TEKtube Tubular Products X-ID®:
What are X-ID® tubes? X-ID® tubes are internally-enhanced tubes that significantly increase the heat transfer for gas flowing inside the tube compared to an equivalent plain tube. This heat transfer enhancement is provided by the helical ribs that are embossed on the inside of the tube. Heat transfer enhancement in X-ID® tubes is not only due to a surface area increase, but more importantly due to a complex boundary layer separation-reattachment phenomenon.
X-ID® performance has been extensively researched both at the Fintube Test Facility and through research performed at an accredited American university .
The X-ID® tube is produced by first embossing the rib pattern on the un-rolled strip of metal which is then rolled and welded into a tube in an ERW tube mill. It is important to note that the specified minimum wall thickness of the tube is the tube thickness under the rib, i.e. the rib thickness is above the minimum wall thickness. For the past ten years, ASME has recognized that X-ID® tubes are manufactured to SA-178A specifications. X-ID® tubes meet all the material, dimensional and testing (destructive and non-destructive) requirements set forth in the SA-178A specifications. In a typical tube in the convection section of a firetube boiler, 90% of the resistance to heat transfer from the flue gas to the water on the shell side, is on the inside of the tubes. Decreasing this resistance (i.e. increasing inside heat transfer coefficient) by using X-ID® tubes significantly increases the overall heat transfer in the convection section.
Benefits of X-ID®
The footprint and space requirements for an X-ID®-tubed boiler are less than those of an equivalent boiler equipped with plain tubes, since each X-ID® tube has an I.D. heat transfer rate which is 85% greater than the plain tube. Cost savings of up to 20% on the entire boiler package can be obtained with X-ID® tubes.
Due to the increased heat transfer from X-ID® tubes, X-ID® boilers are more efficient than similarly-rated plain tube boilers, leading to significant fuel savings. Lower fuel consumption also results in lower emissions of NOx and other pollutants.
Alternately, an X-ID® boiler can be designed with the same shell size as a plain tube boiler, resulting in significantly larger steam space and disengagement heights. This is important in processes that require rapid ramp-up to full production or need high quality steam even with rapidly varying loads.
Special boilers, like Ohio Specials or Detroit Specials, where the fire-side surface area is limited, are ideal candidates for X-ID® tubes. Remember that although fire-side surface in X-ID® tubes goes up about 25%, the heat transfer rate goes up 85%. X-ID® Ohio Special boilers therefore have higher boiler efficiency than a similarly rated plain tube boiler.
Fouling, when the boiler is fired with natural gas or #2 oil as a fuel, is not a concern. In fact, due to the additional turbulence created near the wall, X-ID® tubes are self-cleaning to a great extent and have been shown to foul less than plain tubes. X-ID® air-heater tubes have been used downstream of wood-burning and coal-burning boilers without encountering any fouling problems. Regular tube brushes can be used to clean X-ID® tubes.
Feedback from a burner manufacturer suggests that higher flue gas re-circulation rates (used to lower NOx), while still keeping the flame stable, was achieved in X-ID® boilers. The thought is that X-ID® tubes help break up oscillations in the system that might be responsible for flame instability.
Rolling X-ID® tubes is no different than rolling plain tubes. No special tools are needed, nor is the wear and tear on the roller-expanders any different than for plain tubes. The ribs roll off and the tube acts like a plain tube. X-ID® tubes can be rolled and expanded, flared, beaded and seal welded. We have experience rolling tens of thousands of X-ID® tubes in tubular air pre-heaters. These tubes, made to A-513, are unannealed and much harder than X-ID® boiler tubes made to SA-178A.