Energy Required to Evaporate Water

[ATT55]

Energy required to evaporate water.

Given 3 evaporating scenarios:
1. Glass filled with 50cc of water at 20C; the water is heated to 60C
2. Glass filled with 50cc of water at 20C; the water is heated to 100C
3. 50cc of water at 20C wiped over a large plate to create 50micron thickness layer, plate temperature is 60C.

How much energy is required to evaporate all the water in all those scenarios (neglecting losses to the environment)?

From one side I would think that if all the water evaporate, the net energy required to do it is the same in all the cases, from the other side I can say that evaporating water at 60C takes less energy then 100C, because the water doesn’t boil.

[Unacceptable reference deleted by the Mentors]

Is it correct?

 

[berkeman]

How are you evaporating water at 60C?

 

[Vanadium 50]

More importantly, why do you think the liquid-gas phase change latent heat depends on whether it "boils"? (This is not a trick question)

 

[ATT55]

How are you evaporating water at 60C?

If you put a glass of water inside a oven at 60C, the water will evaporate. Correct?

 

[ATT55]

More importantly, why do you think the liquid-gas phase change latent heat depends on whether it "boils"? (This is not a trick question)

The latent heat depends on the temperature, but are right- this might be the reason.

 

[Drakkith]

Heating the water and getting it to phase change are two different things that require different amounts of energy. Evaporating 50cc of water requires the same energy no matter what temperature the water is at. But that energy has to come from somewhere. Heating the water to a higher temperature evaporates the water faster because the higher temperature more often gives water molecules that random 'kick' of energy they need to break the hydrogen bonds and escape into the environment as a gas.

If we could isolate that 50cc of water, you would find that even at 60 degrees C some of the water evaporates. This takes energy, which was given to the evaporating molecules by the rest of the water, which means that each time a molecule evaporates it steals a little bit of energy from the rest of the water, cooling it off. This can be seen in spectacular form when you put water in a vacuum chamber and then rapidly drop the pressure. The water both boils AND freezes at the same time.

 

[russ_watters]

Are you including the initial heating in the answer? And what is the final temperature of the vapor? Room temp?

Bottom line, if the starting and ending conditions are the same for each case then the energy required to get there is the same regardless of the path taken. Different starting or end point, and you need a different amount of energy.

 

[ATT55]

Thanks all, so the net energy required is the same also for the following evaporation scenarios?

• Glass filled with 50cc of water at 25C; the water is heated to 60C
• Glass is filled with 50cc of water at 25C; the glass is placed inside a vacuum chamber.

Does the net energy required to generate and maintain the vacuum is same as the energy required to heat the water to 60C?

 

[russ_watters]

Energy required to generate a vacuum, starting with a vacuum chamber filled with air, is an entirely different animal and you'd need to exactly define the process to figure it out.

However, if you're really asking if we can reduce the energy input for a process like distillation this way, the answer is yes.

 

[sophiecentaur]

How are you evaporating water at 60C?

I do it at much lower temperatures than that when I hang out my damp laundry in air at 20C. A breath of wind will speed up the process. There is a whole range of kinetic energies of water molecules at the surface; a proportion can escape at any ambient temperature (and pressure). If the experiment is in equilibrium (eventually ) in a closed container, an equal number of molecules will re-enter the water from the air.

 

[Philip Koeck]

Are you including the initial heating in the answer? And what is the final temperature of the vapor? Room temp?

Bottom line, if the starting and ending conditions are the same for each case then the energy required to get there is the same regardless of the path taken. Different starting or end point, and you need a different amount of energy.

Initially I thought the question was whether the actual heat of fusion can be changed in some way, for example by contact with some surface and/or by changing the temperature.

Is the heat of fusion really independent of temperature?
It seems that it does change a bit according to what I found on the web.

Is there some sort of "catalytic" effect (for example due to contact with a solid) that weakens hydrogen bonds and reduces the energy needed to evaporate the water?

 

[Philip Koeck]

..., from the other side I can say that evaporating water at 60C takes less energy then 100C, because the water doesn’t boil.

When the water starts boiling the vapour has to do expansion work against the external pressure (air plus hydrostatic) and that will use some of the supplied energy.
However, this is a very small difference.

Afterthought: And when it stops boiling you get the same heat back during the compression, I would think.

 

[jbriggs444]

Afterthought: And when it stops boiling you get the same heat back during the compression, I would think.

You've lost me here. What compression?

You had a fluid that was boiling. Everything was at ambient pressure. You've allowed it to stop boiling. Everything is still at ambient pressure. Turning off the burner does not result in an increase in pressure.

