Quote:
Originally posted by unregistered
"The problem with such a simplification is that you may quickly add up total head losses higher than the dead head pressure of the pump."
nope, not if you are referring to the manometer test
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Hey BillA,
Sorry for any confusion. This was directed toward the "one foot per 10 feet of tubing and one foot per elbow" stuff. Adding the typical fittings, tubing, block and radiator of a standard setup would yield a result higher than many pump's dead-head pressures. IMHO, having something so simple yet inaccurate is no better than having something more accurate that one doesn't know how to use.
It isn't a question of smarts so much as it is of experience. Anyone using tables and canned equations (especially engineers such as myself) needs to have a handle on what's really going on in a given situation. Otherwise it's kinda like the computer clichè of "garbage in, garbage out". To this end, verification through testing ala your manometer comments is required.
To suggest it is as simple as applying a fixed rule as first quoted leads to a lot of misunderstandings. Folks that don't know why the relationships that exist between flow rate and head loss are what they are ask questions like, "Why doesn't everyone provide such a simple table for their {insert pump, fitting, tubing, block, radiator, etc. here}?" The answer is that it really isn't quite that simple.
Yeah maybe trying to be more accurate is over-engineering it. Funny thing is though, a lot of people seem to be curious about the finest aspects of their systems with only vague clues about how it all works together. Still others will look at a 1°C temperature difference on a graph and conclude a particular product is superior to another without regard for test conditions and (in)accuracies.
OK, I'll get off my soap box now.
As to the mercury manometer, I made the comment mainly in jest. As I'm sure you know, it's main benefit is allowing a manometer that's only 1/13.61 times as tall as a water manometer. Instead of needing a twelve foot tube to measure your pump's dead-head pressure you could use a one foot tube. Too bad it can be messy to work with those things.