# Psychrometric Processes

When discussing psychrometric processes, it is crucial to understand the fundamental principles that govern the behavior of air-water vapor mixtures. Psychrometrics involves the study of the thermodynamic properties of moist air, which are essential for various applications such as HVAC (Heating, Ventilation, and Air Conditioning) systems, meteorology, and industrial processes.

## Key Concepts in Psychrometrics

1. Dry Bulb Temperature (DBT): This is the temperature of air measured by a standard thermometer. It does not account for moisture content and is a primary indicator of thermal conditions.

2. Wet Bulb Temperature (WBT): This measurement is taken using a thermometer covered with a water-soaked cloth over which air flows. It reflects the cooling effect of evaporation and is always lower than or equal to the DBT.

3. Relative Humidity (RH): This is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). Expressed as a percentage, RH indicates how saturated the air is with moisture.

4. Dew Point Temperature: The temperature at which air becomes fully saturated with moisture and water begins to condense. This is critical for predicting weather phenomena and managing indoor air quality.

## Carta Psicrométrica

A view of psychrometric chart is shown before. It shows the relationship between dry-bulb temperature (the temperature measured by a regular thermometer), wet-bulb temperature (the temperature measured by a thermometer with a wetted bulb), relative humidity (the amount of moisture in the air compared to the maximum amount it can hold at that temperature), and dew point temperature (the temperature at which the air becomes saturated and condensation begins).

The curved lines represent constant relative humidity, while the diagonal lines represent constant wet-bulb temperature.

The following equation represents the heat transfer in a thermodynamic process involving a fluid. Here, 𝑄𝐶​ is the heat added or removed, 𝑚𝑎ma​ is the mass flow rate of the fluid, ℎ1​ and ℎ2​ are the specific enthalpies at the initial and final states, respectively, and 𝐶𝑝,𝑚​ is the specific heat capacity at constant pressure. The term (𝑇2−𝑇1) denotes the temperature change of the fluid from the initial state 𝑇1​ to the final state 𝑇2​. This equation essentially states that the heat transfer 𝑄𝐶​ is proportional to the mass flow rate and the change in enthalpy or, equivalently, the product of the mass flow rate, specific heat capacity, and temperature change. This relationship is fundamental in thermodynamics for calculating the energy required to change the temperature of a fluid in various heating or cooling processes.

$$Q_C=m_a\left(h_2-h_1\right)=m_a C p m\left(T_2-T_1\right)$$

The different psychrometric processes are shown in following. The chart illustrates the relationship between temperature (t) on the x-axis and humidity ratio (ω) on the y-axis. It is divided into different regions, each representing a specific psychrometric process. The eight main processes shown in the image are:

1. Evaporative Cooling: This process involves cooling the air through the evaporation of water, which decreases the dry-bulb temperature and increases the humidity ratio.
2. Resfriamento Sensível: This process cools the air without changing its moisture content, resulting in a decrease in the dry-bulb temperature.
3. Aquecimento Sensível: This process heats the air without altering its moisture content, leading to an increase in the dry-bulb temperature.
4. Heating + Humidity: This process involves heating the air while increasing its moisture content, resulting in an increase in both the dry-bulb temperature and the humidity ratio.
5. Cooling + Dehumidification: This process cools the air while reducing its moisture content, leading to a decrease in both the dry-bulb temperature and the humidity ratio.
6. Dehumidification: This process removes moisture from the air without changing its dry-bulb temperature.
7. Humidification: This process adds moisture to the air without changing its dry-bulb temperature.
8. Heating + Humidification: This process involves heating the air while reducing moisture, increasing the dry-bulb temperature and reducing the humidity ratio.

Psychrometric Processes

## Resfriamento Sensível

Durante este processo, o teor de umidade do ar permanece constante, mas sua temperatura diminui à medida que passa por uma serpentina de resfriamento. Para manter o teor de umidade constante, a superfície da serpentina de resfriamento deve estar seca e a temperatura da superfície deve ser maior que a temperatura do ponto de orvalho do ar. Se a serpentina de resfriamento for 100% eficaz, a temperatura de saída do ar será igual à temperatura da serpentina. Contudo, na prática, a temperatura do ar de saída será superior à temperatura da serpentina de resfriamento.Figura abaixomostra o processo de resfriamento sensível 2–1 em um gráfico psicrométrico. A taxa de rejeição de calor durante este processo é dada por:

## Aquecimento Sensível

During Sensible Hating process, the moisture content of air remains constant and its temperature increases as it flows over a heating coil. The heat addition rate during this process is given by:

$$Q_h=m_a\left(h_2-h_1\right)=m_a c_{p m}\left(T_2-T_1\right)$$

Ondecpmé o calor específico úmido (≈1,0216kJ / kgar seco) emumaé a vazão mássica de ar seco (kg / s)

## Resfriamento e Desumidificação

Quando o ar úmido é resfriado abaixo de seu ponto de orvalho, colocando-o em contato com uma superfície fria, parte do vapor d'água no ar se condensa e deixa o fluxo de ar como um líquido, como resultado, tanto a relação de temperatura quanto de umidade do ar diminui conforme mostrado. Este é o processo pelo qual o ar passa em um sistema de ar condicionado. O caminho real do processo depende do tipo de superfície fria, da temperatura da superfície e das condições de fluxo, mas para simplificar, a linha do processo é assumida como uma linha reta, como mostrado naFigura 8.11. As taxas de transferência de calor e massa podem ser expressas em termos das condições iniciais e finais, aplicando as equações de conservação de massa e conservação de energia conforme abaixo:

By applying mass balance for the water: $$m_a \cdot \omega_a=m_a \cdot \omega_2+m_w$$ By applying energy balance: $$m_a \cdot h_a=Q_r+m_w \cdot h_w+m_a \cdot h_2$$ From the above two equations, the load on the cooling coil, $Q_t$ is given by: $$Q_r=m_a\left(h_1-h_2\right)-m_a\left(\omega_1-\omega_2\right) h_w$$

O 2ndO termo no RHS da equação acima é normalmente pequeno em comparação com os outros termos, portanto pode ser desprezado. Por isso,

$$Q_r=m_a\left(h_1-h_2\right)$$

Pode-se observar que o processo de resfriamento e desumidificação envolve processos de transferência de calor latente e sensível, daí as taxas de transferência de calor total, latente e sensível (Qr,Qeu, eQs) pode ser escrito como:

$$\mathrm{Q}_{\mathrm{r}}=\mathrm{Q}_1+\mathrm{Q}_{\mathrm{s}}$$ where $$Q_1=m_a\left(h_1-h_w\right)=m_a \cdot h_{f g}\left(\omega_1-\omega_w\right)$$ and $$Q_s=m_a\left(h_w-h_2\right)=m_a \cdot c_{p m}\left(T_1-T_2\right)$$

## Fator de calor sensível (SHF)

É definido como a razão entre a taxa de transferência de calor sensível e total (Qt), ou seja,

$$\mathrm{SHF}=\mathrm{Q}_{\mathrm{s}} / \mathrm{Q}_{\mathrm{t}}=\mathrm{Q}_{\mathrm{s}} /\left(\mathrm{Q}_{\mathrm{s}}+\mathrm{Q}_{\mathrm{l}}\right)$$

A partir da equação acima, podemos observar que um SHF de 1,0 corresponde a nenhuma transferência de calor latente e um SHF de 0 corresponde a nenhuma transferência de calor sensível. Um SHF de 0,75 a 0,80 é bastante comum em sistemas de ar condicionado em clima seco normal. Um valor mais baixo de SHF, digamos 0,6, implica uma elevada carga de calor latente, como a que ocorre num clima húmido.

A temperatura, Ts, é a temperatura efetiva da superfície da serpentina de resfriamento e é conhecida como temperatura do ponto de orvalho do aparelho (ADP). Numa situação ideal, quando todo o ar entra em perfeito contato com a superfície da serpentina de resfriamento, a temperatura de saída do ar será igual à ADP da serpentina. Contudo, no caso real, a temperatura de saída do ar será sempre maior que a temperatura do ponto de orvalho do aparelho devido ao desenvolvimento da camada limite à medida que o ar flui sobre a superfície da serpentina de resfriamento e também devido à variação de temperatura ao longo das aletas, etc. , podemos definir umfator de desvio (BPF)pois pode ser facilmente observado que quanto maior o fator de by-pass, maior será a diferença entre a temperatura de saída do ar e a temperatura da serpentina de resfriamento. Quando o BPF é 1,0, todo o ar desvia da serpentina e não haverá resfriamento ou desumidificação.

$$\mathrm{BPF}=\frac{T_c-T_s}{T_a-T_s} ; \mathrm{CF}(\text { Contact Factor })=1-\mathrm{BPF}$$

OndeTctemperatura do ar que sai,Tumaé a temperatura do ar que entra eTsé a temperatura da superfície da serpentina de resfriamento.

## Aquecimento e Umidificação

Durante o inverno é essencial aquecer e umedecer o ar ambiente para maior conforto. Isto normalmente é feito primeiro aquecendo sensivelmente o ar e depois adicionando vapor de água à corrente de ar através de bicos de vapor.

O balanço de massa do vapor de água para o volume de controle produz a taxa na qual o vapor deve ser adicionado, ou seja,mW:

$$m_w=m_a\left(\omega_2-\omega_1\right)$$

onde estouumaé a taxa de fluxo de massa de ar seco. Do balanço energético:

$$Q_h=m_a\left(h_2-h_1\right)-m_w h_w$$

OndeQhé o calor fornecido através da serpentina de aquecimento ehWé a entalpia do vapor. Como este processo também envolve transferência simultânea de calor e massa, podemos definir um fator de calor sensível para o processo de forma semelhante a um processo de resfriamento e desumidificação.

