Freezing moisture in a system by pulling to deep a vacuum?

[Drew]

Can water be frozen by pulling too deep a vacuum on a system? What process takes place? Can too fast a vacuum drop the pressure to quickly, causing a fast evaporation and boiling which drains so much heat (energy) from the water that it causes it to freeze?

[mad fridgie]

Yes you can make ice using a vacuum,
This already used to make ice slurry, just so happens to be called "Vacuum Ice" They normally use steam jet refrigeration, a bloody big venturi.
It can happen when you vac out a system.
When your ice is frozen, it still vapourises, this called sublimation.
Also one of the reasons for freezer burn, or dried product. (This principle is used in freeze driers, the freeze means the product frozen nothing to do with the refrigeration plant.)

[NoNickName]

Yes, you can find also a demo on youtube, where a bottle of water is frozen by pulling a fast vacuum. At first, it starts boiling, and then it's frozen.

[Drew]

Thanks guys. I had a look at you tube. Great. Does anyone know how this happens with regard to a exchange in energy at a molecular level?

[mad fridgie]

Thanks guys. I had a look at you tube. Great. Does anyone know how this happens with regard to a exchange in energy at a molecular level?

When you drop the pressure the boiling/vapourizing point drops. Firstly it uses energy to boil from the surroundings, when this is exhausted, it takes the energy from mass of water itself. If energy is remove you then you change state.(energy in to boil, energy out to freeze)
In roughy terms, you need to freeze approx 7.3 units of water to vapourize 1 unit of water.
I hope this what you were after

[Peter_1]

Yes, you can find also a demo on youtube, where a bottle of water is frozen by pulling a fast vacuum. At first, it starts boiling, and then it's frozen.

Is it possible you were referring to my movie where we put a bottle in a high vacuum chamber in front of the window?http://www.refrigeration-engineer.co...+boiling+water

[lowcool]

how would ice affect your micron reading?

[mad fridgie]

how would ice affect your micron reading?

Ice still excerts a vapour pressure, but less than free water. If the ice has no heat source to vapourize, the ice becomes colder and its vapour pressure reduces. At these conditions your density of the water vapour is 1000s of cuM /Kg, so your average Vac pump has little effect. This is why you should triple evacuate with dry nitrogen, so that you melt the ice and add heat load. I would not think it is a major problem in Aus, but is aproblem in colder climates.

[lowcool]

cheers mf
as they say you learn a new thing everyday.have had refreshment discussions and the thought was that water had to be relatively close to zero degrees so now will have to rehash it over more refreshments
bugger
enjoy your weekend fella

[NoNickName]

Is it possible you were referring to my movie where we put a bottle in a high vacuum chamber in front of the window?http://www.refrigeration-engineer.co...+boiling+water

There is also a similar video from Edwards vacuum machines.

[Drew]

Thanks , That is what i was after !

The water requires energy to evaporate which it takes from the water itself lowering it sensibly until it freezes, encouraged by the low Tassie temps!

[NoNickName]

Thanks , That is what i was after !

The water requires energy to evaporate which it takes from the water itself lowering it sensibly until it freezes, encouraged by the low Tassie temps!

No. The evaporating temperature is proportional to the vapour pressure (like for any refrigerant). The lowest the vapour pressure, the lower the evaporating temperature, and the higher the triple point temperature.

[Drew]

Would the following, with regard to evaporisation, be correct? I hope its not to far off the topic?

In an evaporator/ blower coil we create an enviroment with a low enough pressure for the refrigerant to boil. The relatively warm air that blows over the coil is our heat source and the compressor is the low pressure source?
The energy added by the air is transmitted by conduction through our fins and pipes warming the liquid refrigerant . The molecules within the refrigerant change sensibly, gaining energy, until they vaporise.
This vaporisation requires a lot of energy which is again absorbed from the air via the pipes?
So there is a tiny sensible heat exchange and then a big energy exchange when the refrigerant changes state?

How does this tie up with water in a dish left outside where the water has molecules that have all different energy levels (speed of vibration) and that a few are at 100 C . These few , as long as the vapour pressure permits , will evaporate. Because these hotter molecules have left the water the overall average of the water temperature drops. All i see in this is a few molecules that are already at 100 C leaving the water , Nothing about energy being absorbed in vast quantities?

[mad fridgie]

lets look atthis is a different way.
(no long water as a pure refrigerant)
imagine if AIR is sponge (molucules are the material) and the gaps are a vacuum. If you squeeze the sponge (increase pressure or reduce temperature) the gaps become small or less vacuum.
So if can accept water vapourizes at different temperature, then you must accept that the vacuum within the sponge needs to be filled.
The water either absorbes energy from the ambient to vapourize (boil), what really happens it absorbs water from itself.
The process stops when the sponge is full of water (water vapour pressure = air vapour pressure)
This is how a cooling tower works, or sweat on your body (where heat comes from the body).
next time you hang the washing out, feel it on a hot dry day (when still damp) it will feel cool

[Drew]

So there is a difference betrween boiling and evaporating. Whats happens in a refrigerant heat exchanger? Does the refrigerant evaporate or boil?
I understand how evaporation causes cooling,(due to the loss of fast moving molecules) but im getting confused with whether this is the same process that happens in a refrigerant coil? Or does the added heat from the heat exchanger encourage the refrigerant to boil and this superheated vapour just carries the energy away? Therefore boiling is really just a transfer of energy which leads to a change of state?

Whereas evaporation is a slow process involving molecules close to the surface escaping and taking their energy with them lowering the temp of the fluid.

[mad fridgie]

So there is a difference betrween boiling and evaporating. Whats happens in a refrigerant heat exchanger? Does the refrigerant evaporate or boil?

No they are really the same, they are both changing state liquid to gas/vapour. Both need energy to change. It more to do with terminology and application. ( must admit never thought much about how to apply the wording) Maybe you can look in a dictionary and give the meaning of each word

[Drew]

So energy is reqiured in both cases to evap/ boil the refrigerant into vapour. This energy is avalable from different sources. The energy supplied is used to change state ( break the intermolecular forces) (latent heat of evap). This is the main source of heat absorption in the refrigeration system.

[Drew]

vapourisation, evaporation , boiling : same thing?

[mad fridgie]

Basically yes to both