Quote: "80 percent of the heat transfer took place in the first 20 percent of the tube. So the modern tradition of tall VFT boilers is not as good as a shorter one with a wider diameter."
Depending on how it is fired that is not necessarily the case. My tug is a puffer with tall VFT, meaning the exhaust is ran up the stack, I can state with certainty, the fire is being drawn up the tubes and at times to the upper tube sheet. When the flues are cleaned which isn't very often there is only white ash - no creosote. A natural draft on certain boiler arrangements the "80% in the first 20%" figure can be pointed to with some accuracy, but like everything else steam related, it's not always cut and dried. With natural or induced draft, Flue diameter, number of flues, stack height, outside wind speed, fuel type, draft aperture all have an effect on the velocity of the flue gases.
And no I have no intention of running any sort of test to prove this
. That would be a very involved setup to even arrive at some approximation of what is happening.