14.2 Psychrometric charts and processes
5 min read•july 30, 2024
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
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- Psychrometric charts graphically represent thermodynamic properties of moist air including , , relative humidity, specific humidity, , and specific volume
- The x-axis typically represents dry-bulb temperature, while the y-axis represents moisture content or 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, , and , 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 to determine changes in air properties
- Process lines include horizontal lines for or cooling, vertical lines for humidification or dehumidification, and sloped lines for combined processes
- 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
- 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 , where is heat transfer rate, is mass flow rate of air, is specific heat of air, and and are initial and final dry-bulb temperatures
- Example: Heating 1 kg/s of air from 20°C to 30°C requires kW
Humidification and dehumidification
- Energy required for humidification can be calculated using the equation , where is heat transfer rate, is mass flow rate of air, and and 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 kW
- Energy required for dehumidification can be calculated using the equation , where is heat transfer rate, is mass flow rate of air, and and 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 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
Key Terms to Review (19)
Adiabatic mixing: Adiabatic mixing refers to the process of combining two or more substances without exchanging heat with the surroundings, meaning that the total heat of the system remains constant. This process is significant in thermodynamics as it can lead to changes in temperature and pressure of the mixture while maintaining energy conservation. Understanding adiabatic mixing is crucial for analyzing various atmospheric processes and systems involving gases and vapors.
Adiabatic process: An adiabatic process is a thermodynamic process in which no heat is transferred into or out of the system. During this type of process, any change in the internal energy of the system is solely due to work done on or by the system, making it essential in understanding how systems behave under different conditions.
Dehumidification: Dehumidification is the process of reducing the moisture content in the air, typically through mechanical means or chemical methods. This process is vital in maintaining indoor air quality and comfort, particularly in environments where high humidity levels can lead to discomfort, mold growth, and damage to materials. Dehumidification can be represented and analyzed using psychrometric charts, which visually depict the relationships between various properties of air, including temperature, humidity, and enthalpy.
Dry-bulb temperature: Dry-bulb temperature is the temperature of air measured by a regular thermometer, without considering the moisture content in the air. It serves as a fundamental parameter in psychrometric analysis, helping to define the state of air and its thermal properties. This temperature plays a key role in calculating other important values such as relative humidity, dew point, and enthalpy, making it essential for understanding thermodynamic processes involving moisture in air.
Enthalpy: Enthalpy is a thermodynamic property defined as the sum of a system's internal energy and the product of its pressure and volume, represented by the equation $$H = U + PV$$. This concept is crucial for understanding energy transfer in processes involving heat and work, especially in closed systems, where enthalpy changes can indicate how much energy is absorbed or released during physical and chemical transformations.
Enthalpy Change Equation: The enthalpy change equation is a thermodynamic expression that quantifies the heat transfer during a process at constant pressure, represented as $$\Delta H = H_{final} - H_{initial}$$. This equation connects heat transfer to the internal energy and pressure-volume work of a system, making it essential in analyzing both physical and chemical processes. Understanding this equation allows for the application of property tables and psychrometric charts to determine the energy changes involved in various thermodynamic operations.
Environmental Control Systems: Environmental control systems are designed to manage and regulate indoor environments by controlling factors such as temperature, humidity, air quality, and ventilation. These systems play a crucial role in maintaining comfortable living and working conditions, ensuring occupant health and productivity while also promoting energy efficiency. Understanding how these systems interact with psychrometric processes is vital for optimizing performance in various applications like HVAC (heating, ventilation, and air conditioning) designs.
Humidification: Humidification is the process of adding moisture to air to increase its humidity level, making it more suitable for various applications such as comfort, health, and industrial processes. This increase in moisture content affects the thermodynamic properties of air, which can be analyzed using psychrometric charts that depict the relationships between temperature, humidity, and other properties.
Humidity ratio: Humidity ratio is defined as the mass of water vapor present in a unit mass of dry air, typically expressed in units such as grams of water vapor per kilogram of dry air. This measurement is crucial in understanding air properties and behavior, particularly in processes involving moisture, heat transfer, and thermodynamic cycles. It helps illustrate how much moisture the air can hold at a given temperature and pressure, making it essential for evaluating psychrometric processes like cooling and dehumidification.
Hvac design: HVAC design refers to the process of designing heating, ventilation, and air conditioning systems to provide optimal indoor climate control. It involves calculations and considerations related to air flow, temperature control, humidity levels, and energy efficiency to ensure comfort and health in indoor environments.
Hygrometric scale: The hygrometric scale is a measurement system used to quantify the amount of moisture present in the air, typically expressed as relative humidity. This scale provides critical information about the saturation level of water vapor in the atmosphere, which is essential for understanding various psychrometric processes, such as cooling, heating, and dehumidification. By relating temperature and humidity, the hygrometric scale plays a vital role in applications ranging from HVAC systems to weather forecasting.
Isothermal process: An isothermal process is a thermodynamic process in which the temperature of a system remains constant while the system undergoes a change in volume or pressure. This type of process is crucial for understanding how systems interact with their surroundings and how energy is exchanged in various thermodynamic cycles.
Process lines: Process lines are graphical representations on psychrometric charts that illustrate the changes in state of air as it undergoes various thermodynamic processes. These lines connect points representing different air properties, such as temperature, humidity, and enthalpy, showing how air behaves during heating, cooling, humidification, or dehumidification. Understanding process lines is essential for analyzing and optimizing air conditioning and heating systems.
Psychrometric equation: The psychrometric equation is a fundamental relationship in thermodynamics that describes the interactions between temperature, humidity, and other properties of moist air. It helps in calculating parameters such as the humidity ratio, enthalpy, and dew point, which are essential for understanding air conditioning and ventilation processes. This equation is often used in conjunction with psychrometric charts to analyze and visualize changes in air properties during heating, cooling, and humidifying processes.
Saturated air: Saturated air is a condition in which the air contains the maximum amount of water vapor it can hold at a given temperature and pressure. When air is saturated, it has reached 100% relative humidity, meaning that any additional moisture will lead to condensation, forming clouds or dew. Understanding saturated air is crucial for analyzing psychrometric processes, which illustrate how air and water vapor interact under varying conditions.
Sensible cooling: Sensible cooling refers to the process of removing heat from air without changing its moisture content, resulting in a decrease in air temperature. This process is crucial in various applications such as air conditioning and refrigeration, where maintaining comfort and energy efficiency are important. Sensible cooling can be represented on psychrometric charts, showcasing how the dry bulb temperature decreases while the humidity ratio remains constant.
Sensible Heating: Sensible heating refers to the process of raising the temperature of a substance without changing its phase. This concept is crucial when analyzing how air temperature affects moisture content and overall comfort levels in various environments. Sensible heating is often visualized using psychrometric charts, which depict the relationship between air temperature, humidity, and other properties, enabling better understanding and management of indoor climate control.
Unsaturated Air: Unsaturated air is a type of air that contains less water vapor than it can potentially hold at a given temperature and pressure. This means that the air is not fully saturated with moisture, allowing for evaporation and other processes to occur, which is critical when understanding how humidity affects temperature and weather patterns.
Wet-bulb temperature: Wet-bulb temperature is the lowest temperature that can be achieved through evaporative cooling, measured by a thermometer with a moistened bulb. This temperature is crucial in determining humidity levels and assessing the cooling effects of evaporation in various thermodynamic processes. Understanding wet-bulb temperature helps in applications such as HVAC systems, agricultural practices, and meteorology.