HEAT OF VAPORIZATION

Geo-TH,In · Jan 28, 2016

Today's Physics lesson:
HEAT OF VAPORIZATION:
Energy needed to turn a liquid into a gas. Doesn't matter if it freon, gasoline, LP, water, any
liquid requires heat energy to become a gas, vapor. Pic is of my Farmall C when it was idling this
morning. The white stuff is frost.

John T · Jan 28, 2016

Yep that's same as when you step out of the shower you feel cold because the water on your skin is turning to vapor (evaporating) and its extracting heat off your body so you feel cold (less heat) heat of vaporization. Thermo was my weakest subject at Purdue I do remember heat lost = heat gained. Similar to the gas equation Pv = nRT so when you release gas out of your pressurized compressor tank pressure is reduced so temp gets reduced and it gets cold and icy at the valve.

John T

glennster · Jan 28, 2016

and if you ever got to ride in a hyperbaric recompression chamber, its hot as heck when they press you down, then it gets REALLY cold when they bring yoou back up!!!

David G · Jan 28, 2016

You can also prevent it from changing state by denying it the energy, thus super heated steam and sub cooled liquid.

Geo-TH,In · Jan 28, 2016

E and M, electricity and magnetism was a difficult. Left hand and right hand rule. I enjoyed thermo dynamics.

Geo-TH,In · Jan 28, 2016

Why would you want to prevent gasoline from becoming a vapor?

David G · Jan 28, 2016

You keep propane a liquid by denying it the energy to vaporize.

JMOR · Jan 28, 2016

(quoted from post at 19:19:48 01/28/16) Why would you want to prevent gasoline from becoming a vapor?

o there will still be some in the tank/can when you need it? :wink:

Bill(Wis) · Jan 28, 2016

The venturi section of the carburetor is designed to cause rapid expansion of the fuel air mixture and "expansion is a cooling process". This can get out of hand under certain atmospheric and other conditions and cause carburetor freeze ups. Thus the introduction of carburetor heat mechanisms and fuel additives.

D beatty · Jan 28, 2016

My Farmall B does that in cold weather and it gets worse than that. I have made a jacket around muffler and a tube to top of air intake stack and pull warm air from around muffler. Carb and air cleaner stay warm no matter how cold it is..

Geo-TH,In · Jan 28, 2016

I made a way to recover the heat from the exhaust manifold and send warm air to the intake of my Jubilee. Got the idea from my old 71 Plymouth duster. The farmall seems to work, except for this morning. It was around 30 when I let it idle while checking the air pressure in tires. Engine was running a little rough. Cleared up when I ran the engine faster. It's a little odd, both tractors use the marvel carb, I think.

Richard G. · Jan 28, 2016

I have seen them frost up like that in the summer time.
Normal. Richard in NW SC

fixerupper · Jan 28, 2016

The SC Case has frost 3" above the carb on cold mornings. It doesn't smooth out and purr till the frost is gone.

David G · Jan 28, 2016

My MH44 was real bad about that, no frost now with the fuel injection.

K Effective · Jan 28, 2016

At work, we run 125 pig steam, about 350F, through a venturi- you can check performance of it by feeling the cool spot, IF you know where to touch!

David G · Jan 28, 2016

Are those eductors for vacuum generation?

Rick Kr · Jan 28, 2016

Thanks Jon.
Just the mention of the word Thermodynamics gives me cold sweats.
I hope I never see a partial differential in my life. Ha Ha.

Bob · Jan 28, 2016

Well, HECK, it can be a GOOD thing, as well... ultimate high efficiency furnaces get their efficiency by condensing the water out of flue gasses and recovering the latent heat of vaporization.

BTW, what the heck are you doing running a tractor in such cold temps when you could just sit down with a cup of "coffee" and surf "YT"?

D beatty · Jan 28, 2016

The carb in picture on C is the same one I have on B and it is a Zenith carb.

