Thermolec Application &Technical Notes - Manual (Page 50)

APPLICATION NOTES
Open coil and tubular elements considerations
THERMOLEC manufactures both open coil and tubular duct heaters.
It is widely acknowledged that tubular elements find practical application under certain circumstances (service conditions
include possible contact by personnel, presence of dust or particles in the air flow or atmospheric conditions). Where duct
air heating is the sole function, the open coil heater is superior for the reasons shown in the following comparison:
1-ELEMENT LIFE EXPECTANCY
The life expectancy of the nickel/chromium resistance wire in both the open coil and tubular elements is directly related
to the maximum wire surface temperature experienced during operation. In an open coil heater, the passing air in direct
contact with the wire easily dissipates the heat, thus reducing this surface temperature. In the tubular element, the heat
from the enclosed resistance wire must travel through electrically insulating compacted magnesium oxide powder
surrounding the coil and through a steel sheath before dissipation. To further compound the wire surface temperature
problem of the tubular element, the limited space inside the sheath necessitates the use of smaller gauge resistance wire
wound on a smaller arbor; hence a much higher wattage density per square inch of resistance wire becomes a necessary
design evil. For example, 200 watts per square inch may be required for enclosed elements compared to 40 to 50 watts
per square inch for open coil elements. The net result is that the resistance wire of the enclosed element, with a higher
operating temperature, a higher wattage density and double insulation, may not last as long as the open coil element.
2-SERVICE CONSIDERATION
Contrary to common belief, the tubular element is not as robust and trouble-free as might be expected. Space limitations
within the element casing require electrical clearances to be very small. This is a critical problem for elements with higher
voltages, particularly if the insulating powder is not properly compacted. There always exists a danger that the
magnesium oxide surrounding the element will absorb moisture, become conductive and short circuit the element to the
sheath. Because of this problem, UL requirements have become much more stringent. A separate Standard (UL # 1030)
on tubular elements was published containing strict requirements for the manufacturer of UL listed finned tubular
elements. These requirements attempt to control the problems of magnesium oxide contamination and moisture
absorption through frequently repeated tests. No such standards or tests are required for open coil elements.
3-SAFETY DEVICES
Should an open coil element overheat, a low mass, disc-type primary safety device immediately senses the higher
temperature and switches the power off in a very short time. In the case of the tubular heater, there are multiple time
delays. The thermal inertia of the coil, insulating material and sheath surface must be overcome. During the interim, the
unsafe overheat conditions may damage or simply ruin the tubular element, making a tubular heater a less "forgiving"
heating device.
THERMOLEC