173-Analysis of the reasons for shaft breakage in the MVR heat pump compressor used in multi-effect evaporation crystallizers for soda ash, and a solution using a specialized vane separator

Multi-effect evaporation crystallizers are widely used in the evaporation and concentration crystallization of soda ash solutions, the vacuum evaporation and concentration crystallization of table salt, seawater desalination, wastewater treatment, the extraction of traditional Chinese medicinal materials, and the concentration of other food and pharmaceutical products. The energy consumption during the evaporation process is particularly high; therefore, to reduce energy use, it is crucial to recover the waste heat from the secondary steam in order to lower the production costs for enterprises. As is well known, direct evaporation requires a huge amount of energy. By saving energy, it is necessary to make use of the thermal energy of the secondary steam generated from material evaporation; in other words, the secondary steam produced in the previous evaporation stage is not directly condensed and discharged, but rather used as a heating source for the next stage, and so on. It is not until the secondary steam generated in the last effect, due to its relatively low utility value in terms of temperature and pressure, has little technical and economic value that it is directly condensed and discharged.

In recent years, attention has been paid to and the technology of pressurizing and heating secondary steam for reuse in the next evaporation stage has been applied, in order to efficiently recycle the thermal energy of secondary steam for that purpose without affecting the evaporation equipment in the subsequent stage. Such technologies for pressurizing and heating up the secondary steam from evaporators for reuse, those that use electric drive to compress and heat the secondary steam in the form of mechanical energy, are known as MVR technology ； If steam thermal energy is used to drive the compression and heating of secondary steam through heat exchange, it is called TVR technology. Due to the convenience of using electric drive directly in MVR technology, as well as the ability to achieve higher evaporation efficiency, its application is more widespread than that of TVR.

The last edit to this post was made by luoli519 on 2021-1-15 at 20:11. The core equipment used for pressurizing and heating the secondary steam in a multi-effect evaporation crystallizer for saline-alkaline solutions, based on MVR technology, is the secondary steam heat pump compressor. Many enterprises at home and abroad that use MVR technology for pressurizing the secondary steam in multi-effect evaporation crystallizers for saline-alkaline solutions, as well as for heating such systems, rely on heat pump compressors as the key equipment for this purpose. In almost all cases, these compressors suffer from issues such as impeller corrosion, equipment vibration, humming noises, bearing wear, seal damage, and even shaft bending or breakage after operating for a short period of time. Undoubtedly, all of these heat pump compressors are equipped with comprehensive anti-surge protection devices without exception, yet serious shaft breakage problems still occurred – and this was the case even with heat pump compressors from well-known foreign brands. Let’s all work together to analyze and discuss the main reasons for the severe shaft breakage problems in the secondary steam heat pump compressors used in multi-effect evaporation crystallizers for saline-alkaline solutions. We were the first to adopt the vane separation technology for the secondary steam heat pump compressors in the multi-effect evaporation crystallizers using saline-alkaline solutions, which are used by these well-known foreign brands and suffer from shaft breakage problems, thereby effectively solving the issue of long-term stable operation.

The image below shows a multi-effect evaporation, concentration, and crystallization unit. The protective measures installed in the secondary steam heat pump compressor of the multi-effect evaporation crystallizer used in this device are quite comprehensive, including an automatic anti-surge device, an additional make-up air compressor as a backup, a secondary steam separator, and system temperature compensation mechanisms.

The image below shows a multi-effect evaporation, concentration, and crystallization unit. The protective measures installed in the secondary steam heat pump compressor of the multi-effect evaporation crystallizer used in this device are quite comprehensive, including an automatic anti-surge device, an additional make-up air compressor as a backup, a secondary steam separator, and system temperature compensation mechanisms.

This post was last edited by luoli519 on 2022-12-1 at 16:30. The image below shows a multi-effect evaporation, concentration, and crystallization unit. The protective measures installed in the secondary steam heat pump compressor of the multi-effect evaporation crystallizer used in this device are quite comprehensive, including an automatic anti-surge device, an additional make-up air compressor as a backup, a secondary steam separator, and system temperature compensation mechanisms.

