Question
Download Solution PDFSensible heating or cooling of air is the process of heating or cooling
Answer (Detailed Solution Below)
Detailed Solution
Download Solution PDFConcept:
A psychrometric chart is represented as shown in the figure.
Dry Bulb Temperature: Actual temperature of gas or mixture of gases
Wet bulb temperature: Temperature obtained by an accurate thermometer having a wick moistened with distilled water
Dew point temperature: Temperature at which the liquid droplets just appear when the moist air is cooled continuously.
Relative humidity along saturation line is 100%.
Basic Processes in Conditioning of Air:
Sensible heating: Moisture content of air remains constant so specific humidity is constant, temperature increases as it flows over a heating coil
Sensible cooling: Moisture content of air remains constant so specific humidity is constant, but its temperature decreases as it flows over a cooling coil
Cooling and dehumidification: When moist air is cooled below its dew-point by bringing it in contact with a cold surface, some of the water vapour in the air condenses and leaves the air stream as a liquid, as a result, both the temperature and humidity ratio of air decreases.
Heating and Humidification: During winter it is essential to heat and humidify the room air for comfort. It is done by first sensibly heating the air and then adding water vapour to the air stream through steam nozzles, as a result, both the temperature and humidity ratio of air increases.
Cooling & humidification: Air temperature drops and its humidity increases.
Heating and de-humidification: This process can be achieved by using a hygroscopic material, which absorbs the water vapour from the moisture. If this process is thermally isolated, then the enthalpy of air remains constant, as a result, the temperature of air increases.
Humidity ratio/Specific humidity (ω ) (kg/kg) of a given moist air sample is defined as the ratio of the mass of water vapour (mw) to the mass of dry air (ma) contained in the sample.
\(w = \frac{{{m_w}}}{{{m_a}}} = 0.622\frac{{{P_v}}}{{P - {P_v }}} \)
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