## Resfriamento e umidificação

Como o nome indica, durante esse processo a temperatura do ar cai e sua umidade aumenta. Isto pode ser conseguido pulverizando água fria no fluxo de ar. A temperatura da água deve ser inferior à temperatura de bulbo seco do ar, mas superior à temperatura do ponto de orvalho para evitar condensação (TDPT<T2<T1)

## Aquecimento e Desumidificação

Este processo pode ser conseguido utilizando um material higroscópico, que absorve ou adsorve o vapor de água da umidade. Se este processo for isolado termicamente, então a entalpia do ar permanece constante, como resultado, a temperatura do ar aumenta à medida que seu teor de umidade diminui. Este material higroscópico pode ser sólido ou líquido. Em geral, a absorção de água pelo material higroscópico é uma reação exotérmica, como resultado desse processo é liberado calor, que é transferido para o ar e a entalpia do ar aumenta.

What is the difference between sensible heat and latent heat in psychrometric processes?
Sensible heat refers to the heat energy that is transferred between systems due to a temperature difference, resulting in a change in temperature. Latent heat, on the other hand, is the energy required to change the state of a substance (e.g., from liquid to vapor or vice versa) without a change in temperature. In psychrometric processes, both sensible and latent heat play important roles in determining the conditions of air and its interaction with the environment. Understanding the distinction between these two types of heat is crucial for designing and optimizing HVAC systems.
How does the sensible heat factor (SHF) affect the performance of an HVAC system?

The sensible heat factor (SHF) is a critical parameter in psychrometric processes that determines the proportion of total heat that is sensible heat. A higher SHF indicates that a larger portion of the total heat is sensible heat, which can affect the performance of an HVAC system. For example, a system with a high SHF may require more cooling capacity to maintain a comfortable indoor temperature, while a system with a low SHF may require more dehumidification capacity to control humidity levels. Accurate calculation of SHF is essential for designing and optimizing HVAC systems.

What is the significance of the psychrometric chart in understanding psychrometric processes?

The psychrometric chart is a graphical representation of the relationships between various psychrometric properties, such as temperature, humidity, and enthalpy. It provides a visual tool for understanding the complex interactions between these properties and enables engineers to analyze and design HVAC systems more effectively. By using the psychrometric chart, engineers can determine the state of air, calculate heat transfer rates, and optimize system performance. The chart is an essential tool in the field of HVAC engineering and is widely used in design, analysis, and optimization of HVAC systems.

How do psychrometric processes affect indoor air quality (IAQ) in buildings?

Psychrometric processes play a crucial role in maintaining good indoor air quality (IAQ) in buildings. The temperature, humidity, and air movement patterns in a building can significantly impact IAQ by influencing the growth and spread of pollutants, such as mold and bacteria. For example, high humidity levels can lead to mold growth, while poor air circulation can cause the buildup of pollutants. By controlling psychrometric processes, HVAC systems can help maintain good IAQ by providing a comfortable and healthy indoor environment.

What are some common applications of psychrometric processes in HVAC systems?

Psychrometric processes have numerous applications in HVAC systems, including air conditioning, heating, ventilation, and humidification. They are used in various industries, such as commercial buildings, hospitals, laboratories, and industrial processes. For example, psychrometric processes are used in air conditioning systems to cool and dehumidify air, while in heating systems, they are used to warm and humidify air. In ventilation systems, psychrometric processes are used to control the airflow and temperature of the air. Understanding psychrometric processes is essential for designing and optimizing HVAC systems for various applications.

How do psychrometric processes interact with other building systems, such as lighting and plumbing?

Psychrometric processes interact with other building systems, such as lighting and plumbing, in complex ways. For example, lighting systems can generate heat, which affects the psychrometric conditions of the air. Similarly, plumbing systems can introduce moisture into the air, which affects the humidity levels. Understanding these interactions is crucial for designing and optimizing building systems that work together efficiently and effectively. By considering the interactions between psychrometric processes and other building systems, engineers can create more sustainable, comfortable, and healthy indoor environments.

What are some common challenges and limitations of psychrometric processes in HVAC systems?

Psychrometric processes in HVAC systems can be challenging to design and optimize due to various factors, such as non-linear relationships between psychrometric properties, complex air flow patterns, and uncertainty in system parameters. Additionally, psychrometric processes can be limited by factors such as equipment capacity, energy efficiency, and maintenance requirements. Understanding these challenges and limitations is essential for designing and optimizing HVAC systems that meet the required performance, efficiency, and sustainability standards.

How can psychrometric processes be optimized for energy efficiency and sustainability in HVAC systems?

Psychrometric processes can be optimized for energy efficiency and sustainability in HVAC systems by using various strategies, such as optimizing system design, selecting energy-efficient equipment, and implementing control strategies that minimize energy consumption. Additionally, using renewable energy sources, such as solar power, and incorporating energy recovery systems can further improve the sustainability of HVAC systems. By optimizing psychrometric processes, engineers can create more energy-efficient and sustainable HVAC systems that reduce energy consumption and environmental impact.