Thermodynamic Question What am I Missing

[kilowatt3]

It's called the Joule-Thomson effect, an expanding gas cools according to ideal gas laws. It's handy for some applications like cooling thermal optic sensor chips in smart bombs and Stinger missiles, because it doesn't need any machinery, just a vessel of compressed gas expanding through an orifice. It is not the process that makes air conditioners and heat pumps work as asserted in Post #4. Those work on a reverse Carnot thermodynamic cycle, a closed continuous cycle, as opposed to the J-T effect where the working fluid is lost.

You're half right.

The effect that the OP noted is because of the Joule-Thomson effect; the expanding air from the tank cools off enough to freeze the entrained moisture as it passes through the drain valve.

This IS the "process that makes air conditioners and heat pumps work as asserted in Post #4".

The Carnot cycle is a theoretical model that describes the general principle, but it's actually not possible to build a Carnot engine (or reverse-Carnot refrigerator).

A modern mechanical refrigeration system has a JT valve in it, whether they call it that or not, and that's where the actual cooling takes place.

 

[crstrode]

Some info is taught in science Class, others in Home Economics.

Never the twain shall meet.

 

[DWalt]

A simple experiment. Put some rubbing alcohol on your skin. It feels cool because as it evaporates, it absorbs heat from your skin. That is the latent heat of vaporization.

 

[robvious]

in deference to Rusty...
"They said there wouldn't be any math... "
I do find this discussion fascinating... pokes at my geek side

 

[Imissedagain]

Roll a quart of 3.2 beer in lotsa layers of wet newspaper and leave in the sun. Turn often and as the water evaporates, and just before the inner layers dry, the beer will be cooled.

Sadly we didn't have room service at Lowry during tech school days in '66.
Broken promises by the Air Force recruiter.

 

[ggibson511960]

Expansion Valve

You're half right.

This IS the "process that makes air conditioners and heat pumps work as asserted in Post #4".

A modern mechanical refrigeration system has a JT valve in it, whether they call it that or not, and that's where the actual cooling takes place.

The expansion valve in a vapor-compression refrigeration cycle throttles pure liquid refrigerant to a lower pressure. Cooling takes place in the piping and coils downstream of the expansion valve where liquid flashes first to a two-phase mixture, then completely to saturated vapor, and then superheated vapor. There is no J-T effect anywhere in a vapor-compression refrigeration system cycle because there is no vapor-vapor expansion process.

There is one relatively uncommon air conditioning system that does use a J-T valve. Jet aircraft use an air-cycle refrigeration systems to first cool hot high pressure compressor bleed air with airstream flow, then expand in a real J-T expansion valve with no change of state to generate cool air for the cabin. This takes a lot of energy, but jets have it.

Thanks for prompting my long latent thermo brain cells. Thermo was never my strong suit, but I was forced to practice it till it hurt.

 

[pharmer]

......Thanks for prompting my long latent thermo brain cells. Thermo was never my strong suit, but I was forced to practice it till it hurt.

This conversation was started by accident. I learned to bleed the tank on the driveway instead of the enclosed space in the garage. Ears would ring. I never saw the "ice spire" on the grey painted floor of the garage. I too am grateful that at 70 I still don't "know it all." Joe

 

[fordson]

As a results of reading this thread, everyone gets 2 Credit Hours in Thermodynamics/Heat Transfer………

 

[rufgr]

I believe that most air cycle machines actually expand the air in a turbine rather than a j-t valve

 

[ggibson511960]

I believe that most air cycle machines actually expand the air in a turbine rather than a j-t valve

I'm sure you're right. A/C on jetliners pulls a lot of power and it makes sense to recover some of it in an expansion turbine instead of throwing away. A pilot friend of mine told me a tale of flying MATS flights from Vietnam in the war, full of returning GI's. They were forecast to have unfavorable winds on the Saigon to San Francisco leg, necessitating a fuel stop and 2 hour delay in Anchorage. The pilot gave the war weary GI's the option of not running the A/C and extending their flight into a non-stop. They did it, sweating all the way to get home in a hurry. Funny story, but it illustrates how much power the air cycle system pulled out of the engines. If you look closely at wing roots of vintage 707's you can see the air intakes for A/C cooling. An intake hole that large on a 707 at Mach 0.90 would pull a lot of power and shorten range.