Do you truly know all the factors that affect the MVR evaporation system?

Do you truly know all the factors that affect the MVR evaporation system?

ture rise range of the whole vaporization system is within 20-24℃.

When the temperature rise of the vaporization system exceeds the above range, it is not recommended to use the MVR evaporation system.

The main advantages of the MVR evaporator are as follows:
The energy consumption is low, when solution with low salt content is evaporated to approach saturated solution, 1 ton of water needs only about 20-30kW of power consumption to be evaporated(determined by boiling point of the material), and the evaporating temperature can be reasonably selected according to material characteristics.

The evaporation rate is high: it only takes tens of seconds for materials to enter or be discharged from the equipment, thus being conducive to the treatment of thermosensitive media. The evaporation temperature can be controlled low, as low temperature evaporation can reduce corrosion and energy consumption of the material.

Through the use of vapor compressor, the energy cyclic utilization is more complete, and the operating energy consumption of the MVR evaporator is reduced.

Usually, the MVR evaporator is mainly composed MVR steam compressor, heat exchanger, evaporator, separator, condenser, vacuum system, machine pump, instrumentation valves and control system.

The MVR evaporator has high energy utilization rate, high automation degree, stable operation and small floor area, so it is accepted by more and more users. But the design and operation of MVR evaporation system should take into account the influence of numerous factors such as material, altitude, equipment, and operation. The factors that may affect the MVR evaporation system are analyzed and narrated below.

02 The influence of characteristics of evaporation raw material
The main physical property parameters of material include density, solution compositions, specific heat, viscosity, elevation of boiling point, surface tension, heat sensitivity and corrosiveness. Density, solution compositions, constant-pressure specific heat and viscosity greatly affect the heat transfer coefficient of the material side, and different heat transfer coefficients will directly affect the design and calculation of the evaporation area.

Viscosity and surface tension mainly affect the steam-liquid separating process of material as well as the selection of length-diameter ratio of the separator. Besides, they will affect film distribution of the material in the process of film formation and evaporation. The rise of boiling point mainly affects the selection of technological process, the selection of evaporating temperature, temperature graded distribution and the selection of vapor compressor.

Materials with high elevation of boiling point can adopt MVR evaporation process within the temperature rise range of vapor compressor. Single stage high-speed centrifugal compressor and positive-displacement lobed rotor compressor can provide temperature rise of over 20 ℃. The manufacturing technique of high temperature rise vapor compressor is more complicated, and the precision requirement is higher.

The viscosity of material not only affects the heat transfer coefficient but also affects the selection of evaporator type. Materials with high viscosity need to adopt forced circulation or scraper evaporator, so as to prevent coking caused by overly slow material flow. At the same time, forced-circulation evaporator can also more effectively prevent chemical scaling of equipment, it reduces the risk of chemical scaling in heat exchanger through the turbulence effect of high speed fluid.

The heat sensitivity of material requires a short duration of stay of material in the evaporator, otherwise the material will have qualitative change, so the number of effect or series of the evaporator should be reduced to shorten the circulation time of material in the evaporator. If the evaporated material has a requirement on temperature, the evaporating temperature, the evaporator type and the selection of process must be considered in design.

The corrosiveness of some materials is high, for example, the chloride root content in high salinity wastewater is high. The corrosiveness of materials under high temperature is an important factor affecting material selection, especially, the material used for heater of MVR evaporator is usually the best in the system.

03 The influence of altitude of the installation site
Altitude is a physical quantity that reflects the terrain height of a region. High altitude is accompanied by low local air pressure, reduced gage pressure and lower boiling point of solution. The influence of altitude on the evaporation of MVR is directly reflected in the vacuum degree of the system and the thermal parameters of the corresponding saturated vapor.

In order to reduce energy consumption, MVR evaporation is mostly carried out under negative pressure, and the acquisition of negative pressure needs vacuum system. Another function of the vacuum system is to extract the non-condensable gas from MVR evaporator, so as to prevent the enrichment of non-condensable gas, reduce the heat transfer efficiency and prevent evaporation.

