A little history of refrigeration.
Refrigeration is a long known process. In the twelfth century the Chinese used mixtures of saltpeter in order to cool water; in the sixteenth and seventeenth centuries, researchers and authors such as Boyle,Faraday (with their experiments on the vaporization of the ammonia) make the first practical attempts of production of cold.
In 1834, Perkins developed his patent for an ether compression refrigerating machine and in 1835 Thilorier manufactures carbon dioxide by expansion; Tellier built the first compression machine for commercial purposes, Pictet develops a sulfur dioxide compression machine.
Ammonia was the first refrigerant used in refrigeration plants by means of mechanical compression in 1876 by Carl von Linde. Since then, it has been used in large refrigeration plants such as dairies, breweries, trails and other places with great demands of cooling.
To date, ammonia remains the most widely used refrigerant in industrial refrigeration systems for processing and preserving most food and beverages. Ammonia has been at the forefront of advances in refrigeration technology as an essential part of food processing, storage and logistics.
Classified by ASHRAE with R-717, within the group of natural refrigerants, it does not destroy the ozone layer and does not contribute to the greenhouse effect associated with global warming. In fact ammonia, is a compound commonly found in nature. It is essential in the nitrogen cycle of the earth and its release into the atmosphere is immediately recycled. This makes it consistent with international agreements on the reduction of global warming and destruction of the ozone layer.
Ammonia and its advantages.
Ammonia growth is increasing globally, as environmental restrictions on chlorinated or flurochlorinated refrigerants have caused ammonia to emerge as a refrigerant that does not contribute to the depletion of the ozone layer or to global warming. Ammonia is an efficient refrigerant used in food preservation and processing, as well as many other applications in refrigeration and air conditioning. In the food industry the use of ammonia is very widespread, reaching approximately 90% of existing facilities. Ammonia has desirable characteristics as a refrigerant that have been known for about a century, and the most important of all is that the energy efficiency of ammonia is greater than the rest of the refrigerants, so using ammonia reduces the energy consumption of the installation. Also the cost of ammonia is an attractive advantage since it is much smaller than that of any synthetic refrigerant. In general terms costs between 10 and 20% less in installation.
It should be noted that ammonia is corrosive and dangerous when released into the atmosphere in large quantities and therefore requires special precautions. Due to their irritating nature, people can not remain in atmospheres that contain ammonia because it is dangerous to health. Although ammonia can burn in high concentrations, its ignition is difficult and combustion is not maintained if the source of ignition is removed.
An adequate assessment of the environmental impact of refrigerants and refrigeration systems requires consideration of both their direct and indirect impact on global warming. Directly the refrigeration systems contribute to global warming, through the greenhouse effect caused by the leaks of refrigerant gases. Indirectly contribute to global warming by the production of carbon dioxide emissions as a result of the conversion of fossil fuels into the energy required to operate the cooling systems.
The "total equivalent impact of warming" or TEWI, is defined as the sum of these direct and indirect contributions. The TEWI value of ammonia is very low as it does not contribute to global warming by itself. Due to their favorable thermodynamic characteristics, ammonia refrigeration systems use less energy than other common refrigerants. As a result, there is an indirect benefit to global warming due to lower CO 2 emissions from power plants.
|Properties of Ammonia|
|• Autoignition temperature: 690 ° C (1274 ° F) • Lower Flammability Limit (LII): 16% • Upper Flammability Limit (LSI): 25%|
Ammonia as a refrigerant.
Ammonia is a moderate fuel, and considered by experts within the chemical industry relatively as non-combustible. The combustion energy of ammonia is less than its self-igniting energy, this means that ammonia can not be kept on by itself without an external source of ignition, even though the same source has started the fire.
Ammonia in high concentrations is extremely toxic, but its strong odor is an excellent alarm. The concentration of ammonia where its odor can not be supported (about 0.03% by volume), is not harmful, as long as it is exposed to it only for a limited period of time (even after more than one hour, there is no negative effects on the health of people).
The cost of ammonia is much lower than any synthetic refrigerant, generally costing 10 to 20% less in installation. Thermodynamically, ammonia is 3 to 10% more efficient than other refrigerants; As a result, an ammonia cooling system has lower electrical consumption.
The cost of ammonia per se is significantly lower than that of other refrigerants, and a smaller amount is required for the same application as other refrigerants, and being a natural substance, there is no deadline for production or use , unlike other synthetic refrigerants whose use or production is limited to a certain amount of years.
|Table 1: Thermodynamic Properties (-8 ° C)|
|Specific heat (KJ / Kg ° C)||4.65||1.15|
|Thermal conductivity (W / m ° C)||0.55||0.10|
|Table 2: Heat Transfer Coefficient (W / m2 ° C)|
|Condensing on the outside of the tubes||3500-7000||1000-2000|
|Condensing inside the tubes||2500-6000||1000-1800|
|Evaporating on the outside of the tubes (Circulation with pump)||600-6000||300-3500|
|Evaporating inside the tubes (Circulation with pump)||1000-6000||450-1800|
|Table 3: Coefficient of operation or cooling effect *|
|THEORETICAL COP (+ 30 / -15 ° C)|
|COP =||Cooling capacity||=||kWkW|
|* The amount of refrigeration obtained from a machine divided by the amount of energy required to provide this refrigeration (ASHRAE, 1993)|
Ammonia has a heat transfer coefficient greater than R-22, mainly due to its thermodynamic and transport properties. The values for these properties in relation to R-22 are as follows:
• Specific heat of liquid and vapor: 4 to 1
• Heat latent in the vaporization: 6 to 1
• Thermal conductivity: 5.5 to 1
• Viscosity: 0.8 to 1
• Liquid density: 0.5 to 1
The mass flow rate for a given ammonia cooling capacity is 1/7 less than R-22, which has a significant effect on the size of the pipes and on the flow of the liquid.
