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A centrifugal spin on a 500-year-old air compressor design

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Re: A centrifugal spin on a 500-year-old air compressor design

#21

Post by jandrese »

londons_explore wrote: post #18

So by this same reasoning, adding a little water spray to the input of my air compressor, and a water seperator on the output, would reduce energy consumption since the water would absorb heat during the compression stroke making the compression closer to isothermal rather than adiabatic? Do any commercial products do this?

You would have to demineralize the water first or you are going to get deposits building up on the warm parts of the compressor (most of it) where the water evaporates on contact.

It would seem to be better to have a closed loop water cooling system around the compressor head, but this is a very rare feature. So many home compressors treat cooling like "LOL 5% duty cycle for you", it's pretty annoying. Most of the time the cooling fans aren't even running unless the compressor is squeezing air, so the whole thing just soaks in its own heat once the tank is full.

Re: A centrifugal spin on a 500-year-old air compressor design

#22

Post by bluGill »

hinkley wrote: post #5
teuobk wrote: post #2

Interesting. A few questions that come to mind: 1. What is the dew point of the compressed air? Many applications using compressed air require very dry air so as to avoid corrosion issues (e.g., air tools) as well as problems related to heating of water vapor (e.g., tires). For small applications, the dew point is reduced using simple traps, silica gel beads, or molecular sieves, but for even-drier air or larger appl…

I didn't realize that air compression hardware has to deal with water removal but apparently it does. One article I found is suggesting that air at 7 bar can hold about 1/3 as much water. What confused me is that the a wing stall (low pressure above a wing) causes cloud formation, due to vapor condensing out of the air. But 7 bar is ~7 times as much air per cubic centimeter, so that's 14% as much space containing 33%…

All I know is any compressed air tank you have needs water drained from it regularly. Air pipes count as a tank (just a small diameter) so you have to consider where the water will go.

Re: A centrifugal spin on a 500-year-old air compressor design

#24

Post by FiatLuxDave »

jschwartzi wrote: post #15
FiatLuxDave wrote: post #14

I suspect that Carnot is aiming to compete with liquid ring compressors https://en.wikipedia.org/wiki/Liquid-ring_pump , which have much of the same advantages and disadvantages (near isothermal compression, water saturation). I'm guessing the article didn't mention this because every home compressor user wants a less noisy one and they get better propagation of the article around the internet that way. I considered…

Could you do it with mineral or tool oil?

In theory yes, but kerosene has a lower viscosity while still having a low vapor pressure. The low viscosity can be important when the liquid is moving a lot, to prevent losses and heating.

Re: A centrifugal spin on a 500-year-old air compressor design

#26

Post by londons_explore »

jandrese wrote: post #21
londons_explore wrote: post #18

So by this same reasoning, adding a little water spray to the input of my air compressor, and a water seperator on the output, would reduce energy consumption since the water would absorb heat during the compression stroke making the compression closer to isothermal rather than adiabatic? Do any commercial products do this?

You would have to demineralize the water first or you are going to get deposits building up on the warm parts of the compressor (most of it) where the water evaporates on contact. It would seem to be better to have a closed loop water cooling system around the compressor head, but this is a very rare feature. So many home compressors treat cooling like "LOL 5% duty cycle for you", it's pretty annoying. Most of the ti…

I'm more thinking cooling should theoretically ~halve the energy required to compress air...

For some industrial applications, electricity used to compress air is substantial, so there might be big cost savings to be had.

Even if you are pumping up bicycle tyres by hand there are benefits to cooling - by keeping the air cool, the same human muscle power can pump up twice as many tyres.

Re: A centrifugal spin on a 500-year-old air compressor design

#27

Post by dusted »

hinkley wrote: post #5
teuobk wrote: post #2

Interesting. A few questions that come to mind: 1. What is the dew point of the compressed air? Many applications using compressed air require very dry air so as to avoid corrosion issues (e.g., air tools) as well as problems related to heating of water vapor (e.g., tires). For small applications, the dew point is reduced using simple traps, silica gel beads, or molecular sieves, but for even-drier air or larger appl…

I didn't realize that air compression hardware has to deal with water removal but apparently it does. One article I found is suggesting that air at 7 bar can hold about 1/3 as much water. What confused me is that the a wing stall (low pressure above a wing) causes cloud formation, due to vapor condensing out of the air. But 7 bar is ~7 times as much air per cubic centimeter, so that's 14% as much space containing 33%…

When you compress air, the moisture falls out, the amount that falls out depends on pressure and temperature. pretty much all normal compressors simply have a, I don't know the right english word, but.. a valve at the bottom and once in a while (like, once a year or less, depending how much you use it) you take the pressure mostly off the compressor and open this, then the water is blown out.

Re: A centrifugal spin on a 500-year-old air compressor design

#28

Post by londons_explore »

Air compressors need not be noisy.

Your fridge at home likely has a cylinder air compressor in it, pumping gas up to 30 bar (400 psi) and it only makes a quiet humming.

The trick is that air compressor is attached to a big vibration damping weight, mounted on big springs, which are mounted into an oil filled heavy steel box, which is itself mounted onto rubber feet.

Re: A centrifugal spin on a 500-year-old air compressor design

#29

Post by FiatLuxDave »

londons_explore wrote: post #17
FiatLuxDave wrote: post #14

I suspect that Carnot is aiming to compete with liquid ring compressors https://en.wikipedia.org/wiki/Liquid-ring_pump , which have much of the same advantages and disadvantages (near isothermal compression, water saturation). I'm guessing the article didn't mention this because every home compressor user wants a less noisy one and they get better propagation of the article around the internet that way. I considered…

"isothermal" is the reason this system is more efficient right? But is it the theoretically most efficient way to compress air? Is there a (theoretical) way to compress the air adiabatic'ly (letting it heat up as you squeeze it), and do better overall?

Yes ... and kinda yes.

So, if you just want to reach a certain pressure regardless of temperature, you will put in less energy with adiabatic compression, because all the energy you put into the gas stays there. However, in many gas compression applications, the gas is not going to stay hot, but will cool to something closer to the environmental temperature before being used. For these cases, then the hot gas is going to lose some of that energy to the environment. If you compress it isothermally instead of adiabatically, then it still loses heat energy to the environment, but it loses it as it is being compressed rather than after the compression is done. This requires less work.

So, in short, if you want hot compressed gas (like a diesel engine) go adiabatic, but if you want room-temperature compressed gas (like compressed air which sits in a container), go isothermal.

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