The evaporator is a crucial component of the four main parts of a refrigeration system. Low-temperature condensed liquid passes through the evaporator, exchanging heat with the outside air, vaporizing and absorbing heat to achieve a cooling effect. The evaporator mainly consists of two parts: a heating chamber and an evaporation chamber. The heating chamber provides the heat needed for evaporation to the liquid, causing it to boil and vaporize; the evaporation chamber completely separates the gas and liquid phases.
An evaporator mainly consists of a heating chamber and a separation chamber. Based on the structure of the heating chamber and the flow of the solution during operation, indirect heating evaporators commonly used in industry can be divided into two types: circulating type (non-film type) and unidirectional type (film type).
Circulating (non-membrane) evaporator
This type of evaporator is characterized by continuous circulation of the solution within the evaporator to improve heat transfer and facilitate scaling. There are two types of circulation: natural circulation and forced circulation. The former is due to the density difference caused by the different heating levels of the solution at different locations in the heating chamber; the latter is based on external force forcing the solution to circulate in one direction.
1. Central circulation pipe (or standard type) evaporator
In a central circulation evaporator, the heating chamber consists of vertical tube bundles, with the central bundle containing tubes of a larger diameter. The heating surface per unit volume of solution in the thinner tubes is greater than that in the thicker tubes, meaning the former is heated better and more solution evaporates. Therefore, the density of the vapor-liquid mixture in the thinner tubes is lower than the concentration in the thicker tubes. This density difference causes the solution to circulate naturally in a continuous and regular manner, decreasing in size along the thicker tubes and rising along the thinner tubes. The thicker tubes are called downcomers or central circulation tubes, and the thinner tubes are called boiling tubes or heating tubes. To promote good solution circulation, the cross-sectional area of the central circulation tubes is typically 40% to 100% of the total cross-sectional area of the heating tubes. The tube bundle height is 1-2 m, the heating tube diameter is 25-75 mm, and the length-to-diameter ratio is 20-40.
2. Suspended basket type evaporator
To overcome the drawbacks of circulating evaporators, such as easy crystallization, scaling, and difficulty in cleaning, a more rational improvement was made to the standard evaporator structure, resulting in the suspended basket evaporator. The heating chamber, resembling a basket, is suspended from the lower part of the evaporator shell, with an annular channel between the outer wall of the heating chamber and the inner wall of the evaporator replacing the central circulation pipe. The solution rises along the center of the heating pipe and then flows downwards through the annular gap between the outer wall of the suspended basket heating chamber and the inner wall of the evaporator, forming a circulation. Since the area of the annular gap is approximately 100 to 150% of the total cross-sectional area of the heating pipes, the solution circulation speed is greater than that of the standard evaporator, reaching up to 1.5 m/s. Furthermore, the heating chamber of this type of evaporator can be removed from the top for maintenance or replacement, and heat loss is also lower. Its main disadvantages are its complex structure and higher metal consumption per unit heat transfer area.
3. External heating evaporator
The externally heated evaporator has relatively long heating tubes with a length-to-diameter ratio of 50-100. Since the solution in the circulation tube is not heated by steam, its density is greater than that of the solution in the heating tube. The resulting circulation speed of the solution decreases along the circulation tube and rises along the heating tube, with a circulation velocity of up to 1.5 m/s.
Single-pass evaporator
The main characteristic of this type of evaporator is that the solution passes through the heating chamber only once and is discharged as a concentrated liquid without recirculation. When the solution passes through the heating chamber, it flows in a film-like manner on the tube wall, hence it is also commonly called a liquid film evaporator. Based on the different flow directions of the material in the evaporator, single-pass evaporators are further divided into the following types.
