There are different considerations to make when choosing a particular style of heater for a process heating application. The challenge is knowing which heating element is best for a particular application. At JenTer Technologies, we are your source for an array of heater solutions that meet the needs of various industry requirements.
There are ten potential factors to consider when determining whether to use open-coil or other heating elements, such as tubular or finned-tubular process heaters. Factors one through four are air quality, construction, air velocity, and operating temperature. If one type of heater stands out under these first four factors, that heater may be the proper choice. If no heater stands out among these four, then the next six factors must be considered to determine the optimum heater design.
1. Air Quality Impacts Heater Performance
For applications with minimal moisture or conductive particles contaminating the air, open-coil construction is best. Finned tubular can manage most pollutants unless those particles might accumulate between the fins. Tubular construction can be used in applications with more contaminants.
2. Construction Affects Cost
Generally, open-coil heaters are considered to have the most intricate construction. These heaters provide relatively fast thermal response, low heater weight, and low pressure drop. Since they can handle high watt densities, finned-tubular construction is less expensive than tubular, but more expensive than open coil design. Because they need a more conservative design to manage higher temperatures, tubular construction usually costs the most of the three heating element types.
3. Air Velocity versus type of Heating Elements
Tubular construction elements can generally withstand the highest air velocities. Finned tubular elements can handle the midrange air velocities and open-coil can handle the lower air velocities.
4. Operating Temperature Restricts Options
Finned tubular elements generally provide the lowest maximum outlet temperature of the three heating elements. Open-coil and tubular elements can provide higher outlet temperatures.
The above 4 factors are usually adequate for making a heating element selection and often open-coil design turns out to be the optimum choice and most cost-efficient. When the right heating element is not obvious from the first four factors, six additional factors must be considered as given below.
5. Pressure Drop
The pressure drop decreases as the volume occupied by the heating elements reduces. As a result, the lowest pressure drop belongs to heaters with open-coil element design. Tubular elements typically have the highest percentage of space taken up by the heater, and therefore the highest pressure drop. Finned tubular elements usually have a lower pressure drop than tubular, but higher than open-coil.
6. Coil Temperature
Open-coil heating elements operate cooler than coils lodged in sheathed elements since they are directly exposed to the airstream. Tubular heating elements usually operate hotter than finned tubular and open-coil elements. Finned tubular generally run cooler than tubular because the fins produce a heat transfer effect, but they run hotter than open coil. Increasing the number of heating elements used or reducing watt densities in these applications can help keep temperatures in a safe range.
7. Electrical Clearance
Finned tubular and tubular heating elements include relatively small clearances between the sheath and resistance wire and this space is typically filled with insulation in the form of compacted magnesium. Open-coil heaters do not have this insulation and have clearance around the energized parts of the heating element.
8. Airflow Uniformity
In the most ideal application, the airflow is distributed evenly throughout the heater. In the open-coil element design uniform airflow is best for proper operation of the heater. The addition of pressure plates may be used to facilitate a uniform airflow if necessary. The sheath-and-fin design of finned tubular elements make them the most able to handle non-uniform airflows. They tend to dissipate the hot spots. Tubular elements are less tolerant of uneven airflow than finned tubular.
9. Shock Hazard
Finned tubular and tubular elements have their live coil enclosed in a metal sheath, eliminating the risk of shock hazard. Since open-coil heaters are electrically energized, they are not recommended for applications in which personnel or conductive material may contact the element.
10. Thermal Inertia Affects Controllability
Open-coil heaters provide a fast response to step control due to having a low thermal inertia. Fluctuations in temperature are possible as a result unless the controls make up for it. Finned tubular elements provide a slower response due to having a high thermal inertia but can also generate more uniform temperatures with a properly designed control system. Tubular heating elements function somewhere between finned tubular and open-coil designs when it comes to thermal inertia and controllability.
The best type of heating element to choose for a particular process air application depends on various factors as outlined above. It is imperative to perform proper research and testing to find the best solution in each case.
For information about the various custom heating elements, we offer for your applications, give us a call today at 920.928.2022 or send us a message through our contact form.