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[Maximum Spin]

A scroll compressor is a pretty effective compressor design that is simple enough to home-build to reasonable tolerances.

Water is an excellent refrigerant if you're working within its liquid range. You can also push that a bit with the right solutes; seawater is decent (if you are using materials with which it's compatible).

[wierd]

Water has a very high specific heat, which makes it less desirable than some other refrigerants.

[Maximum Spin]

Water has a very high specific heat, which makes it less desirable than some other refrigerants.

Yes, but, you know, it works, and it's a lot easier. It's freely available from the sky.

[wierd]

It will be significantly less efficient as a heat pump, especially in a cold locale, than some other refrigerant.

(You want a refrigerant that rapidly expands and contracts, based on just a teensy bit of thermal energy being provided to it, so that you can soak in and subsequently squeeze out, thermal energy using a compressor.  The high specific heat of water means it takes much more total thermal energy being applied to it for it to raise in temperature-- and conversely, more energy must be pulled out of it to make it lower in temperature.  Something like difluoroethane boils on contact with human skin, but can be liquid at relatively low pressures.  It would make a significantly better refrigerant for use in a cold-weather heatpump.)

[bloop_bleep]

Water sounds like a good idea, if you think it can work. That's one less thing on the design budget.

I'm thinking about this a bit more quantitatively now, particularly how powerful the compressor should be. Am I right in thinking the temperature reduction factor is roughly equivalent to the pressure increase factor of the compressor? Ideal gas law says so, given same volume (which can be managed by having the high pressure part of the circuit have the same volume as the low pressure part of the circuit). But maybe we're dealing with water, and water as a fluid is either liquid, which may invalidate the use of the ideal gas law, or water vapor, which I've heard doesn't follow the ideal gas law very well because of all the intermolecular interactions.

ninja edit: Hmm, I think maybe I've seen that mentioned somewhere else before, as a solvent. However I can't seem to find it on Amazon or Sigma Aldrich.

[Maximum Spin]

It will be significantly less efficient as a heat pump, especially in a cold locale, than some other refrigerant.

(You want a refrigerant that rapidly expands and contracts, based on just a teensy bit of thermal energy being provided to it, so that you can soak in and subsequently squeeze out, thermal energy using a compressor.  The high specific heat of water means it takes much more total thermal energy being applied to it for it to raise in temperature-- and conversely, more energy must be pulled out of it to make it lower in temperature.  Something like difluoroethane boils on contact with human skin, but can be liquid at relatively low pressures.  It would make a significantly better refrigerant for use in a cold-weather heatpump.)

I feel like you're forgetting that bloop_bleep is a 15-year-old building this in (what I imagine to be) a basement.

ETA:

Water sounds like a good idea, if you think it can work. That's one less thing on the design budget.

I'm thinking about this a bit more quantitatively now, particularly how powerful the compressor should be. Am I right in thinking the temperature reduction factor is roughly equivalent to the pressure increase factor of the compressor? Ideal gas law says so, given same volume (which can be managed by having the high pressure part of the circuit have the same volume as the low pressure part of the circuit). But maybe we're dealing with water, and water as a fluid is either liquid, which may invalidate the use of the ideal gas law, or water vapor, which I've heard doesn't follow the ideal gas law very well because of all the intermolecular interactions.

Typically, a heat pump cycle wants to incorporate a phase change if possible, so the latent heat of the phase change has to be factored in anyway.

ninja edit: Hmm, I think maybe I've seen that mentioned somewhere else before, as a solvent. However I can't seem to find it on Amazon or Sigma Aldrich.

Yeah, difluoroethane is "extremely toxic" and is also a drug of abuse. I recommend against it.

[wierd]

Not TOO toxic, it's the stuff inside compressed air duster.

It's major fault is that despite what the MSDS says, it totally *IS* flammable.

https://en.wikipedia.org/wiki/Gas_duster

Note that the most common substances in them are:

Difluoroethane
Trifluoroethane
Tetrafluoroethane (also known as R134A )

[Maximum Spin]

Not TOO toxic, it's the stuff inside compressed air duster.

It will make your heart incredibly unhappy, which counts in my book.

Does the difluoroethane used in the cans also soak into your skin and kill you, or is that the other one? I'm honestly not sure which isomer is which.

[wierd]

Well, in defense of water as a refrigerant, it's what is inside those copper heatpipes used inside laptops.

However, that does not use a compressor, so much as it relies on capillary action, and something more like what's at work inside a sterling engine's working fluid type setup.


RE: Compressed air duster

I have gotten the stuff on my hands many times, and all it does it make them get risk of frostbite.  They add a bittering agent to it to discourage use as an inhalant. If it was that toxic, inhalant abuse would be more than "Stupid kids asphixiating", and more "Stupid kids killing themselves in seconds."

