Lyrebird_Cycles said:
The answer to this conundrum lies in the fact that boilers were originally rated on the size of the steam engine they could be used to run.
One boiler horsepower meant that the boiler's steam output was sufficient to produce one mechanical horsepower from a 19th century steam engine*. This was calculated as the power required to boil 34.5 pounds of water per hour so it is about 9.8 kW. 10 kW is close enough.
Truly an archaic unit.
* This means the engine had an efficiency of about 7.5%. As far as I can work out the engine used in this calculation would have been one of the later compound engines. James Watt's simple engines were about 3% efficient.
Indeed I was about to say something along the similar lines. I've only ever worked in metric and converted units as necessary.
One BHP denotes ability to produce 34.5 pounds of dry steam per hour at 100°C (212°F), and corresponds to 10 square feet of heated surface, 33479 British thermal units (BTU), or 9.809 kilowatt per hour (equal to more than 13 mechanical horsepower).
As I said before. If the above seems easier and more straightforward than calling a spade a spade (or in this case, power power) and using units that actually have some fundamental foundation, then go for your life. This is why we did away with the Imperial system - too many easily-confused and unnecessarily difficult units. There is a difference between "the horsepower of the boiler" and "the boiler thermal power in horse power". It doesn't even convert neatly to BTUs, which is a joke of a unit as well. Truly stupid. Just like ounces vs fluid ounces. One is a mass measurement, the other volume. There is also a"tonne" of refrigeration that has an equally obfuscated origin... something to do with the energy necessary to freeze water when the moons on Jupiter are full and on Tuesdays.
husky said:
My understanding is that when someone quotes boiler hp it's different to brake hp that most people are used to dealing with is that right? It used to screw me up when calculating boiler capacity hence I only use metric as it makes more sense to me.
Since we have a boiler guru can you shed some light on a qn I have had for a while:
What's the point of an electric boiler? My understanding is that for say 50kw elec boiler output you need basically the same as electrical energy input as in there's no efficiency gains like say a refrigeration cycle where elec input can be 1/3rd the cooling capacity? Is that correct or are there efficiency gains within a boiler that allow greater output than electrical input?(which doesn't make sense to me).
I think we've collectively grounded your first question - "BHP - boiler horse power" is different to the power of the boiler in horse power. Stick to kW.
Onto your second question.
There is no coefficient of performance for boilers like there is for refrige. Boilers always have an efficiency (power out/power in) of less than 100%, typically 80-90%. Greater than 100% would get you in trouble with the 0th and 1st laws of thermodynamics. Larger boilers are typically more efficient. So there can never be more power out than power input; which is why I'm on a little bit of a crusade to get rid of Imperial units - they just lead one to confusion. If one sees "horsepower" then one would assume "0.75xkW" but if it's "boiler horse power" then the efficiency appears to be >100%.
Now, electric boilers are very rarely used in anything large as it's a very expensive way to heat something. What can be confusing is that electric boilers are typically significantly more efficient than combustion boilers. However, this is only because efficiency looks at power in divided by power out. Hold that thought for a sec.
An electric boiler can have immersed elements which transfer nearly all the heat without losses. A combustion boiler has to convert chemical energy into thermal energy in the form of a hot gas, and pass this gas over tubes through which the water runs (water tube boiler, there are also fire tube boilers and other types), so there is an additional heat transfer step. So analysing only the boiler, then electric boilers are more efficient at turning kW input power into kW output power.
If, however, we took into account all the losses from the coal fired boiler in the power station, the generators, transformer and transmission then the overall efficiency of electric boilers is woeful. Thusly, these losses manifest as a significantly higher cost per unit of energy for electricity compared to gas. I've seen this on some sites to be as high as 3:1. As you can see, this clearly offsets the ~8% lower efficiency of the gas boiler.
Gas as a fuel can be switched on from 0 flow to full flow without much hassle - as long as the delivery pipework is up to scratch the the utility supplier infrastructure is working. Try to switch on several hundred kW of electricity suddenly and my guess is you'll have Energy Australia asking for money to upgrade your local substation. Also compare the cost of a gas control valve suitable for 500 kW boiler (~$2k) to the electrical system necessary to turn down 500 kW of electrical power (obviously on/off control and a SSR ain't gonna cut it industrially) - I'd guess you'd be up for $40k.
So tl;dr, "what is the point of electric boilers?"
- To make your morning tea or coffee - ie small applications as they become severely limited by the necessary electrical infrastructure.
