Gas-Fired Thermal Oil Heater Double-Coil Spacing: Balancing Efficiency and Flue-Gas Resistance

01 · DEFINE THE GEOMETRY

“Coil spacing” must name the correct dimension.

A fired double-helical-coil thermal oil heater normally has an inner coil around the radiant chamber and a concentric outer coil. Flue gas can transfer heat in the furnace, in the annular passage between the two coils and in the passage between the outer coil and casing. That three-region interaction is why one spacing value cannot be selected independently of the complete geometry.

In this article, the under-50-mm discussion refers to the radial clear gap between the facing tube surfaces of the inner and outer concentric coils. It does not refer to the axial pitch between neighboring turns of one helix. Drawings, calculations and purchase specifications should identify both dimensions unambiguously.

02 · GAS-SIDE HEAT TRANSFER

A smaller free-flow area increases local flue-gas velocity.

For a fixed flue-gas volume flow, reducing the clear flow area raises the gas velocity. In simplified form, v_g = V̇_g / A_free. Higher velocity normally increases the gas-side Reynolds number and turbulence around the tubes, which can raise the convective heat-transfer coefficient. A compact, well-distributed passage also limits low-velocity bypass zones that carry heat toward the stack without doing useful work.

The useful result is not “the smallest possible gap.” The objective is a balanced overall heat-transfer coefficient and a controlled flue-gas outlet temperature at rated and part load. Tube diameter, coil length, turn pitch, gas properties, excess air, heat-release pattern and the temperature difference between gas and thermal oil all change the result.

Conceptually: Q = U · A · ΔT_lm. Coil spacing mainly influences the gas-side part of U and the flow distribution; it does not replace adequate surface area or correct oil-side circulation.

03 · FLUE-GAS RESISTANCE

The same acceleration that helps convection also costs draft pressure.

As the passage becomes tighter, losses through contraction, tube crossings, turns and the annular flow path increase. For comparable geometry, a useful engineering picture is Δp ≈ K · ρv²/2. Because velocity is squared, a modest reduction in free area can produce a much larger rise in pressure drop.

That pressure drop must be added to the complete system: combustion-air path, furnace, coil passages, air preheater or economizer, dust-control equipment where fitted, ducts and stack. The operating point comes from the system curve and the selected fan curve. Fan input also increases with pressure and volume flow; conceptually, P_fan ≈ Δp_total · V̇ / η_fan.

Excessive resistance can reduce burner capacity, make furnace pressure harder to control, increase electrical consumption and noise, and narrow the operating margin as surfaces foul. A coil arrangement is therefore efficient only when the recovered heat is worth the draft penalty over the full operating range.

04 · THE UNDER-50-MM WINDOW

Why it often works for clean gas—and why it still needs calculation.

Natural gas normally produces much less particulate matter than solid fuels and, when combustion is correctly adjusted, presents a comparatively clean flue-gas service. That makes a compact annular passage practical in many gas-fired heaters. In JIELI THERMAL design work, a radial clear gap below 50 mm is therefore a common starting range for suitable double-coil configurations.

It is accepted only after the calculated velocity, Reynolds number, heat-transfer duty and section pressure drop agree with the burner/fan arrangement. The check must include maximum firing, minimum stable load, cold start, expected excess air, site altitude and any downstream heat-recovery equipment. A 45 mm gap can be appropriate in one heater and unacceptable in another with a different tube diameter, capacity or gas-flow path.

The same raw gap also represents different relative spacing. For a simplified comparison of facing tubes with the same outside diameter, S/D = (D + g)/D; a 50 mm clear gap then corresponds to S/D values of approximately 2.32, 1.98, 1.83 and 1.66 for tube ODs of 38, 51, 60.3 and 76.1 mm respectively. This screening ratio does not replace the real three-dimensional coil geometry, and the gap should always be stated with tube diameter and coil arrangement.

05 · DESIGN WORKFLOW

Optimize the gap by iteration, not by habit.

1. Fix the thermal basis.Confirm useful heat duty, thermal-oil supply/return temperatures, fuel composition, excess air, firing range, ambient conditions and altitude.
2. Lay out both coil layers.Define tube OD, inner/outer coil diameters, turn pitch, active length, supports, casing clearance and the proposed radial gap.
3. Calculate free-flow area and gas properties.Use the temperature-dependent density and viscosity in each flue-gas region; do not use one ambient-air value for the full heater.
4. Calculate heat transfer and pressure drop together.Evaluate local velocity, Reynolds number, convection, radiation interaction, gas-temperature profile and cumulative draft loss.
5. Match the burner, fan and stack.Check the fan curve at rated and part load, furnace-pressure control, reserve for normal aging and the effect of downstream recovery equipment.
6. Check the thermal-oil side.Verify coil flow, velocity, pressure drop, tube-wall temperature, maximum film temperature and trip settings.
7. Validate the buildable geometry.Include bending tolerance, ovality, concentricity, supports, thermal expansion, weld access, inspection and cleaning.

06 · THERMAL-OIL PROTECTION

Gas-side efficiency cannot be separated from film temperature.

A double-coil heater transfers heat to a moving thermal fluid inside the tubes. If circulation is insufficient, local tube-wall and film temperatures can rise even when the bulk outlet temperature appears normal. Excessive film temperature accelerates thermal-fluid degradation and can promote deposits that insulate the tube wall, creating a damaging feedback cycle.

Published modeling of fired helical-coil thermal oil heaters treats flue-gas temperature, oil temperature and tube-wall temperature as coupled variables for the inner and outer coils. That is the correct engineering perspective: a spacing change that increases local heat flux must be checked against oil velocity, fluid limits and protection logic, not judged only by stack temperature.

07 · MANUFACTURING & EXPANSION

A calculated 40 mm gap is not useful if the finished gap varies widely.

Large helical coils accumulate tolerances. Tube ovality, springback after bending, weld shrinkage, support location and casing alignment can change the actual clearance. Inspection should therefore record the minimum, maximum and circumferential distribution of the gap—not only the nominal drawing value.

Supports must hold the coils concentric while allowing the thermal movements predicted for start-up and operation. The minimum hot-condition clearance must prevent contact and vibration, while the maximum clearance must not create an uncontrolled bypass channel. Access for weld inspection and practical maintenance is part of the geometry, not an afterthought.

08 · COMPLETE-SYSTEM VIEW

Coil spacing is one variable inside a complete heating plant.

The heater, burner, combustion air, induced or forced draft, stack, heat recovery, circulation pumps, expansion system, instruments and control logic determine the operating result together. A very compact coil does not compensate for poor burner matching, insufficient oil flow or an undersized fan.

For reliable operation, the final design should document the calculation basis, expected clean and normal-used pressure drop, fan operating points, commissioning measurements and maintenance limits. These values give operators a meaningful baseline for future troubleshooting.