Before a potential application or process can be evaluated for whether heat tracing can satisfactorily handle the job, you need to know what’s needed to complete an evaluation. A complete tracing system analysis should consider all of the following:
- The specific application.
- The tracing system’s functional performance.
- The tracing/pipe system energy performance.
- The tracing system’s installation cost.
There are situations where one, two or all three methods of heat tracing -- electric tracers, steam or thermal fluid tracing systems -- may be used to economic advantage in an industrial plant. Steam may be available and the best choice for tracing in one unit while electric or fluid is the best choice in another. Most large refining and chemical facilities will generally have steam and electric tracing in use throughout the plant. The textile industry will often have steam and thermal fluid heating systems for higher temperatures.
To best determine whether heat tracing is right for your application, and which method might be best, follow these four steps.
Step 1: Describe the Industrial Heat Tracing Application
Compile the following list of information for your prospective suppliers:
- Plant location.
- Climatological data, including minimum and maximum ambient temperatures as well as annual average ambient conditions.
- Process, utilities or service materials to be heated, including properties, specifications, processing hours, heatup requirements and the flow path of the process fluids.
- Method of product temperature control and product temperature-monitoring requirement.
- Energy issues. This includes location, type, quantity, quality and cost; area classification; electric energy cost; voltage; steam energy cost; steam pressure; and heat transfer fluid cost including packaged heater unit.
- Piping details, including materials; lengths, sizes, and grade level; piping and instrumentation diagrams (P&IDs); piping isometrics; and piping line list.
- Insulation type, thickness and weather barrier.
- Labor rates and maintenance hours required.
- Tracing system alternatives under consideration.
Step 2: Determine Heat Tracing Functional Performance
First and foremost, any tracing method considered must be able to meet the functional requirements of the process piping and equipment being traced. The tracing system must heat up and maintain the piping system at the prescribed temperature. A heatup time requirement may be placed on the system, not only for the initial startup but for startups following a turnaround or emergency shut down. The pipe, product, heater and insulation maximum temperature limitations must not be exceeded under normal and abnormal conditions. The temperature control system, if one is necessary, must provide the required accuracy of control. A temperature alarm system also may be required to fulfill safety or production specifications. Operations may require monitoring of the heating system. These considerations are all necessary to arrive at a functional system.
Step 3: Evaluating Heat Tracing or Pipe System Performance
The energy consumption characteristics of a tracing system are primarily a function of the following:
- Insulation system.
- Type of tracing system temperature control.
- Type of heat source.
Insulation. Often used for temperature-maintenance applications, heat tracing systems typically are designed to replace only that heat lost through the thermal insulation. Therefore, energy consumption is directly related to the energy loss characteristics of the insulant, which is a function of the insulation type and thickness. While heat loss reduction and optimization are possible by prudent selection of the insulation type, the insulation type first must be matched to the functional requirements of the application. Once an appropriate insulation material is selected, any efforts to minimize heat loss should then be based on insulation thickness.
Tracing Temperature Control. Temperature control can be accomplished using pipe-temperature-sensing or ambient-temperature-sensing controllers. To minimize energy consumption and costs, pipe-temperature-sensing controllers should be considered.
When there is no material flowing in a piping system, a pipe temperature-sensing controller, which activates and deactivates the tracing system, reduces energy consumption by permitting the tracer to deliver only that energy which is required to maintain the pipe temperature. When flow occurs in the pipe at temperatures above the controller setpoint, the pipe-sensing controller de-energizes the tracing and minimizes energy consumption.
Tracing controllers, which sense ambient temperature rather than pipe temperature, permit continuous energizing of the tracing when the ambient temperature is below the controller setpoint. The result is higher energy consumption by the tracing.
Possible Heat Sources. The energy consumption of parallel and series resistance electric tracers is limited to the Joulian (I2R) heating ability of the cable. Most plants will have electricity available for electric tracing -- either purchased or produced at the plant site (cogeneration).
Steam tracers are a constant-temperature heat source. Their energy consumption is proportional to the steam temperature minus pipe temperature differential. When control schemes are not employed, energy consumption of a steam tracer increases when the process fluid temperature is less than the equilibrium temperature flowing through the process pipe.
In general, a thermal fluid tracing system requires multiple tracing circuits before it can be justified due to the cost of the fluid-handling unit. (The fluid handling units require an expansion tank to provide space for fluid expansion and a net positive suction head for the pump; a circulating pump to keep the hot fluid flowing; and a heater to heat up the liquid to the required temperature and reheat it as it returns from the tracers.) When this approach makes sense, process temperature control can be accomplished via flow control valves for multiple users or by a process temperature sensor that controls the heater for single users. Thermal fluid heaters are either fuel fired, steam heated or heated via electrical resistance heaters. The total installation cost, energy costs and the intended operating pattern should be considered when selecting the type of heater for the system.
Step 4: Installation Cost Considerations for Industrial Heat Tracing
The installation costs of steam, fluid and electric tracing are strongly a function of:
- Piping complexity.
- Temperature maintenance, control and monitoring requirements.
- Area classification.
Piping Complexity. Electric tracing cables are normally more flexible than tubing and thus installation time is less for regular objects such as valves, pumps, filters, elbows and flanges. As a tradeoff, however, the number of electric circuits and controllers will increase as the complexity increases and will thus increase the cost of an electric tracing comparison to an uncontrolled steam tracer.
Temperature Maintenance/Control Monitoring. The installation of pipe-sensing temperature control and monitoring devices can be as simple as an indicating on/off mechanical thermostat, or it can be as sophisticated as a microprocessor-based control package. The relative costs of steam, electric or thermal fluid tracing systems are related to some degree by the control/monitoring applied to each system. In the case of steam tracing, control and monitoring devices are available but are seldom used. Steam tracing efficiency will depend in large measure on keeping malfunctioning steam trap energy losses at a minimum.
Area Classification. In hazardous areas, watt per foot outputs may be limited in order to comply with runaway temperature restrictions. This may result in multiple passes of heater cable, which will result in increased installation costs. A constant temperature heater such as steam generally does not fall under the jurisdiction of these runaway temperature restrictions and thus will enjoy the installation cost benefit resulting from installing fewer passes of tracer.
It is important to understand that there is no one single heat-tracing method that is best for every situation. The specific application under consideration, with its particular requirements, should be the determining factor as to which heat tracing method to employ.
This article is adapted from “Relative Merits and Limitations of Thermal Fluid, Electric and Steam Heat Tracing Systems” from Thermon Manufacturing Co., San Marcos, Texas.