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RESEARCH PAPERS: Thermodynamic Properties

# Analysis of Enthalpy Approximation for Compressed Liquid Water

[+] Author and Article Information
Milivoje M. Kostic

Northern Illinois University, DeKalb, IL 60115-2854kostic@niu.edu

J. Heat Transfer 128(5), 421-426 (Nov 01, 2005) (6 pages) doi:10.1115/1.2175090 History: Received December 13, 2004; Revised November 01, 2005

## Abstract

It is custom to approximate solid and liquid thermodynamic properties as being a function of temperature only, since they are virtually incompressible, and Pdv boundary work may be neglected. Furthermore, in classical literature, for isothermal compression processes, a general “improvement” and correction for liquid enthalpy approximation is given by adding the “pressure correction,” $vdP$, to the corresponding saturation value. It is shown that such correction given for isothermal processes is generally valid for isentropic processes only. Analysis of water real properties, over the saturation temperature range and a wide pressure range up to $100MPa$, shows that the recommended corrections are only beneficial for higher pressures at smaller temperatures (below $200°C$), insignificant for smaller pressures at most of the temperatures, about the same but opposite sign (thus unnecessary) for intermediate temperatures and pressures, and more erroneous (thus counterproductive and misleading) for higher temperatures and pressures, than the corresponding saturation values without any correction. The misconception in the literature is a result of the erroneous assumption, that due to incompressibility for liquids in general, the internal energy is less dependent on pressure than enthalpy.

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## Figures

Figure 1

Isothermal and isentropic compression of saturated liquid water

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