 

[Philip Koeck]

You've lost me here. What compression?

You had a fluid that was boiling. Everything was at ambient pressure. You've allowed it to stop boiling. Everything is still at ambient pressure. Turning off the burner does not result in an increase in pressure.

By compression I mean reduction of volume, not increase of pressure.
When the water stops boiling the vapor bubbles undergo an isobaric compression.

 

[Chestermiller]

The water vapor at 60 C is dispersed in air, which also started out at 20C . You can’t have water at 60C while the air in the mixture stays at 20C. The air in the final mixture must also be at 60C; part of the heat added had to go into heating the air.

 

[ATT55]

Energy required to generate a vacuum, starting with a vacuum chamber filled with air, is an entirely different animal and you'd need to exactly define the process to figure it out.

However, if you're really asking if we can reduce the energy input for a process like distillation this way, the answer is yes.

Yes, perhaps the right question is: In which evaporation process is the total energy consumption lower and why — heating water to 60°C or placing it in a vacuum chamber? (at both starting temperature is 25C)

From a physical process perspective, why does it take less energy to evaporate water using a vacuum compared to heating the water?

 

[russ_watters]

Yes, perhaps the right question is: In which evaporation process is the total energy consumption lower and why — heating water to 60°C or placing it in a vacuum chamber? (at both starting temperature is 25C)

This question is not answerable as stated, for the reason I gave.

From a physical process perspective, why does it take less energy to evaporate water using a vacuum compared to heating the water?

Look at a steam table or phase diagram. I'm not sure how to say it in non-scientific words. I guess I'd say you are moving your state closer to the dividing line between phases(or vice versa).

 

[Vanadium 50]

is the total energy consumption lower

Are you really talking about energy or about time?

I don't see an energy calculation anywhere, which makes it hard to discuss. Not because of the details of the process, but because to do the calculation at all you need to completely specify the initial and finl states and what the boundary of the system's configuration is.

 

[ATT55]

Look at a steam table or phase diagram. I'm not sure how to say it in non-scientific words. I guess I'd say you are moving your state closer to the dividing line between phases(or vice versa).

Ok thanks- so why does moving the state closer to the dividing line between phases makes the total energy consumption lower?. scientific words are ok, as well as intuitive explanation.

 

[ATT55]

Are you really talking about energy or about time?

I don't see an energy calculation anywhere, which makes it hard to discuss. Not because of the details of the process, but because to do the calculation at all you need to completely specify the initial and finl states and what the boundary of the system's configuration is.

I'm talking about the total energy consumption.

Let’s say you have a glass with 50cc of water at room temperature and you need to evaporate it completely and return it empty, while using as little energy as possible, and let’s also assume that the duration is not important here.

You have to choose whether to place the glass in an oven heated to 60C or to place it in a vacuum chamber. What would you choose and why? What other initial and final conditions are missing in this scenario?

 

[Philip Koeck]

I'm talking about the total energy consumption.

Let’s say you have a glass with 50cc of water at room temperature and you need to evaporate it completely and return it empty, while using as little energy as possible, and let’s also assume that the duration is not important here.

You can choose whether to place the glass in an oven heated to 60C or to place it in a vacuum chamber. What would you choose and why? What other initial and final conditions are missing in this scenario?

The cheapest way would be just to let it stand there and wait.

 

[ATT55]

The cheapest way would be just to let it stand there and wait.

And if you have to choose between the two?

 

[Philip Koeck]

And if you have to choose between the two?

Then it depends how much energy you need to pump down the vacuum chamber.
(The energy needed to heat water from 20 to 60 is fixed.)

In the space station, for example, creating a good vacuum requires almost no energy.

 

[russ_watters]

Ok thanks- so why does moving the state closer to the dividing line between phases makes the total energy consumption lower?. scientific words are ok, as well as intuitive explanation.

What's wrong with the answers I already gave you? Did you look at a steam table? What did you see? Here's a steam table if you haven't found one:
https://www.efunda.com/materials/water/steamtable_sat.cfm
Maybe the answer you need is "enthalpy".

This is starting to feel like and endless string of "why?" questions that doesn't go anywhere. You're going to have to do more/better work here to make it productive.

I'm talking about the total energy consumption.

Total energy consumption of what? Are you including heat passively drawn frm the room without you doing anything(just letting the glass sit there)?

You still aren't fully describing your starting and ending states and constraints or which energy you are asking about. And they seem to keep changing. We're not you. We can't define your problem for you because we don't know what you are really trying to do.