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Double Effect Absorption Refrigeration Systems

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The energy efficiency of absorption can be improved by recovering some of the heat normally rejected to the cooling tower circuit. A two-stage or two-effect ARS accomplishes this by taking vapors driven off by heating the first stage concentrator (or generator) to drive off more water in a second stage. Many ARS manufacturers offer this higher efficiency alternative.

The double-effect ARS takes absorption to the next level. The easiest way to picture a double-effect cycle is to think of two single-effect cycles stacked on top of each other (as shown in Figure 3.50). Note that two separate shells are used. The smaller is the first stage concentrator. The second shell is essentially the single effect ARS from before, containing the concentrator, condenser, evaporator, and ARS. The temperatures, pressures, and solution concentrations within the larger shell are similar to the single-effect ARS as well. The cycle on top is driven either directly by a natural gas or oil burner, or indirectly by steam. Heat is added to the generator of the topping cycle (primary generator), which generates refrigerant vapor at a relatively higher temperature and pressure. The vapor is then condensed at this higher temperature and pressure and the heat of condensation is used to drive the generator of the bottoming cycle (secondary generator), which is at a lower temperature. If the heat added to the generator is thought to be equivalent to the heat of condensation of the refrigerant, it becomes clear where the efficiency improvement comes from.

For every unit of heat into the primary generator, two masses of refrigerant are boiled out of solution, or generated: one in the primary generator and one in the secondary generator. In a single-effect cycle only one mass is generated. Therefore, in a double-effect system, twice the mass flow of refrigerant is sent through the refrigerant loop per unit of heat input, so twice the cooling is delivered per unit of heat input. Using this approach a double-effect system has a  COP that is roughly twice that of a single-effect cycle. However, this simplifying assumption does not account for cycle inefficiencies and losses. In actuality, a single-effect system has a COP of about 0.65, and a double-effect system has a COP of about 1.0. Note that the reuse of the vapors from the first stage generator makes this machine more efficient than single stage absorption chillers, typically by about 30%.

double-effect-ars

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November 26th, 2009 at 10:48 pm

Single Effect Absorption Refrigeration Systems

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As stated earlier, in ARS, an absorber, generator, pump and recuperative heat exchanger replace the compressor. Like mechanical refrigeration, as shown in Figure 3.49, the cycle begins when high-pressure liquid refrigerant from the condenser passes through a metering device (1) into the lower-pressure evaporator (2) and collects in the evaporator pan or sump. As before, the flashing that occurs at the entrance to the evaporator cools the remaining liquid refrigerant. Similarly, the transfer of heat from the comparatively warm system water to the now-cool refrigerant causes the latter to evaporate (2), and the resulting refrigerant vapor migrates to the lower-pressure absorber (3). There, it is soaked up by an absorbent lithium-bromide solution. This process not only creates a low-pressure area that draws a continuous flow of refrigerant vapor from the evaporator to the absorber, but also causes the vapor to condense (3) as it releases the heat of vaporization picked up in the evaporator. This heat—along with the heat of dilution produced as the refrigerant condensate mixes with the absorbent—is transferred to the cooling water and released in the cooling tower. Of course, assimilating refrigerant dilutes the lithium-bromide solution and reduces its affinity for refrigerant vapor. To sustain the refrigeration cycle, the solution must be reconcentrated. This is accomplished by constantly pumping (4) dilute solution from the absorber to the generator (5), where the addition of heat boils the refrigerant from the absorbent. Once the refrigerant is removed, the reconcentrated lithium-bromide solution returns to the absorber, ready to resume the absorption process. Meanwhile, the refrigerant vapor liberated in the generator migrates to the cooler condenser (6). There, the refrigerant returns to its liquid state as the cooling water picks up the heat of vaporization carried by the vapor. The liquid refrigerant’s return to the metering device (1) completes the cycle.

