Indirect refrigeration systems

This article focuses on the contribution that indirect refrigeration systems can make in the protection of the environment.

by Ing. Ernesto Sanguinetti R. *

Although indirect refrigeration systems are not new because they are used since the time that "artificial ice" was first produced, they have now found a remarkable expansion for two main reasons:

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1.-The worldwide concern about the harmful effects on the Earth's atmosphere produced by synthetic refrigerants such as Chlorinated Fluoro Carbonados (CFC), Hydro Chlorinated Fluoro Carbonados (HCFC) and Hydro Fluoro Carbonados (HFC).

2.-The increase in energy consumption, parallel to the increase in its cost.

The first reason is favoring the use of natural refrigerants such as ammonia (NH3), carbon dioxide (CO2), hydrocarbons HC (Propane, Isobutane and others), some new synthetic coolants such as Hydro Fluoro Olefins (HFO), as well as mixtures of HFO with HFC, of ​​HFO with HC.

All this regulation of elimination of harmful refrigerants is contemplated in the so-called Montreal Protocol and its amendments (the last one in Kigali-Rwanda of the 2016) and the Kyoto Protocol with its amendments (the last COP21 in Paris-France of the 2015) to eliminate those refrigerants that damage the ozone layer and that produce global warming or "greenhouse effect" respectively.

The second reason is encouraging the use of more efficient refrigeration machines, to spend less energy achieving equal or greater cooling effect, which is a challenge for the manufacturers of refrigeration and air conditioning equipment. But it is also encouraging the use of secondary liquid refrigerants because the energy expended in pumping a liquid is much less than that which is spent in a compressor that compresses refrigerant in refrigeration equipment. If an efficient refrigeration equipment is combined to cool a secondary fluid, the resulting positive effect is greater.

Direct and indirect cooling systems
Direct systems: those that cool or freeze a substance by the "evaporation" of the refrigerant fluid circulating in a closed circuit by a refrigeration equipment, generally of the vapor compression type. In the following diagram, the refrigerant circulating inside the coil of pipes connected to a condensing unit cools directly the liquid contained in the insulated container.

Figure 1.

Indirect systems: those that cool or freeze a substance under the influence of a fluid that has previously been cooled by the refrigerant that is "evaporated" generally in a vapor compression refrigeration system. In the following diagram, the refrigerant circulating in the coil of pipes connected to the condensation unit directly cools the water or the glycol contained in the insulated container N ° 1 and this fluid indirectly cools the liquid contained in the container. No. 2 insulated vessel circulating through another coil.

Figure 2.

The indirect refrigeration system is widely used in milk cooling plants, in fishing vessels that use recirculated seawater (RSW), in the manufacture of ice in blocks or bars, in ice skating rinks, in air conditioning using water chillers or "chillers". Now it is being given more importance, using more and more in agroindustrial plants and especially in supermarkets. The indirect refrigeration system employs what we call "Secondary Refrigerants", of which we will list the most important ones and explain some of them.

Main secondary refrigerants used
A. Water: It is widely used because it has high specific heat, good thermal conductivity, it is not toxic, it does not harm the environment, it is available in abundance (but it depends on the area), it is relatively cheap. Its drawbacks: it must be as soft as possible (it must be treated if it has many magnesium and calcium salts), free of sand and other solids (it must be filtered) but the biggest of all is that it must be used in applications above 0 ° C because it freezes.

B. Pickles: They are used in many applications where liquid must be kept at temperatures below 0 ° C. They are mixtures of water with salt uniformly diluted throughout the liquid mass.

The water mixture is used with Sodium Chloride (NaCl) which is the common salt, being called the Sodium Chloride Brine mixture, which is not toxic. The more salt is added to the water, the mixture or solution decreases its freezing point, reaching the maximum concentration where the lowest possible temperature or eutectic point is reached and the concentration is called eutectic concentration (-21.1 ° C with 23% in weight of salt). It is characterized because if more salt is added to the solution, the freezing temperature begins to rise again. It has the disadvantage that it is corrosive, so neutralizing substances must be applied to avoid this effect. The behavior of the Sodium Chloride Brine is shown graphically:

Figure 3.

The water mixture is used with Calcium Chloride (CaCl2) which is a granulated, hygroscopic and bitter salt, being called Calcium Chloride Brine. Similarly, the more salt is added to the water, the freezing point decreases, reaching the eutectic point, the maximum concentration being the eutectic concentration (-55 ° C with 30% by weight of salt). Likewise, if more salt is added, the freezing temperature begins to rise again, having a behavior similar to the previous graph of Sodium Chloride Brine but lower temperatures are achieved. It is also corrosive, so neutralizing substances must be applied to avoid this effect.

C. Glycols: Rarely, cigars are used, it is normal to mix them with water. They are divalent alcohols because they have two groups of oxydyls in their molecules and are somewhat sweet in flavor. When they are mixed with water, they become corrosive, which is why they are marketed with incorporated inhibitors.

There is ethylene glycol or ethylene glycol, which is toxic by ingestion and by inhalation and therefore in food applications is prohibited, being used much as antifreeze, for example, in radiators of cars, trucks. In concentrations up to 50% by weight, it is possible to freeze products at -35 ° C because their eutectic point is reached at -67 ° C with a concentration by weight of 70%.

Figure 4.

It is used more propylene glycol or propylene glycol, which is viscous, biodegradable, low toxic by ingestion and by inhalation, therefore it is used in food applications. In concentrations up to 50% by weight, products can be frozen at -35 ° C because their behavior is similar to ethylene glycol.

D. Alcohols: They are cryoscopic organic compounds (lower the freezing temperature of water if mixed with it), the best known being methyl alcohol which is flammable and toxic that excludes it from applications that have to do with food, ethyl alcohol that is colorless and it is mixed with water in any proportion and since it is not toxic it can be applied to food refrigeration, it is volatile and freezes at -114 ° C; Glycerin, which is a viscous, colorless and sweet liquid, mixes with water in all proportions, freezes at -40 ° C.

E. Carbon Dioxide: It has been used more and more in the supermarket Food Cold system. We show schematically the use of CO2 as secondary refrigerant to be "cooled" previously by another refrigerant to be pumped as liquid and indirectly cool a cold room or food preservation cabinets. It is an application where what we commonly call an evaporator is really a cooler, because there is no refrigerant evaporation. It is used in medium / high temperature applications.

Figure 5.

In cascade cooling it can also be considered that indirect cooling is used even though the 2 fluids used evaporate. It is being increasingly used in the refrigeration of Supermarkets or Food Cold systems.

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

Figure 6.

Ammonia, which is a refrigerant that does not harm the ozone layer (ODP = 0), nor produces global warming (GWP = 0) and has excellent thermodynamic properties, is not a recommended refrigerant for use in direct refrigeration of food, therefore in the agro-industry and in Supermarkets it is beginning to be used as a Primary Coolant as replacement of an HFC to cool "in cascade" to a Secondary Coolant such as CO2 (ODP = 0, GWP = 1) that cools food products . For safety, ammonia is confined in the engine room away from the public and food products.

It is shown schematically the use of these two refrigerants that do not produce damage to the terrestrial atmosphere, that is to say, they take advantage of their properties to produce refrigeration and at the same time protect the environment.

Figure 7.
             
* Ing. Ernesto Sanguinetti R.- Manager of Engineering Division-Cold Import SA Lima, Peru.

Quoting products and technical services for Latin America

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