This means that only 1/7 of the liquid needs to be pumped for a given cooling capacity, as a result, it is a smaller pump that uses less power, and in smaller pipes.
Refrigeration Plants with Ammonia
Compression refrigeration plants (see Figure 1) consist of an evaporator, in which the refrigerant (ammonia) is evaporated producing cold; a compression system for conveying low pressure steam from the evaporator to the high pressure condenser; and the condenser in which the refrigerant condenses the heat by generally dissipating the heat through cooling towers.
|Figure 1. Compression refrigeration|
Absorption refrigeration plants (see Figure 2) require a cooling fluid and an absorbing fluid. The most commonly used refrigerant / absorber pairs are the water / lithium bromide pair and the ammonia / water pair. In plants that use the first pair, the coolant is water so these plants are used for applications at temperatures above 0 ° C, mainly used for air conditioning.
|Figure 2. Absorption refrigeration|
Ammonia / water refrigeration plants use it as a refrigerant and have the application range from 0 ° C to -70 ° C.
In absorption refrigeration plants the mechanical compressor is replaced by a chemical or thermal compressor. The low pressure steam from the evaporator, instead of being compressed by a mechanical compressor, is absorbed by a dilute solution of ammonia and water in the absorber. The solution whose concentration has increased is pumped to the desorber, where it will be heated to its boiling point.
Since ammonia is the most volatile component in the desorber, ammonia vapor is produced, which condenses into the condenser, thus closing the refrigeration cycle. The heat produced in the condenser and the absorber is usually dissipated by cooling towers, whereas the heat supplied in the desorber is waste heat from, for example, a cogeneration plant.
Uses of ammonia in refrigeration
With increasing regulations on the use of chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), and phasing out of CFCs and HCFCs, other refrigerants were actively investigated for use in systems existing refrigeration systems. Alternative refrigerants should have thermodynamic characteristics similar to halocarbons and be safe for humans and the environment. Ammonia is an alternative refrigerant for both refrigeration systems and air conditioning equipment. It can also be used in both new and existing equipment. It has a low vaporization point (-33 ° C), a zero ozone depletion potential when released into the atmosphere and a high latent heat of vaporization (9 times higher than R-12). In addition, ammonia in the atmosphere does not contribute directly to global warming. These characteristics result in a highly energy efficient refrigerant with minimal environmental problems. With a pure economic analysis, without regulatory and necessary burdens, the ammonia would find applications more ample than those at the moment enjoys. Comply with the design and kicking codes assuming ammonia safety your applications increasing. Applications of ammonia-based refrigeration systems include thermal storage systems, coolers for air-conditioning applications, supermarkets, etc. These characteristics result in a highly energy efficient refrigerant with minimal environmental problems. With a pure economic analysis, without regulatory and necessary burdens, the ammonia would find applications more ample than those at the moment enjoys. Comply with the design and kicking codes assuming ammonia safety your applications increasing. Applications of ammonia-based refrigeration systems include thermal storage systems, coolers for air-conditioning applications, supermarkets, etc. These characteristics result in a highly energy efficient refrigerant with minimal environmental problems. With a pure economic analysis, without regulatory and necessary burdens, the ammonia would find applications more ample than those at the moment enjoys. Comply with the design and kicking codes assuming ammonia safety your applications increasing. Applications of ammonia-based refrigeration systems include thermal storage systems, coolers for air-conditioning applications, supermarkets, etc.
Ammonia as a refrigerant has the great advantage of being able to produce refrigeration at temperatures up to -70 ° C. To achieve these temperatures, multi-stage compression systems are required, so these plants are relatively complex. Thus the continuous operation of these plants is problematic, since there are practically no oils compatible with ammonia, which have lubricating qualities at the temperature of the compressors and a low viscosity at -60 ° C.
The oil that usually accumulates in evaporators can only be decanted if the temperature is temporarily raised. All this makes compressor plants more expensive and requires very strict maintenance in order to guarantee the necessary reliability. Especially at these temperatures absorption refrigeration plants have great advantages compared to the compression refrigeration plants. On the one hand they can reach temperatures up to -70 ° C in a single step and on the other hand do not need lubricating oils so they can operate continuously without the need for stops.
Traditionally, ammonia absorption refrigeration plants have always been used in industrial sectors where refrigeration is required at low temperatures and where the availability of continuous refrigeration is of great importance; in these sectors trigeneration can usually be applied.
In the trigeneration plants the heat produced by the cogeneration systems is used to cover the consumption of heat and to propel an absorption cooling plant to cover the demand for cold.
These plants, when combining the heat and cold supply, have a great flexibility, obtaining an optimal use of the heat generated in the cogeneration. In general, the demands of refrigeration at low temperatures are usually relatively constant and usually have a high thermal inertia.
The absorption cooling plant can be regulated in such a way as to consume all surplus heat (usually steam), giving priority to the direct consumption of steam, thus achieving a high utilization of the heat produced in cogeneration