Working principle of lithium bromide refrigerators and maintenance precautions

This post was last edited by zhangya*ong on 2015-8-12 at 22:35. Regarding the differences between lithium bromide chiller units and electric refrigeration units, we know that the concepts of cooling and heating are not entirely precise. According to the law of conservation of energy, the processes of cooling and heating are actually processes of energy transfer, and energy is transferred from one space to another mainly through \"heat transfer\" and \"mass transfer\"； Among the more common water-based environmental air conditioning systems, they can be divided into two main types based on their working principles: absorption-type and mechanically compressed-type. The working principle of bromine-chloride units is driven by thermal energy, with heat transfer occurring between various heat exchangers to achieve the desired operating conditions ； Electrical refrigeration relies mainly on an electric motor to drive the compressor to perform work. To complete this process, the first method uses an aqueous solution of the lithium salt \"lithium bromide\" (in fact, it is the water in the solution) as the working medium, while the second method uses Freon as the working medium – and it is accomplished through a series of simple or complex heat exchanges and mass transfers. It should be understood that they exist in the form of physical or chemical energy; therefore, when humans use energy to power machinery, they are actually making use of the conversion between these two types of energy. The key difference between lithium bromide absorption refrigeration systems and electric compression refrigeration units lies in the fact that lithium bromide converts chemical energy into thermal energy; since the heat source has a higher temperature than the surrounding environment, heat transfer is required to transfer this thermal energy ； In electric refrigeration, mechanical pressure is used to liquefy the Freon gas, and the property of liquid Freon to absorb a large amount of heat during evaporation is utilized to transfer thermal energy; in terms of principle alone, the latter is much more efficient at transferring thermal energy than the former. According to measurements, a single-effect lithium bromide unit can produce only 0.8–0.9 units of cooling capacity for each unit of thermal energy input, while a double-effect unit achieves only 1.1–1.2 units ； Electrical refrigeration units can be divided into two main categories based on the type of compressor: speed-type and volume-type. The former is represented by centrifugal compressors, while the latter includes reciprocating types (which are further divided into piston and rotor types), as well as screw and scroll types among other major models. If water cooling is used in all cases, under standard operating conditions, due to differences in the type of heat exchangers employed and the structure of the compressors, their energy efficiency ratios are 1:6.0–9.0 for centrifugal compressors, while for other types of compressors they range from 1:3.6–4.0, 1:4.0–4.8, and 1:4.2–5.0 respectively. Nevertheless, in the previous years, the central air conditioning unit market in the mainland was still dominated by lithium bromide units. The reasons for this are as follows: Lithium bromide units are mainly used in areas where there is no thermal energy or where inexpensive energy sources are available. From the perspective of energy efficiency, there is a significant gap between them and electric cooling systems. I. It is because the construction of power infrastructure keeps up with demand, resulting in a severe power shortage and a supply-demand imbalance; as a result, small coal-fired boilers that are highly polluting and inefficient are used to generate steam locally in order to power lithium bromide steam turbines. II. It is also related to the energy structure in the mainland, where coal serves as the primary energy source: in today’s environment-friendly era with high standards, many cities in the north have combined heat and power systems or centralized heating networks using heavy-duty boilers, which result in relatively lower pollution. During summer, lithium bromide units can be used to generate chilled water for industrial processes; this approach takes environmental considerations into account and avoids the high costs associated with electricity generated from burning coal as a secondary energy source ； Even for lithium bromide direct-fired engines that use fuel, oil prices were low at that time, resulting in relatively low operating costs ； Even now, the proposed West-to-East Gas Pipeline project will significantly improve the energy structure and reduce gas costs; moreover, some users will continue to use gas-fired engines – primarily due to price factors influencing the decisions of those making such choices. III. Several major manufacturers of lithium bromide-based heating systems took advantage of the strong market demand and the temporary difficulty in resolving the imbalance between electricity supply and demand to launch intensive promotional campaigns, thereby misleading a considerable number of users. (The situation has now changed significantly: policies encourage electricity use, electricity capacity fees have been reduced to a large extent; even for heating, it is possible to apply for the use of energy-storage electric boilers, and corresponding peak-and-valley electricity pricing policies have been introduced.) IV. The technological characteristics of lithium bromide units mean that the larger they are, the lower the cost per unit of cooling capacity; moreover, they can provide both cooling and heating functions. Their structural design makes them relatively inexpensive in terms of initial investment for larger-scale projects. So why has the market share of lithium bromide units declined significantly in recent years? This is due to several obvious disadvantages: A. The operation of these units requires a high degree of vacuum inside them. Maintaining such a vacuum serves three purposes: First, lithium bromide units rely on the physical property of water, which boils at 4 degrees Celsius under a high-purity vacuum environment; it is through the evaporation of water that heat is absorbed, and after a series of complex heat and mass transfer processes, cooling is achieved. Without a vacuum, the technical requirements cannot be met ； Secondly, since the lithium bromide solution is inherently alkaline, in the presence of air, oxygen atoms can easily combine with steel structural components, leading to rapid and widespread corrosion. The consequences of such \"ulcers\" appearing on normal structures are easy to imagine. Thirdly, due to the complex structure of lithium bromide units, many auxiliary components that affect the unit’s performance need to be replaced or repaired frequently. The working fluid (lithium