single-effect-ars

Written by sam

November 26th, 2009 at 10:45 pm

Water Lithium Bromide Absorption Refrigeration Systems

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These ARSs utilize a combination of water (as the refrigerant) and lithium bromide (as the absorbent), as the working fluid. These systems are also called absorption chillers and have a wide range of application in air conditioning and chilling or precooling operations and are manufactured in sizes from 10 to 1000 tons, leading to the lowest evaporation temperature of 4°C (with a minimum pressure of 0.8 kPa) because the water is used as the refrigerant. In practical applications the temperature is 5°C. Low-pressure steam is the main energy source for these H2O-LiBr absorption systems. Despite their COPs less than unity, cheap energy can make these systems economically competitive with much higher COP values for vaporcompression systems. In practical H2O-LiBr ARSs the evaporator and absorber are combined in a shell at the lower pressure side and the condenser and generator are combined in another shell at the higher-pressure level. A liquid-liquid heat exchanger is arranged to increase system efficiency and hence to improve the COP. Its operating principle is the same as that of other ARSs. In the H2O-LiBr ARS, crystallization (which is a solidification of the LiBr) appears to be a significant problem. The crystallization lines are shown on the pressure-temperature and enthalpy-concentration charts. Dropping into the crystallization region causes the formation of a slush, resulting in blockage of the flow inside the pipe and interruption of the system operation. In order to prevent this problem, practical systems are designed with control devices to keep the condensation pressure artificially high. Note that absorption chillers are classified into two categories as follows:

• Single stage (single effect) ARS: Units using low pressure (135 kPa or less) as the driving force. These units typically have a COP of 0.7.
• Double stage (double effect) ARS: Units are available as gas-fired (either direct gas firing, or hot exhaust gas from a gas-turbine or engine) or steam-driven with high pressure steam (270 to 950 kPa). These units typically have a COP of 1.0 to 1.2. To achieve this improved performance they have a second generator in the cycle and require a higher temperature energy source.

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November 26th, 2009 at 10:42 pm

Gas Diffusion Absorption Refrigeration Systems

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The two-fluid absorption refrigeration system succeeded in replacing a compressor which requires a large amount of shaft work by a liquid pump with a negligible energy requirement compared to the refrigeration effect. By addition of a third fluid, the pump is removed, completely eliminating all moving parts. This system is also called the von Platen-Munters system after its Swedish inventors. This type of system is shown in Figure 3.48. The most commonly used fluids are ammonia (as refrigerant), water (as absorbent), and hydrogen, a neutral gas used to support a portion of the total pressure in part of the system. Hydrogen is called the carrier gas. The unit consists of four main parts: the boiler, condenser, evaporator and absorber. In gas units, heat is supplied by a burner, and when the unit operates on electricity the heat is supplied by a heating element. The unit charge consists of a quantity of ammonia, water and hydrogen at a sufficient pressure to condense ammonia at the room temperature for which the unit is designed. This method of absorption refrigeration is presently used in domestic systems where the COP is less important than quiet trouble-free operation. In the system shown in Figure 3.48, the cold ammonia vapor with hydrogen is circulated by natural convection through a gas-gas heat exchanger to the absorber, where ammonia vapor comes in contact with the weak solution from the separator. At the low temperature of the ammonia and hydrogen, absorption of the ammonia occurs and hence hydrogen alone rises through the heat exchanger to the evaporator, while the strong solution flows down by gravity to the generator.

three-fluid-ars

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November 26th, 2009 at 10:40 pm

Ammonia Water Absorption Refrigeration Systems

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In practical ARSs the utilization of one or two heat exchangers is very common. Figure 3.47 represents a practical absorption refrigeration system using a working fluid of ammonia as the refrigerant and water as the absorbent, with two exchangers. As can be seen from the figure, in addition to two heat exchangers, this system employs an analyzer and a rectifier. These devices are used to remove the water vapor that may have formed in the generator, so that only ammonia vapor goes to the condenser.

The system shown in Figure 3.47 utilizes the inherent ability of water to absorb and release ammonia as the refrigerant. The amount of ammonia vapor which can be absorbed and held in a water solution increases with rising pressure and decreases with rising temperature. Its operation is same as the system given in Figure 3.46, except for the analyzer, rectifier, and heat exchangers. In the absorber, the water absorbs the ammonia at the condenser temperature supplied by circulating water or air, and hence a strong solution (about 38% ammonia concentration) occurs.

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amonia-water-ars

Because of physical limitations, sometimes complete equilibrium saturation may not be reached in the absorber, and the strong solution leaving the absorber may not be as fully saturated with water as its pressure and temperature would require. This strong solution from the absorber enters the solution pump (the only moving part of the system), which raises its pressure and delivers the solution into the generator through the heat exchanger. Pumped strong solution passes into generator via heat exchanger where strong solution is preheated before being discharged into ammonia generator. Note that the pumping energy required is only a few percent of the entire refrigeration energy requirement. The generator, which is heated by an energy source (saturated steam or other heat source via heating coils or tube bundles), raises the temperature of the strong solution, causing the ammonia to separate from it. The remaining weak solution (about 24% ammonia concentration) absorbs some of the water vapor coming from the analyzer/rectifier combination and flows down to the expansion valve through the heat exchanger. It is then throttled into the absorber for further cooling as it picks up a new charge of the ammonia vapor, thus becoming a strong solution. The hot ammonia in the vapor phase from the generator is driven out of solution and rises through the rectifier for possible separation of the remaining water vapor. Then it enters the condenser and is released to the liquid phase. Liquid ammonia enters the second heat exchanger and loses some heat to the cool ammonia vapor. The pressure of liquid ammonia significantly drops in the throttling valve before it enters the evaporator. The cycle is completed when the desired cooling load is achieved in the evaporator. Cool ammonia vapor obtained from the evaporator passes into the absorber and is absorbed there. This absorption activity lowers the pressure in the absorber and causes the vapor to be taken off from the evaporator. When the vapor goes into liquid solution it releases both its latent heat and a heat of dilution. This energy release has to be continuously dissipated by the cooling water or air.

