Thermal Balance and Temperature Control

An HR system must be in a state of thermal equilibrium, which means that heat losses must be compensated for by heating. In the ideal case, an HR system will be in an isothermic state. 

The thermal equilibrium of any HR system is governed by variable and fixed values of all components of the thermal balance of the mould/injection moulding machine/ambient conditions system. The fact that variable factors are always present means that automatic temperature control and regulation of the HR system must be used to keep deviations from the required temperature to a minimum. The following initial conditions must be fulfilled  

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The advantage of the piezoelectric balance is that the considerably higher measurement sensitivity provides a mass resolution of about 0.1 pg, against 25 pg for the mechanical balance. Owing to the smaller size of the balance, its temperature control is simpler, and the overall size of the thermal analyser is appreciably smaller. The mass of a fully equipped STA 429 thermal analyser is about 450 kg, and that of a STA 449 analyser, 120kg (Fig. 14.1). 

When the energy flows in and out of a compartment do not balance, the energy difference accumulates and the temperature increases or decreases. The changes in core and skin temperature then in turn alter the physiological control signals to restore balance and thermal stability. 

These points are explained in detail in this chapter. In a first section, the general aspects of reaction engineering for batch reactors are briefly presented. The mass and heat balances are analysed and it is shown that a reliable temperature control is central to the safety of batch reactors. The different strategies of temperature control and their consequences on reactor safety are explained in the following sections. For each strategy, the design criteria and the safety assessment procedure are introduced. The chapter is closed by recommendations for the design of thermally safe batch reactions. 

In cases where heat recovery was practiced, the overall thermal efficiency was assumed to be 507o. The major heat loss was the hot flue gases, but other losses included sensible heat plus the unburned fixed carbon in the ash, and radiation losses from the incinerator unit. Figure 1 shows a summary of the mass and energy balances for a metric ton refuse input to the incinerator. Some auxiliary fuel consumption was assumed (based on discussions with system designers and the actual operating experience of users) for startup, temperature control, and pilot burners in the secondary combustion chambers. 

Another aspect of equipment process control relates to temperature. The mechanical component of CMP is based on friction and thus is a thermal energy source. The understanding of friction under shear force conditions is central to CMP performance. The subject is covered in Chapter 3 on friction. Often different materials in the planarization process have their own energy-balance reactions, and some are exothermic in their own right. Since there is always some form of chemical interaction to complement the mechanical actions in CMP, temperature will have an effect on the process rate. Some processes, like copper planarization, are heavily chemical in nature, and thus control variations are sensitive to temperature fluctuations. 

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