Operating principle of MVR evaporator

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[Edit this paragraph] MVRMVR is the abbreviation for mechanical vapor recompression technology. MVR is an energy-saving technology that reuses the energy of the secondary steam it generates, thereby reducing the need for external energy sources. As early as the 1960s, Germany and France had successfully applied this technology in fields such as chemicals, food, papermaking, pharmaceuticals, seawater desalination, and wastewater treatment. 　　During the multi-effect evaporation process, the secondary steam from one effect of the evaporator cannot be used directly as a heat source for that same effect; it can only serve as a heat source for the subsequent effects. If it is to serve as a heat source in this process, additional energy must be supplied to it in order to raise its temperature (pressure). A steam jet pump can only compress a portion of the secondary steam, whereas an MVR evaporator can compress all of the secondary steam in the evaporator. The solution circulates within the heating tubes in a falling film evaporator, driven by a material circulation pump. The initial steam is heated externally by fresh steam, which causes the solution to boil and generate secondary steam. This secondary steam is drawn in by a turbocharged fan; after being pressurized, its temperature rises, and it then enters the heating chamber as a heat source for cyclic evaporation. After normal startup, the turbine compressor draws in secondary steam, which is then pressurized to become heating steam, thus enabling continuous cyclic evaporation. The water that evaporates ends up as condensed water and is discharged. In its operation, the evaporator compresses steam at a low temperature using a compressor, thereby increasing its temperature and pressure as well as its heat content; subsequent to this, the steam enters a heat exchanger where it condenses, allowing for full utilization of the steam’s latent heat. Apart from starting the machine by driving it, no steam is required throughout the evaporation process. The secondary steam that comes out of the evaporator is compressed by a compressor, resulting in an increase in pressure and temperature as well as an increase in its heat content. This steam is then sent to the heating chamber of the evaporator where it is used as heating steam to keep the liquid material in a boiling state, while the heating steam itself condenses into water. In this way, the steam that would otherwise have been wasted is fully utilized; the latent heat is recovered, and thermal efficiency is improved. The economic efficiency of the generated steam is equivalent to that of 30-effect multi-effect evaporation. To make the manufacture of evaporation units as simple and easy to operate as possible, single-effect centrifugal recompressors are often used, or high-pressure blowers or turbine compressors may also be employed. These machines have a high volumetric flow rate in the compression ratio range of 1:1.2 to 1:2. For low evaporation rates, piston compressors, vane compressors, or screw compressors can also be used. Evaporation equipment is compact, occupies little space, and requires minimal area. The cooling system can also be eliminated. For existing factories that need to expand their evaporation equipment and face insufficient steam and water supply capacity as well as limited space, especially in cases where low-temperature evaporation requires chilled water for condensation, this approach can result in cost savings while also achieving good energy-saving effects. Principle of mechanical vapor recompression Due to cost considerations, single-stage centrifugal compressors and high-pressure fans are commonly used in mechanical vapor recompression systems. Therefore, the following explanation is intended for such designs. A centrifugal compressor is a volume-controlled machine, meaning that regardless of the inlet pressure, the volumetric flow rate remains almost constant. And the change in mass flow rate is proportional to the absolute suction pressure. 　　Energy change diagram The compression cycle of a single-stage centrifugal compressor is depicted on a hysteresis diagram. Power required for a single-stage centrifugal compressor: For example, compressing saturated water vapor coming from the evaporator from an inlet condition of p1=1.9 bar, t1=119 ℃ to p2=2.7 bar, t2=161℃ (compression ratio Π= 1.4). The compression cycle follows the polytropic curve 1–2, with an increase in the specific enthalpy of the steam of Δhp. For the specific enthalpy h2 of the steam, it is determined using the equation for the efficiency within the compressor (isentropic efficiency); at this temperature, it enters the heater of the evaporator. Based on the amount of vapor inhaled, kg/hr. The HP unit represents the effective compressive work, in kJ/kg. hs is the isentropic compression work, in kJ/kg. 　　