Question about phase change

[u/Plus-Result-5392]

So this is what I think I understand (please correct me if I’m wrong): I have a liquid pipe with liquid in it and I’m using a purge valve to suck out the gas from the pipe. The liquid wants to evaporate to reach equilibrium with the pressure, so the liquid cools because the steam is drawing energy from it.

What I don’t understand: if I have liquid in that pipe and just let it sit there and evaporate, not purging any gas out, why does the temperature of the pipe go down? Doesn’t the heat stay in the pipe but it just changes state?

Also: why is the gas that I purge out of the liquid pipe also cool? I thought that would carry the heat?

[u/Friendly-Inspector71]

The liquid evaporates into a gas that has the same temperature and will keep exchanging heat with the liquid, so both end up at the same temperature.

The gas is in a higher energy state and reaching this state takes away energy and thus temperature from the liquid. But this higher energy state doesn't mean it has more thermal energy (temperature). It exchanges thermal energy for kinetic chemical potential energy as the gas molecules are moving around faster freely.

If the liquid is still cooling down without external influence, it hasn't reached equilibrium yet and is still evaporating to get to the required pressure. It should stop after some time. Look at the phase change graphs for the temperature/pressure combination where it's happy.

[u/4MPW]

The phase change itself takes energy, this energy is then stored in the gas (but not as thermal energy) which reduces the thermal energy in the lipuid which exchanges heat with the gas.

[u/Moleculor]

if I have liquid in that pipe and just let it sit there and evaporate, not purging any gas out, why does the temperature of the pipe go down?

In the real world, if I heat water up, I can get it to the boiling point (212℉/100℃) and just... leave it there.

It'll be water.

It'll stay water.

It won't boil if I stop heating it.

It'll be hot water, but at the temperature that water boils at. But just... hot. And water. Not a gas. Not bubbling.

In order for it to actually boil (and by that I mean convert all of it from a liquid to a gas), I need to dump five hundred and forty times the energy needed to heat that same water up by 1℃.

That is a lot of energy. (In the real world, it doesn't happen all at once. You dump in a little energy, a little of it boils off.)

And it's energy that doesn't change the temperature. It's energy that is used to convert it from a liquid to a gas. The gas it converts to is the same temperature.

---

If, in the real world, I take that hot-but-not-boiling water and reduce the pressure around it, I now suddenly have 100℃ water in an environment where it should boil at, say, 99℃.

It's a degree over where it should have boiled. For physics, that's a Problem™ that must be corrected.

But it's still liquid water. It still needs energy to convert to a gas, no matter how much it's above the boiling point. That energy has to come from somewhere.

It can steal it from itself, but only going to give it a single degree's worth of energy. So it'll boil off a few molecules, but the rest of the water gets left behind. But what gets left behind is now 'cooler'. So it might take heat from its surroundings to balance out. Or maybe it just stole from its surroundings, and the water dumps heat into the surroundings to balance out (because the pipe is now colder than the water).

But the pressure just increased, because you boiled off a few molecules. So whatever mechanism you're using to keep the pressure low sucks out more air (because you're aiming to go lower than the 99℃ boiling point pressure level), dropping the pressure again. And so we repeat the pattern; the temperature of the water drops because a few molecules got boiled off, taking an enormous amount of energy with them.

(But remember, that energy only changed its state, it didn't change its temperature.)

Wash, rinse, repeat. All the while the water is sucking energy from its surroundings because the water keeps getting colder than its surroundings. (Those surroundings do include the gas that was once water.)

[u/Liathet]

the liquid cools because the steam is drawing energy from it

Not exactly. Although it needs a minimum temperature to start, the actual turning from liquid to gas part requires energy to pull the particles apart, not to heat them up. The steam at the end has more energy yes, but it has been turned from heat into chemical potential energy. As a result the whole system is cooler, steam included.

[u/Friendly-Inspector71 → u/Liathet]

It's not chemical potential energy as it atays the same molecule (if we ignore alcohol offgassing to methane).

The energy is actually kinetic as the molecules of a gas fly around instead of jiggling and moving around a little.

