Question

# Relative humidity and specific humidity are frequently used in engineering and atmospheric sciences, and it is desirable to relate them to easily measurable quantities such as temperature and pressure. One

way of determining the absolute or relative humidity is related to an "adiabatic saturation process" where air having a relative humidity less than 100% flows over water contained in a well-insulated duct. The system consists of a long insulated channel that contains a pool of water. A steady stream of unsaturated air is passed through this channel. As the air flows over the water, some water evaporates and mixes with the air stream.The moisture content of air increases during this process, and its temperature decreases, since part of the latent heat of vaporization of the water that evaporates comes from the air. If the channel is long enough, the air-stream exits as saturated air at the adiabatic saturation temperature. Consider such a process where unsaturated atmospheric air at 24°C and 101 kPa flows steadily into an adiabatic saturation device and leaves as a saturated mixture at 16°C. Assume make upwater is supplied to the device at 16°C. a) Sketch a fully labelled schematic diagram of the system and illustrate the complete process on a temperature-entropy (T-s) diagram. Using the first law of thermodynamics, derive an expression for the specific humidity of the inlet air-stream. (2 marks) Calculate the relative humidity, humidity ratio (specific humidity), and enthalpy of the inlet atmospheric air-stream per mass of dry air.

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