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CHILDREN'S SCIENTIFIC LABORATORY
How to do without freon. Children's Science Lab
Directory / Children's Science Lab Do you know what freon is? We are sure that after reading this question, the first thing you will remember is refrigerators. And if you delve into your memory, then remember about the ozone holes. The connection between them is direct: the more freon escapes from faulty units, the greater the threat to the ozone layer of the planet. But, unfortunately, it is not yet possible to completely eliminate freon from use. After all, there are more than a billion refrigerators on earth ... Nevertheless, they are hastily looking for a replacement for the ill-fated substance - a gas that could act as a refrigerant. Meanwhile, refrigerating machines have long been known that use the most harmless and cheap substance - air. Why are they forgotten?
It turns out that not everything is so simple. Recall that when a gas is compressed, it heats up, and when it expands, it cools. These two facts are enough to understand how a refrigeration machine works according to the method proposed by Joule. The figure shows one of many such designs. Look - two cylinders are "planted" on one crank. The left, compressor, compresses air, the right, expansion, works from compressed air (we did not show details such as valves and spools). So, a portion of air is compressed in the left cylinder. At the same time, there was an increase in its internal energy - the temperature and pressure increased. But in order to get cold, that is, the temperature is lower than that of the environment, internal energy must be taken from the air. Do this in two steps. First, air is sent to the heat exchanger. (In this design, this is a tube blown by a fan.) Here the air cools to a temperature close to the ambient temperature. Note that its pressure does not change. Next, a portion of air enters the expansion cylinder, where it performs physical work and is cooled to a temperature lower than that of the environment. The cold air is pushed into the refrigerator compartment during the next piston stroke. It is clear that due to the work done during the expansion of air, part of the energy expended on its compression is returned and the machine as a whole becomes more economical. It would seem that everything is fine. However, air-cooling machines operating according to this principle are most advantageously used to obtain temperatures below -90 ° C. In home refrigerators, these temperatures are not needed. Meanwhile, as practice shows, when a moderate cold is received, the expansion cylinder returns only an insignificant part of the energy expended on compressing the air. So maybe stop using it altogether? If you go for it, the refrigeration machine is extremely simplified, turning into a device that does not contain moving parts. Here is one of the options - the so-called vortex tube, designed in 1931 by J. Rank. It is set up quite simply. Air is pumped in from the left (see diagram), and "miracles" begin in the resulting vortex. Its outer part, heating up, moves to the right and, having met with a conical insert, comes out. But not all come out. Approximately one third of the air turns back (the flow goes in two layers) and, being more and more cooled, enters the left branch pipe for cold air. Temperature drop can reach about 85 degrees!
A way out seems to have been found, but, unfortunately, the energy consumption of vortex tubes is unacceptably high. Therefore, they are used only where low weight and reliability are most important, and compressed air is plentiful, as, for example, in aviation. More perfect pulsating pipes. Here's how the device, proposed in 1962 by low-temperature specialist Gifford, works. Imagine a pipe with two taps. At its deaf end is a heat exchanger. In our case (see fig.) this is a coil through which water flows. If you open the top valve for a moment, then a portion of air will enter the pipe and begin to expand at supersonic speed. This phenomenon, known as the "shock wave", was usually of interest only to the creators of rocket and space technology, but, as you see, it suddenly found a more prosaic application. So, the shock wave, propagating in the pipe, compresses the air in front of it. From compression, it heats up and gives off excess heat to the water. If you open the second tap, the cooled air will begin to expand, and its temperature will drop significantly. This is the flowchart of the process.
In the simplest devices such as the described pipe, many side effects arose that made the work uneconomical. But it looks like a solution has been found. The Moscow company "Novid-Ecoholod" as a result of more than ten years of research conducted under the guidance of Ph.D. B. G. Kuznetsova, brought the pulsating pipe to a very high perfection. In the modern version, it is a pipe of variable section, all dimensions and proportions of which fully correspond to the best course of the cooling process. The ability to find them correctly is the main secret of the company. Equally important is the rotating spool, one of the system components that best manages processes. Air coolers manufactured by Novid-Ecoholod are capable of delivering a jet of chilled air with a temperature of -60 °C and below. With its help it is especially convenient to quickly freeze products on a moving conveyor. Speed in this case is not just convenience. There is also a new quality here. Biologists have long noticed that very rapid cooling does not destroy living tissues. Small lizards, for example, after that can come to life even a few years later, and very tiny crustaceans that have undergone such a procedure in natural conditions manage to wake up after ... a thousand-year sleep! So it's no surprise that quick-frozen food looks fresh on the table! The firm "Novid-Ecoholod" without much difficulty receives even minus 120 ° C from its refrigerators. The result is curious. Meat carcasses, for example, can easily be stored in an unequipped wagon for up to seven days, and if the wagon is provided with the simplest thermal insulation, then all ten. There are other advantages as well. Everyone must have seen how tens of meters of icy metal pipes stretch along the walls of the refrigeration chamber of an ordinary warehouse or store, through which freon circulates. This is not the case with an air chiller. And yes, it is small in size. You can deliver and install it almost anywhere. The jet of cold air from the machine can be directed to any place. For example, the figure shows an inflatable warehouse for perishable vegetables, spread out right in the field. The benefits of such an installation become especially obvious, given that with a plentiful harvest, up to 80% of the products often die in the field. Given the huge demand for such equipment, the company has already built several installations for the rapid freezing of fish on fishing vessels. By the way, due to the lack of such installations in our country, up to 50% of the fish caught is now lost!
Now a few words about the efficiency of air chillers in relation to the food industry. It is useful for a future store owner or farmer to understand the difference between parade figures, which are usually given in the characteristics of freon refrigerators, and what is obtained in practice. The thing is that they are usually tested with clean pipes that have not yet had time to be covered with a crust of ice. Under these conditions, for the first hours of operation, freon refrigerators "prove" their exceptional efficiency. However, then the pipes are covered with frost and ice. For a month of operation of the refrigerating chamber filled with goods, the thickness of the "fur coat" increases so much that the energy consumption increases by 1,5-2 times. There are no such pipes in air coolers, and there are no similar problems. Therefore, for long-term storage, air coolers are unrivaled in terms of economy. Author: A.Vargin
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