Difference between Min & Avg Tube Wall Thickness of Heat Exchangers 1

[Venkatachalam Ponnusamy]

Want to understand the difference between minimum & average Tube wall thickness. My basic understanding is if we order for minimum wall thickness then there won't be any negative tolerance is allowed in wall thickness. whereas if we order for Avg wall thickness then there will be both Positive & Negative tolerance is allowed while manufacturing the tubes, hope my basic understanding is correct.

why customer wants most of the time minimum thickness based not average, is there any impact on heat transfer rate for min & Avg wall thick. Also we have price advantage for Average tube wall thickness.
Is any standard like ASME/TEMA defines on Tube wall thickness (whether Min or Avg).

Request you to share your thoughts and experiences..

 

[r6155]

See ASME II SA-450 General Requirements for Carbon and Low Alloy Steel Tubes.

Regards

 

[EdStainless]

Minimum wall tubes are 10% thicker, 10% heavier, 10% more expensive, and will have 10% resistance to heat transfer in the tube wall (however this is usually a minor factor).
We often saw tubes order to odd average walls so that they would meet some specific min wall requirement. In the world of stainless steel people order exactly the size that they need, often not a std gage.
More typical was to see 0.083" min wall (0.083"-0.100") or 0.095" avg wall 0.085"-0.104") interchanged as identical (though they are not quite).

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P.E. Metallurgy, consulting work welcomed

 

[davefitz]

The astm/asme spec for each grade of tube and pipe specifies allowable variance in wall thickness. The actual delivered variation varies between maufacturer, and generally japanese vendors have the smallest variance in thickness, and additional finishing ( cold finish) can reduce variations in wall thickness.

For the case of very long, small diameter tubes ( as used in supercritical furnace waterwall panels) the variation in wall thickness has enormous impact on heat transfer to individual tubes, as the fL/d effect on an individual tube's ability to draw water in a heated circuit with over 500 tubes in parallel is just as important as is the variation in heat flux around the furnace. You may recall that the tubes are all operating under an equal pressure drop boundary condition, while the pressure drop caused by tube wall friction is proportional to (fL/d)*(W^2)/d^4. Thus the change in flow to the small diameter individual tube will be inversely proportional to the tube diameter to the 5th power , rapidly compromised by wall thickness variations.

For short tubes the variation in wall thickness is not as significant as other variations in the heat exchanger( external flow distribution , header flow unbalance, cleanliness) that impact heat transfer, but for long, small diameter tubes that have a fluid flowing that has a major change in specific volume over its length, then variations in wall thickness are critical. It is noted that the apparent wall thickness can also be affected by accumulations of inside corrosion deposits. This was an especially difficult problem prior to 1980, when the supercritical furnaces had to deal with internal buildup of iron oxides ( spalled off HP feedwater heater tubes) but after 1980 and the common use of combined oxygenated feedwater treatment eliminated the spalling of iron oxides, which it reduced the impact on availability of these units.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick