Heating water in a vacuum

  • #1
Hey guys,

I have been scouring the internet to find an answer but I can not.

The problem is: I want to heat water to boiling point with the least amount of energy required. I found out that if you apply vacuum into an chamber with water (room temp), the boiling point can be as low as room temperature. Now, I understand that once the chamber is pressurized the water is once again at room temp.
My question is: Can a small amount of heat applied during the vacuumed state (28.92 HG) be enough to boil the water when the chamber is pressurized? if so, how much energy will be required?
 

Answers and Replies

  • #2
.Scott
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If the water and its environment is already at room temperature, you don't need a vacuum, just a low H2O partial pressure. That is, dry air.
Under those conditions, the normal heat from the surrounding environment is all you will need to evaporate the water.
 
  • #3
If the water and its environment is already at room temperature, you don't need a vacuum, just a low H2O partial pressure. That is, dry air.
Under those conditions, the normal heat from the surrounding environment is all you will need to evaporate the water.
How would you create the low H2O partial pressure? will the vapors be above room temp?
 
  • #4
.Scott
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How would you create the low H2O partial pressure? will the vapors be above room temp?
Wait for a warm sunny day and then move the water outdoors.
 
  • #5
russ_watters
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My question is: Can a small amount of heat applied during the vacuumed state (28.92 HG) be enough to boil the water when the chamber is pressurized?
No. Since the boiling point is lowered when in a vacuum, you can't raise the temperature of the water at all: applying heat just makes it boil faster. Then when you raise the pressure, it will just be cold water.
 
  • #7
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The simplest answer is " Yes and No depending on your desired result."
Boiling point is when the vapor pressure of the liquid is equal to pressure exerted by the surrounding atmosphere.
Heating the liquid is a description of adding thermal energy. One common unit is a calorie which is the energy required to raise 1cc of water 1 degree C.
As you add thermal energy the vapor pressure increases. This must be balanced by how much energy is needed to "adjust" the atmosphere.

Do you want to boil the liquid or to heat it? Better definition of your goal is up to you.

My initial impression, with no calculations, is that one would want the smallest volume of water and the least energy utilized by a vacuum pump. This gives the most increase in temp and the lowest effort against ambient conditions.

Does this help in your calculations.
 
  • #8
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The (specific) heat of liquid evaporation depends on many factors, although in practice changes are not very important. Obviously starting from low temperature and heating liquid to the boiling point is accompanied with lowering heat of evaporation. Pessure dependence is smaller. Behaviour of such a system is described by Clapeyron-Clausius equation.
If you want to minimise energy consumption in evaporation, look for boiler construction. In industry there are many "thermal bridges", lacks of insulation, etc. Lowering pressure needs input of mechanical energy, in some cases alo thermal one. When you are interested in collecting distilate, you need lower temperature in condenser, which generally consumes more energy and money than putting more heat to evaporator. Pressurising low pressure vapor is a difficult task.
Lowering pressure in distillation is frequently used to overcome difficulties in fully removing solvent from solution, especially when solute is heat-sensitive. It also accelerates the distillation. In laboratories there are rotary evaporators, designed to work under lowered pressure.
Have a good day,
zbikraw
 
  • #9
Baluncore
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The problem is: I want to heat water to boiling point with the least amount of energy required.
Do you define the "boiling point" of the water as 100°C, or as the pressure reduction needed to boil water at the ambient temperature?

Two reason why we boil water to make tea is to drive dissolved gasses from the water and to speed the reaction. At low pressures, the dissolved gasses are removed at lower temperatures, but the tea is cold and takes longer to make. Dissolved gasses can interfere with the chemistry of the tea making ceremonial effusion.
 
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  • #10
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Do you define the "boiling point" of the water as 100°C, or as the pressure reduction needed to boil water at the ambient temperature?

Two reason why we boil water to make tea is to drive dissolved gasses from the water and to speed the reaction. At low pressures, the dissolved gasses are removed at lower temperatures, but the tea is cold and takes longer to make. Dissolved gasses can interfere with the chemistry of the tea making ceremonial effusion.
In tea preparation, boiling water is needed to dissolve many componets from a "biological matrix" of dried tea leafs. In ambient temperatureextraction, composition of tea is quite different
Temperature-dependent quantities must be reported with reference to temperature of mesurement, for example Bp1bar of water is 100 deg Celsius, and water density in 4 deg C is d4=1.
zk
 
  • #11
Baluncore
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In tea preparation, boiling water is needed to dissolve many componets from a "biological matrix" of dried tea leafs. In ambient temperatureextraction, composition of tea is quite different
I beg to differ. It actually requires hot water to make a good cup of tea.
Water boils when it changes from a liquid to a gas. That boiling can be at an ambient temperature if the pressure is reduced.
 
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  • #12
Update: Thank you all for sharing your knowledge with me! I have moved on from boiling in a vacuum to induction heating. So far, I have been gathering information on how to build a Royer Oscillator induction heater. I figure with the 3 x 1300mah 11.1v batteries I am trying to power from, this is the best route. If, I am wrong please let me know, I just can't seem to find a more energy efficient heat source.
 
  • #13
I beg to differ. It actually requires hot water to make a good cup of tea.
Water boils when it changes from a liquid to a gas. That boiling can be at an ambient temperature if the pressure is reduced.
In tea preparation, boiling water is needed to dissolve many componets from a "biological matrix" of dried tea leafs. In ambient temperatureextraction, composition of tea is quite different
Temperature-dependent quantities must be reported with reference to temperature of mesurement, for example Bp1bar of water is 100 deg Celsius, and water density in 4 deg C is d4=1.
zk
Do you define the "boiling point" of the water as 100°C, or as the pressure reduction needed to boil water at the ambient temperature?

Two reason why we boil water to make tea is to drive dissolved gasses from the water and to speed the reaction. At low pressures, the dissolved gasses are removed at lower temperatures, but the tea is cold and takes longer to make. Dissolved gasses can interfere with the chemistry of the tea making ceremonial effusion.

Hey, thanks for getting back

so my main goal is to heat water to +/- 100 c, so that steam can be generated, now I am looking for a result that will sustain heat enough to maintain BP with the lowest amount of energy possible. The ingredient I am trying to apply steam to is freeze dried food.

not a fan of tea haha
 
  • #15
Did you know that freeze drying is done in a vacuum? https://en.wikipedia.org/wiki/Freeze-drying
I guess I should clarify, I am rehydrating freeze dried. I am trying to build an external backpack frame that would include an induction heater. So you can heat water as you hike, so you can eat on the go.
 
  • #16
Baluncore
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Heating water is energy expensive.

You might reverse the freeze dry process, save the electrical power and avoid the freezing. The presence of air in the food will block and prevent the rapid entry of steam. You might use a form of vacuum impregnation as follows:
1. Place the food in a chamber. Use a hand pump to pull a vacuum, and so remove air from the chamber contents.
2. Open a valve that allows a set volume of clean water to enter from a closed chamber. That water will immediately boil at ambient temperature and move as cool steam throughout the dry food. The water will condense into the full depth of the food because there is no air to block the immediate flow of water vapour throughout the food.
 
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  • #17
Heating water is energy expensive.

You might reverse the freeze dry process, save the electrical power and avoid the freezing. The presence of air in the food will block and prevent the rapid entry of steam. You might use a form of vacuum impregnation as follows:
1. Place the food in a chamber. Use a hand pump to pull a vacuum, and so remove air from the chamber contents.
2. Open a valve that allows a set volume of clean water to enter. That water will immediately boil at ambient temperature and move as cool steam throughout the dry food. The water will condense into the full depth of the food because there is no air to block the flow of water vapour throughout the food.
My original design was exactly that. I would create a tubberware that the food would seat in, and the device I am trying to develop will apply steam into the said vacuumed chamber. However, the number of components need to achieve such result is too high to be considered "efficient" (especially in the backpack frame design, lighter the better)
 
  • #18
Baluncore
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So far, I have been gathering information on how to build a Royer Oscillator induction heater. I figure with the 3 x 1300mah 11.1v batteries I am trying to power from, this is the best route.
I would create a tubberware that the food would seat in, and the device I am trying to develop will apply steam into the said vacuumed chamber.
You do not need heat to introduce steam into a vacuum chamber.

The batteries will probably weigh more than the water you will need to carry. Why carry both water and electrical energy when you can use a hand operated vacuum pump to cold boil the water directly into the dry food. You might consider integrating a filter and a reverse osmosis hand pump into the device, as used in life boats to produce clean drinking water.
 
  • #19
You do not need heat to introduce steam into a vacuum chamber.

The batteries will probably weigh more than the water you will need to carry. Why carry both water and electrical energy when you can use a hand operated vacuum pump to cold boil the water directly into the dry food. You might consider integrating a filter and a reverse osmosis hand pump into the device, as used in life boats to produce clean drinking water.
This is awesome, I do want to eliminate the power source (if the battery dies the device will just be a junk you have to carry) Can you heat food by cold boiling? I suppose directly applying water into the pores in a vacuum state would be a fast way to hydrate something.. What do you suppose for the pressure needed to achieve hydration.

info:
i.e: Any hard root freeze dried vegetables need 15 min soak time (5min in boiling water), need 1:1 water to solid
 
  • #20
Baluncore
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info:
i.e: Any hard root freeze dried vegetables need 15 min soak time (5min in boiling water), need 1:1 water to solid
I presume that is 1:1 by initial volume.

The word soak is the key. It takes 5 minutes because the air in the dry food blocks the free entry of the water, which must then displace the air by entering by capillary action. Vacuum impregnation by water is an inverse of freeze drying.

So vacuum impregnation by cold water should be significantly faster, but you must then heat the food if you need it hot. One advantage of using hot water is that the food may still be warm at the end of the process. However, heating is probably best done quickly after slow rehydration, that way less heat will be lost waiting for chemistry to happen.
 

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