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The use of thermal properties cp and p in design problems is illustrated in the examples
given in Chapter 6.
The expansion coefficient O^ at constant pressure is given by [4]
The linear expansion coefficient Ce11n is approximately
The isothermal compression coefficient yK is defined as [4]
av and yK are related to each other by the expression [4]
2.3 Enthalpy
Equation 2.4 leads to
(2.8)
(2.9)
(2.10)
(2.11)
(2.12)
As shown in Figure 2.4, the measured data on h = h(T) [6] for a polymer melt can be
expressed by the polynomial
(2.13)
The specific enthalpy defined as the total energy supplied to the polymer divided by
throughput of the polymer is a useful parameter for designing extrusion and injection
molding equipment such as screws. It provides the theoretical amount of energy required
to bring the solid polymer to the process temperature.
Values of this parameter for different polymers are given in Figure 2.5 [4].
If, for example, the throughput of an extruder is 100 kg/h of polyamide (PA) and the
processing temperature is 260 0 C, the theoretical power requirement would be 20 kW.
This can be assumed to be a safe design value for the motor horse power, although
theoretically it includes the power supply to the polymer by the heater bands of the
extruder as well.