### Re: Methods of Cooling and Heating

Date: Thu Jun 5 03:06:39 2003
Posted By: Joseph Weeks, Engineer
Area of science: Physics
ID: 1048603160.Ph
Message:

Your question points out some of the major challenges with a design question; what is the basis for your design and what are the constraints of the design. You have stated that in one container you wish to boil water, in the other freeze the water. The methods that one might use to accomplish these goals depend upon a wide variety of variables, such as:

1. Is the system to operate continuously, intermittently, or one time?

2. Are there any constraints on the size of the containers? The approach you might take to cooling a couple grams of water may be completely different from what would be used to cool a swimming pool.

3. What are the sources of energy available to accomplish your objective? Are you going to be located next to a running stream of water, a gasoline engine, a car battery, or a standard electrical outlet?

4. How quickly do you need to accomplish your objective?

If you think about it, I'm sure you can come up with other variables, and likewise also determine some of the constraints on your design. The more broadly you look at a design problem, the more likely you may be able to develop potentially a new and perhaps improved design.

Your specific question is: How can I freeze water? Generally, there are three general methods that come to mind. You can cool/freeze an object by using a physical process, a chemical process, or by using a semiconductor junction that cools when electric current is passed through it.

So, first lets consider cooling by a strictly physical process. Through applying work, gas can be compressed from a low pressure to a higher pressure. If you have pumped up a bicycle tire using a hand pump, you may feel the base of the pump get hot after a few minutes of pumping. This process is called adiabatic compression; the gas is compressed quickly enough that no heat transfer happens between the gas and the outside world. If the compression ratio (the relative volume of the gas before and after compression) is quite high, lets say 15 to 1, the gas will get very hot. In fact the gas would be hot enough that it would tend to boil water.

If the high pressure gas is allowed to cool back to room temperature, we can pass the gas through a nozzle to reduce the pressure. As the gas expands, it adsorbs heat. It may or may not adsorb enough heat to freeze water. In fact, the very first air conditioning was performed by first compressing, and then decompressing air.

The other form of physical process that can be very effectively used for cooling is to recognize that it takes a great deal of energy for a material to vaporize from a liquid to a gaseous form. A number of materials have been used as so called liquid refrigerants including ammonia and Freon. Most liquid refrigerants have a high vapor pressure. So, if liquid Freon or liquid ammonia is placed in an open container at atmospheric pressure, the Freon or ammonia rapidly begins to evaporate. As it evaporates, it adsorbs heat from its surroundings.

If we capture this Freon or ammonia vapor and compress it (just like we did with the air), the Freon or ammonia will get very hot. When passed through some coils, the Freon or ammonia will cool down, and, because of the high pressure, turn back into a liquid. Now all we have to do is to reduce the pressure on the Freon or ammonia, and it will begin to evaporate again and the cycle will repeat itself. This is the process that your refrigerator or freezer uses to cool its contents.

The next method of cooling (or heating) involves using a chemical change. Some materials adsorb a lot of heat when they dissolve. Other materials release heat when they dissolve. This is referred to as the “heat of solution.” Materials like sodium hydroxide or sulphuric acid give off quite a bit of heat when they dissolve in water; they have a positive heat of solution. Ammonium nitrate has a negative heat of solution; it cools water when it is dissolved in it. This approach is very useful for providing “cool packs” for immediately treating an injury. A small vial of water in a pouch containing ammonium nitrate (or similar material) is broken; begins dissolving the ammonium nitrate, and cools the pack down. If you then took the ammonium nitrate solution and heated it to remove the water, the process could be reversed.

Of course, moving around solid ammonium nitrate is not as easy as pumping around a couple of solutions. Therefore, the standard method of cooling by a chemical process involves water and a lithium bromide solution. A description of this process can be found at: nicorinc.com This process involves the low pressure vaporization and adsorption of water by the lithium bromide, followed by using external heat to remove the water from the lithium bromide solution. It is a chemical process because the concentration of the lithium bromide solution changes. Frankly, it is not easy to understand this type of cooling process. The advantage of using this type of cooling process, however, is that most of the energy needed for cooling does not need to be in the form of electrical energy. If you wanted, you could potentially use a campfire to power this type of refrigeration process.

The final type of cooling process that comes to mind uses the Peltier effect. Back in 1834, a French watchmaker, Jean Peltier, found that if he passed current through the junction of two dissimilar metals, like copper and iron, the junction would get cold. If he reversed the direction of electricity flow, the junction would get hotter. You can read about the Peltier effect and thermoelectric coolers at: enertron-inc

Thermoelectric coolers are a great way of providing cooling for smaller containers and devices. As long as you pass direct electrical current through the cooler, one surface gets cold and the other gets hot. You can actually stack one cooler on top of another in order to decrease the minimum temperature and increase the maximum temperature. Unfortunately, it is difficult to manufacture thermoelectric coolers that are very large. But if all you want to cool is a few drink cans, an insulated box with a thermoelectric cooler may be just the ticket, especially if you want to use the 12 direct current power available in your car.

Of course there are other methods that might be used, but tend to be less practical. If your container was carefully insulated, you may be able to get it to radiate its excess heat into space, particularly at night. This is the reason that puddles of water freeze in the fall, even though the air temperature remained above freezing during the night.

I hope I have given you a few cool ideas to think about.

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