When I was in college, I stumbled upon a ASME paper advocating that the U.S. should switch to the metric system and touting the benefits of doing so.
It was dated in the 1880s
AnthonyVWhen I was in college, I stumbled upon a ASME paper advocating that the U.S. should switch to the metric system and touting the benefits of doing so. It was dated in the 1880s
But the U.S. did switch to the metric system, a decade before that (even before joining the CGPM), and its standards have been nominally metric-based ever since. I suspect that as long as base measurements for engineering under SI are either in mm or km, there won't be much groundswell for practical engineers here to utilize it -- there's enough trouble trying to do things with non-graphics-based 'drafting' programs and such to consider adopting and then trying to use utterly inconvenient units that leave orders-of-magnitude ambiguity in the coefficients.
With the advent of good calculators, any reason for a formal switch to SI on 'this side of the pond' isn't nearly as important as it previously might have been. It's only when people start forgetting rigorous attention to their calculations, or leaving out defining their units (both of which I thought were still part of any engineer's fundamental education) that trouble with spacecraft and such crops up.
I never learned thermo in anything other than metric ... but it's cgs, and will remain resolutely so, whatever mise en pratique silliness Sevres might cook up next.
RME AnthonyV When I was in college, I stumbled upon a ASME paper advocating that the U.S. should switch to the metric system and touting the benefits of doing so. It was dated in the 1880s But the U.S. did switch to the metric system, a decade before that (even before joining the CGPM), and its standards have been nominally metric-based ever since. I suspect that as long as base measurements for engineering under SI are either in mm or km, there won't be much groundswell for practical engineers here to utilize it -- there's enough trouble trying to do things with non-graphics-based 'drafting' programs and such to consider adopting and then trying to use utterly inconvenient units that leave orders-of-magnitude ambiguity in the coefficients. With the advent of good calculators, any reason for a formal switch to SI on 'this side of the pond' isn't nearly as important as it previously might have been. It's only when people start forgetting rigorous attention to their calculations, or leaving out defining their units (both of which I thought were still part of any engineer's fundamental education) that trouble with spacecraft and such crops up. I never learned thermo in anything other than metric ... but it's cgs, and will remain resolutely so, whatever mise en pratique silliness Sevres might cook up next.
AnthonyV When I was in college, I stumbled upon a ASME paper advocating that the U.S. should switch to the metric system and touting the benefits of doing so. It was dated in the 1880s
In all my engineering classes in the late 1970s and 1980s, the English system was the dominant system of units. Thermo, fluids, heat transfer, and mechanics was taught using the english system with some SI. Machine design and strength of materials was taught exclusively using the English system with a homework problem or two in SI
I got to enjoy Thermo in both English and Metric sometimes both in the same problem ...
Clearly, SI is easier mathematically but I have no feel for it. I admit I may be old fashioned (I'm 58) but I have a better feel for one pound per square inch than one newton per square meter, as an example.
As I recall, a few items - viscosity of fluids (centipoises ?), some flow rates of quasi-solid materials (thixotropic ?), and some biological reactions (as in sewage treatment plants) were in metric units.
Haven't touched some of those in years (decades ?), so I may well be incorrect, and will welcome any corrections.
- PDN.
My classes in thermo and fluid mechanics were done in both SI and English Engineering units. English engineering was easier when pressures were pounds per square foot, density in slugs per cubic feet and velocity in feet per second. Pressure is a funny one as units still in use include psi, pascals, inches of mercury, millimeters of mercury (Torr), bar and atm. Velocity is another one as the most commonly used units are km/hr and mph.
A few of the at least somewhat common metric/English hybrids are heat rate for power plants in terms of BTU per kwhr, HO scale at 3.5mm/foot and 1 lbf at 1 mph equalling 2W. I still remember 1 kwhr equalling 3412 BTU.
erikem . . . HO scale at 3.5mm/foot and 1 lbf at 1 mph equalling 2W. I still remember 1 kwhr equalling 3412 BTU.
