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The bad and the good of CO2

One of the coolants that most market is getting is CO "and that is why it is important to know its characteristics in order to make the best choice.

by Ing. Ernesto Sanguinetti Remusgo *

CO2 was formerly called carbon dioxide, but a few years ago, the International Union of Pure and Applied Chemistry made changes in inorganic nomenclature by establishing the use of the name of Oxide for anhydrides of inorganic acids such as carbonic, sulfuric, nitric and So terms like carbon dioxide instead of carbon dioxide, sulfur trioxide instead of sulfuric anhydride, nitrogen oxide instead of nitric anhydride are now being used.

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The bad
Carbon dioxide is considered a greenhouse gas, which absorbs and emits infrared radiation.

Solar radiation at frequencies of visible light passes mostly through the atmosphere to heat the surface of our planet, part of that energy is absorbed by the surface and part is emitted at lower frequencies of infrared thermal radiation out of the planet. For thousands of years that balance was maintained so the average temperature of the atmosphere remained unchanged and as we know with very defined seasons throughout each year according to geographical location. The incoming solar radiation in the atmosphere must be compensated for by the outgoing radiation, because if the incoming radiation is greater than the outgoing radiation, global warming occurs and if otherwise the global cooling occurs.

However, the activities that have led to the development and human progress since the Industrial Revolution, mainly with the burning of fossil fuels, tree felling and forest burning, are unbalancing the exchange of heat causing the so-called "greenhouse effect" or "warming global".

The "greenhouse effect" is a process by which the thermal radiation emitted by the surface of the earth is absorbed by the atmospheric greenhouse gases (located at the height of the clouds) and is re-irradiated in all directions. Part of this radiation is returned to the surface and the lower atmosphere, resulting in an increase in the average surface temperature compared to what would be in the absence of such gases.

Most experts indicate that the increase in the atmospheric concentration of CO2, and therefore the CO2-induced greenhouse effect, is the main reason for the increase in global average temperature since the middle of the last century. The main greenhouse gas responsible for heating is CO2, but methane, nitrous oxide, ozone and other gases, such as refrigerants used in refrigeration and air conditioning equipment, also contribute. The CO2 is the most worrying, because it exerts a greater influence on the heating than all the other gases combined, and because it has a long life. It is estimated that the mass of CO2 emitted into the atmosphere: about 50% will take 30 years to disappear, an 30% will remain one hundred or two hundred years and the remaining 20% will last several centuries.

That is why corrective measures are being taken through the much discussed Kyoto Protocol.

The good
CO2 is so common and frequent in our lives that many of the things we consume and do in our day to day, we do not know or do not realize that they work or are made with this compound:

It is used as a fire extinguishing agent because it makes contact with the oxygen of the air difficult.

In the food industry, it is used in carbonated drinks: soda, beer, champagne. Also to preserve vegetables in controlled atmospheres and like "dry ice" to preserve ice cream.

In medicine: insufflation agent in laparoscopic surgeries, contrast agent in blood vessel radiology, in CO2 laser, treatment of cranial wounds and ulcers, aesthetic treatments, treatment of circulatory problems. As "dry ice" it is used for the long-distance transport of biological specimens, for cryopreservation, for the storage of blood platelets without requiring the use of electromechanical freezers.

In agriculture, it can be used as fertilizer: plants can not absorb it by the roots but can be added to lower the pH, avoid deposits of lime and make more available soil nutrients. As insensitive agent of pigs or chickens, those that before being slaughtered are exposed to CO2. There is no residue left in the meat.

It is used to create artificial fog and appearance of boiling water, in special effects in the cinema, television and spectacles. Also in refrigeration it is used as "dry ice" for the preservation of some frozen products and as refrigerant in steam refrigeration machines.

Use in refrigeration
Damage to the Ozone Layer and Global Warming has led the world of refrigerants used in refrigeration and air conditioning equipment to take a new path through so-called "green refrigerants." Inside them is the CO2 that is presented as a novelty and therefore many people believe that it is a new refrigerant but it is not.

By the year 1,750 the CO2 is identified as a substance other than air because the Scot Joseph Black noted that the calcium carbonate present in the limestone, when heated, produced a gas called "fixed air". This "fixed air" or carbon dioxide was denser than air and did not serve to achieve or maintain the fire. Subsequently it was found that carbon dioxide, now carbon dioxide, occurs during animal respiration and fermentation.

By the year 1,772, the English chemist Joseph Priestley published a document describing a process of sulfuric acid dripping on the chalk to produce carbon dioxide, which gas passed through water contained in a vessel and shaking the vessel so that There is more contact between water and gas, obtained water with tiny particles of gas distributed in its breast. Without realizing it he had invented carbonated water.

