Thermal Oil Heater Coil Diameter, Flow Direction and Gas Pockets

What the small-tube “piston effect” describes

JIELI’s engineering notes use “piston effect” as a practical description of liquid carrying a gas pocket through a relatively small tube. In an appropriate flow regime, the gas occupies enough of the cross-section that flowing liquid can push it along, including through a downward section.

In a larger bore, liquid may pass below a pocket while buoyancy holds the gas near the upper wall. A bend or local peak can then retain gas. This is a useful design warning, but tube diameter alone does not determine the flow regime.

DN65 and DN80 are observations, not universal boundaries

The field notes report stronger gas-carrying behaviour in DN65-and-smaller passages and weaker behaviour around DN80 and above in the arrangements observed. Treat those sizes as prompts for closer review, not a physical cutoff at which gas transport disappears.

DN is a nominal size. Actual internal diameter, gas quantity, oil viscosity, density, surface tension, pipe inclination and flow rate all influence transport. Even two DN65 tubes can have different internal bores and velocities because of wall thickness.

Experimental air–water research also finds that gas accumulation in downward pipes depends on diameter, inclination and air flow. This supports reviewing several variables together; it does not validate a numerical clearing velocity for hot thermal oil. Reference: Pothof and Clemens, experimental study of downward two-phase flow.

Why 1.0–1.5 m/s is not a universal safety limit

JIELI’s notes identify approximately 1.0–1.5 m/s as a useful observed range for carrying gas in certain smaller passages. They also highlight the benefit of adequate velocity in convection tubes. These values must remain tied to the fluid and geometry involved.

A velocity above 1.5 m/s does not, by itself, demonstrate safe operation of every downward coil. Check at least two different requirements: whether the liquid can transport gas through the local geometry, and whether the flow keeps the heated tube and fluid film within their allowable temperatures.

Use v = Q/A with the actual tube internal area and actual flow in each parallel path. Total pump flow divided by the number of paths assumes equal distribution; that assumption needs verification. The existing thermal oil flow and pipe-size guide explains the heat-balance calculation.

Review convection and radiant sections separately

A smaller convection passage may transport gas through a downward run when the local velocity and venting arrangement are adequate. Confirm this for the actual operating range, including startup and reduced flow. Do not approve the route solely from its nominal diameter.

A larger radiant-section tube deserves particular attention where the route rises and then turns downward. If gas collects at the high point, it can reduce liquid contact with part of the heated wall and impair cooling. Under substantial heat flux, the resulting local temperature rise can damage the fluid or tube; burn-through is a possible failure outcome, not an inevitable result of every downward bend.

Review the coil’s actual heat-flux distribution, fluid-film temperature, circulation balance and gas-release path together. A route that is satisfactory in a lightly heated section may be unsuitable in a more intensely heated section.

Questions to put on the coil-design review

• Where could gas collect in each pass, return bend and header?
• Can gas reach a designed release point during filling and operation?
• What are the minimum local velocity and flow distribution across the full operating range?
• Are wall and fluid-film temperatures acceptable at the governing heat flux?
• How are low flow and abnormal temperatures detected and protected?

Route changes, a vent connection or a different tube arrangement may be needed when these checks expose a pocket that normal circulation cannot reliably clear. Increasing pump flow is only one possible response and must be checked against pressure loss and equipment limits.

When evaluating a thermal oil heater manufacturer, ask for coil-pass drawings, actual tube internal diameters, minimum circulation, pressure loss and fluid-film temperature limits. Review JIELI’s manufacturing and inspection evidence alongside the calculation basis; photographs alone do not verify the proposed coil design.