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Evaporators: Types and uses

Evaporators are used to convert liquids into gaseous state via heat transfer. All of the basic evaporators have mechanism to separate liquid and vapor by means of a heat transfer. The raw material fed into the evaporator and passes through heated tubes. The heat evaporates the water within the raw material.

Application of evaporators

Evaporators are used widely in processing industries that work with fluids and chemicals. They work best in low-temperature applications in food and pharmaceutical industries and in effluent treatment. Apart from manufacturing reliable and cost-effective evaporators for over two decades, Shachi has recently introduced the new self-cleaning evaporator in India in collaboration with Klaren International BV from Holland for effluent treatment.

Types of evaporators

Evaporators are distinguished into different types based on the way they achieve the heat transfer. For instance, the working of forced circulation evaporators relies on pumps to circulate the feed whereas falling film evaporator design calculations rely on gravity. Based on their designs, evaporators can be classified into following types:

Forced circulation evaporators consist of tubular heat exchangers for concentrating a feed. Forced circulation evaporator is deployed in cases where there is a high chance of feed or product liquor slating or scaling, or when the feed has high viscosity and the thermal and flow properties of the process liquor are poor, rendering forced circulation necessary. In the forced circulation evaporator mechanism, the feed is pumped at high velocity through the tubes to ensure high heat transfer coefficient. Forced circulation evaporator working is best suited for crystallization of slurries and other solutions.

The working of forced circulation evaporator achieves high velocities and low residence time in a small space due to high heat transfer. This prevents the feed from reacting with the vessel of the forced circulation evaporator. Thus, forced circulation evaporator mechanism greatly reduces or eliminates altogether, the possibility of scaling. Rapid evaporation makes forced circulation evaporators suitable for processing thermonuclear substances. . The use of pumping in forced circulation evaporator mechanism also makes forced circulation evaporators the best fit for viscous preparation.

The rising film evaporator is essentially a shell and tube type heat exchanger. The liquid to be evaporated is fed into the evaporator from the bottom of the tube. There is a condensation of steam on the outer surface of the vertical tubes, and the liquid within the evaporator boils up into vapor. As the liquid progresses up the tube, the volume of vapor being generated increases leading to a higher central core velocity. This velocity in turn forces the liquid to the remaining tube wall, creating a thin liquid film which moves rapidly. The rapid movement of the film leads to high heat transfer coefficient and reduction in residence time of the material. Rising film evaporators are best suited to process materials that have mild scaling tendencies, but they cannot handle heavy scaling and heat sensitive materials.

A falling film evaporator is built to solve the problems rising film evaporator faces. Falling film evaporator is best suited to evaporate heat sensitive liquids. As seen in the falling film evaporator diagram here, the feed enters the evaporator from the top and is distributed through the heating tubes. The liquid flows through tubes and forms a thin film on the walls of the tube while being partially evaporated. The heat is provided through the heating medium (generally steam) to ensure high heat-transfer coefficient. The liquid and the vapor then flow downwards under the pull of gravity. In the falling film evaporator working, the co-current vapor flow serves to augment the downward movement of the liquid. Concentrated product and its vapor are then separated from each other at the bottom of the falling film evaporator unit.

A standard process followed while material selection which takes into account the feed properties ensures high & efficient heat transfer. The distributor head of the falling film evaporator which feeds into the tubes is designed to ensure uniform wetting of the tube surfaces thus avoiding any crusting which is the root of major maintenance problems pertaining to falling film evaporators. Shachi’s experience in falling film evaporator design calculations clubbed with high end design capabilities and meticulous manufacturing practices ensure that the falling film evaporator carries out your process in exactly the required way.

Agitated thin film evaporators consist mainly of jacketed shells and high-speed rotors. Working principle of the agitated thin film evaporator involves the feed entering the evaporator from the top. As shown in the agitated thin film evaporator diagram here, the feed is then agitated and spread out into a thin film on the shell walls by the fast spinning rotor. The liquid is distributed in the form of a thin film on the heated wall where the heat transfer takes place. Further on, the rotor inside the heating shell wipes the fluid film from the walls. Hence, agitated thin film evaporator is also called as Wiped film evaporator. Agitated thin film evaporators are used mostly for fluids that possess high sensitivity and high viscosity.

Agitated thin film evaporator working principle enables businesses to achieve extremely high and specific evaporation rates. The vapors generated within the ATFD evaporator after heating flow concurrently are cleaned using an entrainment separator. The turbulence helps the thorough heat transfer and mechanically ensures that the entire mass undergoes the process to produce high yields from 30% to 80% concentration possible in a single pass. Intensive mixing process within the ATFD evaporator also serves to protect temperature-sensitive products from overheating.

Multi effect evaporator operation is a process where multiple evaporators are used, and the water vapour discharged from one effect is used as the heating medium for the next effect which has a lower boiling point. Multi effect evaporator is widely used for concentrating aqueous fluids, and enables businesses to obtain the product at the lowest temperature. One or more bodies operating at the same boiling temperature are defined as one ‘effect’ of a multi-effect evaporator.

In the multi-effect evaporator, the first effect is heated with raw steam and the other effects are arranged in an order of subsequently reducing boiling points. The multi effect evaporator working principle enables the operation to reuse the latent heat in the water vapor. As the vapor pressure of the second effect relative to the first effect is lowered, the boiling point of the second effect in the multi effect evaporator is also lowered. The feed moves from high pressure to low pressure in multi effect evaporator, which eliminates the need to pump the feed. The feed arrangement in a multi effect evaporator can be done in forward, backward, mixed or parallel manner.

The operating principle of the self-cleaning evaporator is based on the circulation of solid cleaning particles through the tubes of a vertical shell and tube heat exchanger. The fouling liquid flows upward through the tube bundle of the heat exchanger which incorporates specially designed inlet and outlet channels. Solid particles are fed to the fluid through the inlet channel.

A proprietary distribution system is employed to ensure a uniform division of particles over all the tubes. The particles are fluidized by the upward flow of liquid, where they create the mild scouring effect on the wall of the heat exchanger tubes, thereby removing any deposit at an early stage of fouling formation. After the tube bundle the particles disengage from the liquid in the separator and are returned to the inlet channel and the cycle is repeated.

Procuring reliable and cost-effective evaporators in India

The efficiency of evaporators is determined to a considerable extent by the design and capabilities of the evaporator. Factors such as construction materials, energy efficiency and carbon footprint of the evaporator should be considered when choosing an evaporator suitable for your application. Shachi works to deliver exact results as per your requirements on these fronts.

Backed by experience of over two decades, Shachi has successfully installed 350+ projects in 5+ countries for over 225 clients. Infrastructure and documentation that meets global standards ensures that Shachi’s designs give an edge to the customer. Our design process begins with a thorough study of customer processes and requirements, and evaporation solutions are designed using advanced analysis with CFD technology.

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