Absorption Refrigerating Systems

Although the principle of absorption refrigeration cycle was known already in the early 1800s, first invented by a French engineer Ferdinand P.E. Carre in 1860, intermittent crude absorption of ammonia apparatus based on the chemical affinity ammonia water, produced ice on a limited scale. The first five ARS units Carre produced were used to make ice, up to 100 kg per hour. In the 1890s, many large ARS units were produced for chemical and petroleum industry. Development of the ARSs slowed to a full stop to 1911 as a vapor compression refrigeration systems, came to the forefront. After 1950, large ARSs gained popularity. In the 1970s, the market share of ARSs plunged due to the oil crisis and, consequently, decisions of the government. Due to rising energy prices and environmental impact of refrigerants during the last decade ARSs received increasing attention. Thus, many companies are focusing on ARSs and now research and development, while the market demand is increasing dramatically.

ARSs have experienced a lot of UPS and downs. The system, which was the forerunner of the steam-compression refrigeration machines in the nineteenth century, and ammonia water systems enjoy a variety of applications in domestic refrigerators and large industrial enterprises of the chemical and processing industries. They were energized by steam or hot water is generated from natural gas, oil boilers and electric space heaters. In the 1970s there has been a transition from the direct combustion of oil and natural gas stroke a blow at the request of the ARSs but at the same time opened up other possibilities, such as the use of heat derived from solar collectors to energize these systems.

The concept of absorption cooling was developed long before the advent of electric drive refrigerators. In recent decades, the availability of cheap electricity did absorption systems are less popular. Today, improvements in technology absorption, growth of cost and environmental impact of generating electricity contribute to the growth of popularity of absorption systems. ARSs for industrial and domestic use, attracts a growing interest around the world, because the following advantages in comparison with other systems of cooling:

* silent operation
* high reliability,
* long service life,
* effective and efficient use of low-potential energy sources (e.g. solar energy, waste, energy, geothermal energy),
* simplicity of control performance
* no Cycling losses during the on-off operations,
* ease of implementation, and
* Meeting the variable load easily and efficiently.

Recently, there has been increasing interest in the industry (Fig. 3.45) and the internal use of the ARSs for meeting air conditioning and cooling demands in the alternative, due to trends in the world for the rational use of energy sources, environmental protection, and prevention of the depletion of the ozone layer, as well as reduce pollution. There are a number of applications in various industries, where ARSs occupied, including:

* food industry (meat, dairy products, vegetables and freezing and storage of food, fish industry, freeze drying),
* chemical and petrochemical industry (liquefaction if gas separation processes),
* cogeneration combined production of heat and cold (trigeneration plants),
* entertainment sector (compactors),
* Systems of ventilation and conditioning
* refrigeration equipment, and
* cold storage.

Absorption cycle is a process by which, primarily, the effect produced by using two liquids and some of the heat, not the electric input, and in the more familiar vapor compression cycle. In ARSs, secondary fluid (i.e. absorbent) are used for the transmission and absorption primary fluid (i.e. refrigerant), which evaporates in the evaporator. Success of the process of absorption depends on the choice of the correct combination of refrigerant and absorbent. The most commonly used refrigerant and absorbent combinations in ARSs were ammonia-water and lithium bromide water. Lithium-bromide water, steam, available for air conditioning and refrigeration applications (more than 4BC, due to the crystallization of water). Ammonia-water is used for cooling and low freezing temperature applications (below 0VC).

Absorption cycle uses heat-driven difference in the concentration move refrigerant vapors (usually water), from the evaporator in the condenser. High concentration of side cycle absorbs refrigerant vapors (which, of course, dilutes that material). The heat is then used to ward off these refrigerants, thereby increasing the concentration again.

Both vapor compression and absorption refrigeration cycles of execution of the removal of heat by evaporation of the coolant low pressure, and the rejection of heat through the condensation of the refrigerant at very high pressures.

Extensive research to find the right chemicals for ARSs were conducted with the use of solubility of measure for the binary systems. While this information is useful as a rough method for selecting suitable for binary systems, more sophisticated research, now, it seems, you should be familiar with the basics of absorption phenomena.

During the last decade of numerous experimental and theoretical studies ARSs have been taken to develop alternative fluids like R22-dimethyl ethertetraethylene glycol (DMETEG), R21-DMETEG, R22 - dimethylformamide (DMF), R12-dimethylacetamide, R22-dimethylacetamide, and R21-dimethyl ether. Previous studies have shown that ammonia, R21, R22, and methylamine promise as refrigerants, and organic glycol, certain amines, esters and other fulfill the conditions for a good absorbents. Recently, environmental issues have led some alternative working fluid to the fore, e., R123a-ethyltetrahydrofurfurylether (ETFE), R123a-DMETEG, R123a DMF. and R123a-trifluoroethanol, due to CFCs, ozone depletion effects.

The efficiency of the cycle and operational characteristics of the ARS depend on the thermophysical properties of the refrigerant, absorbent materials, and their combinations. The most important properties for the choice of the working fluid vapor pressure solubility, density, viscosity, and thermal stability. Knowledge of these required properties to define and other physical and chemical properties and parameters that affect the performance, size and cost.

Please note that ammonia will be quickly corrode copper, aluminum, zinc, and all these alloys of metals, therefore these metals can not be used where ammonia is present. Of common materials only from steel, cast iron and stainless steel can be used in ammonia ARSs. Most plastics are also resistant to chemical action of ammonia, hence plastic suitable for the seat, pump parts and other small parts of the system.