[bloop_bleep]

"precipitates fatal cardiac arrhythmia" -- oof. Also, the solvent I was thinking of earlier probably was dichloromethane, which when I was looking at its Wikipedia article I started considering as a refrigerant too, since it's volatile and the article mentioned its use in a heat engine, until I got to the part about it being an inhalation risk and a possible carcinogen... maybe a pass from me too. Also, could you explain why a phase change is desired? Is it because of the huge thermal energy absorption/radiation that's required for a phase change, compared to temperature changes below or above the boiling point? But how is that different from doing an equal thermal energy movement without a phase change? If I'm using water, I'm not sure if I'm going to be able to boil it with my palm-sized 12V DC motor...

They add a bittering agent to it to discourage use as an inhalant.

Well, that might be useful if the thought were to strike me of huffing the bottle wholesale, but as you can imagine I'm a bit more worried about an accidental exposure. 

[wierd]

You want a phase change, because it means you can use the heat of enthalpy.

https://en.wikipedia.org/wiki/Enthalpy


It takes energy to cause a liquid to become a vapor-- this is the enthalpy.  If you compress this vapor, it will force the heat to leave so that it can condense again.  Ideally, you want a substance that is liquid at room temperature and pressure, but which evolves into a gas rapidly as the temperature rises.  This allows you to exploit the phase change.


To use WATER as the refrigerant-- instead of COMPRESSING the water, you should consider using a vacuum pump instead.  Pressure is greatly reduced, causing the water to seek to become a vapor at the ambient temperature.  This will cause it to "become very cold", as the thermal energy needed to become vapor is rapidly released due to the sudden drop in pressure.  On the "hot" side, you compress the vapor, forcing condensation.  This will make it expel the heat it rapidly pulled from solution on the low pressure side.

See also, "Vacuum freezing"

[Maximum Spin]

Also, could you explain why a phase change is desired? Is it because of the huge thermal energy absorption/radiation that's required for a phase change, compared to temperature changes below or above the boiling point?

Yeah, pretty much.

But how is that different from doing an equal thermal energy movement without a phase change?

It's not, except insofar as doing it with a phase change is a lot easier.

[wierd]

A vacuum pump variation, using water with an antifreeze agent (like isopropyl alcohol), would allow you to still use phase changes.

You would want to prevent the refrigerant from freezing in the line, but you DO want it to boil in there freely.  The gas pulled from the vacuum line would be getting pushed into the compressor chamber, to force recondensation.

It would then be effective down to the freezing temperature of the refrigerant. (which, if you add the antifreeze to, could be very cold indeed.)

[bloop_bleep]

Ok, so because you get more thermal energy to a degree Kelvin across a phase change than elsewhere, the fluid has to go through a smaller temperature cycle in terms of degrees than if you were doing it elsewhere. I assume this smaller temperature change makes it easier on the compressor, since it has to change the nRT value less, and so has to change the PV value less, so it has to increase the pressure less. I hope this is the right way of thinking about it. It's nice to know it's not strictly necessary, I just might need more powerful components if I don't use a phase change.

You want a phase change, because it means you can use the heat of enthalpy.

https://en.wikipedia.org/wiki/Enthalpy


It takes energy to cause a liquid to become a vapor-- this is the enthalpy.

Interesting, I usually only think of enthalpy in the context of chemical reactions, but I guess it can be applied to other state changes too.

If you compress this vapor, it will force the heat to leave so that it can condense again.  Ideally, you want a substance that is liquid at room temperature and pressure, but which evolves into a gas rapidly as the temperature rises.  This allows you to exploit the phase change.

But doesn't the compression increase the heat too? Or do you mean that it makes it easier for the heat to leave in the condenser or radiator?

ninja edit:

A vacuum pump variation, using water with an antifreeze agent (like isopropyl alcohol), would allow you to still use phase changes.

You would want to prevent the refrigerant from freezing in the line, but you DO want it to boil in there freely.  The gas pulled from the vacuum line would be getting pushed into the compressor chamber, to force recondensation.

It would then be effective down to the freezing temperature of the refrigerant. (which, if you add the antifreeze to, could be very cold indeed.)

Ah, so you mean instead of making the pressure cycle atmospheric pressure-high pressure, make the pressure cycle low pressure-atmospheric pressure, which pushes the cycle lower down the state diagram to where the gaseous state of water is more accessible. Interesting idea.

Thanks a lot for all your help so far, guys!

[Maximum Spin]

Compression is (in principle) adiabatic, so it doesn't increase the heat content, just the temperature; the higher-temperature refrigerant now loses heat to its surroundings because those surroundings are at a lower temperature and, when decompressed adiabatically again, drops to a lower temperature than it started at because it lost heat.