Thermolec Application &Technical Notes - Manual (Page 52)

6-WEIGHT
Open coil duct heaters weigh considerably less than comparable tubular heaters, hence they offer additional cost savings in
packing, shipping, installation and installation hardware.
7-WIRE SURFACE TEMPERATURE
A well-designed open coil element operates "in the black", at a temperature range of 750ºF to 900ºF. The tubular element is
designed for a surface temperature of similar magnitude, but this requires the surface temperature of the inner coil portion
to often exceed 1600ºF. Element durability is sacrificed in the normal design of the sheathed element.
8-SUSCEPTIBILITY TO MOISTURE
The structure which supports tubular elements must allow for the movement of the elements as they expand and contract
from heating and cooling.
Moisture, when present, can easily be absorbed from the terminal end into the core of the element where it combines with
the moisture-thirsty magnesium oxide powder into a conductive form of magnesium oxide. When this occurs, the enclosed
element may develop an internal short and fail, becoming a hazard.
Open coil elements wire are electrically insulated by thick ceramic bushings which have five to ten times the electrical
clearances of the typical tubular type element. The open coil element has been proven reliable in air with 100% relative
humidity in laboratory tests, and in years of less-than-ideal field service conditions. Testing proved that a 600V open coil
heater, after total immersion of the elements in water, functioned without fault.
9-PRICE
Heaters equipped with tubular elements are considerably more costly in materials, tooling and labor to produce than are
comparable open coil heaters, hence they must be sold at a higher price. The only exception could be mass-produced
heaters of standard wattage increments; however, such heaters do not give the design engineer the flexibility of choosing
from an infinite variety of electrical ratings and dimensions as in the case of the open coil heater.
10-CONCLUSION
In summary, the advantages of the open coil heaters over the tubular element heaters for duct heating applications include:
• Longer life
• More kilowatts per square foot
• Lower wire surface temperature
• Less maintenance
• Increased safety and larger electrical clearances
• Less pressure drop
• Lighter weight
• Lower shipping costs
• Less sensitivity to moisture
• More flexibility in size and capacity
• Lower cost
• Smaller size
APPLICATION NOTES
THERMOLEC