The heat introduced into the absorption system in the generator (from steam heat) and the evaporator (from actual refrigeration operation) has to be rejected to outside. One heat ejection occurs in the ammonia condenser and other heat ejection occurs in the ammonia absorber. Reabsorpion of ammonia into weak solution generates heat and unfortunately this heat has to be rejected so the absorption process can function. Aqua ammonia consists of water and ammonia. Water can easily absorb ammonia and stay in solution under normal temperature, hence the absorber has to be cooled with cooling water or air. Evaporated ammonia in the generator is passed through the distilling column where the ammonia is concentrated into nearly pure ammonia vapor before going into the condenser. Once ammonia is turned into liquid it is let down into the evaporator, low pressure side, where ammonia is again turned into vapor, by evaporation, while picking up heat from the confined refrigerated space. Ammonia vapor is then absorbed in the absorber to complete the cycle.

For ammonia-water ARSs, the most suitable absorber is the film-type absorber for the following reasons :

* high heat and mass transfer rates,
* good overall performance, and
* large concentration rates.

Written by sam

November 23rd, 2009 at 5:26 am

Absorption Refrigeration Systems Basic Design

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It is considered that the ARS is similar to the vapor-compression refrigeration cycle (using the evaporator, condenser, and throttling valve as in a basic vapor-compression refrigeration cycle), except that the compressor of the vapor-compression system is replaced by three main elements, an absorber, a solution pump, and a generator. Three steps, absorption, solution pumping, and vapor release, take place in an ARS.

In Figure 3.46, a basic ARS, which consists of an evaporator, a condenser, a generator, an absorber, a solution pump, and two throttling valves, is schematically shown. The strong solution (a mixture strong in refrigerant), which consists of the refrigerant and absorbent, is heated in the high- pressure portion of the system (the generator). This drives refrigerant vapor off the solution. The hot refrigerant vapor is cooled in the condenser until it condenses. Then the refrigerant liquid passes through a throttling valve into the lowpressure portion of the system, the evaporator. The reduction in pressure through this valve facilitates the vaporization of the refrigerant, which ultimately effects the heat removal from the medium. The desired refrigeration effect is then provided accordingly. The weak solution (weak in refrigerant) flows down through a throttling valve to the absorber. After the evaporator, the cold refrigerant comes to the absorber and is absorbed by this weak solution (i.e. absorbent), because of the strong chemical affinity for each other. The strong solution is then obtained and is pumped by a solution pump to the generator, where it is again heated, and the cycle continues. It is significant to note that the system operates at high vacuum at an evaporator pressure of about 1.0 kPa; the generator and the condenser operate at about 10.0 kPa.

basic-ars

Written by sam

November 23rd, 2009 at 5:21 am

Absorption Refrigeration Systems

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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),
* HVAC,
* refrigeration, and
* cold storage.

(a) An ARS of 2500 kW at -15°C installed in a meat factory in Spain. (b) An ARS of 2700 kW at -30°C installed in a refinery in Germay. (c) An ARS of 1400 kW at -28°C installed in a margarine factory in The Netherlands

(a) An ARS of 2500 kW at -15°C installed in a meat factory in Spain. (b) An ARS of 2700 kW at -30°C installed in a refinery in Germay. (c) An ARS of 1400 kW at -28°C installed in a margarine factory in The Netherlands

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.

Written by sam

November 22nd, 2009 at 6:16 pm

Twin Refrigeration System

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The twin refrigeration system is a new refrigeration technology that solves the problems of conventional vapor compression refrigerators. A no-frost cooling system is the latest craze, but conventional no-frost features reduce energy efficiency and humidity. To overcome this problem, a new refrigeration system, named the Twin Refrigeration System (see Figure 3.44a) has been developed by Samsung. Here are the primary features of this new system:

• Two evaporators and two fans: The evaporators and fans of the freezer and the refrigerator operate independently to achieve the necessary temperature in each compartment. This minimizes unnecessary airflow from one compartment to another. It eliminates the need for a complicated air flow system which would lead to energy loss. • Turbo fans: Newly developed turbo-fan and multiple-scroll air distribution duct system minimizes the air path.