It is certainly good that comprehensive protection measures are in place for the secondary steam heat pump compressors in multi-effect evaporation crystallizers, but if these measures are not appropriate, some of the protective devices can instead cause obstacles and problems to the smooth operation of the equipment. For example, among the various seemingly comprehensive protection measures installed in the secondary steam heat pump compressors of the multi-effect evaporation crystallizers used by many plant owners, it is common practice to use simple separators such as traditional wire mesh demisters at the inlet of the heat pump compressor’s secondary steam. Although the screen separator has a simple structure and low cost, it has poor adaptability to actual complex and variable operating conditions, limited operational flexibility, high operating pressure drop, and is prone to being clogged by salt and alkali crystals that precipitate from the secondary steam. This leads to insufficient air supply to the heat pump compressor, causing the anti-surge protection device to activate frequently. As a result, a large amount of secondary steam is lost, the device struggles to operate smoothly on a continuous basis, its efficiency is very low, and it is difficult to maintain its operation.

This post was last edited by luoli519 on 2021-1-16 at 11:59. The owner is very troubled by the fact that the multi-effect evaporation, concentration, and crystallization unit cannot operate continuously; people involved are under great psychological and mental stress, and they turn to whatever solution is available in a hurry. As a result, many suppliers at home and abroad came forward to recommend their own types of separators and demisters, such as swirl plate demisters, vane separators, Chevron baffle separators, swirl tube demisters, and so on. These suppliers even claimed with confidence that their products would solve the problem completely. As a result, the incident mentioned in the title of this post occurred. Next, let’s take a look together at what these demisters actually are and what their performance is like. After all, gas-liquid separation is a pure precision kinetic separation technique; it requires the use of specialized kinetic separation technologies to conduct precise calculations and configure system platforms in order to develop accurate solutions, so as to be implemented reliably. It cannot be resolved by non-specialized kinetic separation companies at home and abroad through experience, guesswork, or even arbitrary decisions.

This post was last edited by luoli519 on 2021-1-16 at 11:42. The image below shows the \"cyclone plate demister\" installed inside the secondary steam separator shell of the secondary steam heat pump compressor MVR used in many multi-effect evaporation crystallizers both domestically and internationally. Cyclone plate demisters were first used abroad in the mid-20th century for the preliminary separation of large-sized particles and liquid droplets present in air streams; the size of particles that can be separated by these devices is typically 80–100 microns or more in equivalent diameter. Around 2005, an engineer from a domestic design institute introduced it to China for promotion while on a short-term research visit in Europe. However, this engineer did not have access to a computational design model platform for the dynamic separation process of the counter-current swirl plate demister, nor did he have a comprehensive understanding of the practical limitations of this type of demister. By promoting it to various companies based on experience and rough estimates, he was being imprudent and irresponsible, with an emphasis more on economic considerations. The swirl plate demister is essentially a variant of the blade separator and the Chevron baffle demister; in other words, the flat blades and baffles are arranged at an inclined angle following a certain spiral pattern. Due to the diagonal arrangement of the spirals, as the air flows past the swirl plates, it undergoes a slight low-speed, large-diameter spiral motion, resulting in a mild centrifugal separation effect; however, this effect is not very significant. The advantage of the swirl plate demister is that it can effectively prevent clogging in screen demisters and the issue of insufficient MVR air supply caused by high pressure drops.

This post was last edited by luoli519 on 2022-12-1 at 16:31. The image below shows vane separators installed in the compressor separation tanks of various domestic and international companies; these separators were adapted and installed inside the secondary steam separator housing at the inlet of the MVR compressor used in multi-effect evaporation crystallizers. Leaf separators were first used abroad in the mid-to-late 20th century for separating gas streams containing particulates, droplets, gels, and other entrained substances. It has the same advantages as the swirl plate demister, as it can effectively prevent clogging in the mesh demister and the issue of insufficient MVR air supply caused by high pressure drops. Domestically, foreign-introduced vane separators were not widely used until the beginning of this century. However, the vane separator is also a purely dynamic separator; to ensure that it can exert an effective separation efficiency under specific operating conditions, it is necessary to use precise dynamic separation techniques, along with a system platform for calculation and configuration design, taking into account a full range of parameters such as the gas flow composition, operating temperature, operating pressure, gas compression eccentricity factor, gas viscosity, gas density, and gas kinetic momentum, as well as parameters related to droplets such as their composition, apparent density, viscosity, surface tension, and concentration in the flow. Non-professional dynamics separation technology companies lack systems and platforms for accurate calculations related to dynamics separation technology as well as for configuration design; as a result, they are unable to determine accurately the various physical property parameters under different actual operating conditions, such as the flow compression eccentricity factor, flow viscosity, flow density, and flow kinetic momentum, as well as the apparent density, viscosity, and surface tension of droplets. Therefore, it is impossible for them to develop accurate and reliable technical solutions that ensure the effective and reliable separation efficiency of blade separators in specific applications.