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Old 07-05-2012, 02:11 PM   #1
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Default Why do air conditioners use expansion valves?

I understand the answer to the question - the compressor takes the low pressure/low temperature fluid and compresses it to a high pressure/high temperature fluid, at which point the fluid is cooled by an external ambient air source. The high pressure fluid is then forced against an expansion valve which it forces open to escape into the low-pressure area beyond the valve. This expansion is what causes the fluid to cool to temperatures below ambient before it goes through a heat exchanger to cool internal ambient air.

But an expansion valve seems like a huge waste of potential energy to me. What am I missing? Why isn't the high pressure fluid moved through a turbine to recapture a significant amount of the energy required to compress the fluid in the first place?

If the high-side pressure were roughly 3x the low side pressure, the turbine would flow a volume equal to roughly 1/3 that of the compressor. The expansion of the fluid as it passed through the turbine would then be able to do a significant amount of the work required by the compressor. If there's a problem with controlling the pressure differential, you simply set the turbine to flow about 95% of the required fluid and let the other 5% pass through a waste (expansion) valve. You then run an insulated low-pressure pipe from the compressor to the cold side of the system.

It seems to me that someone has obviously thought of this before - and maybe this system is actually in widespread use and I just don't know about it - am I simply imagining efficiency gains that aren't real? Is it too complex?

I just feel like the "expansion valve" should be a long obsoleted technology in modern air conditioners, but since that isn't the case, I've obviously got something wrong. What principals of fluid dynamics am I misunderstanding?
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Old 07-05-2012, 02:31 PM   #2
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The reason expansion valves exist is to serve as a point of periodic leakage and failure. This will cause you to not only disassemble your entire dash to replace a $5 part, but also to replace the receiver/drier, refrigerant and miscellaneous O-rings, thereby ensuring that the rest of your air conditioning system is maintained in tip top condition.
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Old 07-05-2012, 02:58 PM   #3
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Quote:
Originally Posted by fooger03 View Post
I understand the answer to the question - the compressor takes the low pressure/low temperature fluid and compresses it to a high pressure/high temperature fluid, at which point the fluid is cooled by an external ambient air source. The high pressure fluid is then forced against an expansion valve which it forces open to escape into the low-pressure area beyond the valve. This expansion is what causes the fluid to cool to temperatures below ambient before it goes through a heat exchanger to cool internal ambient air.

But an expansion valve seems like a huge waste of potential energy to me. What am I missing? Why isn't the high pressure fluid moved through a turbine to recapture a significant amount of the energy required to compress the fluid in the first place?

If the high-side pressure were roughly 3x the low side pressure, the turbine would flow a volume equal to roughly 1/3 that of the compressor. The expansion of the fluid as it passed through the turbine would then be able to do a significant amount of the work required by the compressor. If there's a problem with controlling the pressure differential, you simply set the turbine to flow about 95% of the required fluid and let the other 5% pass through a waste (expansion) valve. You then run an insulated low-pressure pipe from the compressor to the cold side of the system.

It seems to me that someone has obviously thought of this before - and maybe this system is actually in widespread use and I just don't know about it - am I simply imagining efficiency gains that aren't real? Is it too complex?

I just feel like the "expansion valve" should be a long obsoleted technology in modern air conditioners, but since that isn't the case, I've obviously got something wrong. What principals of fluid dynamics am I misunderstanding?
Like Scott said. Cheap replacemeable part. Imagine if your A/C had a turbo in it that went bad. how much is that to replace mr mechanic? oh. no more a/c!

Turboexpander - Wikipedia, the free encyclopedia
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Old 07-05-2012, 04:39 PM   #4
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Deeeerp. You can replace the expansion valve by only removing the glovebox. I've done it. At least in the Miata. Other cars are not so fortunate.
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Old 07-06-2012, 11:26 AM   #5
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Complexity. A turbine would add a lot of expense, and the AC is relatively cheap to run without it, so there is no point.
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Old 07-06-2012, 11:28 AM   #6
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Most manufacturers run orifice tubes, even simpler than TXVs.
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Old 07-06-2012, 12:01 PM   #7
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Quote:
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Most manufacturers run orifice tubes, even simpler than TXVs.
Pish, even simpler than that, a lot of cheap refrigerators just use a crimped section of line. Like literally some chinese kid takes a pair of pliers and crushes the line.

