Large scale equipment (500L +)

[husky]

Roughly 5 HP.

BTW I like the concept of defiantly wanting steam but I think you meant definitely.

LOL, yep good spot on the typo! definitely want min 50kw(or 5 boiler hp). Given the relatively low cost of the boiler vs the cost of the piping and equip you would be silly not to go a bigger boiler for future capacity for only a few $ more.

 

[klangers]

LOL, yep good spot on the typo! definitely want min 50kw(or 5 boiler hp). Given the relatively low cost of the boiler vs the cost of the piping and equip you would be silly not to go a bigger boiler for future capacity for only a few $ more.

66 hp!

Yes, good to oversize by up to 50%. The boiler is low cost but the gas or electricity supply isn't. If you take it too far you'll suddenly find yourself needing to apply for to be an industrial utility consumer and all sorts because your plant has the potential to suck heaps of gas/electricity.

 

[husky]

1 horse power is approximately 750 watts = 0.75 kW. So 50kW = 66 horse power (an interestingly enough, an actual horse is capable of significantly more than 1 horse power for short bursts).

However, if you're running steam calcs in horse power then you're either a masochist who enjoys excruciating unit conversions in Imperial, or aren't familiar whatsoever in metric. Seriously, the Imperial units for thermodynamic work are painful. Do yourself a favour and do thermodynamic calcs in metric.

Boiler capacity is typically listed in kg/hr of steam at a particular service pressure. This equates to a full load power. The boiler only boils off what it needs to in order to maintain the setpoint pressure. So if you can only condense 15kW of steam at your load, the boiler can only boil off that amount too.

There is also the whole condensate removal and return process which is critical to the performance of a steam system. I won't even begin to start that here.

So, typical process to size a boiler is as follows:

1. Work out your connected loads . Bear in mind that with saturated steam, temperature is proportional to pressure (which is actually the reason why steam is so bloody useful). So, you need to keep in mind not just the kW of your load, but also the pressure. Lower pressure steam needs larger pipework (lower density, kinda like high voltage electricity) and visa versa, so forgetting to understand your delivery pressure will screw you.
2. Work out the sequencing of your connected loads. It is very rare for everything to be running full load simultaneously.
3. Convert all kW values to pressure and mass flow rate (otherwise you can't size pipework)
4. Determine your peak, average and minimum demand in kg/hr at boiler discharge pressure.
5. Boilers have to be sized on worst case, ie peak demand +10%. Accidentally blowing out your boiler will be a rude expensive shock. If your peak is significantly higher than average, you may wish to revisit step 2.

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).

 

[good4whatAlesU]

I always presumed one advantage was something to do with the even spread of heat (steam jacket) versus the comparative point source of electric element.

Edit: and minimisation of cleaning etc.

 

[Lyrebird_Cycles]

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.

 

[husky]

I always presumed one advantage was something to do with the even spread of heat (steam jacket) versus the comparative point source of electric element.

Edit: and minimisation of cleaning etc.

It's definitely a better heating source but if committing to a boiler I never understood going electric where you still need all the infrastructure to get a lot of electrical power to the boiler when you could go gas. Unless like in refrigeration, you need less electrical power input to get a given output.

 

[malt junkie]

The advantage is as I said before instant exchange of (stored) energy(pressurised steam), so while your mashing your boiler will be storing that ready for when you need to ramp to boil.

Go back to the flash boiler (no pressure) steam injection directly into a 40L mash had a 5°c rise in just a few minutes, the phase change from vapour to liquid expels/exchanges the heat faster than any other method at very high efficiency.

 

[klangers]

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.

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.

 

[husky]

Great answer, exactly what I was after basically reconfirming my purely theoretical thoughts. COP was the missing link in my question and since a boiler COP(if there was such a thing) would basically = 1(likely slightly under after efficiency losses) I don't understand why anyone would use an elec boiler.

 

[Lyrebird_Cycles]

It is theoretically possible to build an electrical boiler with a COP greater than 1, eg a heat pump. As I understand it, the large "lift" involved means that the COP won't be anywhere near that available from a normal heat pump so the extra capital expense isn't worth it.

 

[klangers]

It is theoretically possible to build an electrical boiler with a COP greater than 1, eg a heat pump. As I understand it, the large "lift" involved means that the COP won't be anywhere near that available from a normal heat pump so the extra capital expense isn't worth it.

Yeah. The issue is that there is no known refrigerant with high enough efficiency and with the right p/t phase transitions. To boil water you need a heat source significantly greater than 100 degrees, unless you have a massive and expensive heat exchange surface. Then if you want to raise a useful pressure of steam (no offense to noted success of flash boilers in previous posts above - but industrial steam needs greater pressures) you need temperatures several hundreds of degrees. The most efficient boilers currently known - in power stations - raise supercritical steam to in excess of 500 deg C. You'd need a refrigerant which can be compressed to a pressure where it will condense and release latent heat at >100 degC, but also expanded evaporate below ambient temperature so that the "cool" can be released, without requiring more work input to achieve those pressures than available thermal output.

That said there are off the shelf hot water "boilers" aka domestic hot water units that use heat pumps and achieve great COP. But they aren't trying to heat anything beyond 70 degrees.

EDIT: My beery idea for the evening: A heat pump mash heater? With in-mash condenser coil? Now we're talking efficiency!