Steve in VA · Jan 29, 2016

Yes, it is 3:45 in the morning but I've been pondering this issue and I believe that attributing this frosting to LHoV is incorrect. Please forgive me if I drone on. LHoV is the energy required for a phase change and we frequently picture this as heating water on a stove or consumed by a freezer to make ice cubes. But, LHoV is not a fixed and constant value. Consider attempting to boil water at a high elevation or a liquid boiling in the vacuum of space. As ambient pressure changes, the partial pressure changes and the energy required to complete a phase change varies.

Our internal combustion engines are essentially air pumps with inclusion of a small quantity f fuel to achieve ignition. The fuel enters the carburetor with a given energy in its liquid phase. Inside the carburetor pressure drop is achieved through the venturi. At this reduced pressure the fuel has sufficient energy to achieve phase change and become a vapor. If we could achieve a sufficient drop in partial pressure then the fuel would draw heat from its environment but I believe this is not what we see.

Consider a CO2 fire extinguisher. The CO2 is stored as a gas under fairly high pressure. When we pull the trigger, the gas is released, pressure drops, and the nozzle will commonly frost. Since there is no phase change, LHoV isn't the driver. Of course there are many factors incuding ambient temperature and humidity levels.

But maybe I'm wrong.

Ultradog MN · Jan 29, 2016

Some smart people here - thanks Steve.
The simplest solution of course, would be to buy a diesel.

Steve in VA · Jan 29, 2016

No doubt. I confess that I have both gas and diesel. I tend to develop attachments to older equipment and just can't bring myself to get rid of something just because its not the latest. That's not unusual for the folks here.

Geo-TH,In · Jan 29, 2016

Read up on the ideal gas lows. PV=nRT will explain the CO2. Look inside a diesel, compress air real fast and the temp goes up. Expand air the opposite happend, cooling. Has nothing to do with HoV

Geo-TH,In · Jan 29, 2016

Bob, 30 degrees isn't cold. I can only drink so much coffee and surf YT. I have a bad case of cabin fever. No snow to push, need seat time on tractor. After last flood in gravel pit, I've been working on making new flower beds above the last flood for the boss to have a place to plant her flowers. Have about 15 acres to play in. I been using my little terramite, the farmall attached to a 10K dump trailer to move dirt and remove brush. I like playing in the dirt which makes me a dirty old man.

FBH44 · Jan 29, 2016

I think Bernoulli's Principle applies. It strictly applies to incompressible flow and gets complicated when appled to compressible flow [air flowing thru a carb.] Basically, the rapidly accelerating air, containing some constant amount of energy, zips thru the venturi, where it suddenly increases its velocity and drops its static pressure. That sudden drop in static pressure pulls gasoline out of the bowl, thru the jet, and into the accelerating air stream. There is a simultaneous drop in temperature, which depends, but it can be so cold outside and enough moisture outside, it condenses moisture into liquid water on the outside of the carb. If suffiencltly cold it can directly form ice. The vaporization of the liquid gasoline into gas gasoline will also drop temperature in the air stream. I think the mosture can also form inside the carb, downsteam of the venturi.
It would be ineresting to know exactly what happens inside the carb at every point, under all conditions. Betcha those old carb engineers did a lot of research.

Geo-TH,In · Jan 29, 2016

Yes, definitely diesel if you can get them started without plugging them in. As a kid, Dad's diesels sat all winter, NEVER STARTED. No thank you. My gasser always starts. This was rare seeking frost on carb.

jacksun65 · Jan 29, 2016

A coupla years ago I was at a tractor auction and there was an JD 60 with an ice on the carb it was leaking fuel and the wind blowing over it created the Ice. Oh it was in july.

K Effective · Jan 29, 2016

Are those eductors for vacuum generation?

More for sustaining vacuum- these take a suction from the steam space of post-turbine condensers to remove non-condensable gasses, kinda purifying the steam so to speak.

We do have another, much larger set of eductors used for initial vacuum establishment in those same condensers, for turbine start-up.

HEAT OF VAPORIZATION

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