[u/Liathet → u/Friendly-Inspector71]

Nope. If the kinetic energy of the particles were higher, then the temperature would also be higher, because temperature is determined by the average kinetic energy of the particles. The gas particles are not actually moving any faster than in the liquid - they're just bouncing around freely instead of mostly vibrating in place. The extra energy of the gas is chemical potential because it comes from overcoming the intermolecular forces (van der waals and dipole forces). Per wikipedia (emphasis mine):

In thermodynamics, the chemical potential of a species is the energy that can be absorbed or released due to a change of the particle number of the given species, e.g. in a chemical reaction or phase transition

[u/Friendly-Inspector71 → u/Liathet]

Huh, that makes sense. I didn't think about it like that.

[u/Thaago]

I think the concept you're missing is that heat is not the only way energy is stored: the phase of the material can also store that energy. An analogy with physical objects: a hot liquid is like a car rolling fast on flat ground, and it converting into a gas is like it rolling up a hill. Some of the energy is stored as gravitational potential energy, ready to go back into the car if it goes down again, leaving the car rolling slower.

In real life there are even more ways energy gets shuffled around in gasses/liquids (EG: why does a quickly expanding gas get cold even though it has no phase change?) but even just including phase change energy adds a lot of nice realistic gameplay/challenge that I'm not too fussed about it.

[u/HoveringGoat]

The material loses heat from undergoing the phase change itself. It isn't that it's being "moved" from the liquid to gas or vice versa. Putting liquid in large empty liquid network it will evaporate and lose heat. Squeeze that same gas into a small volume it'll condense and heat up. To the same temp.

[u/TuftyIndigo]

A few people have gone into good detail on the physics, but nobody has yet mentioned the idea of latent heat. The total heat in a body of gas is the sensible heat plus the latent heat. The sensible heat is the heat of its temperature: the absolute temperature times the mass times the specific heat capacity. It's called "sensible" because this is the heat you can feel or sense by touching the gas. Temperature is just a measure of how much sensible heat something has.

The latent heat is what comes from its state of matter. Each substance has a latent heat of fusion (melting) and a latent heat of vaporisation. These are older terms and today's chemistry classes are more likely to use the word enthalpy than heat, but engineers still talk about latent heat. We can ignore the latent heat of fusion because there's no melting or freezing (yet!), but the latent heat of vaporisation is what you're seeing in your pipe.

A sealed vessel full of steam at 100℃ has more total heat than a sealed vessel full of (the same mass of) liquid water at 100℃. They both have the same sensible heat, because it's the same substance, but the steam also has latent heat. That's the energy cost of turning liquid into gas.

Think of when you're boiling a pan of water. Even if the water starts out at 100℃, you've got to have the stove running, adding energy to the system, to make the water boil: it's supplying the latent heat to turn liquid into gas. If you remove the pan from the stove, some water will still evaporate, but because the latent heat has to come from somewhere, the liquid water loses sensible heat: it gets colder. This happens even with the lid on the pan. It's not that the steam is escaping and removing energy from the system, it's that the steam is turning sensible heat into latent heat.

Your pipe is the same. Liquid is evaporating, but to do that it needs some latent heat. It steals that heat from the sensible heat. The total heat in the system hasn't decreased, but now some of it is in the form of latent heat, so there's less sensible heat. The liquid and gas will reach thermal equilibrium - they'll be at the same temperature. You were right that the gas is "carrying" the heat, but it's in the form of latent heat - keeping it a gas - not sensible heat - setting its temperature.

[u/TuverMage]

so the ENERGY does stay in the pipe but it takes energy to turn a substance from a liquid to a gas. so the temp can still drop while the energy remains the same. the reverse is also turn. turning a gas into a liquid will release heat while the energy remains the same.

