Sheet D-00 · Field notes
Field Notes
Things I have had to work out, written down while they were still fresh. Codes, thermal design, manufacturability, and the parts of the job that never make it into a textbook.
The ΔT you assume is the plant you buy
Airflow is inversely proportional to delta-T, so the number you assume across a data hall decides how much cooling plant you buy. Most take it on faith.
Colebrook, and why I stopped using the explicit fits
Swamee-Jain is a good approximation and it is free. Iterating the real Colebrook equation is also free, converges fast, and removes an error term.
Rule #1 is already doing the work
Most drawings I review are covered in flatness and straightness callouts that the size limits already enforced. Here is what Rule #1 actually controls.
Sixty per cent out of the build time, and nothing given back
A redesign that took 60% of the manufacturing time out of a heat exchanger line, with none of the saving bought by accepting a worse machine.
Thermal stress does not care how long your pipe is
Length appears in the expansion equation and vanishes from the restrained stress equation. That cancellation is why shortening a hot run does not help.
A workshop that needed no eyes
My father lost his sight at seventy-five and kept working, because his workshop had been ordered for so long it no longer needed eyes to operate.
Why I write these
A note is a design review with myself.
If I cannot write down why a decision was right, I do not understand it yet — I have just got used to it. Writing is where that shows up, which is uncomfortable and useful in roughly equal measure.
If something here is wrong
Tell me. I would rather be corrected in public than be confidently wrong in a design review. Corrections get made, dated and credited.