MVR evaporator

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MVR evaporator is short for mechanical vapor recompression. MVR is a 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 industries such as chemicals, pharmaceuticals, papermaking, wastewater treatment, and seawater desalination. The working principle of an MVR evaporator is that steam at a low temperature is compressed by a compressor, resulting in an increase in its temperature and pressure as well as in its enthalpy. Afterwards, it enters a heat exchanger where it condenses, thereby making full use of the latent heat of the steam. Apart from starting the engine, no steam is required throughout the evaporation process. 　　The solution is in a falling-film evaporator, where it circulates within the heating tubes via 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. During the multi-effect evaporation process, the secondary steam generated in one of the evaporators cannot be used directly as a heat source for that same evaporator; it can only serve as a heat source for the next stage or subsequent stages. 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). Steam jet pumps can only compress a portion of the secondary steam, whereas MVR evaporators can compress all of the secondary steam in the evaporator. 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 aimed at 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 diagram: The compression cycle of a single-stage centrifugal compressor is depicted in 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 compression work, in kJ/kg. hs is the isentropic compression work, in kJ/kg. 　　The isentropic efficiency (internal efficiency) of a compressor depends, among other factors, on the specific polytropic compression work in hp, which is determined by the polytropic index 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 up to 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. Technical parameters of MVR: 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). The principle of mechanical vapor recompression involves the secondary steam that comes out of the evaporator being compressed by a compressor, which increases its pressure and temperature as well as its enthalpy. This compressed 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 back 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 stages of 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 within a 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 capabilities 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. Reasons for using mechanical vapor recompression: ■ Low energy consumption per unit ■ Gentle evaporation of the product due to low temperature differences ■ Short residence time of the product since a single-effect system is commonly used ■ Simple process with high practicality ■ Excellent performance under partial load operation ■ Low operating costs. Technical features of MVR: MVR flow diagram 1) Low energy consumption and low operating costs ； 　　2) Small land area required ； 　　3) Fewer supporting utility facilities, resulting in lower total project costs; 4) Stable operation with a high degree of automation ； 　　5) No native steam required ； 　　3) The short product residence time is achieved due to the use of a single effect. 4) It features a simple process, high practicality, and excellent performance under partial load conditions. 5) Operating costs are low. 6) Evaporation can take place at temperatures below 40°C without the need for refrigeration equipment, making it particularly suitable for heat-sensitive materials. Application scope of MVR:　　 Diagram of MVR 1) Evaporation concentration 　　2) Evaporation crystallization 　　3) Low-temperature evaporation