ENERGY, WORK, AND HEAT ThermodynamicsExample:Determine the specific P-V energy of 15 lbm of steam at 1000 psi in an 18 ft3tank.Solution:Using Equation 1-14PnPVmPn(1000 lbf/in.2) (144 in.2/ft2) (18 ft3)15 lbmPn172,800 ft lbf/lbmSpecificEnthalpySpecific enthalpy (h) is defined as h = u + Pn, where u is the specific internal energy (Btu/lbm)of the system being studied, P is the pressure of the system (lbf/ft2), and n is the specific volume(ft3/lbm) of the system. Enthalpy is usually used in connection with an "open" system problemin thermodynamics. Enthalpy is a property of a substance, like pressure, temperature, andvolume, but it cannot be measured directly. Normally, the enthalpy of a substance is given withrespect to some reference value. For example, the specific enthalpy of water or steam is givenusing the reference that the specific enthalpy of water is zero at .01°C and normal atmosphericpressure. The fact that the absolute value of specific enthalpy is unknown is not a problem,however, because it is the change in specific enthalpy (Dh) and not the absolute value that isimportant in practical problems. Steam tables include values of enthalpy as part of theinformation tabulated.WorkKinetic energy, potential energy, internal energy, and P-V energy are forms of energy that areproperties of a system. Work is a form of energy, but it is energy in transit. Work is not aproperty of a system. Work is a process done by or on a system, but a system contains no work.This distinction between the forms of energy that are properties of a system and the forms ofenergy that are transferred to and from a system is important to the understanding of energytransfer systems.HT-01 Page 18 Rev. 0
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