulation does not replace low-flow trips, temperature protection or correct firing control. Do not reduce an existing heater’s flow simply to obtain a larger ΔT.
4. Lower flow can reduce piping size; higher flow has a pumping cost
At a chosen pipe velocity, lower volume flow allows a smaller internal diameter and can reduce pipework and oil inventory. It does not automatically mean a smaller heater or less heating surface.
At the same outlet temperature, a larger ΔT means a lower inlet temperature. That can increase the gas-to-oil temperature difference, but the heat-transfer coefficient, radiant duty and permitted local heat flux must still be checked. Material savings cannot be inferred from ΔT alone.
Pump power depends on both flow and pressure loss: Pshaft ≈ V̇ × Δp/ηpump, using m³/s and Pa for watts. Increasing flow through an unchanged circuit raises resistance as well as flow. Compare actual pump and circuit curves, including cold-start viscosity and any secondary circulation pumps.
5. Compare these five design values
- Duty and temperatures: useful output, inlet/outlet temperatures and the design ΔT.
- Fluid data: density, specific heat, viscosity and bulk/film temperature limits.
- Coil hydraulics: tube internal diameter, parallel paths, velocity, pressure drop and minimum flow.
- Thermal margin: local heat flux, wall/film temperature and response to reduced flow.
- Operating cost: main and secondary pump power, pipe size, oil inventory and operating hours.
JIELI’s preference for smaller ΔT and adequate circulation is a design choice aimed at stable heat transport and thermal margin. Judge the result from the complete thermal and hydraulic calculation, not the country of manufacture or pump flow alone.
Related: thermal oil circulation pump selection · thermal oil heater selection guide · thermal oil heater range.