Heater Types

The weakest link in the sample transport system is often the selection and installation of the heat trace medium and temperature control scheme.

There are three primary tracing mediums: electric, steam and re-circulated fluids. Consistent temperature maintenance is a critical aspect of the sample transport system, yet its design and installation is often treated without the same care. Heat tracing is required for process and stack gas analyzers to maintain a specific process temperature as well as freeze protection. Many of the specifications in the marketplace today overcompensate for common failures related to poor designs or installation. Likewise, sample transport systems that are field traced and insulated pose additional problems when compared to a pre-engineered, pre-traced system like O’Brien Analytical Tracepak® and Stackpak™.

Electric heat trace products come in a wide range of heater types, accessories and temperature control schemes. There are also a host of safety issues related to electrical code compliance and approvals. Electric heat tracers can be categorized by several factors: output characteristics, the ability to cut-to-length, and temperature limits. Each heater will have its own performance and application limitations.

Output Characteristics: The output of the heater will be either fixed or variable. Fixed output heaters are referred to as ‘constant wattage,’ the output does not vary with temperature. Variable output heaters are referred to as ‘self-limiting,’ ‘self-regulating,’ ‘variable resistance’ or ‘power-limiting.’ In all cases the output varies with the temperature of the heater; as the temperature increases the output decreases. The value and slope of the change varies widely within this family of heater types.

The Ability to Cut-to-Length: This refers to the ability to cut a heater to a desired length in the field without specialized training or tools. If a heater cannot be cut in the field it is referred to as ‘fixed’ length. In order to shorten a fixed length heater it requires specialized training and / or the addition of a transformer or a controller with power limiting capabilities. Cut-to-length heaters are divided into two subgroups: those that can be cut anywhere and those that must be cut at specific locations or nodes. The second subgroup is called zone heaters. The zone length is consistent for a given heater but may vary from one output to another.

Temperature Limits: Each heater will have a maximum maintain temperature as well as a maximum exposure temperature. Exposure temperatures can be rated as power on or power off and a heater may have a higher ‘intermittent’ exposure temperature rating.

* Nominal output of variable resistance heaters is rated at 50°F (10°C).
** Some resistance and/or zone heater selections may carry hazardous area approvals. Consult factory if hazardous area approval is required.
= Re-circulated fluid or steam tracing has its own set of design and installation issues requiring consideration such as pressure, trapping, and system recovery. The discussion of steam and fluid tracing is not covered in this design guide. For more information on steam and fluid tracing visit our website at http://www.obcorp.com.