Psychrometric charts are essential tools for understanding air-conditioning processes. They visually represent the properties of moist air, allowing us to analyze and manipulate air conditions for comfort and industrial applications.

These charts help us track changes in temperature, humidity, and energy as air undergoes various processes. By mastering psychrometric charts, we can design efficient air-conditioning systems and solve real-world HVAC problems with confidence.

Air-conditioning processes analysis

Interpreting psychrometric charts

• Psychrometric charts graphically represent thermodynamic properties of moist air including dry-bulb temperature, wet-bulb temperature, relative humidity, specific humidity, enthalpy, and specific volume
• The x-axis typically represents dry-bulb temperature, while the y-axis represents moisture content or humidity ratio of the air
• Lines of constant relative humidity, wet-bulb temperature, enthalpy, and specific volume are plotted on the chart, allowing determination of various air properties at different conditions
• The chart is divided into regions such as cooling, heating, humidification, and dehumidification, based on processes that can occur in each area

Analyzing air-conditioning processes

• Analyzing air-conditioning processes involves plotting initial and final states of air on the psychrometric chart
• Follow appropriate process lines to determine changes in air properties
• Process lines include horizontal lines for sensible heating or cooling, vertical lines for humidification or dehumidification, and sloped lines for combined processes
• Adiabatic mixing processes are represented by straight lines connecting initial states of two air streams

Psychrometric processes

Sensible heating and cooling

• Sensible heating increases dry-bulb temperature of air without changing moisture content, represented by a horizontal line on the psychrometric chart
  • Example: Heating air from 20°C to 30°C at constant humidity ratio
• Sensible cooling decreases dry-bulb temperature of air without changing moisture content, also represented by a horizontal line
  • Example: Cooling air from 30°C to 20°C at constant humidity ratio

Humidification and dehumidification

• Humidification adds moisture to air, increasing humidity ratio while maintaining constant dry-bulb temperature, represented by a vertical line on the psychrometric chart
  • Example: Adding moisture to air at 25°C, increasing humidity ratio from 0.010 to 0.015 kg water/kg dry air
• Dehumidification removes moisture from air, decreasing humidity ratio while maintaining constant dry-bulb temperature, also represented by a vertical line
  • Example: Removing moisture from air at 25°C, decreasing humidity ratio from 0.015 to 0.010 kg water/kg dry air

Combined processes

• Cooling and dehumidification occurs when air is cooled below its dew point temperature, decreasing both dry-bulb temperature and humidity ratio, represented by a line with negative slope
  • Example: Cooling air from 30°C and 50% relative humidity to 15°C and 90% relative humidity
• Heating and humidification occurs when air is heated and simultaneously humidified, increasing both dry-bulb temperature and humidity ratio, represented by a line with positive slope
  • Example: Heating air from 20°C and 30% relative humidity to 30°C and 50% relative humidity
• Adiabatic mixing combines two air streams with different properties to obtain a mixed air state, represented by a straight line connecting the two initial states
  • Example: Mixing 1 kg/s of air at 20°C and 50% relative humidity with 2 kg/s of air at 30°C and 30% relative humidity

Air properties calculation

Locating initial and final states

• To calculate air properties, locate the initial state on the psychrometric chart based on given dry-bulb temperature and humidity ratio or relative humidity
• For sensible processes, follow the horizontal line from initial state to desired dry-bulb temperature and read corresponding air properties at final state
• For humidification or dehumidification, follow the vertical line from initial state to desired humidity ratio and read corresponding air properties at final state

Following process lines

• For combined cooling and dehumidification, follow the appropriate process line (e.g., 100% saturation line or apparatus dew point line) from initial to final state and read corresponding air properties
  • Example: Following the 100% saturation line from 30°C and 50% relative humidity to 15°C and 90% relative humidity
• For combined heating and humidification, follow the appropriate process line (e.g., a line with a specific slope) from initial to final state and read corresponding air properties
  • Example: Following a process line with a slope of 2000 J/kg per °C from 20°C and 30% relative humidity to 30°C and 50% relative humidity

Adiabatic mixing calculations

• When calculating properties for adiabatic mixing, locate initial states of the two air streams on the psychrometric chart
• Connect initial states with a straight line and find mixed air state based on mass flow ratio of the two streams
  • Example: Mixing 1 kg/s of air at 20°C and 50% relative humidity with 2 kg/s of air at 30°C and 30% relative humidity results in a mixed air state at approximately 26.7°C and 36.7% relative humidity

Energy requirements for air-conditioning

Sensible heating and cooling

• Energy required for sensible heating or cooling can be calculated using the equation $Q = m × c_p × (T_2 - T_1)$, where $Q$ is heat transfer rate, $m$ is mass flow rate of air, $c_p$ is specific heat of air, and $T_1$ and $T_2$ are initial and final dry-bulb temperatures
  • Example: Heating 1 kg/s of air from 20°C to 30°C requires $Q = 1 × 1.005 × (30 - 20) = 10.05$ kW

Humidification and dehumidification

• Energy required for humidification can be calculated using the equation $Q = m × (h_2 - h_1)$, where $Q$ is heat transfer rate, $m$ is mass flow rate of air, and $h_1$ and $h_2$ are initial and final enthalpy values of the air
  • Example: Humidifying 1 kg/s of air from 25°C and 50% relative humidity to 25°C and 70% relative humidity requires $Q = 1 × (56.28 - 50.44) = 5.84$ kW
• Energy required for dehumidification can be calculated using the equation $Q = m × (h_1 - h_2)$, where $Q$ is heat transfer rate, $m$ is mass flow rate of air, and $h_1$ and $h_2$ are initial and final enthalpy values of the air
  • Example: Dehumidifying 1 kg/s of air from 25°C and 70% relative humidity to 25°C and 50% relative humidity requires $Q = 1 × (56.28 - 50.44) = 5.84$ kW

Combined processes and adiabatic mixing

• For combined cooling and dehumidification, total energy required is the sum of sensible and latent cooling loads, determined using the psychrometric chart and equations mentioned above
  • Example: Cooling and dehumidifying 1 kg/s of air from 30°C and 50% relative humidity to 15°C and 90% relative humidity requires approximately 20 kW of total cooling capacity
• For combined heating and humidification, total energy required is the sum of sensible heating load and energy needed for humidification, determined using the psychrometric chart and equations mentioned above
  • Example: Heating and humidifying 1 kg/s of air from 20°C and 30% relative humidity to 30°C and 50% relative humidity requires approximately 15 kW of total heating and humidification capacity
• When calculating energy requirements for adiabatic mixing, use mass flow rates and enthalpy values of initial air streams and mixed air state to determine energy balance of the system
  • Example: Mixing 1 kg/s of air at 20°C and 50% relative humidity with 2 kg/s of air at 30°C and 30% relative humidity results in a mixed air state with an enthalpy of approximately 52.8 kJ/kg, requiring no additional energy input or output