In the decade of the 1960's a model was formulated for the calculation of thermodynamic properties of water for industrial use, and was known as the IFC-67 model. Keywords: thermodynamic properties, water and steam, calculation algorithms This paper presents the IAPWS-IF97 model in summarized form, and results obtained in applying it to the thermodynamic properties of water, including enthalpy, entropy, internal energy, and volume, in the states of compressed liquid, liquid-vapor equilibrium, and superheated steam, using calculation algorithms developed using common, universally available computational techniques. The new model is oriented toward facilitating calculations employing computers, improving accuracy, and improving consistency among the model's limiting regions as well as contributing towards improved process design and simulation. The latter model had been used for 30 years for the calculation of thermodynamic properties of water for industrial, scientific, and academic applications. Was recommended before replacing the current model which had been developed in the 1960's (IFC-67). This new model, termed IAPWS-IF97, was adopted beginning in 1997, but an evaluation and changeover period 2ġDepartment of Chemical Engineering, Engineering Faculty, Universidad de Antofagasta, Antofagasta, Chile,ĢDepartment of Chemistry, Basic Sciences Faculty, Universidad de Antofagasta, Antofagasta, Chile,īeginning in 2000 the International Association for the Properties of Water and Steam (IAPWS) recommended the use of a new model for calculation of the thermodynamic properties of water and steam. Soc., 51, Nº 2 (2006), pags: 891-900ĬALCULATION OF THE THERMODYNAMIC PROPERTIES OF WATER USING THE IAPWS MODEL Q is positive for the cold water, because heat was added, and negative for the hot water. The heat transferred from the hot water to the cold water is therefore:Ĭalculate the change in entropy for the hot and cold water using the equation: The process is irreversible - any process involving a transfer of heat from a higher-temperature region to a lower-temperature region is irreversible.Īssuming no heat is exchanged with the surroundings or the environment, what is the change in entropy in the mixing process?įirst, determine how much heat is involved. Is this process reversible or irreversible? The colder water is then poured into the warmer water, and the system is allowed to come to equilibrium. In one the water temperature is 17☌, while in the other it is 37☌. You have two styrofoam containers of water. Time moves in the direction of increasing entropy. This is why the glass of spilled milk never spontaneously transforms itself back into an upright full glass of milk - that would decrease the entropy.Įntropy is often called time's arrow. The entropy of a closed system is constant for reversible processes and increases for irreversible processes. The Second Law of Thermodynamics states that: In these there is no change in entropy in a closed system. The entropy postulate connects the concept of entropy with such processes:Įntropy Postulate: If an irreversible process occurs in a closed system, the entropy S of the system always increases. This is an example of an irreversible process. Even though Newton's Laws and The Laws of Conservation of Energy and Conservation of Momentum would be obeyed when you played the film backwards, the probability that all the milk and the glass would spontaneously come together to form a full glass of milk is incredibly small. If you videotaped the spill and then played the film backwards, it would be obvious to you that the film was running backwards. If you spill a glass of milk, what the glass and the milk droplets do is governed by the laws of physics. ΔS = nR ln(V f / V i) + nC V ln(T f / T i) If the heat transfer takes place over a range of temperatures then, as long as ΔT is small compared to the absolute temperature T, the change in entropy is approximately:įor an ideal gas, it can be shown that the change in entropy is given by: If the heat transfer takes place at a single temperature, the change in entropy is simply: The change in entropy is the heat added divided by the temperature at which the transfer took place. On the other, a change in entropy is easy to determine.Įntropy changes whenever there is a transfer of heat. Unlike P, V, and T, which are quite easy to measure, the entropy of a system is difficult to calculate. The symbol for entropy is S, and the units are J/K.Ī container of ideal gas has an entropy value, just as it has a pressure, a volume, and a temperature. Entropy is in some sense a measure of disorder.
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