bromide solution) must be replaced after 3–5 years, and the interior of the unit often needs to be cleaned. All these factors mean that even minor faults can lead to exposure of the interior of the unit to air, with the direct result being corrosion throughout the unit. B. Lithium bromide units are highly prone to two types of failures: \"refrigerant contamination\" and \"crystallization\". The former occurs when the liquid level in the high-pressure generator becomes too high, causing small droplets containing lithium salts to splash into the refrigerant water cycle, which reduces the purity of the water. As a result, the water cannot boil under low temperatures and high vacuum conditions, and thus the unit can no longer function properly. Both of these failures occur frequently. To date, no suitable method has been found for controlling the liquid level effectively, both domestically and internationally; currently, the only reliable approaches involve the use of metal probes or motor sensors. Yet even these methods have a high failure rate, requiring frequent replacement. Once they fail, it leads to \"refrigerant contamination\"“ ； \"Crystallization\" occurs due to a certain balance between the various heat exchangers in the unit; if blockages occur as a result of external factors or issues within the unit, it can even lead to internal leaks, forcing the unit to be shut down for major repairs. Since lithium bromide units contain 12–22 heat exchangers, precise automatic control also relies on sensors. Compared to electric refrigeration units, lithium bromide units have a more complex structure, numerous control points, and a large number of sensors, which increases the likelihood of failures. Once a balance is disrupted, crystallization occurs, and recovery is slow; in severe cases, this can lead to internal leaks or the destruction of the heat exchangers. C. Due to the frequent occurrence of these phenomena, lithium bromide units require a high level of skill from the personnel in charge of them; even with automatic control systems, they are usually unreliable for the reasons mentioned earlier. Experienced operators can prevent problems before they occur and take action promptly in case of faults, but such incidents still happen frequently ； If individuals who have no understanding of the working principles and basic operating procedures of the units are assigned to these positions, it often leads to serious problems, resulting in a decrease in cooling capacity and a shortened lifespan; in most cases, the units cannot reach the expected service life of 15–20 years. Moreover, many of the operators used by users lack experience ； Thus, the impact and losses suffered as a result are immeasurable. The above has discussed some of the inherent weaknesses of lithium bromide units, but everything has multiple aspects, and it is necessary to consider both advantages and disadvantages comprehensively: if there is a large amount of waste heat available or cheap energy sources can be used, then the economic benefits of using such units are significant ； Furthermore, with proper maintenance and a high level of responsibility among the staff, the expected service life can also be achieved ； Therefore, it cannot be generalized completely. Electric refrigeration units also have their limitations: for example, air-cooled heat pump units that provide both cooling and heating consume relatively high amounts of energy, and they are not suitable for use in areas where the average winter temperature is below zero degrees Celsius ； Certain types of refrigeration compressors and their operating principles are not suitable for use in large or small systems. For example, among the four main types of refrigeration compressors, reciprocating and scroll compressors are more appropriate for small-scale units, while screw compressors are suitable for medium-sized projects. Centrifugal compressors require a capacity of at least 500,000 Btu; they are generally not used in projects with a capacity of less than 1.5 million Btu. This is because electric power is needed to drive the motor regardless of whether it is operating or not, and since the load on refrigeration units used in environmental air conditioning systems changes frequently, having too few compressors can lead to waste. In terms of structure, electric refrigeration compressors with piston reciprocating and screw types can be classified as hermetic, fully hermetic, and open-type ； Screw compressors also come in single-screw and twin-screw types ； Single-stage and multi-stage centrifugal oil pumps ； The basic configurations of vortex compressors are largely similar ； In terms of energy efficiency alone: the most power-efficient are centrifuges and open-type twin-screw compressors; followed by semi-hermetical screw and vortex compressors; while reciprocating compressors are the least efficient ； In terms of service life: the most durable type is the open twin-screw design, followed by fully enclosed twin-screws, screw compressors, multi-stage centrifugal compressors, semi-enclosed twin-screws, and single-screw or single-stage centrifugal compressors. The reciprocating type has the shortest service life; as a result, their maintenance costs per unit of time increase in order, and this is determined by the working principles and manufacturing processes of each type of compressor. However, the reason why certain technologies and processes are still sold in large quantities even by units that are not the most efficient from a technical standpoint is, aside from cost differences, also due to the fact that most brand manufacturers rely on a distribution agency system; there are deviations in the way these agencies recommend products – in other words, consumers do not end up purchasing the products with the best technical and economic performance. Both lithium bromide chillers and electric refrigeration units have their respective areas of application; there is some overlap between them as well as potential for substitution. The specific situation needs to be considered on a case-by-case basis. By adopting a scientific approach and carefully selecting the appropriate unit, it is still possible to achieve more results with less cost. With the development of modern technology to date, the results of research in basic sciences and material mechanics are generally shared. Whether a machine is durable and convenient for users to use – choosing a well-known brand implies a certain level of assurance. The key is that brand-owned companies have mature management systems and corporate cultures, which enables them to introduce the best products to the market at each stage; however, users may not choose such products, mainly because new materials and manufacturing processes result in higher costs for these new products.

Coming to the HVAC sector, there is a lot of knowledge available regarding maintenance and principles.