Copper tubes
Copper tubes are rolled and fabricated into active parts and ocasionally into cooling parts of the coil. Hence, the importance of the its properties cannot be denied. About copper tubes there are couple of points to consider:
Tube material
The copper in the tubes of coil’s active part must be of at least 99.90% of purity and 58 MΩ/m of conductivity with the most possible impurity of phosphorous, oxygen and iron. These element’s presence will drastically lower the conductivity and may cause coil overheat and explosion. Aside from chemical composition of the coil’s active part, must be annealled via heat operations under special vaccum condition after bending and forming. Otherwise, during rolling and production process or mechanical tensions, tiny cracks may appear on the surface and little by little these cracks may get wider leading in major leaks and sometimes decaying. Hence, Araz Trans Co. applies purified copper of the ETP-ASTM C11000 grade for coil’s active parts and ocasionally for special orders OFC-ASTM C10100.
Tube shapes
Amoung quality factors of the tubes used for coil construction, shape and cross-section of the tubes has the most impact on the operation and life cycle of the active part. Aside from cross-section of the conductor being adequate for the current, the point mostly been neglected is the shape of the tube. The shape must be in a way that handle the current considering penetration depth dou to proximity effect (fully related to the frequency) and proper cooling and water flow from the inside. Tubes with internal rectangular canal (90 degrres corners), would face congestion and seimentation and consequently more erosion on the corners particularly with low quality water. Henceforth, tubes with curved cross-section are better for this intention.
About penetration depth in the conductor duo to proximity effect, it can be said that a magnetic field caused by conductor’s current will induce eddy currents on outer side of the melt opposing coil’s current. According to proximity effect between coil and melt, the current in the both part will tend to flow near both surfaces.
Considering above-mentioned issues, normally the current density on the inside surface of the coil conductor (facing melt) will be higher than the other sides. Then, more thickness on this side will lead in more efficiency of the coil. According to the literature it has been suggested that the inner side of the copper tube to have thickness at least two times of the penetration depth caused by proximity effect.
The relationships and the table below show the depth of current penetration inside the conductor due to the proximity phenomenon and also the minimum optimal value for the thickness of the side facing the melting of the copper section.
The results of the relationships indicate that the effect of the proximity phenomenon in the conductors has an inverse relationship with the frequency of the electric current passing through it. The lower the frequency of the electric current of the coil, the more thickness is needed for the side of the copper profile to be melted.
The reason for using D-shaped cross-section profiles in the coils of some European manufacturers’ network frequency furnaces is the same.
| مقطع | شکل | مورد استفاده |
|---|---|---|
| foursquare section
with foursquare water passage |
 |
Active part of most
Chinese Indian and Turkish furnaces |
| foursquare section
with circular water passage |
 |
Ajax-TOCCO
Active part of Active part of Ajax-TOCCO Company |
| foursquare section
with elliptical water passage |
 |
Active part of midrange
frequency furnaces of German companies Otto-Junker and ABP |
| D-type section with
circular water passage |
 |
Active part of grid
frequency furnaces of German companies Otto-Junker and ABP |
| Circular section with
circular water passage (pipe) |
 |
Active part of some
Chinese furnaces and cooling part of some German furnaces |
| Asymmetric foursquare
section with circular water passage |
 |
Novel design for active
part of midrange frequency furnaces by Araz Trans |
Considering point about the quality and occasions of the copper tubes to be used in the coil of the induction furnaces, below table summarizes the consideration by Araz Trans Co. All the copper tubes are available by us.