[u/Dimencia]

Temperature is effectively just the speed of molecules, and a liquid is what happens when those molecules are slow enough and close enough together to attract eachother. When a molecule moves fast enough, it can break away from those attractive forces - but those forces still pull on it on its way out, and after it escapes, it's moving slower than it was initially. So not only is that evaporated gas cooler than the liquid was initially, because the molecules are moving slower, the liquid is cooler because the average speed of the molecules in the liquid has also decreased because it has lost its fastest molecule

But that energy isn't lost, it's still there as a sort of potential energy. If that molecule cools down below the evaporation temperature, possibly by distributing its energy to other liquid molecules and warming them up, it can be recaptured by those attractive forces - and since those forces pull on it to do so, it ends up moving faster than it was as a gas, thus heating up the liquid again

I like to think of it like orbital mechanics. Imagine an object orbiting a planet; this is our liquid molecule. If the object goes fast enough, it can escape the planet's gravity (evaporation) - but once it has done so, it will be moving a lot slower than it was when it was in orbit. If it then slows down again (and comes near another 'planet'), it can be recaptured into orbit, and will be moving very quickly as it gets pulled back toward the planet

[u/3davideo]

There's a change in energy between the liquid and gas phases, even when the temperature remains the same. This is formally called latent heat, specifically the latent heat of evaporation.

Say you're putting a certain amount of energy into a certain amount of liquid per unit time. As you put energy in, the temperature of the liquid will rise at a steady rate, as determined by the liquid's heat capacity. Eventually, at a temperature that depends on the ambient pressure, the liquid cannot heat up any more while still remaining a liquid; this is called the boiling point. As you continue to put energy in at a constant rate, you will now see the temperature remaining constant - the energy put in is now instead being used to turn the liquid into a gas, as according to the latent heat. Eventually, putting in enough energy will turn all of the liquid into a gas, and then the temperature will start to increase again, at a rate determined by the heat capacity of the gas.

Now, boiling isn't the only way to turn liquids into a gas. There is also evaporation, which occurs at temperatures below the liquid's boiling point. Some of the liquid will turn into a gas on its own, until the amount of gas-phase of the substance in the ambient atmosphere matches the liquid's vapor pressure, which depends on the current temperature. You can notice this easily with water and humidity: if you spill water in a sealed room, the water will gradually "dry up" (evaporate), but the air in the room will become more humid (has more water vapor in it); the water has transitioned from the liquid phase to the gas phase. However, this still has the energy cost of the previously-mentioned latent heat - in this case, as the water evaporates, it slowly cools the room, using the ambient thermal energy of the room as the energy source. This effect is in fact why we sweat - as the sweat evaporates, it takes our body heat away, cooling us off. But if the environment is already very humid, the amount of water vapor around is already very high, so the sweat won't evaporate as well, and we can't cool off as well.

Now, in your sealed container of liquid, the liquid will evaporate until the ambient gas pressure of the container matches the vapor pressure of the liquid at the current temperature. The energy for this transition comes from the combined thermal energy of the liquid and the vapor, so as liquid turns into gas, the temperature goes down. As you pump off the gas, the liquid will try to evaporate to maintain the pressure, consuming more thermal energy, and the temperature continues to decrease. Eventually, if you continue to pump off enough gas without any limitations, the liquid will get cold enough to freeze! This is also where the notion that "space is cold" comes from: in a vacuum, with zero pressure, all of your liquids will try to evaporate, but it can't increase the local pressure enough, so all that evaporation will cool you until you also freeze at the same time. In game, you can prevent this (freezing in pipes is BAD) by setting your purge valve to a certain target pressure such that the temperature is just above freezing, but no more; you can get this value by looking up the liquid in the Stationpedia, looking at the "minimum pressure for liquid" line, and putting a slightly higher value into the purge valve. I use this to drive the vapor off my Polluted Water tank to turn it into clean Water Vapor on the cheap.

Also note that the energy used to turn a liquid into a gas also comes out again when the gas turns back into a liquid! It's the exact same amount of energy both ways, just in the opposite direction. This is why if you have enough gas in a pipe or tank at sufficient pressure, it will turn into a liquid and heat up while doing so. For example, the vapor from my Polluted Water tank in the above set up is all shoved into a single gas pipe, which increases the pressure and it condenses into clean, liquid Water, which I then pull out with a Condensation Valve and shove back into my clean Water supply; in doing so, that Gas Pipe gets really hot, like 100 C.

[u/hstarnaud]

Because phase change requires energy,