Engineering task and calculation objective
Module BEHE designs heat tracing for pipelines. Tracer tubes run parallel to the product pipe underneath the insulation and compensate its heat losses so that the medium stays at temperature — indispensable for pour-point-critical products such as sulfur, bitumen or fatty acids, for crystallizing solutions, and for freeze protection of water and condensate lines. The calculation is based on the design method of W. W. Blackwell (Estimate heat-tracing requirements for pipelines, 1982) and on Groeneveld/Gerdts, Begleitheizungssysteme für Industrieanlagen (1973).
From the geometry of the product pipe, insulation and tracer tubes, the temperatures of the product and the ambient air, and the wind speed, the module calculates the external heat transfer coefficient, the overall heat transfer coefficient and the total heat loss of the line. From these follow the required number of tracer tubes and the heating medium demand — typically saturated steam as the heating medium. For single-phase heating fluids, a distinction is made between tracers installed with or without heat transfer cement, and the nominal size can be optimized via the flow velocity.
This makes it possible to calculate and document a steam heat tracing system: heat loss of the insulated pipeline, tracer count and steam consumption for the design of the heat tracing system in plant engineering.
Standard and calculation basis: Estimate heat-tracing requirements for pipeline W. Wayne Blackwell, Ford, Bacon & Davis / Texas Inc. September 1982; Begleitheizungssysteme für Industrieanlagen Wilhelm Groeneveld / Gustav F. Gerdts KG 1973
Calculation workflow
- Capture the geometry of pipe and insulation: The outside diameter and length of the product pipe, the inside and outside diameter or thickness of the insulation, its thermal conductivity, the gap between insulation and tracer tube, and the outside diameter of the tracer tubes are entered.
- Define the boundary conditions: Governing are the product temperature to be maintained, the lowest ambient air temperature to be assumed, and the wind speed. From wind and temperature difference, the external heat transfer coefficient at the insulation surface is determined.
- Calculate the heat loss of the insulated line: From the conductive resistance of the insulation layer (cylindrical geometry) and the external surface resistance follows the overall heat transfer coefficient; multiplied by the temperature difference between product and air temperature and the pipe length, this gives the total heat loss that the tracing must cover.
- Determine the number of tracers: The duty transferable by one tracer tube depends on the heating medium temperature and the thermal coupling — with heat transfer cement, the transfer to the product pipe is several times better than with loose installation in the air gap. From the heat loss and the tracer duty follows the required number of tracer tubes.
- Optimize heating medium demand and nominal size: From the heat duty to be delivered, the heating medium demand is calculated — for saturated steam via the enthalpy of vaporization, for single-phase fluids via mass flow and temperature drop. The nominal size of the tracers is checked against allowable flow velocities and adjusted if necessary.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Product temperature | ϑP | °C |
| Air temperature outside | ϑL | °C |
| Tube length | l | m |
| Inside diameter of insulation | Dinsu,i | m |
| Outside diameter of insulation | Dinsu,o | m |
| Thermal conductivity of insulation material | λ | W/(m·K) |
| Outside heat transfer coefficient | αa | W/(m²·K) |
| Overall heat transfer coefficient | k | W/(m²·K) |
| Total heat loss | Q | W |
| Insulation thickness | (Dinsu,o - Dinsu,i) / 2 b | m |
| Inlet temperature | ϑe | °C |
| Outlet temperature | ϑa | °C |
| Mean temperature | ϑm | °C |
| Specific heat capacity | cp | J/(kg·K) |
| Density | ρ | kg/m³ |
| Vapour pressure of the steam | pD | Pa |
| Steam temperature | ϑD | °C |
| Heat of evaporation | Δhv | J/kg |
| Outside diameter | D | m |
| Gap between insulation and heating tubes | s | m |
| Required mass flow of the fluid | m | kg/s |
| Outside diameter of the heating tubes | d | m |
| Number of heating tubes without cement | n1 | - |
| Flow velocity of the fluid | w1 | m/s |
Calculation options
heating fluid
Fluid · Saturated steam
Worked example
A 25 m long DN 100 product line is insulated with 50 mm of mineral wool (insulation inside 133 mm, outside 233 mm). The product is to be held at 80 °C; the design ambient temperature is −10 °C with wind (external heat transfer coefficient 20 W/(m²·K)). This worked example calculates the heat loss that the heat tracing must cover.
Given values
| Pipe length L | 25 m |
| Insulation inside di / outside da | 133 mm / 233 mm |
| Thermal conductivity of insulation λ | 0.05 W/(m·K) |
| Product temperature (holding temperature) | 80 °C |
| Ambient air temperature | −10 °C |
| External heat transfer coefficient αa | 20 W/(m²·K) |
Solution
Conductive resistance of the insulation layer (per meter)
RIso = ln(da/di) / (2π·λ) = ln(233/133) / (2π · 0.05) ≈ 1.785 (m·K)/W
External surface resistance (per meter)
Ra = 1 / (π·da·αa) = 1 / (π · 0.233 · 20) ≈ 0.068 (m·K)/W
The insulation resistance clearly dominates — typical for well-insulated lines.
Heat loss per meter and total
q = (TP − TL) / (RIso + Ra) = (80 − (−10)) / (1.785 + 0.068) ≈ 48.6 W/m
Q = q · L = 48.6 · 25 ≈ 1,214 W ≈ 1.2 kW
The heat tracing must supply this power in steady state; it is then used to determine the tracer count (with/without heat transfer cement) and the steam demand.
Result
| Heat loss per meter | ≈ 48.6 W/m |
| Total heat loss Q | ≈ 1.2 kW |
All values are illustrative. The applicable standard and project-specific boundary conditions remain authoritative.
Frequently asked questions
What is the benefit of heat transfer cement in tracer installation?
Without heat transfer cement, the tracer tube contacts the product pipe only along a line; the heat must be transferred mainly across the air gap underneath the insulation, which severely limits the heat transfer. Heat transfer cement establishes an areal, well-conducting connection between tracer and pipe and increases the transferable duty per tracer tube by roughly a factor of three to five. This reduces the required number of tracers — important for large pipe diameters or high holding temperatures.
Does the heat tracing keep the product at temperature or heat it up?
The classical design is a heat maintenance calculation: the tracer duty covers the steady-state heat loss at the design ambient temperature and thus maintains the product temperature. Heating up a cooled-down, stagnant line requires considerably more power (heating pipe steel and contents against the ongoing losses) and much longer times — if this case is also to be covered, it must be calculated separately with a specified heat-up time.
What role does the wind speed play?
The external heat transfer coefficient at the insulation surface increases markedly with wind speed. For well-insulated lines, the insulation resistance dominates, so the wind influence on the total loss remains moderate; with thin insulation, at supports and at valves, however, it can be considerable. Design is based on the unfavorable combination of lowest air temperature and design wind, not on mean values.
Why is saturated steam usually used as the heating medium?
Saturated steam releases its heat at constant temperature through condensation — the tracer temperature is thus defined over the entire length and the duty transferred per meter is nearly uniform. The heating temperature can be set via the steam pressure. Attention must be paid to condensate removal (slope, steam trap per tracer run) and, for temperature-sensitive products, to the risk of local overheating, which may argue for a lower steam level or hot water.