• Inverting compressor: Variable compressor PRM according to the condition of the refrigerator 4-step control is utilized.
• High-efficiency fan motors: Brushless DC variable motors are employed.
• High-efficiency insulation: The insulation material is cyclo-pentane. It helps minimize heat penetration, due to its low thermal conductivity.
• CFC-free: All these new refrigerators use R-134a and R-600a only, and are free of CFC and HCFC. Therefore, they are environmentally benign.

As seen in Figure 3.44a, the system has both freezer and refrigerator compartments which are controlled independently due to each compartment’s separate evaporator and precise control unit. These features also eliminate inefficient air circulation between the compartments. The result is considered a technological ingenuity, due to the following:

• high humidity preservation,
• ideal constant temperature storage,
• high energy savings,
• no mixed odors between compartments.

(a) A Twin refrigeration system and its components. Comparison of (b) a twin refrigeration system with (c) a conventional no-frost system (Courtesy of Samsung Electronics).

(a) A Twin refrigeration system and its components. Comparison of (b) a twin refrigeration system with (c) a conventional no-frost system (Courtesy of Samsung Electronics).

 

Written by sam

November 22nd, 2009 at 6:06 pm

Three Stage Cascade Refrigeration System

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Cascade refrigeration cycles are commonly used in the liquefaction of natural gas, which consists basically of hydrocarbons of the paraffin series, of which methane has the lowest boiling point at atmospheric pressure. Refrigeration down to that temperature can be provided by a ternary cascade refrigeration cycle using propane, ethane and methane, whose boiling points at standard atmospheric pressure are 231.1 K, 184.5 K and 111.7 K (Haywood, 1980). A simplified basic diagram for such as cascade cycle is shown in Figure 3.40. In the operation, the compressed methane vapor is first cooled by heat exchange with the propane in the propane evaporator before being condensed by heat exchange with the ethane in the ethane evaporator, so reducing the degree of irreversibility involved in the cooling and condensation of the methane. Also, because of the high temperature after compression, the gas leaving each compressor passes first through a water-cooled after cooler. In large-scale plant of this type, the compressors become rotary turbo-machines instead of the reciprocating type ones.

three-stage-cascade-refrigerating-system

Written by sam

November 22nd, 2009 at 6:00 pm

Two Stage Cascade Refrigerating System

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A two-stage cascade system employs two vapor-compression units working separately with different refrigerants, and interconnected in such a way that the evaporator of one system is used to serve as condenser to a lower temperature system (i.e. the evaporator from the first unit cools the condenser of the second unit). In practice, an alternative arrangement utilizes a common condenser with a booster circuit to provide two separate evaporator temperatures.

In fact, the cascade arrangement allows one of the units to be operated at a lower temperature and pressure than would otherwise be possible with the same type and size of single-stage system. It also allows two different refrigerants to be used, and it can produce temperatures below -150°C. Figure 3.38 shows a two-stage cascade refrigeration system, where condenser B of system 1 is being cooled by evaporator C of system 2. This arrangement enables to reach ultralow temperatures in evaporator A of the system.

A practical two-stage cascade refrigeration system.

A practical two-stage cascade refrigeration system.

For a schematic system shown in Figure 3.39, the condenser of system I, called the first or high-pressure stage, is usually fan cooled by the ambient air. In some cases a water supply may be used but air-cooling is much more common. The evaporator of system I is used to cool the condenser of system II called the second or low-pressure stage. The unit that makes up the evaporator of system I and the condenser of system II is often referred to as the inter-stage or cascade condenser. As stated earlier, cascade systems generally use two different refrigerants (i.e. one in  each stage). One type is used for the low stage and a different one for the high stage. The reason why two refrigeration systems are used is that a single system cannot economically achieve the high compression ratios necessary to obtain the proper evaporating and condensing temperatures. It is clear from the T-s diagram of the two-stage cascade refrigeration system as shown in Figure 3.39 that the compressor work decreases and the amount of refrigeration load (capacity) in the evaporator increases as a result of cascading (Cengel and Boles, 1998). Therefore, cascading improves the COP.

schematic-twostage-cascade-refrigerating-system

Written by sam

November 22nd, 2009 at 5:51 pm

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