Evaporators

Evaporation plants consist of a system for supplying the initial input and discharging the concentrated product, a heated surface, a system for supplying and removing steam, and a vacuum system for creating the required pressure. The initial solution in liquid form passes over the heated surface and a portion of the moisture in the product is evaporated away. The steam is removed and the concentrated solution is fed to the next evaporator (in multi-stage units) or is discharged.

In the food industry, produces are also subjected to evaporation in order to boost their nutritional value, extend their shelf life, and lower storage and transport costs due to the reduced weight and volume. The content of soluble solids within concentrated juice is increased to 70–75%, while the volume is reduced by 5–6 times compared to the original input. Concentration can be the initial step in producing a dried product, and it is also an economical method for processing perishable fruits and vegetables during harvest periods.

Dry-matter content after evaporation for dairy products:

Evaporators are available in various versions:

• Recirculating
• Falling film evaporators
• Vacuum devices
• Continuously running evaporators with a serpentine construction
• Multi-section evaporation plants

Evaporation can be carried out at reduced pressure (vacuum) or atmospheric pressure. In vacuum-type plants, boiling happens at a lower temperature than the ordinary boiling point for atmospheric pressure (160–320 hPa, 55–70°C). These devices are meant for processing fruits, producing tomato paste, purees, concentrated juices, milk; for liquids that cannot undergo boiling at atmospheric pressure because their nutritional value would be lost or their physicochemical properties would be changed; caramelization of sugar; and for producing acrylamide (a carcinogen). Boiling at reduced temperature will, on the contrary, preserve to the maximum the product’s taste, natural color and consistency, vitamins, and minerals. Such evaporators are used when making condensed milk, jam, marmalade, fillings, courgette and eggplant caviar, and tomato paste from concentrated puree. Evaporation at atmospheric pressure is used to save on energy costs, for example when boiling salt solutions in order to produce table salt.

The movement of the boiling liquid within the device can be of film or circulating type. In film evaporators, the solution is evenly sprayed over the heated surface and flows down along the inner walls in the form of a thin film. Such evaporators consist of tubing that is 4–8 m long and covered with steam jackets. When the liquid hits the heated surface, part of it evaporates and the vapor is separated from the concentrated solution. These devices work quite quickly and efficiently, and they allow you to avoid exposing products to high temperatures, so they are best suited for concentrated citrus juices, dairy, and other food products that demand delicate treatment.

Circulating vacuum evaporators are used when processing a small quantity of a product or when a small degree of concentration is required, e.g. in the making of yogurt, when the dry content in the milk is increased from 13–14.5% to 16.25%.

In evaporators of circulating cycle, the solution heated to boiling temperature is fed at high speed into the vacuum chamber, thus forming a thin rotating layer on the wall. During this process of rotation, part of the liquid evaporates and the vapor is removed by means of a vacuum pump. The concentrated solution gradually loses its velocity and drops down to the bottom of the chamber, after which it is discharged.

In plants with forced circulation, centrifugal pumps are used. Machines of this type are suitable for applications where the initial liquid product may lead to salinization or scaling, and the solutions are of such high viscosity that natural circulation is not possible. A centrifugal pump allows you to control the time that the product is exposed to heat, which is important when working with viscous liquids such as tomato juice and mango juice. The heating surface can be located inside or outside the plant. While these evaporators are among the most widely used, they are generally less economical than other types due to the operating and maintenance costs of the pumps.

According to the number of sections, evaporators can be single- or multi-section (single or multi-stage). Multi-section (multi-stage) systems are more cost-effective than other types, because the secondary steam produced in each section is fed to heat the next section, where the solution will boil at a lower pressure. This method provides significant savings on heating costs, and therefore these plants are widely employed in the food industry. There can be as many as seven stages, but after this the cost savings from reduced energy usage do not justify the use of additional machinery.

According to how the evaporation process runs, evaporators can be either continuous or batch-mode. In continuous plants, the supply of the input and the discharge of the concentrated solution, and the supply of heating steam and the removal of the secondary steam and condensate, all run continuously. In plants that have a periodic or batch mode of operation, the initial solution is evaporated to the required degree of concentration, and after the concentrated solution is discharged, a new batch is fed to the evaporator. Usually in industry, continuously operating plants are used. Plants that operate in a periodic mode are suitable for small enterprises.

The use of water steam as a heating medium is highly effective due to the specific heat of condensation and the heat transfer coefficient.

Food products have differing sensitivity to heat. Production of plum jam in a vacuum-type plant is done at a temperature of 90°C, meat bouillon is evaporated in vacuum-type plants at a temperature of 60°C, apple and cherry juices can withstand heating to 45–55°C, while evaporation of citrus juices is done at lower temperatures.  To obtain a high-quality product at the end and to avoid any damage from heat, the evaporator for your production process should be carefully chosen based on temperature and retaining time. Boiling in vacuum evaporators happens at lower temperatures, so these evaporators are suitable for concentrating solutions that have a high boiling point at atmospheric pressure or would lose important properties at high temperatures.