Engineering Model of High Pressure Moist Air

Abstract

Abstract. The article deals with the moist air equation of state. There are equations of state discussed in the article, i.e. the model of an ideal mixture of ideal gases, the model of an ideal mixture of real gases and the model based on the virial equation of state. The evaluation of sound speed based on the ideal mixture concept is mentioned. The sound speed calculated by the model of an ideal mixture of ideal gases is compared with the sound speed calculated by using the model based on the concept of an ideal mixture of real gases. The comparison of enthalpy end entropy based on the model of an ideal mixture of ideal gases and the model of an ideal mixture of real gases is performed. It is shown that the model of an ideal mixture of real gases deviates from the model of an ideal mixture of ideal gases only in the case of high pressure. An impossibility of the definition of partial pressure in the mixture of real gases is discussed, where the virial equation of state is used. Basic parameters of moist airThis section covers only a few selected parameters of moist air. The detailed description can be found e.g. in [1]. Let us consider the moist as a binary mixture of dry air and water vapour. An amount of water vapour can change from zero (it is dry air) to some maximum, which depends on the temperature and pressure. Humidity ratio is defined as the ratio of mass of water vapour to the mass of dry airwhere V is volume, ρ v is water vapour density (absolute humidity) and ρ a is dry air density. Let x is the humidity ratio of saturated moist air at the same temperature t and pressure p. Degree of saturation is defined as the ratio of air humidity ratio x to humidity ratio of saturated moist airThe value of saturated humidity ratio x is limited in the atmospheric moist air, e.g. that is in the case of the pressure p = 98000 Pa and temperature t = 25 • C. There can by a different story in some technical application. Consider the case of the same pressure p = 98000 Pa and at the temperature t = 150 • C, where is water vapour always superheated for arbitrary vapour pressure p v ≤ p, therefore moist air can not be saturated for arbitrary humidity ratio x. In such a situation is appropriate to define x = ∞ and degree of saturation will be zero. Relative humidity ϕ R is frequently used parameter in psychrometrics. Relative humidity is defined as the ratio of absolute humidity ρ v to absolute humidity of saturated e-mail: tomas.hyhlik@fs.cvut.cz moist air ρ v at the same temperature t and pressure pAbsolute humidity of moist air ρ v expresses weight of water vapour in cubic meter of moist air. The partial pressure of water vapour in the saturated moist air is not in general equal to saturated water vapour pressure p sat at given temperature, which is related to pure water vapour [2]. Partial pressure of water vapour iswhere f e is so called enhancement factor, which has the value of 1.0041 at standard atmospheric pressure 101325 Pa and at temperature 0 • C. In general, it is possible to say that the value of enhancement...