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This paper derives equations for the calculation of enthalpy and enthalpy difference as a function of both temperature and pressure that can be used when a full equation of state description is not available for a liquid. A result is the inclusion of the "volume expansivity", usually a small effect, but missing in most other literature. In the end, the equations are easy to implement in the analysis of data or predictions of component performance. This paper also displays the relative importance of pressure drop to change in temperature when computing heat transfer rates. For example, a pressure drop of 140 kPa (20 psid) accounts for 1% of the enthalpy difference for a stream of water undergoing a 2.8°C (5°F ) change in temperature. Although this might not appear to be significant, it can be important when computing system energy balance closures for performance standards that call for tight energy balance closure, plus or minus 3% being typical.

Citation: ASHRAE Transactions -- Volume 119, Part 2, Denver, CO.