Heating and Cooling Time

The thermal time constant, defined in Equation 9.5, is useful for calculating heating and cooling times, which often take up a considerable amount of the cycle time of batch and semi-batch reactions. We start from Equation 9.4 that, after variable separation, combines with Equation 9.5, to become 

From Equation 9.9, different practical useful forms can be derived  

Obviously all these expressions are also valid for cooling, then (Tc T). They are useful in process design, for calculating cycle times, where heating and cooling phases are often time-consuming. Of course, their use requires knowledge of the overall heat transfer coefficient U. Its determination is described in Section 9.3. 

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Overcurrent protection. To provide a thermal replica protection, the relay is set according to motor s heating and cooling (/ - 1) curves supplied by the motor manufacturer. If these curves are not available, they can be established with the help of motor heating and cooling time constants, as in equations (3.2) and (3.4). A brief procedure to establish the motor thermal curves when they are not available is explained in Section 3.6. 

This value, together with the heating and cooling times of the reactor that are summarized in Table VIII, was used to correct the rate constant data in Table VII. This correction is based on the recognition that for second order kinetics... 

Wall thickness fluctuations of up to 5% may occur as a result of the uneven temperature in the molten polymer during rotation. Also, because of rate of cooling which is not always reproducible, deviations in the dimensions of the finished product may amount to 5%. Requirements for this process are that the material can be molten completely, that the melt is of sufficiently low viscosity, and that the molten polymer does not degrade too rapidly. Besides plasticized PVC, high and low density polyethylene (HOPE and LDPE) are often used, as well as copolymers of PE such as EVA (ethylene-vinyl acetate). Cycle times vary between 3 and 40 min, depending on the wall thickness. Cycle time is predominantly controlled by the heating and cooling time. 

Simulator Exercise The temperature controllers have an on/off switch and allow set-point adjustment between 0 and 500 °E Heating and cooling time is compressed for user-friendliness. 

Thickening plastic panels to meet stiffness requirements is an expensive use of any plastic (or other material, for that matter) because inefficiently large quantities of the material are used, and increased heating and cooling times raise the cost of fabrication. 

Factors such as the time of exposure to certain temperatures, heating and cooling times are collected by some authors produce significant changes in the residual stresses mortar subjected to heat [10, 50]. 

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