Altitude affects the final vacuum. For example, the final vacuum of the water ring vacuum pump is -0.094Mpa at the altitude of 0m, but at the altitude of 1,500m, its final vacuum is -0.08Mpa. For heat source vapor, the temperature of vapor with gage pressure of 0.1Mpa is 99.6℃ at the altitude of 0m and 95.1℃ at the altitude of 1,500m. Thus it can be seen that altitude will affect the thermal parameters of vapor, so the design of MVR evaporation should take altitude into full consideration.

High altitude also has an impact on the selection of water pump motor. When the altitude exceeds 1,000 m, high-altitude machine should be selected in case that the water pump motor cannot operate according to design parameters, thus affecting water pump flux and the lift.

04 The influence of system design parameters
Process parameters are the basis for the design of MVR evaporator, and the understanding and mastery of characteristic parameters of raw materials can better avoid design risks and ensure the normal operation of the system. Material concentration, compositions, treatment capacity and evaporation capacity determine the size of equipment, the number of effect and level, model selection of vapor compressor, the size of evaporation area, the material of heat exchanger and the cost of equipment investment.

Input concentration and discharging concentration directly affect the calculation of heat transfer and the selection of evaporating temperature, and thus affect the selection of inlet and outlet temperatures of the vapor compressor. Due to evaporation intensity, the concentration of discharging will also affect model selection of the evaporator, the design of evaporation process and the temperature of input and discharged materials. MVR evaporator should achieve the maximization of energy utilization in the process design, so the condensate outlet temperature should be reduced to the minimum by process change on the premise of meeting the inlet and outlet temperatures of material.

05 The influence of the selection and operation of the main equipment
a.The influence of charge-in change
Feed volume: after the design of evaporation system is completed, and the heat exchanger area, heat transfer coefficient and vapour pressure are determined, the whole system has its inherent operating flexibility, that is, the feed volume is within a certain range in normal operation. According to the law of conservation of energy, the maximum rate of heat transfer of the equipment is a constant value. Excessive feed volume will affect the heat transfer coefficient of evaporation, change the evaporation capacity and reduce the concentration of discharging. When the feed volume is overly low, the material side flow of the evaporators with various effects will be greatly reduced, which will increase the evaporating temperature, and even cause dry ashing and coking. At the same time, overly low feed volume means that the vapour produced by evaporation decreases, which will also affect the operation of vapor compressor and even cause damage to the compressor.

Feed concentration: feed concentration changes will cause changes in material physical properties, thus directly affecting the changes of heat transfer coefficient and boiling point elevation. If the raw material quantity remains unchanged, the evaporation capacity will be reduced and cannot reach the originally designed one, besides, material crystallization may be dissolved out, resulting in coking or tube plugging.

Feed temperature: for evaporator feed, generally bubble point feed is required. If the feed temperature is overly low, part of the evaporator area will be occupied for increasing material sensible heat, rather than latent heat and reduce evaporation capacity. So the design usually sets a vapor preheater before feed solution enters the evaporator, so that once the temperature of supplied materials is overly low and cannot meet bubble point feed through the preheater, fresh vapor can be replenished for final heating with the preheater.
b. The influence of operating condition change of the vapour compressor
MVR vapour compressor is the core equipment of the evaporator. The main parameters include flow, inlet and outlet temperatures (temperature rise) and electric current. The vapour compressor can also be chosen according to temperature rise and vapour flow, and there are multiple choices, such as centrifugal fan, stand-alone high speed centrifuge and positive displacement lobed rotor compressor.

According to the law of conservation of energy, the total power required by the compressor should be equal to the difference between water vapor inlet and outlet total enthalpies. The power is provided by the motor, and the shaft power can be calculated by the total power required by compressing dividing efficiency value. Then the optimal motor can be selected according to the shaft power and the motor power. The difference between the maximum power provided by the motor and the power required by vapour temperature rise is big in some cases, thus resulting in large compressor surplus.

