Vacuum cooling
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Vacuum cooling is a rapid
cooling Cooling is removal of heat, usually resulting in a lower temperature and/or phase change. Temperature lowering achieved by any other means may also be called cooling.ASHRAE Terminology, https://www.ashrae.org/technical-resources/free-resources/as ...
technique for any
porous Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
product that has free water and uses the principle of
evaporative cooling An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning sy ...
. Vacuum cooling is generally used for cooling food products that have a high water content and large porosities, due to its efficacy in losing water from both within and outside the products. This is the most widely used technique for rapid cooling of food products which has been proven to be one of the most efficient and economical methods of cooling and storage of vegetables, fruits, flowers, and more. This cooling technology not only largely improves the product quality, but also increases the
shelf life Shelf life is the length of time that a commodity may be stored without becoming unfit for use, consumption, or sale. In other words, it might refer to whether a commodity should no longer be on a pantry shelf (unfit for use), or no longer on a ...
of the product and, at the same time, it reduces the cooling costs compared to the conventional cooling methods available.


Principle

The technology is based on the phenomenon that as the
vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases ...
on a liquid reduces, its
boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...
reduces. The boiling point of a liquid is defined as the temperature at which the vapor pressure of the liquid is equal to the external pressure. When the pressure put onto a liquid is reduced, the vapor pressure needed to induce boiling is also reduced, and therefore the boiling point of the liquid decreases. By reducing pressure, boiling water is possible at lower temperatures. This rapid evaporation of moisture from the surface and within the products due to the low surrounding pressure absorbs the necessary
latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be understo ...
for
phase change In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic State of ...
from the product itself. This latent heat required for evaporation is obtained mostly from the
sensible heat Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system, and some macroscopic variables of the body or system, but leaves unchanged certain other macroscopic vari ...
of the product and as a consequence of this evaporation the temperature of the product falls and the product can be cooled down to its desired storage temperature.


Process

An airtight chamber is maintained by removing air from the inside of the chamber using a
vacuum pump A vacuum pump is a device that draws gas molecules from a sealed volume in order to leave behind a partial vacuum. The job of a vacuum pump is to generate a relative vacuum within a capacity. The first vacuum pump was invented in 1650 by Otto v ...
. The products to be cooled are kept in that airtight chamber. As the pressure is reduced the boiling point of water reduces and water starts to evaporate, taking heat from the product. As a consequence of this evaporation, the product temperature begins to decrease. This cooling process of the products continues until it reaches the desired product temperature. For maintaining a steady cooling process, it is necessary to evacuate the chamber continuously. Other factors that determine the cooling process are the surface area of the product that is available for
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
as well as the product's sensitivity to losing water.


Advantages

As the product is cooled uniformly throughout the body without any temperature gradient in the body, the shelf life of the product increases. Cooling the product through vacuum cooling takes roughly a quarter of the energy of other traditional cooling methods.


Disadvantage

Sometimes excess moisture loss during the cooling process will deteriorate the product's quality and therefore there is a limit to the cooling process. This problem is to be taken care of by maintaining the required pressure, temperature, and time of cooling.


References

* H.M. Ozturk, H.K. Ozturk, Effect of pressure on the vacuum cooling of iceberg lettuce Int. J. Refrig. Revue Int. Du. Froid, 32 (3) (2009), pp. 402–410 * {{cite journal, journal=Critical Reviews in Food Science and Nutrition , volume=52, issue=11, year=2012 , title=Vacuum Cooling of Meat Products: Current State-of-the-Art Research Advances, doi=10.1080/10408398.2011.594186, last1=Feng , first1=Chaohui , last2=Drummond , first2=Liana , last3=Zhang , first3=Zhihang , last4=Sun , first4=Da-Wen , last5=Wang , first5=Qijun , pages=1024–1038 , pmid=22823349 , s2cid=34953281 * http://issuu.com/coldmax/docs/coldmax_web_uk/1 Cooling technology Applied and interdisciplinary physics Engineering thermodynamics Industrial gases Food preservation