Air-conditioning Processes (Module 3)

Fundamentals of Air-Conditioning Processes

A thermodynamic system is described as undergoing a process whenever it transitions from one specific state-point to another. Within the context of psychrometric analysis, this change in state is typically induced by adding or removing heat, performing work on or by the fluid, adding or removing mass—most commonly in the form of water vapor—or mixing two fluids that exist at dissimilar states. At any given moment, the condition of the air can be represented as a single point on a psychrometric chart. When conditions change, this point moves across the graph in a direction dictated by which specific properties of the external air are being altered.

Most processes encountered in modern air conditioning practice are comprised of single actions, combinations, or sequences of specific fundamental processes. These include sensible cooling and heating, humidification, dehumidification, and the mixing of air streams. The behavior of air during these transitions is governed by the principles of mass and energy conservation, which allow for the precise calculation of heat exchange and moisture content changes.

Sensible Cooling and Heating Processes

Sensible cooling occurs when air is passed over a cooling coil, typically a finned-tube heat exchanger. In these systems, cool water or refrigerant flows through the internal tubes while external fins increase the surface area for heat transfer. If the air is cooled but remains at a temperature above its dew point ($T_{dewpoint}$), no condensation occurs. This specific process, where the temperature is reduced without changing the actual quantity of water in the air, is defined as sensible cooling. On a psychrometric chart, this is represented by a horizontal line moving to the left, parallel to the abscissa. Since the humidity ratio ($w$) remains constant, the relative humidity ($RH$) necessarily increases because the saturation point decreases with lower dry-bulb temperatures.

Sensible heating involves passing air over a heating coil or using electrical strip heaters. Similar to sensible cooling, the humidity ratio remains unchanged, and the process is represented by a horizontal line moving to the right on the psychrometric chart. As the dry-bulb temperature increases, the relative humidity decreases. Common applications for these processes include space heating loads, sensible cooling loads, the operation of fans that move air, and radiant heating or cooling devices. Because the saturation point of air is a function of dry-bulb temperature ($DBT$), the relative humidity (which is the absolute humidity divided by the saturation point) must change if the temperature changes while absolute humidity remains constant.

The governing equations for sensible heating and cooling are derived from mass and energy balances. The mass balance for dry air is expressed as $m_{a1} = m_{a2} = m_a$. The mass balance for moisture is $m_{a1}w_1 = m_{a2}w_2$. The energy balance depends on whether heat is added ($Q_{in}$) or removed ($Q_{out}$). For heating, the energy balance is $m_a h_1 + Q_{in} = m_a h_2$, which simplifies to $Q_{in} = m_a(h_2 - h_1)$. For cooling, the reverse applies: $Q_{out} = m_a(h_1 - h_2)$.

Humidification and Sequential Heating/Humidifying

Humidification is the process of introducing moisture into an airstream. In winter, this is frequently necessary because cold outdoor air that infiltrates or is brought in for ventilation becomes excessively dry when heated to internal comfort levels. In summer, humidification is often a component of evaporative cooling systems. Methods to achieve humidification include using spray washers, passing air over a pool of water, or the direct injection of steam.

Heating and humidifying is a simultaneous process that increases both the dry-bulb temperature and the humidity ratio of the air. This is represented on a psychrometric chart as a line sloping upward and to the right. The total heat gained ($Q$) can be divided into sensible and latent heat portions. A horizontal movement on the chart represents the sensible heat change (constant humidity ratio), while a vertical movement represents the latent heat change (constant dry-bulb temperature). This process is best analyzed as a sequence: first, the air undergoes sensible heating from state $1$ to state $0$ as it passes through a heat exchanger; second, it undergoes humidification from state $0$ to state $2$.

The governing equations for heating and humidifying require accounting for the added mass of water ($m_w$). The mass balance for dry air remains $m_{a1} = m_{a2} = m_a$. The moisture mass balance is $m_{a1}w_1 + m_w = m_{a2}w_2$. The energy balance is expressed as $m_{a1}h_1 + m_w h_w + Q_{in} = m_{a2}h_2$, where $h_w$ is the enthalpy of the added water or steam.