Remember a valve like this is a "constant" enthalpy element and the constant enthalpy is what is required for the system to work. In order for a turbine to extract energy from the refrigerant it would need to lower the enthalpy of the refrigerant and then the ac wouldnt work.
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Old 07-06-2012, 12:07 PM   #8
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Auto manufacturers. Lol.
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Old 07-06-2012, 12:08 PM   #9
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What is the goal of recovering said energy?
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Old 07-06-2012, 12:46 PM   #10
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wiki link from y8s says 6-15% reduction of power consumption vs. an expansion valve.
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Old 07-06-2012, 01:16 PM   #11
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damn, i figured the reduction in power consumption would be more significant than that - I was thinking in the 40-60% realm. The goal would be less fuel use or, in the case of a home refrigeration system, less electric use.
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Old 07-06-2012, 01:18 PM   #12
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So you're looking for more efficiency, not waste recovery? I aas trying to think of another application than just radiating the waste heat to atmosphere.

A big thing with cryo services right now is using cooling waters, etc. to help heat or vaporize liquids to gases. I just can't think of an automotive equivilant for waste heat recovery.

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Old 07-06-2012, 01:27 PM   #13
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Quote:
Originally Posted by cymx5 View Post
So you're looking for more efficiency, not waste recovery? I aas trying to think of another application than just radiating the waste heat to atmosphere.

A big thing with cryo services right now is using cooling waters, etc. to help heat or vaporize liquids to gases. I just can't think of an automotive equivilant for waste heat recovery.
If there were any good waste heat recovery uses in a car, there would be much more to gain from the cooling system. I think BMW had some kind of design for that a while ago, but I never heard anything.
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Old 07-06-2012, 02:51 PM   #14
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Correct - I want to use the pressure differential from the hotside to the coldside to do work via positive displacement pumps to assist in compressing the fluid from the coldside to the hotside.

If I was trying to find a way to use waste heat, we wouldn't use an a/c compressor at all, we would simply put this magical system in the passenger compartment and swipe the energy from the air in the passenger compartment - unfortunately, we haven't yet developed power generation technology that is so efficient that it cools air, instead, all of our thermal energy based power is generated by temperature differentials of significance, the temperature differentials experienced in an automobile aren't great enough to produce any practically measurable energy.
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Old 07-06-2012, 03:01 PM   #15
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Quote:
Originally Posted by fooger03 View Post
Correct - I want to use the pressure differential from the hotside to the coldside to do work via positive displacement pumps to assist in compressing the fluid from the coldside to the hotside.

If I was trying to find a way to use waste heat, we wouldn't use an a/c compressor at all, we would simply put this magical system in the passenger compartment and swipe the energy from the air in the passenger compartment - unfortunately, we haven't yet developed power generation technology that is so efficient that it cools air, instead, all of our thermal energy based power is generated by temperature differentials of significance, the temperature differentials experienced in an automobile aren't great enough to produce any practically measurable energy.
Actually there is a way to use heat and a tiny liquid pump to do AC, its how the massive office buildings do AC, and the way that industrial freezers work. Look up the lithium bromide and water cooling system.

But again, you cant generate power in place of the expansion valve. It requires the enthalpy to remain constant and when a turbine makes power it lowers the enthalpy.
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Old 07-06-2012, 03:20 PM   #16
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You may have to forgive me, as I might not yet fully comprehend "enthalpy"

How does expansion via valve differentiate from expansion via pump? If the compressed air is forced to do work in order to expand vs. just expanding, that would make me believe that the end result of the expansion is cooler - because it takes energy to do work.
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Old 07-06-2012, 03:26 PM   #17
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enthalpy is basically the energy of a thermodynamic system able to do work. that's a temperature difference, pressure, etc.
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Old 07-06-2012, 03:26 PM   #18
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Enthalpy is like the fluids ability to do thermodynamic work. It is made of up not only the pressure and temperature but also the quality (the % of the fluid that is a liquid in the mass of fluid, aka a 0.5 quality would mean that you had 50% gas and 50% liquid). The quality is the real big kicker in making an ac system work. Any work that you would take out of the working fluid through the turbine would have to be added, in addition to the normal amount of work, by the compressor.
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Old 07-06-2012, 03:57 PM   #19
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Any work that you would take out of the working fluid through the turbine would have to be added, in addition to the normal amount of work, by the compressor.
I still think this is wrong, but I'm not smart enough to prove it.
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Old 07-06-2012, 04:20 PM   #20
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So what's the point of a/c? I just bought a hot Asian bitch to ride in the passenger seat with a mouth full of dry ice, and she blows cool air on me. Once I get to my destination, she gets my peepee to its destination, with her sore and burned mouth.
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