In the majority of applications all three types of heat transfer occur simultaneously but one type may be more important than others. Steady-state heat transfer takes place when there is a constant temperature difference between two materials. The amount of heat entering a material equals the amount of heat leaving, and there is no change in temperature of the material. This occurs for example when heat is transferred through the wall of a cold store if the store temperature and ambient temperature are constant and in continuous processes once operating conditions have stabilised. However in the majority of food processing applications the temperature of the food and/or the heating or cooling medium are constantly changing and unsteady-state heat transfer is more commonly found. Calculation of heat transfer under these conditions are extremely complicated but are simplified by making a number of assumptions and in some cases using prepared charts to give approximate solutions. Example Calculate the rate of heat transfer across glass panes which consists of two 1.6mm (0.0016m) thick glass separated by 0.8 mm (0.0008m) layer of air. The heat transfer coefficient on one side which is at 21 oC is 2.84 W/m2.K and On the opposite side which is at -15 oC is 11.4 W/m2.K The thermal conductivity of the glass is 0.52 W/m.K and The thermal conductivity of air is 0.031 W/m.K. Solution: Using the diagram (right) there are 5 resistances to heat transfer: R1 = the convective resistance to one surface exposed to air. R2 = the conductive resistance of one layer of glass R3 = the conductive resistance of the air layer between the glass R4 = the conductive resistance of the other layer of glass R5 = the convective resistance of the opposite surface exposed to air.
In the majority of applications all three types of heat transfer occur simultaneously but one type may be more important than others. Steady-state heat transfer takes place when there is a constant temperature difference between two materials. The amount of heat entering a material equals the amount of heat leaving, and there is no change in temperature of the material. This occurs for example when heat is transferred through the wall of a cold store if the store temperature and ambient temperature are constant and in continuous processes once operating conditions have stabilised. However in the majority of food processing applications the temperature of the food and/or the heating or cooling medium are constantly changing and unsteady-state heat transfer is more commonly found. Calculation of heat transfer under these conditions are extremely complicated but are simplified by making a number of assumptions and in some cases using prepared charts to give approximate solutions. Example Calculate the rate of heat transfer across glass panes which consists of two 1.6mm (0.0016m) thick glass separated by 0.8 mm (0.0008m) layer of air. The heat transfer coefficient on one side which is at 21 oC is 2.84 W/m2.K and On the opposite side which is at -15 oC is 11.4 W/m2.K The thermal conductivity of the glass is 0.52 W/m.K and The thermal conductivity of air is 0.031 W/m.K. Solution: Using the diagram (right) there are 5 resistances to heat transfer: R1 = the convective resistance to one surface exposed to air. R2 = the conductive resistance of one layer of glass R3 = the conductive resistance of the air layer between the glass R4 = the conductive resistance of the other layer of glass R5 = the convective resistance of the opposite surface exposed to air.
sample_self_learner_basic_engineering.docx