Evaporation crystallization technology originated in the 1930s. After research and practice, Switzerland produced the first simple MVR evaporation crystallization system in the early 20th century. MVR is short for Mechanical Vapor Recompression. It is an energy-saving technology that reuses the energy of secondary steam generated by the system itself through compressor work, eliminating the need for live steam. This technology is now widely used in the chemical, food, papermaking, pharmaceutical, seawater desalination, and wastewater treatment industries.

The working process of MVR is to compress low-temperature steam through a compressor, increasing its temperature and pressure and increasing its enthalpy. Then, it enters the heat exchanger to fully utilize the latent heat of the steam. Compared with other evaporation technologies, the MVR system also has advantages such as small footprint, high degree of automation, and low operating costs. The compressor, as the core equipment, is the key to achieving these advantages. Therefore, compressor selection is one of the factors that need to be considered in MVR system design.

Steam compressors are divided into two types: Roots steam compressors and centrifugal steam compressors. Centrifugal steam compressors are further divided into low-speed centrifugal blowers and single-stage high-speed centrifugal steam compressors.

When designing an MVR system, the parameters required for steam compressor selection mainly include steam processing capacity, temperature rise, and inlet temperature or pressure.

1. Matching of steam processing capacity and compressor type

The steam processing capacity is the amount of secondary steam generated by the MVR evaporation crystallization system. This parameter is related to the water inflow, TDS, and salt output. There are three main types of steam compressors used in MVR systems: Roots steam compressors, low-speed centrifugal blowers, and single-stage high-speed centrifugal steam compressors. Roots compressors are positive displacement compressors with smaller processing capacity, while centrifugal compressors have relatively larger processing capacity. Therefore, the appropriate type of compressor should be selected based on the system's processing capacity.

2. Temperature rise of the compressor

The temperature rise value is a very important selection parameter. The temperature rise selection of the steam compressor is related to the boiling point elevation and effective heat transfer temperature difference of the MVR evaporation crystallization system's material. For a certain wastewater, the boiling point elevation of the material is fixed. The size of the designed effective heat transfer temperature difference determines the high or low temperature rise of the steam compressor, and the effective heat transfer temperature difference is inversely proportional to the area of the heat exchanger. Therefore, the larger the designed effective heat transfer temperature difference, the higher the temperature rise of the steam compressor, and the smaller the area of the heat exchanger; conversely, the smaller the designed effective heat transfer temperature difference, the lower the temperature rise of the steam compressor, and the larger the area of the heat exchanger. If the area of the heat exchanger is large, the initial equipment investment will be higher. If the temperature rise of the steam compressor is chosen to be higher, the area of the heat exchanger will be smaller, and the initial equipment investment will be lower, but the operating cost will be higher. Therefore, the selection of the compressor temperature rise parameter needs to be determined by considering the project experience, water quality characteristics, and balancing investment costs and operating costs. For different compressors, the temperature rise range is different. Usually, the temperature rise of Roots compressors is relatively high, about 20℃. The temperature rise of low-speed centrifugal blowers is relatively low, generally not exceeding 10℃. If the temperature rise is high, a multi-stage series connection can be used. With the improvement of compressor manufacturing technology, the use of single-stage high-speed centrifugal compressors is increasing. Single-stage high-speed centrifugal steam compressors can also be selected for systems with large processing capacity and high temperature rise.

3. Inlet temperature or pressure of the compressor

Inlet temperature or pressure is also an important selection parameter. Different compressors have different requirements. Roots compressors have certain requirements for inlet temperature, generally 80-85℃, while centrifugal compressors have a wider range of inlet temperatures. The inlet temperature or pressure of the compressor depends on the evaporation temperature of the MVR system. Generally, we control it in a slightly positive or slightly negative pressure state according to the material characteristics. In addition, for saturated steam, the higher the temperature, the greater the density, and the smaller the volume flow rate entering the steam compressor, which can reduce the investment and operating costs of the steam compressor. This is also a factor to be considered in the design of the compressor inlet temperature.

In addition, the adjustment method of the compressor also needs to be considered. The MVR system is relatively stable and has certain operational flexibility. Industrial wastewater is characterized by volatility. System design needs to consider that the compressor maintains high-efficiency operation under small-range gas volume fluctuations to save electricity consumption and adapt to the flexible and stable operation of the MVR system under different operating conditions. In this case, variable frequency regulation is more suitable for system requirements. Variable frequency regulation has a certain adjustment range and a wider high-efficiency area, and can maintain high-efficiency operation of the compressor under variable operating conditions. At the same time, variable frequency regulation startup has linear and smooth characteristics, effectively avoiding the impact on the power grid. In addition, for the evaporation system, it can operate at low speed during startup to assist the entire system cycle and reach the design operating point faster.

In addition to considering the requirements of system parameters, engineering system design should also combine the characteristics of the equipment. The organic combination of the two can ensure the reliability, stability, and economy of the entire system.