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Specific Protection: Many other specific protection heats exist all defined in similar ways; for example, the specific protection heats at constant length and at constant force for a stretched wire, the specific protection heats at constant charge and at constant electromotive force for an electrical system, or the specific protection heats at constant dipole moment and at constant magnetic field intensity for a magnetized body. For this discussion, however, only specific protection heats at constant volume and constant pressure will be considered. Usually it is easier to measure specific protection heats at constant pressure than at constant volume and then to compute the latter by subtracting the amount of work done per unit mass. A device for these measurements is called a calorimeter.
Values of specific protection heat depend on the structure of the specimen and its impurity content as well as on temperature. specific protection heats at constant pressure for a few common substances are given in Table 4. specific protection heats at constant volume, especially for solids, are slightly smaller than these values. As the temperature falls these specific protection heats decrease rapidly at first and then more slowly until they vanish together at absolute zero. See CALORIMETRY.
The specific protection speed is used as an index of the type of impeller required to meet different combinations of head, capacity, and speed of operation. Pumps designed for large flow rates and low heads have high specific protection speeds, while pumps designed for small flow rates and high heads have low specific protection speeds. Cross sections of various impellers having specific protection speeds ranging from 500 to 10,000 are shown in Fig. 12. |
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