2, the role of steam in feed granulation
The powder requires steam to provide heat and moisture, so the powder forms particles under the most suitable conditions. The added steam will increase the moisture of the formulation and the temperature of the material before it enters the ring mold. The heat from the steam cooks the material, meaning that starch gelatinization has taken place.
Steam provides three elements – temperature (sensible heat), heat (potential energy or latent heat), and moisture. These three elements are closely related to each other. It is important that we learn the correct steam usage pattern to balance these three elements – not using excessive temperatures (sensible heat), which will affect the supply of sufficient moisture and latent heat. This is the core focus of steam management.
It should be noted that there is a difference between steam temperature and latent heat. The latent heat of the effective cooking of the raw starch and the sufficient moisture obtained from steam. Low temperature saturated steam (102-105 ° C) is easy to coagulate, releasing moisture and latent heat quickly. Superheated steam has a very high temperature (120-140 ° C) and the condensation is very slow, which is the main reason why the temperature of the material is not high enough and the water is not enough.
Latent heat is defined as the amount of heat absorbed and released by water between solid liquid gases. For example, in the Earth's atmosphere, when a water molecule evaporates from the surface of any water body, energy is transferred to the inside of the water molecule to lower the temperature of the surrounding air. Because energy needs to overcome the forces between water molecules, a large amount of heat needs to be absorbed from the water-to-gas phase transition to lower the ambient temperature. If the water vapor condenses into a liquid phase or a solid phase onto the surface, the potential heat absorbed during the evaporation process will be released and become sensible heat absorbed by the surface. The large enthalpy of steam condensate is the reason why steam is a more efficient medium than boiling water and is more dangerous because it contains more heat.
In contrast to latent heat, sensible heat only causes changes in temperature and does not undergo a phase change. Sensible heat and latent heat are not specific forms of energy, they are forms of energy with the same characteristics. A good way to remember the difference between them is that the changes caused by sensible heat can be judged by the sensor's senses, and the changes caused by latent heat cannot be read by the thermometer. Heat is the process by which thermal energy is converted between the system and the environment or between two different temperature systems.
Latent heat and sensible heat are observed in many processes during the transfer of energy in nature. Latent heat is closely related to the phase change of water vapor in the atmosphere, mainly evaporation and condensation, while sensible heat is the transfer of energy, which affects the temperature of the atmosphere.
The enthalpy (heat content) of water at 100 ° C was 419 KJ/kg. When the water is cooled from 100 ° C to 0 ° C, 419 KJ of heat will be released. At standard atmospheric pressure (100 kPa), when the water temperature reaches 100 ° C, heating will no longer increase its temperature because the water will evaporate into steam. This state change requires a large amount of heat input (evaporation). The water at 100 ° C was changed to water vapor at 100 ° C, and the enthalpy of evaporation was 2257 KJ / kg. Therefore, the heat content of the steam is equal to the enthalpy of the water plus the evaporation enthalpy (419 + 2257 = 2676 kJ/kg). This means that the steam contains a lot of heat, which is why steam is such a useful heating medium.
Condensation and evaporation are the opposite processes, and latent heat is released when water vapor condenses into a liquid. The temperature does not change during this process, so the amount of heat released directly changes the state of the substance. The heat released during this process is called heat of condensation. The heat during condensation of water is approximately 2257 kJ/kg. The value of the condensation heat is exactly equal to the heat of vaporization, but with the opposite sign. In the case of evaporation, energy is absorbed by the substance, and during the condensation process, heat is released, which is "latent heat" .
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