Naked Science Forum
General Science => General Science => Topic started by: doppler1 on 29/03/2010 12:32:57
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If I am talking about compressing air or pressurizing water to the same pressure to supply a driving system, lets say 80 bar, which would be the more efficient or cost effective medium to us, water or air? supply line could be 150mm or 6inch.
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Air, I think.
Air can be compressed, a lot. You can put a bit of work into it, although, if you are not careful, most of the energy you spend will escape as heat, in which case, you won't get anything like as much work out as you put in.
Water does not compress very much at all. So you won't be able to put much work into it before you get to 80 bar.
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Actually, when you think about hydraulic systems, I think pressurising a fluid might be easier as all you'd need is a single ram. Pressurised water is certainly used for most pressure testing purposes, such as steam boilers, and the Comet airliner was famously pressure tested with water to find the cause of the early models breaking up in flight (and which lead to a much greater understanding of metal fatigue).
When you say you want to use it for a driving system, what exactly do you mean? Are you talking about actually delivering the drive power via air or water pressure? Hydraulic drive systems have been around for a long time now and have been used in quite high power applications, certainly > 1000HP. I'm not aware of any compressed air drive systems though, and if you are talking about delivering drive power then I think you'd run the risk of wasting a lot of the power due to the low viscosity and resultant heating of air.
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"I'm not aware of any compressed air drive systems though,"
Never heard of a pneumatic drill or air driven tools?
Weird!
Anyway, as usual, without knowing more about the application it's impossible to say what the answer is.
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"Anyway, as usual, without knowing more about the application it's impossible to say what the answer is.
That's true. You can't spend much energy compressing water, because it hardly compresses at all, but that's not quite what the question was [:D]
Rats! Caught again.
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"Anyway, as usual, without knowing more about the application it's impossible to say what the answer is.
That's true. You can't spend much energy compressing water, because it hardly compresses at all, but that's not quite what the question was [:D]
Rats! Caught again.
Actually, you can spend lots of energy compressing water, but you need a lot of water or a lot of pressure.
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"Anyway, as usual, without knowing more about the application it's impossible to say what the answer is.
That's true. You can't spend much energy compressing water, because it hardly compresses at all, but that's not quite what the question was [:D]
Rats! Caught again.
Actually, you can spend lots of energy compressing water, but you need a lot of water or a lot of pressure.
That is very true. I remember something about a gun that (I think) operated on compressed water. I'll post a link if I can find it.
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Thanks for the inputs guys. In actual fact, I would say that about 80% of the tooling used in underground mines in South Africa are driven by pneumatics which is pressurized in a compressor house on surface and piped down the mines to the work face. I agree that water is a far more efficient means for driving tools and equipment and should not need extra lubrication. It also has a cooling effect on the equipment but it seems that the problem presents itself in the following way: for water to drive anything, a differential needs to be present and the greater the differential,the more driving force and energy available, there are water driven tools in use in some of the mines but to create a differential water has to be dumped on the footwall and this water then has to be processed through the very expensive cycle of being collected, flocculated, pumped out of the mine (which requires energy), treated on surface and chilled before it can be re-entered into the cycle. Oil hydraulic supply systems, in my experience, are usually on a high pressure but low flow rate requirement which creates a few problems as well.
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If pressurizing water is more efficient than using compressors to mobilize air, why not use water to pressurize the air? If we have a sealed off chamber filled with air and we start to pump water into the chamber, the water displaces the air and compresses it until the required pressure is reached, thanks to the difference in density, the air will always sit at the top off the chamber and the water at the bottom so you could put a solenoid valves or something on the top to tap the air pressure of. Once the charge as been exhausted, the chamber is simply allowed to fill with atmospheric air and the water is gravitationally left to run out again. Once the tank is full of air, it is sealed again and re pressurized. The tank could also feed an accumulator to supply seamlessly
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Any help on this?
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Doppler,
I don't think we are quite sure what your question is - at least I know I'm a bit confused.
Is your question something like:
Which is more energy efficient for mining; hydraulic or pneumantic drilling equipment?
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Thanks Geezer, not really for the mining side of things. What I am trying to establish is which system actually requires more energy to perform the same function. I know that running a water pump consumes energy...and a fair amount at that but is quite efficient thanks to the properties of water. A compressor also requires energy to run but would it use more or less than a water pump to achieve the goal pressure? I am asking because I am lookng at a new control system and could use some outside inputs on this as far as efficiency is concerned. Is this better or still murky?
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OK - That helps. Is the output power of the system continuous or intermittent, and if it is intermittent, what sort of duty cycle do you expect?
Also, is the output power constant or continuously variable, and what do you expect the maximum power output to be?
Also, what's the primary energy source? Is it coming from an electric power grid, or is it being produced from an internal combustion engine locally?
There may be more questions! [;D]
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As to the use you contemplate, hydraulic uses less power, but is more complex piping wise, and does not tolerate leaks well. It can generate very high forces in a small space, and is perfect for precision control, although the control systems are slightly more expensive. If you are just worried about power, the hydraulic only uses power when moving, as it will hold position when control valves are closed, and the pump will shut off when it has provided the reservoir with the neded pressure. As a plus you get the ability to have multiple types of operating fluids, you are not restricted to mineral oils, but can use many fluids with the right choice of seals
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Thanks Geezer and SeanB, this is really helpful. I am just getting the finishing touches on my vertical column water hammer preemptive device but it sparked a few other ideas which can be employed on pneumatic and water service systems but have a single control system which operates both units simultaneously. I want to ensure that I use the most energy and cost efficient source of supply so that the end users do not waste unnecessary money. Anyway, I know what to do now and thank you for your input.
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If you are dealing with water you could possibly power the device from the water pressure alone, just integrate a fine filter in the water supply to the unit to keep muck out, and possibly add extra pressure transducers/switches to detect low pressure.
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If it's a hammer type device you're talking about then air is probably a better bet as you can use an 'open' system and dump the air after use and don't need the power levels of hydraulic systems. Having less mass too, air is easier to accelerate than a fluid system, so you'd be able to run the hammer at a higher rate, for a given power source, than with hydraulics.
It's still really not very clear exactly what you're trying to do though.
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Thanks all. Not to worry. I have everything I need now and will let you know how it works out and exactly what I am trying to do once I have tested it suitably but water driven is the way I will go because it is readily available and has less waste.