Absorption Refrigeration Systems
Although the principle of the absorption refrigeration cycle has been known since the early 1800s, the first one was invented by French engineer Ferdinand P.E. Carre in 1860, an intermittent crude ammonia absorption apparatus based on the chemical affinity of ammonia for water, and 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 manufactured for chemical and petroleum industries. The development of ARSs slowed to a standstill by 1911 as vapor compression refrigeration systems came to the forefront. After 1950, large ARSs gained in popularity. In 1970s the market share of ARSs dropped rapidly due to the oil crisis and hence the government regulations. Due to increasing energy prices and environmental impact of refrigerants, during the past decade ARSs have received increasing attention. So, many companies have concentrated on ARSs and now do research and development on these while the market demand increases dramatically.
ARSs have experienced many ups and downs. The system was the predecessor of the vapor-compression refrigeration system in the nineteenth century, and water-ammonia systems enjoyed a variety of applications in domestic refrigerators and large industrial installations in the chemical and process industries. They were energized by steam or hot water generated from natural gas, oil-fired boilers, and electrical heaters. In the 1970s the shift from direct burning of oil and natural gas struck a blow at the application of the ARSs but at the same time opened up other opportunities, such as the use of heat derived from solar collectors to energize these systems.
The concept of absorption refrigeration developed well before the advent of electrically driven refrigerators. In the last decades, the availability of cheap electricity has made absorption systems less popular. Today, improvements in absorption technology, the rising cost and the environmental impact of generating electricity are contributing to the increasing popularity of absorption systems. ARSs for industrial and domestic applications have been attracting increasing interest throughout the world because of the following advantages over other refrigeration systems:
* quiet operation,
* high reliability,
* long service life,
* efficient and economic use of low-grade energy sources (e.g. solar energy, waste energy, geothermal energy),
* easy capacity control,
* no cycling losses during on-off operations,
* simpler implementation, and
* meeting the variable load easily and efficiently.
Recently, there has been increasing interest in the industrial (Figure 3.45) and domestic use of the ARSs for meeting cooling and air conditioning demands as alternatives, due to a trend in the world for rational utilization of energy sources, protection of the natural environment, and prevention of ozone depletion as well as reduction of pollution. There are a number of applications in various industries where ARSs are employed, including:
* food industry (meat, dairy, vegetables and food freezing and storage, fish industry, freeze drying),
* chemical and petrochemical industry (liquefying if gases, separation processes),
* cogeneration units in combination with production of heat and cold (trigeneration plants),
* leisure sector (skating-rinks),
* refrigeration, and
* cold storage.
The absorption cycle is a process by which the refrigeration effect is produced through the use of two fluids and some quantity of heat input, rather than electrical input as in the more familiar vapor compression cycle. In ARSs, a secondary fluid (i.e. absorbent) is used to circulate and absorb the primary fluid (i.e. refrigerant), which is vaporized in the evaporator. The success of the absorption process depends on the selection of an appropriate combination of refrigerant and absorbent. The most widely used refrigerant and absorbent combinations in ARSs have been ammonia-water and lithium bromide-water. The lithium bromide-water pair is available for air-conditioning and chilling applications (over 4°C, due to the crystallization of water). Ammonia-water is used for cooling and low temperature freezing applications (below 0°C).
The absorption cycle uses a heat-driven concentration difference to move refrigerant vapors (usually water) from the evaporator to the condenser. The high concentration side of the cycle absorbs refrigerant vapors (which, of course, dilutes that material). Heat is then used to drive off these refrigerant vapors thereby increasing the concentration again.
Both vapor compression and absorption refrigeration cycles accomplish the removal of heat through the evaporation of a refrigerant at a low pressure and the rejection of heat through the condensation of the refrigerant at a higher pressure.
Extensive studies to find suitable chemicals for ARSs were conducted using solubility measurements for given binary systems. Although this information is useful as a rough screening technique for suitable binary systems, more elaborate investigations now seem necessary to learn more of the fundamentals of the absorption phenomena.
During the last decade, numerous experimental and theoretical studies on ARSs have been undertaken to develop alternative working fluids, such as R22-dimethyl ethertetraethylene glycol (DMETEG), R21-DMETEG, R22- dimethylformamide (DMF), R12-dimethylacetamide, R22-dimethylacetamide, and R21-dimethyl ester. Previous studies indicated that ammonia, R21, R22, and methylamine hold promise as refrigerants, whereas the organic glycols, some amides, esters, etc. fulfill the conditions for good absorbents. Recently, environmental concerns have brought some alternative working fluids to the forefront, e.g. R123a-ethyltetrahydrofurfurylether (ETFE), R123a-DMETEG, R123a-DMF. and R123a-trifluoroethanol, because of the CFCs’ ozone depletion effects.
The cycle efficiency and the operating characteristics of an ARS depend on the thermophysical properties of the refrigerant, the absorbent, and their combinations. The most important properties for the selection of the working fluids are vapor pressure, solubility, density, viscosity, and thermal stability. Knowledge of these properties is required to determine the other physical and chemical properties, as well as the parameters affecting performance, size, and cost.
Note that ammonia will quickly corrode copper, aluminum, zinc, and all alloys of these metals, therefore these metals cannot be used where ammonia is present. From common materials only steel, cast iron, and stainless steel can be used in ammonia ARSs. Most plastics are also resistant to chemical attack by ammonia, hence plastics are suitable for valve seats, pump parts, and other minor parts of the system.