We use jam, condensed milk, milk powder, dried cheese powder in day-to-day life, but have you ever wondered how these things are processed? The food industry is booming and ready to eat meal is one of the reasons behind this. Evaporation is one of the important processing which is used by the food industries for the preparation of food products. In the food industry, raw materials are used which contains more water than required in the final product. The easiest way to remove water/concentrate the food product is to use evaporators.
An evaporator is defined as a heat exchanger intended to heat a given product and separate water by evaporation. Evaporators are widely used in the food manufacturing industry to remove the portion of water from food products. This helps in increasing the total solid content in food products and reduction in the bulk weight which ultimately helps in the facilitation of the transport.
The process of evaporation is different from drying and dehydration as the liquid obtained from it is still in a form of non-solid liquid. Evaporation only serves as the first step in making the finished product into concentrated liquid and dried goods. Powdered ingredients like dairy whitener, whey powder etc. are prepared by spray drying. Hence, we can say that powdered products are a combination of both techniques.
Evaporation is a process in which simultaneous heat and mass transfer process occur resulting in the separation of vapour from a solution. Evaporation occurs where molecules obtain enough energy to escape as vapour from a solution. The rate of escape of the surface molecules depends primarily upon the temperature of the liquid, the temperature of the surroundings, the pressure above the liquid, surface area and rate of heat propagation to the product. It is thus a thermal separation or thermal concentration process that begins with a liquid product and ends up with a more concentrated, but still liquid and still pumpable concentrate as the main product from the process.
Reason for Installation of Evaporators
There are several reasons for concentrating food liquids:
- To reduces costs for storage and transportation.
- To induce crystallization.
- It may be used as an economical preparatory step for subsequent dehydration as in the cases of spray drying, vacuum drying, freeze-dried, crystallization and mixing.
- To reduce water activity to increase microbiological and chemical stability.
- Recover valuable substances and by-products from waste streams.
Components of Evaporator System
- A heat source: It is generally comprised of a heat exchanger for the evaporation of water.
- An evaporation vessel: It drives out water from the product as vapor.
- A vapor separation vessel: It separates vapor and product.
- A vacuum system: It draws water vapor out of the separation vessel. This vacuum also reduces pressure in the evaporation vessel, which reduces the boiling point.
Evaporators may operate singly, or several may operate in series. Each one is referred to as an effect and in multiple-effect systems, the product output from one effect is the feed for the following effect. Similarly, higher-temperature vapor driven out of the product in one effect is used to heat a lower-temperature product in another. Efficiency is gained by using multiple-effect systems.
Types of Evaporators Used
- Circulation or Vertical Tube Evaporators
Tubes carrying the steam internally are placed vertically in the bottom of the cylindrical evaporator, chamber. It is popularly known as Calandria evaporator. As a single pass through the tube heat exchanger is usually not sufficient to achieve the required degree of concentration, the product is returned through the circulation tube.
- Horizontal Tube Evaporators
In certain conditions, if, for, example, there is limited vertical space for installation, the tubular heat exchanger can be used as a horizontal evaporator. To ensure that both the top and the bottom tubes are covered with liquid, the evaporator must be operated in a flooded condition, i.e., the feed inlet must be positioned above the uppermost tube. Steam enters a chest on one end of the tubes, moves through the tubes and the condensate is removed from the chest at the opposite end while the vapour is removed from the top of the cylindrical chamber.
- Plate Evaporators
The plate evaporator is characterized by a large heat exchange surface occupying a relatively small space. Like the plate heat exchanger, it is constructed from profiled plates with the condensing steam used as a heating medium and the evaporating product passing between alternate pairs. The capacity of a plate evaporator can be altered by changing the number of plates.
- Forced Circulation Evaporators
When highly viscous products are to be evaporated, the forces which normally move the liquid along are not sufficient to transport the product satisfactorily. Centrifugal or positive pumps are used for the forced circulation of viscous liquids. The operation is only economical if the product has been pre-concentrated on a simpler evaporator.
- Falling Film Evaporators:
The heat exchange surface in a falling film evaporator consists of a bundle of tubes down which the product flows. Depending on the design, the tubes are 4-10 m long and have a diameter of 25-80 mm. The falling film evaporator has become of great importance to the dairy industry. The relatively short contact times and the possibility of accurate temperature control ensure mild evaporating conditions.
Maintenance Tips to Improve Energy Efficiency
Regular maintenance will help ensure that equipment serves a long, useful life and operates efficiently. The following suggestions will help keep your evaporators operating efficiently:
- Prevent air leaks into the evaporators to minimize venting rates. Air is a non-condensable gas and must be vented to keep evaporation vessel pressure from increasing.
- Clean heat transfer surfaces to allow the most efficient use of energy to evaporate water from the product.
- Inspect and repair or replace wet or damaged insulation as it is found. Also, make sure insulation is the appropriate thickness.
- Keep the vapor separation vessel clean to maintain product yields and pressure profiles.
- Prevent water leaks into the system to avoid diluting the product, which defeats the process.
Evaporators are mainly used to concentrate a weak solution by using a certain heating medium. Most commonly used heating medium is steam and generally, vacuum-based evaporator systems are preferred in the food industry as it lowers the boiling point and thus increases the process efficiency and lower downs the energy usage. Selection of material concentration, product flowability, product characteristics, heat sensitivity, capacity is some of the important factors which are required to be considered for the designing of evaporators. Evaporators must be designed in a way that they can also discard non-condensable gases and material that originate as dissolved material in the evaporator or from leaked air into the evaporator.
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