The application of carbon dioxide in refrigeration systems dates back many years:

  • The CO2 was liquefied (at high pressures) on 1823 by Humphry Davy and Michael Faraday.
  • In 1,834 Charles Thilorier describes what is solid carbon dioxide after uncovering a pressure vessel containing CO2 liquid.
  • At 1,850, Alexander Twining was the first person to propose CO2 as a refrigerant and mentions it in a British patent for that year.
  • At 1,867, Thaddeus SC Lowe experimented with CO2 on balloons with military use, also designed an ice machine using CO2 and towards 1,878 built a machine to transport frozen meat on boats.
  • Time passed and cooling systems with CO2 had some roots between 1,925 and 1,933 because it was the preferred choice for use on ships instead of NH3 or ammonia.
  • Between 1,929 and 1,935, Chlorofluorocarbon (CFC) refrigerants appear and the application of CO2 as a refrigerant decreased abruptly because these new refrigerants had good cooling capacity, were non-toxic and flammable and had very low working pressures for any temperature range .

Phase diagram
It is important to know the behavior of CO2 and therefore it is interesting to compare it with a fluid with which we are very familiar: water.


Imagen 1
Water Triple Point Conditions:
PRESSURE = 611.73 Pa = 0.00611 bar = 0.089 psia
TEMPERATURE = 273.16 ° K = 0.0098 ° C (0.01 ° C is considered)
Critical Water Point Conditions:
PRESSURE = 22.1 MPa = 221 bar = 3,205.3 psia
TEMPERATURE = 674.4 ° K = 374.2 ° C
It is observed that at atmospheric pressure we can move from solid to liquid and then to steam as the temperature increases, because its Triple Point is below.

It is also observed that its Critical Point is at very high pressure and temperature, being difficult to reach under normal conditions of use.

The graph also shows that the saturation curve separating the Solid-Liquid phases has slight slope to the left. It is exclusive behavior of water because any other substance that exists in nature will have slope to the right as we will see for CO2. Thanks to this property is that you can only "skate" on the water ice.


Imagen 2
CO2 Triple Point Conditions:
PRESSURE = 518 KPa = 5.18 bar = 75.13 psia
TEMPERATURE = 216.55 ° K = - 56.56 ° C (considered -56.6 ° C)
Conditions of the CO2 Critical Point:
PRESSURE = 7.382 MPa = 73.82 bar = 1,070 psia
TEMPERATURE = 304.1 ° K = 31 ° C

It is observed that at atmospheric pressure we can go directly from solid to vapor (Sublimation process) as the temperature increases, because its Triple Point is above. It also indicates that care should be taken when handling it in equipment because it easily passes from liquid to solid (dry ice) in low temperature applications.

Its Critical Point is at high pressure, but its temperature is very close to the ambient temperature, being easy to reach under normal conditions of use. Due to this property, it is possible to work in the refrigeration equipment, following a thermodynamic cycle with the condensation process below the Critical Point (Subcritical condition) as well as with the condensation process on the Critical Point (Supercritical or Transcritical condition) as an explanation Can be displayed as follows:

3 Image. Figure showing the Subcritical and Transcritical Thermodynamic Cycles for CO2

For countries with high and relatively high ambient temperatures, which are approximately between the parallels 40 ° latitude North and 35 ° latitude South, it is advisable to work with Subcritical cycles not to exceed the 31 ° C of condensation temperature of CO2 or refrigerant R-744 according to Nomenclature of ASHRAE.

Note that the capacitor in a transcritical cycle is actually a coolant cooler and that condensation itself is done in the expansion device.

As a reference we show schematically the use of CO2 or R-744 as a secondary refrigerant "cooled" by another refrigerant to be pumped as liquid is an application where the evaporator becomes a cooler. This is only used in medium / high temperature applications.


Imagen 4
Also, as a reference, we show schematically the use of CO2 or R-744 as a secondary refrigerant "cascade" with another refrigerant to fulfill a subcritical thermodynamic cycle. In this case the R-744 evaporator is used for medium / low temperature applications.


Imagen 5
When using parallel compressors or compressor racks that allow varying system capacity or one of the compressors is "Inverter" or "Digital" it is better to use electronic expansion valves instead of thermostatic expansion valves because they modulate flow Of refrigerant and are adapted to the variations of thermal load achieving great energy savings as a whole.

Supermarkets are taking advantage of these cooling systems very well, being able to combine both systems explained.

* Ing. Ernesto Sanguinetti R. is the Manager of the Engineering Division at Cold Import SA in Lima, Peru. It can be contacted through email titosangui@gmail.com

Comments  

Alfredo Diaz Santa C
# Alfredo Diaz Santa C 08-08-2017 18: 40
As clear and didactic as when he received his classes of Thermal Processes II at UNI.
75 Promotion.
Vicentecabestany
# Vicentecabestany 24-08-2017 08: 16
Congratulations, very clear and definitory.

Thank you
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