The isentropic efficiency (internal efficiency) of the compressor depends, among other factors, on the specific polytropic compression work in hp, which in turn is determined by the MVR energy flow diagram, the polytropic exponent κ and the molar mass M of the gas being drawn in, as well as the inlet temperature and the desired pressure rise. For the actual coupling power of prime movers (electric motors, gas turbines, turbines, etc.), a larger margin for mechanical losses is taken into account. A single-stage centrifugal compressor with an impeller made of standard materials can achieve a vapor pressure rise with a compression factor of 1.8; if higher-quality materials such as titanium are used, the compression factor can reach as high as 2.5. As a result, the final pressure p2 is 1.8 times the suction pressure p1, or up to 2.5 times it; this corresponds to an increase in the saturated steam temperature of about 12–18 K, with a maximum increase of up to 30 K, depending on the suction pressure. In terms of evaporation technology, it is common practice to express the pressure based on the corresponding boiling point temperature of water. In this way, the effective temperature difference is directly represented. Reasons for using mechanical vapor recompression: 1) Low energy consumption per unit. 2) Gentle evaporation of the product due to low temperature differences. 3) Short residence time of the product since a single-effect system is commonly used. 4) Simple process with high practicality. 5) Excellent performance under partial load operation. 6) Low operating costs. By using relatively little energy, namely the mechanical energy of the compressor impeller in a heat pump setup, this energy is added to the process heating medium and enters a continuous cycle. In this case, steam is not required as a heating medium. Mechanical vapor recompressors – Design and functional range Machines used for gas compression operate based on the principle of positive displacement or dynamic principles. In positive displacement machines, the moving parts of the machine separate the suction chamber from the pressure chamber, resulting in a decrease in the volume of the working chamber and an increase in gas pressure. In the case of reciprocating compressors, such a process is achieved through the movement of the piston within the cylinder. In dynamic machines, gas energy is supplied through the high-speed rotation of the impeller blades. The gas is first accelerated and then decelerated by a diffuser located downstream of the impeller. In this way, high speed is converted into pressure energy. Depending on the direction in which the fluid flows through the impeller, the corresponding equipment is referred to as an axial, mixed-flow, or centrifugal compressor. The most suitable type of compressor depends on the operating conditions of the relevant application. The key parameters are the pressure rise to be achieved and the flow rate of the steam to be compressed. P is the ratio of the final pressure p2 to the intake pressure p1, defined as the compression ratio. Since evaporation units often operate under vacuum conditions, with a moderate load on the heating surfaces and small temperature differences, centrifugal recompressors are typically used. 　　Dynamic operation compressors Mixed-flow centrifugal types Centrifugal fans Single-stage centrifugal compressors Multi-stage centrifugal compressors Design details of centrifugal compressors Gas outlet Integral volute Allows a final pressure of up to 60 bar Inlet guide vane Used for continuous adjustment To achieve maximum part-load efficiency Gas inlet Highest-quality pinion shafts Ensure safe operation Maintenance-free pinion bearings Optimal wetting properties Robust gearbox Compact design Helical gears Direct drive for the main oil pump Provides reliable lubrication for bearings and gears Semi-open design of the impeller Allows the highest pressure ratio per stage Technical features: Schematic of the MVR principle 1) Low energy consumption, low operating costs ； 　　2) Small land footprint ； 　　3) Fewer utility facilities required, resulting in lower total project costs; 4) Stable operation with a high degree of automation ； 　　5) No native steam required ； 　　6) It can be evaporated at 40°C without the need for freezing equipment, making it particularly suitable for heat-sensitive materials. Application areas: 1) Evaporation concentration 2) Evaporation crystallization 3) Low-temperature evaporation. Technical parameters: 1) 23–70 kWh of electricity is required to evaporate one ton of water ； 　　2) It enables low-temperature evaporation at temperatures of 17–40°C (no chilled water system required)