Principles of heat transfer

Food Biotechnology

1. BY ASSIST. PROF. BERCIYAL GOLDA. P Principles of Heat Transfer

2. CONTENTS Introduction Principles of Heat Transfer Conduction Convection Radiation  

3. Introduction Heat kills microorganisms by changing the physical and chemical properties of their proteins . When heat is used to preserve foods, the number of microorganisms present, the microbial load , is an important consideration. Various types of microorganisms must also be considered because different levels of resistance exist. For example, bacterial spores are much more difficult to kill than vegetative bacilli. In addition, increasing acidity enhances the killing process in food preservation.

4. Heat transfer is important in food processing. In the unit operations several issues of steady and unsteady heat transfer occur. So for instance heat transfer processes are important for the hygienic, nutritional and sensory quality of the food product.

5. Principles of Heat Transfer Heat  is transferred  via three processes:  C onduction (solid material ) Convection (liquids and gases ) Radiation ( electromagnetical waves ) 1. Conduction   Conduction  is heat traveling through a solid material. On hot days, heat is conducted into your home through the roof, walls, and windows. Heat-reflecting roofs, insulation, and energy efficient windows will help to reduce that heat conduction .

6. Heat transfer occurs because of the transfer of vibration energy from one molecule to the adjoining molecules closely associated in a solid mass. Free electrons in molecular level might carry the thermal energy and electrical energy with in a system. Physical movement of body does not take place in conduction. Example:- heating of metal body

7. 2. Convection Convection  is another means for the heat from your walls and ceiling to reach you. Hot air naturally rises, carrying heat away from your walls and causing it to circulate throughout your home. As the hot air circulates past your skin (and you breathe it in), it warms you . Convection is associated with fluids since heat transfer in a system occurs due to the mass movement within the system. Example:- boiling of water

8. 3. Radiation Radiation  is heat traveling in the form of visible and non-visible light. Sunlight is an obvious source of heat for homes. In addition, low-wavelength, non-visible infrared radiation can carry heat directly from warm objects to cooler objects. Infrared radiation is why you can feel the heat of a hot burner element on a stovetop, even from across the room. Older windows will allow infrared radiation coming from warm objects outside to radiate into your home; shades can help to block this radiation. Newer windows have low-e coatings that block infrared radiation. Infrared radiation will also carry the heat of your walls and ceiling directly to your body.

9. Radiation is the transfer of thermal energy in the form of electromagnetic spectrum in vacuum. Generation of electromagnetic spectrum occurs at high temperature range. Example:- Microwave heating, heating of bread crust inside baking oven

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