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Q & A: closed-cycle evaporative cooling

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Most recent answer: 01/07/2015
Q:
Is there anyway to create a vacuum in a sealed metal container by means of a chemical reaction? My goal is to take copper tubing and put some water in it, then form a vacuum in it (or lower the pressure inside the copper tubing as much as physically possible, then seal the copper tubing by means of either welding or soldering)I would like the water inside the sealed copper container to be able to boil/evaporate at as low of a temperature as possible (preferably somewhere in te range of 10-15 Celsius)My thought was maybe there are two chemicals that could be added at opposite ends of the container, then seal the container tilt and allow the chemicals to mix and then have chemical reaction from chemical A and chemical B take place which would form the vacuum or a very low pressure environment.Are there any chemicals reactions like this or a way to make this possible?If not, What is my best possible course of action to create a sealed copper container that container water and is sealed at a very low pressure inside of it (low enough pressure to allow boiling/evaporating at 10-15 degrees Celsius)Thank you for your time!!Also my metal container does not necessarily have to be copper but it does need to be something with a very good thermoconductivity. This item is for an experiment I would like to do, My idea that led to this question;1.)A copper container with a flat bottom will contain water and is sealed at a very low pressure to allow boiling at 10-15 degrees Celsius.2.) the main container holding the water will taper to a small tubing at the top which will then slope over and down (so that the water vapor will condense and flow down the copper tube)3.) Right after the downward slope starts for the condensed water it will go round and round in a circular coil.4.) aluminum fins will completely surround the downward coil.5.) A fan will be placed at the top of the coil/fins assembly blowing air through them to assist in cooling of the water going through the coil.6.) At the bottom of the coil the water will empty back into the main chamber.The main chamber will be more thin compared to the rest of the surface area and will sit on/above a hot surface and there will be a thermal paste applied between the two surfaces to aid in heat transfer.My thought is that the heat source will heat the inner liquid causing it to evaporate/boil and due to the low pressure this will occur at much lower temperatures than normal. Since water evaporating does a great job at dissipating heat it will cool the hot surface in a never ending loop. Heat source -> water evaporates-> vapor goes into the top where it condenses and flows down the spiral coil of the copper tubing -> mean while the water is cooled so aid in it returning to its liquid state -> water is now back in the main container and cycle repeats.No moving parts needed just the low pressure to aid in the lowering of the boiling temperature.My thought is that this device will be able to continuously dissipate heat once the water starts to boil, so by lowering the pressure as much as I possibly can I will be able to maintain the cooling temperature to around the temperature I am able to get the water to boil at.I apologize for this not being a simple question but anything you have that can help me with this I would greatly appreciate it, if you see something wrong with my thinking please let me know as well.I got this idea from 1) watching videos of people boiling water at very low temperatures &2) reading about "phase change coolers"Thank you for your time! I very excited to get your response!!
- Alan (age 28)
jenison, michigan, ids
A:

Your basic idea is to use the evaporating liquid as an efficient way of getting heat from the hot surface out to the cooling coils and their cooling fins, from which the heat is blown away by a conventional fan. Essentially, this is a replacement for solid metal cooling devices, in which simple thermal conduction carries the heat away to the cooling fins. I'm not sure which is overall more efficient, with regard to price, weight, volume, etc. 

As for the idea of lowering the boiling point of water to 10°-15° C by lowering the pressure, it certainly can be done. The idea of using some chemical reaction to reduce the pressure inside a sealed container is also feasible, although I don't know what reaction would be best. It would seem simpler just to use a different fluid, one with about the desired boiling point at atmospheric pressure. For example, chloroethane has a boiling point of 12.3°C. One point to be aware of is that there will not be a unique boiling point in this sort of device, because it has constant volume rather than constant pressure. The more of the liquid that has boiled into vapor, the higher the pressure and thus the higher the boiling point.

Mike W.


(published on 01/07/2015)

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