Use heat tracing to maintain process temperature and avoid heat loss.
Developments in heat tracing equipment designs, materials and controls have made heat trace systems an efficient method of avoiding heat loss and providing temperature control in process equipment. In general, the broader range of choices means that overall system design has been simplified. Here are some tips for successful heat trace system design. However, remember that this information is offered as a guideline. Consult your heat trace manufacturer's literature for complete formulas and data.
The first step in designing a heat trace system is to determine the heat loss from each pipe or tank to be traced. Most manufacturers have a design guide that shows step-by-step calculations for pipe and tank heat losses. If manual calculation is not for you, most manufacturers also have computer-automated design programs that will take your pipe or tank information and calculate heat loss for you.
Second, to determine the heat loss from each pipe or tank to be traced, you will need to know the desired maintenance temperature, the minimum startup temperature, the minimum ambient temperature and the nominal size of the pipe to be heated.
Third, select the correct cable for your application. Self-regulating cables are the most popular and can be used on plastic or metal pipe for freeze protection. They can be effective in pipe process temperature maintenance installations up to 300°F (149°C). Other choices include steam tracing and zone heaters.
Fourth, determine which jacket material will provide the desired level of mechanical, chemical and corrosive protection for your application. Consider the environment in which the tracing will be used. Is there moisture and high humidity? Aqueous inorganic chemicals? Organic chemicals, acids and corrosives? The cable supplier you select can help you choose jacketing suited for the operating environment.
Fifth, select the cable output rating that matches your application. The watts per foot (W/ft) output should be higher than the watts per foot heat loss calculated for your application. If the calculated heat loss exceeds the output rating, consider whether it is most practical to spiral wrap the cable, run two parallel lines of cable, or select a thicker insulation. Remember to adjust the cable output for supply voltage if necessary. Check the original manufacturer's output ratings for use with supply voltage other than nominal.
Sixth, determine the total cable length. When measuring the total length of pipe for a heat trace system, it is important to remember that inline equipment such as valves, flanges and pipe supports requires additional heat tracing to maintain proper system operating temperatures. Always follow your manufacturer's recommendations with respect to additional cable allowances for inline equipment.
Seventh, after you know your total cable length, size the circuit breakers for adequate protection. The National Electric Code requires the use of ground fault equipment protection (GFEP) circuit breakers with a 30 mA trip level for heating cable installations. Thermal magnetic style circuit breakers are recommended because they minimize the possibility of nuisance tripping at low temperature that can occur with magnetic circuit breakers.
Eighth, select the thermostat control device that matches your application for operating voltage, switching current, setpoint range, sensor input, remote monitoring and alarm requirements. Ambient-air-sensing controls often are used in freeze-protection applications. By contrast, pipe-wall-sensing controls typically are used in process maintenance applications where precise temperature control is required.
Finally, select the appropriate cable connection accessories, including power connection boxes, splice and tee connections, pipe straps, cable end seals and tape wrap. Consult with your heat trace system manufacturer for design guides that provide complete specifications and data tables.
This article is provided by Pittsburgh-based Chromalox, a manufacturer of heat tracing, heaters, sensors and controls. For more information, call (800) 443-2640; e-mail firstname.lastname@example.org or visit www.chromalox.com.
It's a Wrap
January 1, 2009