TIP 1: Construction Affects CostOpen-coil type heaters generally are considered the most cost-effective construction. Open-coil offers quick thermal response, low pressure drop and low heater weight. Finned-tubular construction is more expensive than open coil but less expensive than a tubular design because it can accommodate higher watt densities. Tubular construction generally is the most expensive because it requires a more conservative design to handle the higher temperature ratings.
TIP 2: Process Air Quality Affects Heater PerformanceOpen-coil construction is best for applications where no conductive particles or water spray contaminate the air. Tubular construction can be used with nearly any type of air contamination. Finned tubular also can handle most contaminants unless there is a possibility that the particles could build up between the fins.
TIP 3: Operating Temperature Narrows ChoicesOpen-coil and tubular designs can be used for outlet temperatures up to 1,200 oF (649oC). Finned tubular designs are recommended for a maximum outlet temperature of 600oF (316oC).
TIP 4: High Air Velocity Rules Out Open CoilWhile all three types can withstand high air velocities, only tubular and finned tubular design can handle applications of more than 2,500 ft/min. Finned tubular can handle velocities of 5,000 ft/min, and tubular elements are suitable for applications up to 8,000 ft/min.
STILL DON'T KNOW?Frequently, these four factors are sufficient to make the appropriate selection. For example, air velocity was a key factor in a 4.3 MW heater built to process agricultural products. Applications included drying barley for malt production and dehydrating alfalfa for livestock feed. High air velocity precluded the use of open-coil heaters. The greater watt density capacity of the finned tubular design also reduced the number of elements required, keeping cost comparable to that of an open-coil heater and minimizing the weight of this mega-heater.
In contrast, open-coil heaters satisfied another application for a customer manufacturing electric ovens for drying and curing solid-state components. The customer was not burning anything off the product so the air remained pure. The uncontaminated air permitted the use of an open-coil heater.
Although open-coil construction frequently proves to be the best -- and most affordable -- solution, it is not always the best choice. For example, tubular elements were specified for a 13', 550 kW batch oven used to dry sealants and cure adhesives on components wrapped in fiberglass for the aerospace industry. With fiberglass filings in the airstream, tubular elements were best for this application.
In some cases, a determination cannot be made on the first four factors. When that occurs, six other criteria should be considered.
TIP 5: Coil Temperature Varies by Heater StyleOpen-coil elements, exposed directly to the airstream, run cooler than coils embedded in sheathed elements. Finned tubular elements run hotter than open coil but cooler than tubular due to the heat transfer effect of the fins. Tubular elements run hotter than open coil or finned tubular. In these applications, temperatures are kept within safe limits by using reduced watt densities or by increasing the number of elements used.
TIP 6: Pressure Drop Is Lowest with Open-CoilThe less space occupied in the heater by elements, the lower the pressure drop will be. Therefore, heaters with open-coil elements have the lowest pressure drop. Finned tubular elements will have a higher pressure drop than open coil. Tubular elements have the highest pressure drop due to the higher percentage of space occupied in the heater.
TIP 7: For Severe Applications, Remember Electrical ClearanceTubular and finned tubular elements have relatively small clearances between their live parts (the resistance wire and the sheath), but this space is filled with compacted magnesium oxide, which acts as insulation. Open-coil heaters are better suited to withstand severe applications because there is larger clearance between the live parts of the heating element and ground.
TIP 8: Tubular Elements Enclosed in Sheath, Reducing Shock HazardBecause the live coil is enclosed in a metal sheath, shock hazard due to accidental contact is eliminated in tubular and finned tubular elements. If the possibility exists that conductive material or personnel may come in contact with the element, open-coil heaters are not recommended because they are electrically live.
TIP 9: Airflow Uniformity Affects Heater SelectionA perfect application has airflow distributed uniformly throughout the heater. Open-coil heaters are not tolerant of hot spots and must have uniform airflow to operate properly. If necessary, pressure plates can be used to even out the airflow. Finned tubular elements are the most tolerant of nonuniform airflow situations. Hot spots tend to be dissipated by the sheath-and-fin design. Tubular elements are more tolerant than open coil of uneven airflow but not to the same degree as finned tubular elements.
TIP 10: Thermal Inertia Influences ControllabilityOpen-coil heaters have a low thermal inertia, which results in quick response to step control. This can cause temperature fluctuations unless the controls compensate. Finned tubular elements have a high thermal inertia, which results in slower response but can produce more uniform temperatures if the control system is properly designed. Tubular elements would react somewhere between open-coil and finned tubular designs.
Every application is unique and warrants an individual solution. In the long run, it is worth the time to research the best possible type of construction for each process air application.