Cooling and Dehumidification

When air is cooled below its dew point temperature ($T_{dewpoint}$), condensation occurs, and moisture is removed from the airstream. The resulting air is at both a lower temperature and a lower humidity ratio than the incoming air. This process, involving both sensible and latent cooling, is represented on the psychrometric chart as a line sloping downward and to the left. The process assumes simple cooling followed by condensation, during which the air is assumed to remain saturated. This is a critical process for air-conditioning systems in hot, humid climates. It requires a cooling coil surface temperature lower than the dew-point of the incoming air, typically between $40^{\circ}F$ and $50^{\circ}F$ for standard indoor conditions.

The governing equations for cooling and dehumidification include the mass balance of dry air ($m_{a1} = m_{a2}$) and the mass balance of moisture ($m_a w_1 - m_a w_2 = m_w$, where $m_w$ is the rate of water removed). The energy balance is $m_a h_1 = m_a h_2 + m_w h_w + Q_{out}$. The refrigeration capacity of the coil is given by $Q_{out} = m_a h_1 - m_a h_2 - m_w h_w$.

The performance of the cooling coil can also be evaluated using the bypass factor ($b$), which is the ratio of the amount of air that does not contact the coil ($m_{ab}$) to the total amount of supply air ($m_a$). The total air mass is the sum of bypassed air and cooled air ($m_a = m_{ab} + m_{ac}$). The bypass factor can be calculated using temperatures, where $t_d$ is the apparatus dew point: $b = \frac{t_2 - t_d}{t_1 - t_d}$.

Heating and Dehumidification (Chemical Dehydration)

Heating and dehumidification, often called "chemical dehydration," is used extensively in industrial environments and modern air-conditioning for precise humidity control. This process involves bringing air into contact with a desiccant or sorption material. Water vapor molecules are captured through either absorption (where physical or chemical changes occur in the material) or adsorption (where no physical or chemical changes occur). As moisture is removed, the heat of condensation enters the air stream, causing the air temperature to rise. This process is essentially adiabatic or isenthalpic.

On a psychrometric chart, dehumidification by solid desiccants is shown as a line of increasing dry-bulb temperature and decreasing humidity ratio. Liquid desiccants follow a similar path, though if internal cooling is used, the process line can move from warm and moist to cool and dry. Typical applications include desiccant heat wheels used for dew point control or latent heat recovery in ventilation streams within warm, humid climates.

Evaporative Cooling and Adiabatic Saturation

Evaporative cooling is an adiabatic process, meaning there is no net heat loss or gain ($Q = 0$). When a hot air stream passes through a wet spray or a wetted medium (like a wick, matting, or plates), the sensible heat of the air is used to vaporize the water. This causes the air's dry-bulb temperature to fall while the humidity ratio rises. Because the sensible heat lost is converted into latent heat in the added vapor, the total enthalpy and the wet-bulb temperature remain constant.

The process moves upward along a line of constant enthalpy on the psychrometric chart. Hot, dry air has a significantly higher capacity for evaporative cooling than hot, humid air. For instance, hot dry air might experience a $24^{\circ}F$ temperature drop (e.g., from point $D$ to $E$), whereas hot humid air might only see a $12^{\circ}F$ decrease (e.g., from point $F$ to $G$). This hardware usually includes an adiabatic saturation chamber designed to maximize the interface surface area between the water and the air.

Mixing Process (Steady Flow)

In air handling systems, it is common to mix two streams of air with different properties and flow rates, such as combining outdoor ventilation air with return air from a room. In most cases, heat loss from the mixing chamber is negligible, making the process adiabatic. The resulting state of the mixture on a psychrometric chart must lie exactly on the straight line connecting the two initial state-points. The final position on that line is determined by the ratio of the mass flow rates of the incoming streams.

The governing equations for the mixing process are derived from steady-state mass and energy balances. The dry air mass balance is $m_{a3} = m_{a1} + m_{a2}$. The moisture balance is $m_{a3}w_3 = m_{a1}w_1 + m_{a2}w_2$. Assuming the process is adiabatic, the energy balance is $m_{a3}h_3 = m_{a1}h_1 + m_{a2}h_2$. By substituting necessary equations and making standard assumptions, the mixture's dry-bulb temperature ($DBT$) can be approximated as $m_{a3}t_3 = m_{a1}t_1 + m_{a2}t_2$. Examples of this include dual-stream systems (double duct or multi-zone) and supply air-recirculated air mixing in fan-powered terminals.