SL-08-039 -- Thermodynamic Properties for Humid Air — An Engineering Correlation

Air-conditioning systems operate with air at humidity values below saturation; therefore, air properties are functions of pressure, temperature, and humidity. Perez Galindo et al. (2007) presented a model of saturated air properties, based on polynomial curve fits of required variables, whose only inputs are pressure and temperature but needed to be completed to include other humidity contents.

This work presents the development of a similar model for dry air properties and procedures to calculate humid air properties at any humidity content. The procedures were based on the use of the degree of saturation and include provisos for the three most common humidity-related measurements: relative humidity, wet-bulb temperature, and dew-point temperature.

Model results were evaluated, in the range of 0.0 to 1.0 relative humidity, with respect to values calculated with the real gas model predictions for dry and saturated air properties. Errors were found to be within ±0.2% of the most precise model for the prescribed ranges of –40°C to 50°C (–40°F to 122°F) and 77.059 to 101.325 kPa (11.176 to 14.696 psia). These correspond to data in ASHRAE psychrometric charts 1, 2, 5, 6, and 7 (ASHRAE 1992).

Units: Dual