I didn't know about the 1 lb. force @ 1 mph = 2 Watts, but it correlates pretty well (1/2% or so) with what I often use: 1 HP (550 ft.-lbs./ sec.) = 746 Watts. Both are also rail-related when working on electrification studies.
Btu is one that's hard to know intuitively (at least for me). "It is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. One kilowatt-hour (kWh) of electricity contains 3,412 Btu."*
IIRC, 1 ton of heating/ cooling capacity** = 12,000 BTU, so my 3-ton ground-source ("geothermal") heat pump will be using about 10.5 kW at full power (~$1.27 per hour, $30 per day). Fortunately, that happens about never . . . ~$5 - $6 per day is a more typical average figure for the heating season, maybe $1 - $2 / day during the cooling season.
**"A ton is the cooling capacity of an air conditioning system. One ton is equal to the amount of heat required (288,000 Btu) to melt one ton of ice in a 24-hour period. A one-ton air conditioner is rated at 12,000 Btu per hour (288,000/24). A two-ton unit would be rated at 24,000 Btu per hour."*
* https://www.nwnatural.com/uploadedFiles/ConnectToGas/StepsToGetGas/FAQ/FAQs_The_Definition_of_Cool.pdf
Paul_D_North_Jr I didn't know about the 1 lb. force @ 1 mph = 2 Watts, but it correlates pretty well (1/2% or so) with what I often use: 1 HP (550 ft.-lbs./ sec.) = 746 Watts. Both are also rail-related when working on electrification studies.
Or discussing the efficiency of electric transmissions on diesel loconotives.
You're forgetting that a heat pump is called that for a reason, that is uses mechanical work to pump heat from a colder location to a warmer location (same thing a fridge or freezer). I suspect that a ground sourced heat pump would be good for a coefficient of 3, which means the heat output is three times the mechanical work in. Running full tilt, your heat pump would consume closer to $10/day and your bill suggests that the system is running ~50% of the time.
The traditional reason for expressing power plant heat rate in BTU/kwhr is that coal is spec'ed on the basis of BTU/lb, sold by the ton, while electricity is sold by the kwhr.
- Erik
Erik -
You're quite right - I do know that heat pumps don't create heat (or cold), but merely move it from one place to another. I overlooked that principle in my haste to finish the post early this morning. [As supplemental info, the coefficient is said to be ~4, so it (WaterFurnace) would be running 65 to 80 % of the time, which seems about right for our experience on the coldest winter night. (The rest of the winter electric bill difference might be explained by running the water heater a little more, cooking warmer foods, running the clothes dryer instead of outside line drying, etc.). At other times - such as bright sunny days - the passive solar heating alone supersedes it (typ. 71 deg. in that zone, with the thermostat set at 68 deg.), and/ or when the fireplace insert/ stove is burning. Someday I want to put usage meters on each 220v device in the house (heat pump, water heater, cooktop, oven, and clothes dryer, etc.) and measure how much each actually consumes during the winter vs. summer.]
Back to railroading: I wonder what the 'ton' rating was for the ice blocks for the early air conditioning in the Pullman cars and cooling function in the refirgerator cars; and then for the later mechanical systems in each ?
There's more to a ground-source heat pump than just effectively 'moving heat with a lower expenditure of energy'. The ground source can be treated as a near-infinite source and sink of heat energy at from 55 to 57 degrees F depending on latitude and soil conditions ... and this is good for two things: you can pretemper the air exchange, and you are no longer trying to 'force' the heat into already very hot air in summer or very cold air in winter, a major cause of inefficiency in "conventional' HVAC heat-pump setups (and a reason why most honest ones are dual-fuel).
If I remember correctly, the circ pump on the ground loops in a Water Furnace is a fractional-horsepower synchronous AC motor. and this small amount of power alone accounts for an enormous saving of effective compressor energy any time the outside climate goes out of temperate conditions.
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