Some HVAC engineering calculations, such as real-time simulations and control system models, need simple and precise equations for properties estimation. Since the simplest model available has large errors as the variables of interest, v, h, and s, approach zero and the most precise model calculations are iterative and lengthy, a simple but precise model was developed.

This model, based on polynomial curve fits of the enhancement and compressibility factors as functions of temperature and pressure, is described and its parameters calculated. Model prediction errors were found to be well within ±0.2% of the most precise model for prescribed ranges of -40°C to 50°C (-40°F to 122°F) and 77.059 to 101.325 kPa (11.176 to 14.695 psia). These correspond to data in ASHRAE psychrometric charts 1, 2, 5, 6, and 7.

Units: Dual