Evaporator is used for condensation of whole and skim milk in production of canned milk products, whey, juices and other liquids.

The purpose of evaporation is concentration of non-volatile solids of the condensed liquids by the removal of water from it by evaporation. During evaporation the ratio between individual components of the liquid is preserved. The driving force for evaporation is the temperature difference between heating medium (heating steam) and boiling of the concentrated product.

Steam vapor is used for heating the liquid to the boiling point and is called heating or primary steam unlike secondary steam that is generated from the evaporated product. Evaporation occurs during boiling, i.e. in conditions where the steam pressure of the product is equal to the pressure in the working capacity of the main body (housing) of evaporator.

Evaporation is carried out under the excess pressure and under vacuum. At the excess pressure of secondary steam has high temperature that allows it to be used for heating in various heat exchangers operating under less pressure. In this case, the efficiency of evaporator increases, but increase in temperature and pressure of secondary steam increases the consumption of heating steam and cost of the plant.

The evaporation is carried out in evaporator, with condensing steams - in condenser. Constant low pressure in the system is maintained by using a dry- or water-air pump or by system of ejectors for air suction from the system.

Evaporation may be a primary process or a preliminary processing operation before the drying process.

Evaporation is always an energy-consuming process for any production.

The choice of evaporator depends on the application and the original characteristics of the product. Technological modes of evaporation in the production of canned milk are different.

As the concentration proceeds, physical and mechanical product properties change: thermal conductivity and heat capacity reduce, viscosity increases, etc.

The conditions of heat transfer from the heating surface of the main body to the boiling product improve that is taken into account when determining processing modes as well as at calculating, designing and operation of evaporator.

Evaporators with thermal compression of secondary steams are the most common in the dairy industry. Consumption of main steam for evaporation in a single-effect evaporator with thermal compression is 0.5-0.6 kg/kg of evaporated moisture, and in a double-effect one is 0,35-0,40 kg/kg.

 

Types of evaporators.

  • with mechanical vapor recompression (MVR)
  • with thermal vapor recompression (TVR)
  • with combination of mechanical and thermal recompression.

 

For mechanical vapor recompression you need a compressor or air blower to compress steam; steam cleaners are used for thermal vapor recompression.

Plants with mechanical recompression use electricity but consume almost no steam, not considering steam that is required to start the plant that allows increasing the production economy.

Evaporators with thermal vapor recompression use ready steam.

Evaporators are complete with heat pump for more comprehensive use of secondary steam heat generated in evaporator.

Steam injectors are installed on a single-effect evaporator or on the first effect of multi-effect evaporators. The main parameter that characterizes the efficiency of the injector is the injection rate. It shows what quantity of secondary steam can be compressed in the injector to the required parameters of 1 kg of heating steam.

 

The classification of evaporators:

  • by the principle of operation (batch and continuous);
  • by the shape and location of heating surfaces (inclined, vertical);
  • by the use of secondary steam heat (with and without using it);
  • by the number of effects (one-, two - and multi-effect);
  • by motion of the condensed liquid (circulation, falling film or rising film).

 

The advantages of evaporation in vacuum evaporator in comparison with evaporation at atmospheric pressure:

  • Evaporation in vacuum at boiling temperature in 60°C of liquids, boiling of which at atmospheric pressure has a boiling point of 100°C and is accompanied by physical and chemical changes with loss of biological value of the product.
  • Reducing heat losses to the environment and as a consequence – reducing the consumption of heating steam.
  • The possibility of using exhausted and secondary steam that results in saving heat, reducing the consumption of steam.
  • Less condensation time and more product output from 1 kg of heating surface - due to greater temperature differences between the heating medium and the evaporated liquid.

 

© Kalinovskiy machine building plant 1934-2017