In terms of design parameters, the flow rate, temperature rise, pressure ratio and efficiency value of the compressor are high. After the design is completed, the operation of the compressor is codetermined by temperature, flow rate and motor. When the compressor inlet temperature is low and the specific volume increases, the maximum quantity of vapour allowed to enter the compressor will increase. However, as the power of the compressor has been fixed, the outlet temperature will be reduced according to the principle of conservation of energy, which is manifested as insufficient difference between inlet and outlet temperatures. When the inlet temperature of the compressor increases, the specific volume decreases, and the mass flow rate of the vapour decreases. Affected by the motor and the efficiency value, the discharge temperature will increase, but due to the influence of impeller and rotate speed, the representation difference between inlet and outlet temperatures will not increase.

The process of starting up the compressor is a process in which the motor frequency gradually increases. In the case of small frequency, flow rate and temperature rise have their respective corresponding value. With the increase of flow rate, temperature rise will decrease in the same frequency, but the compressor will automatically increase the frequency to ensure higher temperature rise. The frequency, flow rate and temperature rise of the compressor will mutually affect and gradually increase to a rated value.

c. The influence of gas-liquid separator
Gas-liquid separator provides material and secondary vapor separation. The design of the separator should take into full consideration factors such as evaporation capacity, evaporating temperature, material viscosity and separator liquid level.

If the separator is designed undersize, increased evaporation capacity will accelerate the rising of secondary vapor, which can cause serious entrainment amount, and may also give rise to increased pressure drop, resulting in that the vapor carries large amount of high concentration materials into the compressor system, thus causing compressor corrosion and declined condensate quality. Once the material is adhered to the surface of impeller inside the compressor, compressor corrosion will occur, impeller’s dynamic balance may be destructed and compressor damage may happen.

Increased material concentration, viscosity and surface tension will cause difficulties in secondary vapor separation and insufficient separation of the separator. The increase of solid circulating rate and liquid level height of the separator will cause serious entrainment amount of secondary vapor. The diameter of the separator ensures enough separation surface for vapor-liquid separating, and the separator height is the factor that enhances the mutual entrainment of vapour and liquid. The larger the height of the separator, the higher the separation efficiency, and the higher the equipment cost. Therefore, in the separator design, the design of demister is very necessary, which can be lowered to the separator height, and ensures minimal follow-up impact of vapour entrainment.

d.The influence of pump
Pump is the main mobile equipment in the evaporation system, which mainly plays the role of transporting liquid. Pumps in MVR evaporation system mostly adopt the form of double mechanical seal to pump out material under negative pressure. The quality of pump mechamical seal, and the flow rate, the suction range and the lift in operation will affect the evaporation system.

If the system adopts forced circulation pump, especially process design with crystal formation, preference will be given to axial flow pump as circulation pump, which is characterized by mass flow and low lift, besides, the pump head design will cause minimum damage to crystals produced by solution in the working process, which is conducive to the growth of crystals in the system.

If the condensate pump can not pump out the condensate water in time, condensate water will be gathered in the evaporator, thus the heat transfer of the evaporator will be severely inhibited. In the evaporator, the original vapour heating will be changed to hot-water heating, the efficiency will be greatly reduced and vapour blocking will occur.

e. Other influences
In addition to the above influencing factors, there are influences of factors such as the selection of pipe diameter, the pipeline run, the choice of valve forms, the selection of valve installation site and operation standardization, all of which have an influence on the whole evaporation system.

The technical principle of MVR evaporation system is very mature, but as a set of system engineering, various links such as design, installation and debugging are very important. The characteristic parameters of material, the details of process design, the notes for equipment selection and the actual operation all affect the system operation. Therefore, in the initial stage of system design, the characteristics and production process of raw materials should be fully understood. all the processes of the evaporation system should be understood, and overall consideration should be given to various factors to ensure the normal, continuous and efficient operation of the equipment.

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