食品工程与装备技术.ppt

Drying,Members： 贾 燕 11811505 孙婵婵 11811503 王子健 11811502 张立辰 11811501 Major：农产品加工及贮藏工程,1 Basic drying theory,1.1 The definition of drying Drying of foods implies the removal of water from the foodstuff. In most cases, drying is accomplished by vaporizing the water that is contained in the food. 干燥的定义 食品干燥，去除食品的水分。在大多数情况下，干燥是通过蒸发而除去食品中的水。,1.2 Three States of Water Pure water can exist in three states, solid, liquid and vapour. The state in which it is at any time depends on the temperature and pressure conditions and it is possible to illustrate this on a phase diagram, as in,Figure 1.1 Phase diagam for water,If we choose any condition of temperature and pressure and find the corresponding point on the diagram, this point will lie, in general, in one of the three labelled regions, solid, liquid, or gas. This will give the state of the water under the chosen conditions. 如果我们选择任何温度和压力条件，并在图上找到对应点，这一点在于，在一般情况下，在标记的三个地区，固体，液体或气体之一。所选择的条件下，这将使水的状态。,2 Drying Equipment,In an industry so diversified and extensive as the food industry, it would be expected that a great number of different types of dryer would be in use. Next several typical of the dryer will be introduced:,2.1 Tunnel Dryers These may be regarded as developments of the tray dryer, in which the trays on trolleys move through a tunnel where the heat is applied and the vapours removed. In most cases, air is used in tunnel drying and the material can move through the dryer either parallel or counter current to the air flow. Sometimes the dryers are compartmented, and cross-flow may also be used.,2.2 Spray Dryers In a spray dryer, liquid or fine solid material in a slurry is sprayed in the form of a fine droplet dispersion into a current of heated air. Air and solids may move in parallel or counterflow. Drying occurs very rapidly, so that this process is very useful for materials that are damaged by exposure to heat for any appreciable length of time. The dryer body is large so that the particles can settle, as they dry, without touching the walls on which they might otherwise stick. Commercial dryers can be very large of the order of 10 m diameter and 20 m high.,2.3 Vacuum Dryers Batch vacuum dryers are substantially the same as tray dryers, except that they operate under a vacuum, and heat transfer is largely by conduction or by radiation. The trays are enclosed in a large cabinet, which is evacuated. The water vapour produced is generally condensed, so that the vacuum pumps have only to deal with non-condensible gases. Another type consists of an evacuated chamber containing a roller dryer,2.4 Freeze Dryers The material is held on shelves or belts in a chamber that is under high vacuum. In most cases, the food is frozen before being loaded into the dryer. Heat is transferred to the food by conduction or radiation and the vapour is removed by vacuum pump and then condensed. In one process, given the name accelerated freeze drying, heat transfer is by conduction; sheets of expanded metal are inserted between the foodstuffs and heated plates to improve heat transfer to the uneven surfaces, and moisture removal. The pieces of food are shaped so as to present the largest possible flat surface to the expanded metal and the plates to obtain good heat transfer. A refrigerated condenser may be used to condense the water vapour.,3 Unit operations and processes,The drying process including the following unit operations： Fluid flow process, including liquid transportation and solid transportation etc. Heat transfer process, including heat conduction, convection and evaporation and condensation, etc. Mass transfer process, the material of transmission, including gas absorption and drying etc. Thermal process, the temperature and pressure change process, including liquefaction, frozen etc.,4 The application of drying,Drying is one of the oldest methods of preserving food. Primitive societies practised the drying of meat and fish in the sun long before recorded history. Today the drying of foods is still important as a method of preservation. Dried foods can be stored for long periods without deterioration occurring. The principal reasons for this are that the microorganisms which cause food spoilage and decay are unable to grow and multiply in the absence of sufficient water and many of the enzymes which promote undesired changes in the chemical composition of the food cannot function without water.,5 The domestic and overseas manufacturers,5.1 The domestic manufacturers For example: 上海奕晟矿山机械有限公司 四川望昌干燥设备有限公司 常