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Maxwell
27th October 2006, 07:05 PM
Hi All,
Can someone explain how a vacuum works?
I know that it reduces the boiling point of water but does that mean it will have the same characteristics?
Like can you put say water and rice in a pot and cook the rice by adding a vacuum to it or would it just evaporate the water?
Does anybody know that thickness a vessel has to be for different vacuums?
How big a vacuum pump is needed to say boil 1 l of water in say 5 mins?
How cost effective would it be?
Capital outlay/running costs?
Cheers
Maxwell

Tony From West Oz
28th October 2006, 01:11 AM
Hi All,
Can someone explain how a vacuum works?Vacuum is "negative pressure". This means that instead of pressure being inside a vessel (like ythe tank of a compressor) the pressure is outside the vessel (vacuum inside). The maximum pressure differential relative to atmospheric pressure, which can be achieved by a vacuum, is 14.7 PSI (Atmospheric pressure)

I know that it reduces the boiling point of water but does that mean it will have the same characteristics?The water will boil at a lower temperature!

Like can you put say water and rice in a pot and cook the rice by adding a vacuum to it or would it just evaporate the water?The water would not be as hot and thus, the rice would not absorb the water as fast. Use a pressure cooker to make things cook faster, increased pressure allows higher cooking temperatures, which shortens cooking time.

Does anybody know that thickness a vessel has to be for different vacuums?Sufficient to withstand 14.7 psi on every surface.

How big a vacuum pump is needed to say boil 1 l of water in say 5 mins? Do you mean, completely evaporate that 1 litre of water? or just bring it to the boil? Remember, as the water evaporates, it causes the temperature to drop. As the temperature drops, the water will require a greater vacuum, to cause it to boil. Adding external heat may be necessary to prevent freezing the water.

How cost effective would it be? Compared to what?

Capital outlay/running costs?
Cheers
Maxwell

All of the above depends on what you are trying to achieve. If you want to cook rice, forget using a vacuum. If you want to remove water from vegetable oil, you may be able to use low grade heat in conjunction with vacuum, to remove the water cost effectively.
Low grade heat may be water at 50 - 80°C. Normally, this would not be hot enough to boil water off, but under a slight vacuum, it will allow a lot of water to boil off.

Solar heated water, engine coolant, waste heat from other sources are low cost heating , especially where the heat needs to be removed. eg, engine cooling.

If removing water from vegetable oil, I recommend that you wash the oil first, to dilute and remove most of any salts or other water soluble contaminants which are in the water you are trying to remove. Once you have the ability to dry the oil, the added steps to wash should be relatively easy to implement.

Maxwell
29th October 2006, 01:55 AM
Hi Tony,
I gather that the vacuum reduces the surface tension of the water allowing it to boil more easierly!
If this is the case then wouldn’t the surface tension on the rice be reduced allowing the water to enter, sort of cold cooking it?

Yes I would like to know how to completely evaporate 1 litre of water
in 5 mins.

Can a vacuum pump be used as a still to separate methanol and water?
Anybody know who would supply a vacuum pump that can handle methanol?
Cheers
Maxwell

dagwill
29th October 2006, 11:02 AM
very interesting subject [I just let the rice cooker take care if the details] But seriously i wondered about the use of vacume in flash drying Where Bio, or SVO etc is heated above 100c then pumped through a spray under pressure the sudden drop in presure causes all moisture to boil off and drift away as steam. I have acouple of large gas bottles that i bought from a test station for $10 Ok so they didnt pass the test but i thought Well if there was a fault they would just crumple. not the danger involved with flanable gas under pressure as they were being tested for. Anyway i thouht if connected the suction side of my compressor to a fitting [welded on the side of the gas bottle] this would reduce the temperature needed to boil off the water as it was pumped through a spray wich sprayed inside the gas bottle[drain tap at the bottom for Bio]. As the compressor kept sucking it would constantly draw off any steam.