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.
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.
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.
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.
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 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.
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