Atmospheric air at 760 mm Hg is at 22C dry bulb temperature and 20C wet bulb temperature. Using the psychrometric chart, determine:
a. Relative humidity.
b. Humidity ratio.
c. Dew-point temperature.
d. Enthalpy of air per kg dry air.
e. Volume of moist air/kg dry air.
The Correct Answer and Explanation is :
To solve this problem using a psychrometric chart, let’s first review the key concepts needed to answer each part of the question.
a. Relative Humidity
Relative humidity (RH) is a measure of the amount of moisture in the air compared to the maximum amount of moisture the air can hold at a given temperature. The RH can be determined using the wet-bulb and dry-bulb temperatures.
On the psychrometric chart, you would locate the point corresponding to a dry-bulb temperature of 22°C and a wet-bulb temperature of 20°C. From the chart, the relative humidity corresponding to these two values is approximately 75%.
b. Humidity Ratio
The humidity ratio is the mass of water vapor per unit mass of dry air. This can be found on the psychrometric chart by locating the intersection of the dry-bulb temperature (22°C) and wet-bulb temperature (20°C). The corresponding humidity ratio is around 0.010 kg of water vapor per kg of dry air.
c. Dew-Point Temperature
The dew point is the temperature at which air becomes saturated with moisture and water vapor begins to condense. From the psychrometric chart, you can trace a line vertically down from the point of intersection of dry-bulb and wet-bulb temperatures (22°C and 20°C) until it reaches the saturation curve. The dew-point temperature is approximately 18°C.
d. Enthalpy of Air per kg Dry Air
Enthalpy is a measure of the total energy in the air. On the psychrometric chart, the enthalpy of air can be found by locating the point of intersection of the dry-bulb temperature and wet-bulb temperature. The enthalpy is the sum of the sensible and latent heat components and for this case is approximately 45 kJ/kg of dry air.
e. Volume of Moist Air per kg Dry Air
The volume of moist air per kg of dry air can be determined from the specific volume (or volume of moist air per unit mass of dry air) shown on the chart. The specific volume corresponding to the given conditions (22°C dry bulb and 20°C wet bulb) is about 0.86 m³/kg of dry air.
Explanation Summary
These calculations are based on the data from the psychrometric chart, which is a graphical representation of the physical and thermodynamic properties of moist air. By knowing the dry-bulb and wet-bulb temperatures, you can easily determine the relative humidity, humidity ratio, dew point, enthalpy, and specific volume of moist air. These properties are crucial in various applications like HVAC system design, environmental controls, and other engineering processes involving air systems.