tailieunhanh - Lecture Thermodynamics: An engineering approach (8/e): Chapter 14 - Yunus A. Çengel, Michael A. Boles

Chapter 14 - Gas-vapor mixtures and air-conditioning. In this chapter, we consider the air–water-vapor mixture, which is the most commonly encountered gas–vapor mixture in practice. We also discuss air-conditioning, which is the primary application area of air–water-vapor mixtures. | Chapter 14 Gas-Vapor Mixtures and Air-Conditioning Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Çengel and Michael A. Boles We will be concerned with the mixture of dry air and water vapor. This mixture is often called atmospheric air. The temperature of the atmospheric air in air-conditioning applications ranges from about –10 to about 50oC. Under these conditions, we treat air as an ideal gas with constant specific heats. Taking Cpa = kJ/kg K, the enthalpy of the dry air is given by (assuming the reference state to be 0oC where the reference enthalpy is taken to be 0 kJ/kga) The assumption that the water vapor is an ideal gas is valid when the mixture temperature is below 50oC. This means that the saturation pressure of the water vapor in the air-vapor mixture is below kPa. For these conditions, the enthalpy of the water vapor is approximated by hv(T) = hg at mixture temperature T. The . | Chapter 14 Gas-Vapor Mixtures and Air-Conditioning Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Çengel and Michael A. Boles We will be concerned with the mixture of dry air and water vapor. This mixture is often called atmospheric air. The temperature of the atmospheric air in air-conditioning applications ranges from about –10 to about 50oC. Under these conditions, we treat air as an ideal gas with constant specific heats. Taking Cpa = kJ/kg K, the enthalpy of the dry air is given by (assuming the reference state to be 0oC where the reference enthalpy is taken to be 0 kJ/kga) The assumption that the water vapor is an ideal gas is valid when the mixture temperature is below 50oC. This means that the saturation pressure of the water vapor in the air-vapor mixture is below kPa. For these conditions, the enthalpy of the water vapor is approximated by hv(T) = hg at mixture temperature T. The following T-s diagram for water illustrates the ideal-gas behavior at low vapor pressures. See Figure A-9 for the actual T-s diagram. The saturated vapor value of the enthalpy is a function of temperature and can be expressed as Note: For the dry air-water vapor mixture, the partial pressure of the water vapor in the mixture is less that its saturation pressure at the temperature. Consider increasing the total pressure of an air-water vapor mixture while the temperature of the mixture is held constant. See if you can sketch the process on the P-v diagram relative to the saturation lines for the water alone given below. Assume that the water vapor is initially superheated. P v When the mixture pressure is increased while keeping the mixture temperature constant, the vapor partial pressure increases up to the vapor saturation pressure at the mixture temperature and condensation begins. Therefore, the partial pressure of the water vapor can never be greater than its saturation