Written by An evaporator is a heat exchanger that transforms the sensible or latent heat of one fluid into the latent heat of the vaporization of another. If this evaporator converts water or any liquid to a vapor state, it is called a shell and tube evaporator. When an evaporator is used at the bottom of a distillation column to produce steam, it is called a reboiler.
When not used to produce steam or as part of a distillation process, it is referred to as a vaporizer. When an evaporator produces main or secondary steam in a cogeneration facility, it is called a steam generator.
Evaporators are used in chemical processing, food production, and other industries. This guide shares the basic details of shell and tube evaporators, their working principles, advantages, and more.
What is a Shell and Tube Evaporator?
The evaporator is a heat exchanger that uses a fluid that may transition from liquid to gaseous form under certain pressure and temperature conditions. These are composed of a series of closely spaced tubes organized in parallel (known as a tube bundle) and housed in the same cylindrical body (known as a shell).
Shell: This is the outer casing of the evaporator, which holds the tube bundle inside. It holds the fluid that is heated and undergoes a phase change (evaporation).
Tubes: The evaporator features a series of tubes inside the shell through which the fluid or liquid flows. The heating medium (steam or hot water) flows through the spaces between the tubes and transfers heat to the liquid inside the tubes which causes evaporation.
How Does a Shell and Tube Evaporator Work?
The basic function of an evaporator is to use heat to vaporize a liquid or fluid in a solution.
First, a heating medium (steam or hot water) is put into the shell. This fluid flows through the space between the inner tubes and the outer shell.
The liquid solution flows inside the tubes. As it comes in contact with the outer surface of the tubes, heat is transferred from the heating medium to the solution inside the tubes. The heat causes the fluid or liquid in the solution to evaporate.
After that, evaporated fluid moves upward and the remaining concentrated solution continues to flow through the tubes. The evaporated fluid is then separated from the liquid and transferred to a condenser where it is either collected for reuse or further processed.
Types of Shell and Tube Evaporators
Single-Effect Evaporators: This is the simplest type where steam is used once to heat the solution. The vapor generated is condensed, and the heat is not reused.
Multiple-Effect Evaporators: These are more energy-efficient systems. In this design, the vapor produced in one effect (or stage) is used to heat the next stage, thus reducing the need for additional steam. Multiple-effect evaporators can be quite complex, with two or more stages of evaporation. They’re commonly used in industries that require large-scale evaporation, like desalination or food processing.
Forced Circulation Evaporators: In these systems, the solution is pumped through the tubes at a high flow rate to prevent the formation of fouling or scaling. These are particularly useful when dealing with liquids that tend to form deposits during evaporation.
Two types of shell and tube evaporators are different:
Shell and tube flooded evaporators.
The fluid to be evaporated is outside the tubes of a shell and tube-flooded evaporator. The tube bundle is submerged in the saturated fluid housed on the shell side. Depending on the heat transmission scenario, boiling can occur by nucleation or film formation. You can ensure operating conditions and a steam overheat of up to 5 °C by adjusting the liquid level.
Shell-and-tube dry expansion and thermosyphon evaporators.
Shell and tube dry expansion and thermosyphon evaporators operate with the fluid to be evaporated circulating within the tubes. Dry expansion is supplied by a thermostatic expansion valve, which allows it to manage overheating. Used in industrial refrigeration, the refrigerant flows inside the tubes (often with enhanced shape) to improve transfer performance and heat exchanger compactness. Thermosyphon evaporators are more commonly employed in distillation operations and are positioned at the bottom of the column, with a manometric head that allows for recirculation.
Importance of Shell and Tube Evaporator
Evaporators are important in a variety of industrial applications because they facilitate the transmission and removal of heat from the substance to be cooled, particularly in refrigeration and air conditioning, but also in power generation and several chemical and petrochemical processes.
Shell and tube evaporators come in a broad range of sizes and forms, and they may be classed based on many criteria such as construction type, refrigerant supply technique, and application.
Key Components of Shell and Tube Evaporators
Tubes: These are the heart of the evaporator, where the actual evaporation takes place. They are often made of materials like stainless steel or titanium to resist corrosion and wear. The tube material and design depend on the nature of the fluids being evaporated.
Shell: The shell surrounds the tube bundle and contains the heating medium. It is designed to withstand the pressure and temperature conditions of the system.
Baffles: Baffles are used to direct the flow of the heating medium in the shell, ensuring better heat transfer efficiency.
Vapor Outlet: After the solution inside the tubes is heated, the vapor is directed to the top of the evaporator, where it is separated and extracted.
Condenser: After vaporization, the vapor is often condensed to recover the solvent. The condenser helps to cool the vapor and convert it back to liquid form.
Conclusion
Shell and tube evaporators are used in industries for heat transfer and evaporation of liquids.
Understanding how these evaporators work can lead to substantial savings in energy costs. These evaporators have multiple benefits in different sectors. Moreover, they need regular maintenance to work longer.