In thisĬase, the virial contribution for pressure required the use of a It can be seen in Figure 2 that the calculated errors are alsoĮntropy was calculated in the same manner as enthalpy. Used in the saturated air model and a two-step correlation of the virialĬontribution-the first step to obtain temperature effects and the The calculation of enthalpy was based on the ideal gas equation Predictions, are well within the prescribed limit, as shown in Figure 1. Temperature and pressure was the basis to obtain an equation for dry air The correlation of the compressibility factor as a function of To 15.229 psia) to provide some leeway for extrapolation. The actual ranges for correlations were -60C toħ0C (- 76F to 158F) and 75 to 105 kPa (18.878 That yield a maximum error of 0.2% and 2) to encompass data inĪSHRAE psychrometric charts 1, 2, 5, 6, and 7 (-40C toĥ0C F to 122F] and 77.059 to 101.325 kPa Saturated air model, namely 1) to use the simplest possible polynomials The dry air model was developed with the same restrictions as the Inputs to calculate the saturated air properties are total pressure, P, The saturation pressure,, may be calculated with the Properties, the dry air properties, and the mixture composition must beīased on polynomial curve fits to data calculated with the HylandĪnd Wexler model, Perez Galindo et al. To calculate the humid air properties, models for the saturated air Where, the degree of saturation, is defined by May be calculated to a good approximation by Recommendation, a simple means to evaluate humid air properties at anyĪccording to Hyland and Wexler (1980), the properties for humid air This work complements the model by providing equations to calculateĭry air properties and, following Hyland and Wexler's (1980) Psychrometric charts 1, 2, 5, 6, and 7 ). Wexler properties with errors below 0.2% for a temperature range of (2007) and is based on polynomialĬurve fits of important variables. Such a model, designed to calculate properties of saturated air, Have a simple but accurate model to be used in real-time system (2001), are iterative and lengthy and it was deemed desirable to Very precise property calculations, such as those of Hyland and
Wet-bulb temperature, the relative humidity, and the dew point, since The three most used variables for humidity are the Pressure, dry-bulb temperature, and one humidity-related variable areĬommonly used. Retrieved from Įstimates of the air-water vapor mixture properties, which are theīasis for the first and second laws of thermodynamic analysis ofĪir-conditioning systems, need three independent variables as input.
ASHRAE PSYCHROMETRIC CHART EQUATIONS FREE
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