8.2 Psychrometric Charts and Humid Air Properties

Psychrometric charts are essential tools for understanding moist air properties. They graphically represent temperature, humidity, and other key factors, making it easier to analyze and design air conditioning systems.

Humid air properties are crucial in HVAC applications. By using psychrometric charts, engineers can determine cooling loads, heating requirements, and optimal conditions for comfort and efficiency in various environments.

Psychrometric Charts and Applications

Graphical Representation of Moist Air Properties

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• Psychrometric charts are graphical representations of the thermodynamic properties of moist air at a constant pressure, typically at sea level (101.325 kPa)
• The horizontal axis of the psychrometric chart represents the dry-bulb temperature, while the vertical axis represents the humidity ratio (also known as moisture content or absolute humidity)
• Other properties represented on the psychrometric chart include relative humidity, wet-bulb temperature, dew-point temperature, specific volume, and enthalpy
• The psychrometric chart is divided into different regions, including the saturation curve (100% relative humidity), the wet-bulb temperature lines, and the dew-point temperature lines

Applications of Psychrometric Charts

• Psychrometric charts are used in various applications, such as HVAC (Heating, Ventilation, and Air Conditioning) systems design, indoor environmental control, and drying processes
  • In HVAC systems, psychrometric charts help in determining the required cooling, heating, humidification, or dehumidification processes to achieve desired indoor conditions
  • For indoor environmental control, psychrometric charts are used to analyze and maintain comfortable temperature and humidity levels for occupants
  • In drying processes (food, pharmaceutical, or material drying), psychrometric charts help in understanding the drying process and optimizing the drying conditions

Humid Air Properties

Determining Properties using Psychrometric Charts

• To determine the properties of humid air using a psychrometric chart, locate the point on the chart corresponding to the given dry-bulb temperature and either the humidity ratio or relative humidity
• Once the point is located, other properties such as wet-bulb temperature, dew-point temperature, specific volume, and enthalpy can be read from the corresponding lines or scales on the chart
  • Wet-bulb temperature can be found by following the constant wet-bulb temperature line from the point to the saturation curve
  • Dew-point temperature can be found by following the horizontal line from the point to the saturation curve
  • Specific volume can be read from the specific volume lines (sloped lines from top-left to bottom-right)
  • Enthalpy can be read from the enthalpy scale (usually located on the top or bottom of the chart)
• Interpolation may be necessary when the point falls between the lines on the psychrometric chart

Calculating Parameters using Psychrometric Chart Data

• The properties obtained from the psychrometric chart can be used to calculate various parameters in HVAC systems, such as cooling load, heating load, and ventilation requirements
  • Cooling load can be calculated using the enthalpy difference between the indoor and outdoor air conditions
  • Heating load can be calculated using the enthalpy difference and the desired indoor temperature
  • Ventilation requirements can be determined based on the desired indoor air quality and the outdoor air conditions
• Psychrometric charts are also used to analyze and design processes involving air-water vapor mixtures, such as evaporative cooling, humidification, and dehumidification
  • Evaporative cooling processes can be analyzed by following the constant wet-bulb temperature lines on the chart
  • Humidification and dehumidification processes can be designed by considering the required changes in humidity ratio and temperature

Temperatures: Dry-bulb, Wet-bulb, Dew-point

Dry-bulb Temperature

• Dry-bulb temperature is the temperature of the air measured by a thermometer freely exposed to the air but shielded from radiation and moisture
• It is the temperature used as the horizontal axis on the psychrometric chart
• Dry-bulb temperature represents the sensible heat content of the air

Wet-bulb Temperature

• Wet-bulb temperature is the temperature measured by a thermometer with its bulb covered by a wet wick and exposed to an airstream
• It represents the temperature of adiabatic saturation and is always lower than or equal to the dry-bulb temperature
• Wet-bulb temperature lines on the psychrometric chart are sloped and run from the saturation curve to the bottom of the chart
• Wet-bulb temperature is an indicator of the air's potential for evaporative cooling

Dew-point Temperature

• Dew-point temperature is the temperature at which the water vapor in the air begins to condense when the air is cooled at constant pressure
• It represents the temperature at which the air becomes saturated (100% relative humidity)
• Dew-point temperature lines on the psychrometric chart are horizontal and parallel to the dry-bulb temperature axis
• The dew-point temperature is a measure of the air's moisture content

Wet-bulb Depression

• The difference between the dry-bulb and wet-bulb temperatures is known as the wet-bulb depression, which is an indicator of the air's humidity
• A smaller wet-bulb depression indicates higher humidity, while a larger wet-bulb depression indicates lower humidity
• Wet-bulb depression is used in determining the effectiveness of evaporative cooling systems

Humidity Ratio vs Relative Humidity vs Specific Volume

Humidity Ratio

• Humidity ratio (w) is the mass of water vapor per unit mass of dry air, expressed in grams of water vapor per kilogram of dry air (g/kg) or pounds of water vapor per pound of dry air (lb/lb)
• Humidity ratio is represented on the vertical axis of the psychrometric chart
• It is a measure of the absolute moisture content of the air

Relative Humidity

• Relative humidity (φ) is the ratio of the actual water vapor pressure in the air to the saturation water vapor pressure at the same temperature, expressed as a percentage
• Relative humidity lines on the psychrometric chart are curved and run from the saturation curve (100% RH) to the bottom of the chart
• Relative humidity is a measure of the air's moisture content relative to its maximum moisture-holding capacity at a given temperature

Specific Volume

• Specific volume (ν) is the volume of the air-water vapor mixture per unit mass of dry air, expressed in cubic meters per kilogram of dry air (m³/kg) or cubic feet per pound of dry air (ft³/lb)
• Specific volume lines on the psychrometric chart are sloped and run from the top-left to the bottom-right of the chart
• Specific volume is a measure of the air's density and is affected by both temperature and moisture content

Relationships between Humidity Ratio, Relative Humidity, and Specific Volume

• As the humidity ratio increases at a constant dry-bulb temperature, the relative humidity and specific volume also increase
  • Adding moisture to the air increases its water vapor content (humidity ratio) and its relative humidity
  • The added moisture also increases the volume of the air-water vapor mixture, resulting in a higher specific volume
• For a constant humidity ratio, as the dry-bulb temperature increases, the relative humidity decreases, and the specific volume increases
  • Increasing the temperature of the air increases its moisture-holding capacity, thus lowering the relative humidity for the same humidity ratio
  • The increase in temperature also causes the air to expand, resulting in a higher specific volume
• Understanding these relationships is essential for analyzing and designing processes involving moist air, such as air conditioning, humidification, and dehumidification systems
  • Air conditioning systems often involve cooling and dehumidifying the air to maintain comfortable indoor conditions
  • Humidification systems add moisture to the air to increase the humidity ratio and relative humidity
  • Dehumidification systems remove moisture from the air to decrease the humidity ratio and relative humidity