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Electrical Trace Heating System Basics
What is Heat Tracing?
Electrical trace heating, or as it is often called - heat tracing, refers to the process of maintaining or raising the temperature of instrument impulse lines, pipes, and even vessels through specially designed cables. In simple terms it the application of a compensating heat source.
When to Use Trace Heating?
Heat tracing is usually considered in the following circumstances:
- when there is a risk of pipes freezing. In cold weather this is especially prevelant in dead legs or when there is little or no flow of a fluid susceptible to freezing in a line. Pipes and impulse lines that freeze can rupture therefore trace heating can prevent this.
- to maintain the temperature of a liquid system. Trace heating is often used in hot water systems.
- to maintain process temperatures for smooth and efficient running of process plant and equipment. For example, heavy, or waxy oils flow better at higher temperature therefore trace heating is often used on these lines.
How does Trace Heating Work?
Every pipe, vessel and impulse line is subject to heat loss when its temperature is greater than ambient temperature. The rate of heat loss can be slowed by the use of thermal insulation, but this does not eliminate it. Electrical trace heating replaces some, or all, of the heat lost from the surface. The amount of heat replaced depends on what is to be achieved, i.e. freeze prevention, or temperature maintenance.
Control of the heat supplied can be from a simple on-off thermostat, e.g. the thermostat energising the heat tracing when temperature falls below the setpoint and de-energises when temperature is a couple of degrees above setpoint, or increasingly common control is supplied from microprocessor-based control and monitoring systems - either stand alone or within the plant control system.
I Have Heat Tracing - Do I Still Need Insulation?
The need for properly installed and well-maintained thermal insulation cannot be overemphasised. Without insulation, heat losses are generally too high to be offset by a conventional heat tracing system.
What Types of Trace Heating Cables are Available
There are three types of heat tracing cables available: constant power cables, constant wattage cables, and self regulating cables. Each type of trace heating cable works differently, and the choice of cable is influenced by intended application.
Constant Power Trace Heating Cable
Constant power heat tracing cable, sometimes referred to as series resistance cable, is made up of a high-resistance wire that is typically insulated and encased in a protective cover. When powered at its operating voltage, thermal energy is produced from the resistance of the wire.
The advantage of a constant power heating cable is that it is generally inexpensive and it is able to maintain very high temperatures (especially mineral insulated cables) for longer lines. Mineral insulated cables are also good for maintaining lower temperatures on lines which can get extremely hot such as high temperature steam lines.
Disadvantages of constant power trace heating cables include:
- they are supplied in specific lengths and cannot be shortened in the field,
- a break or failure anywhere along the length of a constant power cable will cause the entire cable to fail,
- care needs to be taken during installation that the cable is not crossed over itself as this can lead to the cable overheating and eventual burn out.
Constant Wattage Trace Heating Cable
A constant wattage cable is composed of multiple constant electric power zones, made by wrapping a fine heating element around two insulated parallel bus wires. A notch is created in the insulation on the opposing sides of the conductors and a small heating circuit is then produced by fusing the heating element to the exposed conductor wire and this is repeated throughout the entire cable producing the power zones. There is then an inner jacket which separates the bus wires from the grounding braid.
The major advantage of constant wattage trace heating cable is that this cable can be cut to length in the field due to its parallel circuitry. Another advantage is that constant wattage heating tapes can be joined using either a jointing kit or a trace heating junction box.
As with constant power cables, constant wattage cables should not be crossed over itself as this can lead to them overheating and eventual burn out.
Constant wattage cable is always installed with a thermostat to control the power output of the cable, making it a very reliable heating source.
Self Regulating Trace Heating Cable
Self regulating heating cable is most often called a tape rather than a cable i.e self regulating tape, or even self limiting tape. Self regulating tape adjusts heat output depending on the heat loss from the pipe work by varying its conductivity. As pipe temperature falls the electrical conductivity of the semi-conductive polymer core increases causing the tape to increase output. As the pipe temperature increases the conductivity reduces and output decreases.
Self regulating tape uses two parallel bus wires which carry electricity but do not create significant heat. They are encased in a semi-conductive polymer. This polymer is loaded with carbon; as the polymer element heats, it allows less current to flow. The cables are manufactured and then irradiated and by varying both the carbon content and the dosage then different tape with different output characteristics can be produced.
Advantages of self regulating trace heat tape include:
- it can be cut to length in the field,
- it is more energy efficient than the other types as it is able to reduce its output at higher temperatures,
- it cannot over heat itself so will not burn out if inadvertently crossed over itself during installation. This makes it an attractive choice for use in potentailly hazardous areas.
Self regulating tape does have some disadvantages, including:
- it is not as reliable as series or constant wattage cables,
- it has a specific maximum exposure temperature and if subjected to temperatures above this then the tape can be damaged beyond repair,
- it is subject to high inrush currents on starting up so a higher rated contactor is required compared to other trace heating cables.
Typical Heat Trace Installation
Trace heat cables are typically designed to run alongside the piping, 1 foot of cable per 1 foot of pipe.
Valves, flanges, and other components with greater mass require additional cable to ensure compensation for additional heat loss.
A typical heat tracing installation is depicted:
1 - Electrical heat tracing cable
2 - Power connection kit
3 - Control thermostat
4 - In-line/T-splice kit (permits two cables to be spliced together)
5 - Cable end termination
6 - Attachment tape (use on 12" intervals or as required by code or specification)
7 - "Electric Heat Tracing" label (peel-and-stick label to advise line is traced)
8 - Thermal insulation and vapour barrier
Sizing a Heat Trace System
Heat trace cables need to be properly sized for each application. Heat loss calculations are required and these factor in the insulation thickness and its conductivity, the difference in ambient and pipe temperature, and the overall surface area for the pipe. Valves, flanges, and other components with greater mass become heat sinks and this must also be considered in the heat loss calculations.
Once the heat loss is known, then a heat trace cable can be selected with the appropriate wattage output, voltage inputs, and jacket material.
The heat loss calculations and cable selection can be done by hand, however most reputable heat tracing manufacturers have computer programs or apps that can be used.
Can Heat Tracing be Used in a Hazardous Area?
Heat tracing products are available with certification and approval for use in Zone 1, and Zone 2 areas. Self regulating tape is often used in potentially hazardous areas. See our page on Hazardous Areas for more information.
Can Plastic Pipes be Heat Traced?
Yes - but with care. The power output of heat trace cable should be checked to ensure it will not damage the plastic pipe; typically around 12W. Earthing is also important, so ensure the trace heat cable is properly earthed.
Technical Library
The following pages on Control and Instrumentation.com provide more background on topics relevant to heat tracing:
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Technical Bookshop
For those who want to learn more about heat tracing, freeze protection and insulation then the following books from Amazon will be of interest:
