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Archive for the ‘Absorption Refrigeration Systems’ tag

Electrochemical Absorption Refrigeration Systems

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Recently, Newell (2000) has proposed a new electrochemical ARS as shown in Figure 3.52, which consists of four main components. An electrochemical cell is the heat absorber, equivalent to an evaporator in a conventional vapor compression refrigeration system. A fuel cell rejects heat in a manner similar to a condenser in a common vapor compression refrigeration cycle. The third component is a heat exchanger between gas streams and water flow stream. The fourth component is a current pump for elevating the fuel cell’s voltage output to a level sufficient for driving the electrochemical cell. The voltage required is sufficiently low such that the cycle may be one that is conveniently matched for solar photovoltaic cells or other direct current electric energy conversion systems. In fact, the system shown in Figure 3.52 can be used as a thermally driven power cycle by operating the fuel cell at a temperature lower than the electrochemical cell. The voltage supply becomes a load driven by the electric circuit. Lowering component irreversibilities is essential to reach a breakeven operating condition where the fuel cell is generating sufficient power for operation of the electrochemical cell. Newell’s system is based on a water/hydrogen/oxygen fuel cell and electrochemical cell combination. Other combinations are also considered. Each one has its own advantages or disadvantages. The configuration envisioned for the system operates near atmospheric pressure. The components could be operated at nearly uniform pressures with gravitation, surface tension, or low head pumping used for transporting the working fluids within and between components. Water may be moved from the electrochemical cell and fuel cell to external heat exchange surfaces, or the cells could be configured for direct heat exchange with their surroundings.

electrochemical-ars

Written by sam

November 27th, 2009 at 7:12 am

Steam Ejector Recompression Absorption Refrigeration Systems

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The ejector recompression absorption cycle, which has recently been developed by Eames and Wu (2000), is similar to the conventional single-effect lithium bromide absorption cycle. The difference between them is that there is a steam ejector in this novel cycle for enhancing the concentration process. Because of the use of the steam ejector, the performance and the operating characteristics of the novel cycle are different from the conventional cycle.

The steam ejector recompression absorption cycle is shown schematically in Figure 3.51a. In this figure, the expansion of the high-pressure steam causes a low pressure at the exit of the primary nozzle of the steam ejector, therefore, the vapor at point 8 in the concentrator is entrained by the primary flow. The two streams are mixed in the steam ejector and condensed in the heat exchanger of the concentrator. The condensation heat is used to heat the solution in the concentrator. Obviously, the heat of the entrained vapor is recovered by the steam ejector in this process. Water at point 3 splits into two streams; one flows back to the steam generator and the other flows into the condenser. In stable operation, the mass flow rate of the first stream equals that of primary flow while the mass flow rate of the second stream equals that of the entrained vapor. The rest of the cycle is similar to that of the conventional single-effect lithium bromide absorption cycle. Figure 3.51b shows the novel cycle on a P-T-C diagram. As shown in Figure 3.51b, the cycle 6-7-9-10-6 takes up water at the absorber (10-6) and releases it as vapor at the concentrator (7-9). In the conventional absorption cycle, the vapor is condensed at 8′ and the condensation heat is rejected to the surroundings. In the novel cycle, this vapor undergoes a compression process through the ejector to point 2. Since the vapor temperature is greater than the solution temperature in the concentrator, this vapor is used to heat the solution by condensation to point 3. Therefore the heat otherwise wasted is recovered and the energy efficiency is improved.

Eames and Wu (2000) investigated the energy efficiency and the performance characteristics of the novel cycle and the theoretical results showed that the COP of the novel cycle is better than that of the conventional single-effect absorption cycle. The characteristics of the cycle performance show its promise in using high temperature heat source at low cost.

Recently, Kang et al. (2000) have undertaken a study to propose and evaluate advanced absorption cycles for the COP improvement and temperature lift enhancement applications. The characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its applications. The advanced cycles for the COP improvement are categorized according to their heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/absorption heat recovery. In H2O-LiBr systems, the number of effects and the number of stages can be improved by adding a third or a fourth component to the solution pairs. The performance of NH3-H2O systems can be improved by internal heat recovery due to their thermal characteristics such as temperature gliding. NH3-H2O cycles can be combined with adsorption cycles and power generation cycles for waste heat utilization, performance improvement, panel heating and low temperature applications. The H2O-LiBr cycle is better from the high COP viewpoint for evaporation temperature over 0°C while the NH3-H2O cycle is better from the viewpoint of low temperature applications. This study suggests that the cycle performance would be significantly improved by combining the advanced H2O-LiBr and NH3-H2O cycles.

steam-ejector-ars

Written by sam

November 27th, 2009 at 7:07 am

Crystallization Absorption Refrigeration Systems

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Some absorption chillers are notorious for ‘freezing up’ or crystallizing. The basic mechanism of failure is simple enough – the lithium bromide solution becomes so concentrated that crystals of lithium bromide form and plug the machine (usually the heat exchanger section). The most frequent causes are:

• air leakage into the machine,
• low temperature condenser water, and
• electric power failures.

The first two are actually very similar since they both drive the heat input up to the point that crystallization can occur. Whether air leaks into the machine or the condenser water temperature is too low, the water vapor pressure in the absorption chiller evaporator has to be lower than normal to produce the required cooling. This forces the heat input to the machine to be higher to increase the solution concentration. Air leakage into the machine can be controlled by designing the machine with hermetic integrity and routinely purging the unit using a vacuum pump.

Excessively cold condenser water (coupled with a high load condition) can also cause crystallization. While reducing condenser water temperature does improve performance, it could cause a low enough temperature in the heat exchanger to crystallize the concentrate. Sudden drops in condenser water temperature could cause crystallization. For this reason, some of the early absorption chillers were designed to produce a constant condenser water temperature. Modern absorption chillers have special controls that limit the heat input to the machine during these periods of lower condenser water temperatures.

Power failures can cause crystallization as well. A normal absorption chiller shutdown uses a dilution cycle that lowers the concentration throughout the machine. At this reduced concentration, the machine may cool to ambient temperature without crystallization. However, if power is lost when the machine is under full load and highly concentrated solution is passing through the heat exchanger, crystallization can occur. The longer the power is out, the greater the probability of crystallization.

Major absorption chiller manufacturers now incorporate devices that minimize the possibility of crystallization. These devices sense impending crystallization and shut the machine down after going through a dilution cycle. These devices also prevent crystallization in the event of power failure. A typical anti-crystallization device consists of two primary components: (i) a sensor in the concentrated solution line at a point between the concentrator and the heat exchanger, and (ii) a normally open, two-position valve located in a line connecting the concentrated solution line and the line supplying refrigerant to the evaporator sprays.

Written by sam

November 27th, 2009 at 7:02 am

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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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

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