1. Rising film evaporator
Its heating chamber consists of many vertical tubes. Commonly used heating tube diameters are 25–50 mm, with a length-to-diameter ratio of approximately 100–150. The preheated feed liquid is introduced from the bottom of the evaporator, boils and rapidly vaporizes within the heating tubes, generating steam that rises at high speed. Under normal pressure, the suitable outlet steam velocity is 20–50 m/s, while under reduced pressure, the velocity can reach 100–160 m/s or even higher. The solution is carried by the rising steam, rising along the tube wall in a film-like manner and continuing to evaporate. The vapor-liquid mixture is separated in separator 2; the finished liquid is discharged from the bottom of the separator, while the secondary steam is discharged from the top. It should be noted that if the amount of water evaporated from the feed liquid is small, it is difficult to achieve the required steam velocity; that is, rising film evaporators are not suitable for evaporating concentrated solutions; they are also unsuitable for materials with high viscosity, easy crystallization, or easy scaling.
2. Falling film evaporator
If a solution with a high evaporation concentration or viscosity is used, a falling film evaporator can be used. Its heating chamber is similar to that of a rising film evaporator. The feed liquid is added at the top of the heating chamber and flows evenly into the heating tubes through a liquid distributor at the end of the tubes. Under the influence of gravity, the solution flows downwards along the inner wall of the tubes in the form of a thin film and evaporates. To ensure the film is evenly distributed on the walls and to prevent secondary vapors from escaping directly from the top of the heating tubes, a well-treated liquid distributor must be installed at the top of the heating tubes.
Falling film evaporators are also suitable for processing heat-sensitive materials, but they are not suitable for processing solutions that are prone to crystallization, scaling, or high viscosity.
3. Scraped evaporator
The evaporator shell contains a heating steam jacket with rotating blades, also known as scrapers. There are two types of scrapers: fixed and rotor. Fixed scrapers have a gap of 0.5–1.5 mm between the scraper and the inner wall of the shell, while rotor scrapers have a gap that varies with the rotor's rotation speed. The feed liquid is added tangentially from the top of the evaporator (or sometimes onto a scraper plate coaxial with the scraper). Due to gravity, centrifugal force, and the scraping action of the rotating scraper, the solution forms a downward-swirling film on the inner wall of the evaporator, where it is evaporated and concentrated. The finished liquid is discharged at the bottom. This type of evaporator is a single-pass evaporator that utilizes external power for film formation. Its outstanding advantages are its strong adaptability to materials and short residence time, typically a few seconds or tens of seconds, making it suitable for high-viscosity materials (such as tannin, honey, etc.) and materials that are prone to crystallization, scaling, or heat sensitivity. However, its structure is complex, and its power consumption is high, requiring approximately 1.5–3 kW per square meter of heat transfer surface. Furthermore, its throughput is small, and its manufacturing and installation requirements are high.
Evaporator with direct heat transfer
In actual production, evaporators with direct contact heat transfer are sometimes used. These evaporators mix fuel (usually coal gas and oil) with air and burn it in a combustion chamber immersed in a solution. The resulting high-temperature flame and flue gas are directly injected into the solution being evaporated through nozzles at the bottom of the combustion chamber. The high-temperature gas and solution are in direct contact, simultaneously transferring heat to evaporate and vaporize water. The resulting water vapor and waste gas are discharged from the top of the evaporator. The immersion depth of the combustion chamber in the solution is generally 0.2–0.6 m, and the gas temperature exiting the combustion chamber can reach over 1000℃. Because it is direct contact heat transfer, its heat transfer effect is excellent, and its heat utilization rate is high. Since it does not require a fixed heat transfer wall, its structure is simple, making it particularly suitable for the evaporation of easily crystallizing, scaling, and corrosive materials. It has been widely used in waste acid treatment and the evaporation of ammonium sulfate solutions. However, if the evaporated liquid cannot be contaminated by flue gas, this type of evaporator is generally not suitable. Furthermore, the presence of a large amount of flue gas limits the utilization of secondary steam. Furthermore, nozzles are more prone to damage because they are immersed in high-temperature liquids. As can be seen from the above introduction, there are many structural types of evaporators, each with its own advantages, disadvantages, and applicable scenarios. When selecting an evaporator, the first consideration should be its suitability for the process characteristics of the material being evaporated, including the material's viscosity, heat sensitivity, corrosiveness, and whether it is prone to crystallization or scaling. Then, it should be required to have a simple structure, be easy to manufacture, consume little metal, be easy to maintain, and have good heat transfer performance, etc.
