8.3 Air-Conditioning Processes and Systems

Air-conditioning system components

Air-conditioning processes control the thermodynamic state of moist air by manipulating its temperature, humidity, and enthalpy. In a Thermodynamics II context, the focus is on analyzing these processes using mass and energy balances on air-water vapor mixtures, psychrometric charts, and the vapor-compression refrigeration cycle.

Main components and their functions

Each component in an air-conditioning system corresponds to a specific thermodynamic process:

• Compressor: Compresses the refrigerant vapor, raising its pressure and temperature. This is modeled as isentropic compression in the ideal cycle. Common refrigerants include R-134a and R-410A.
• Condenser: A heat exchanger where the high-pressure, high-temperature refrigerant rejects heat to the surroundings (outdoor air or cooling water) at roughly constant pressure, condensing from superheated vapor to saturated liquid.
• Expansion valve (throttling device): Reduces the refrigerant's pressure and temperature through an irreversible, adiabatic process at constant enthalpy. Thermostatic expansion valves and capillary tubes are common types.
• Evaporator: A heat exchanger where the low-pressure refrigerant absorbs heat from the indoor air at roughly constant pressure, evaporating from a liquid-vapor mixture to a vapor. This is the component that actually cools (and often dehumidifies) the air stream.

Air distribution and refrigerant cycle

Cooled and dehumidified air is delivered to the conditioned space through supply ducts, and warm return air flows back to the air handling unit. The refrigerant, now a low-pressure vapor leaving the evaporator, returns to the compressor, and the cycle repeats.

Additional components like air filters, zone dampers, and thermostats maintain air quality and control. The overall system performance depends on proper sizing of every component and how well they work together.

Thermodynamic processes in air-conditioning

Vapor-compression refrigeration cycle

The standard air-conditioning system operates on the vapor-compression refrigeration cycle, which consists of four processes:

1. Isentropic compression (compressor): The refrigerant vapor is compressed adiabatically and reversibly, increasing its pressure and temperature while entropy remains constant. On a T-s diagram, this is a vertical line moving upward.
2. Isobaric heat rejection (condenser): At constant pressure, the high-temperature refrigerant rejects heat to the environment, transitioning from superheated vapor to saturated liquid.
3. Isenthalpic expansion (expansion valve): The liquid refrigerant passes through the throttling device. Pressure and temperature drop while enthalpy stays constant. This process is irreversible, so entropy increases.
4. Isobaric heat absorption (evaporator): At constant pressure, the cold refrigerant absorbs heat from the indoor air, evaporating from a two-phase mixture to a vapor.

Analyzing the air-conditioning cycle

Two diagrams are essential for cycle analysis:

• P-h (pressure-enthalpy) diagrams are the most practical for refrigeration cycle calculations. Each of the four processes appears as a distinct line segment, and you can read off enthalpy values directly to compute heat transfer rates and work input.
• T-s (temperature-entropy) diagrams help visualize where irreversibilities occur (the throttling process shows an entropy increase at constant enthalpy) and where heat is added or rejected.

To find the heat transfer or work for any component, apply a steady-state energy balance. For example, the compressor work per unit mass is $w_{comp} = h_2 - h_1$, and the cooling effect in the evaporator is $q_L = h_1 - h_4$, where the subscripts refer to state points around the cycle.

Air-conditioning system performance

Performance and efficiency metrics

Coefficient of Performance (COP) is the primary metric in thermodynamic analysis. It's the ratio of the desired output (cooling) to the required input (compressor work):

$COP = \frac{Q_L}{W_{net,in}} = \frac{h_1 - h_4}{h_2 - h_1}$

A higher COP means more cooling per unit of work. Typical values for residential systems range from about 2.5 to 5.

Energy Efficiency Ratio (EER) expresses the same idea in mixed units, useful for comparing commercial equipment:

$EER = \frac{\text{Cooling capacity (BTU/hr)}}{\text{Power input (W)}}$

The conversion between them is $EER \approx 3.412 \times COP$.

Seasonal Energy Efficiency Ratio (SEER) averages performance over an entire cooling season, accounting for varying outdoor temperatures and part-load operation:

$SEER = \frac{\text{Total cooling output during the season (BTU)}}{\text{Total electrical energy input during the season (Wh)}}$

Factors affecting performance and efficiency

• Refrigerant choice affects cycle pressures, COP, and environmental impact. R-22 has been phased out due to ozone depletion; R-410A and R-32 are current alternatives with different pressure-enthalpy characteristics.
• Component sizing: An oversized compressor causes short-cycling (frequent on/off), while an undersized one can't meet the cooling load. Both reduce efficiency.
• Compressor type (scroll, reciprocating, rotary) and heat exchanger design (fin spacing, tube diameter) directly influence isentropic efficiency and heat transfer effectiveness.
• Control strategies like variable-speed compressors adjust capacity to match the load, avoiding the efficiency losses of simple on/off cycling.
• Maintenance: Dirty coils increase thermal resistance, low refrigerant charge shifts operating pressures away from design conditions, and clogged filters restrict airflow. All degrade COP.

Principles of heating vs. cooling

Sensible heat transfer

Sensible heating and cooling change the dry-bulb temperature of the air without altering its moisture content (humidity ratio stays constant). On a psychrometric chart, these processes move horizontally along a constant humidity ratio line.

• Heating is achieved by electric resistance heaters, gas furnaces, or heat pumps transferring energy to the air stream.
• Cooling without dehumidification occurs when the evaporator surface temperature stays above the dew point of the air, so no condensation happens.

The sensible heat transfer rate for a steady-flow air stream is:

$\dot{Q}_s = \dot{m}_a \cdot c_p \cdot \Delta T$

where $\dot{m}_a$ is the mass flow rate of dry air, $c_p$ is the specific heat of air (approximately $1.005 \text{ kJ/kg·°C}$), and $\Delta T$ is the temperature change.

Latent heat transfer and humidity control

Latent heat processes change the humidity ratio (moisture content) of the air. On a psychrometric chart, pure latent processes move vertically at constant dry-bulb temperature.

• Dehumidification most commonly occurs when air passes over an evaporator coil whose surface temperature is below the air's dew point. Water vapor condenses out, reducing the humidity ratio. This is why air conditioners produce condensate.
• Humidification adds moisture using steam injection (isothermal, moves nearly horizontally on the psychrometric chart at higher humidity ratio) or evaporative methods (adiabatic, follows a constant wet-bulb temperature line).

The latent heat transfer rate is:

$\dot{Q}_l = \dot{m}_a \cdot h_{fg} \cdot \Delta \omega$

where $h_{fg}$ is the latent heat of vaporization of water (approximately $2501 \text{ kJ/kg}$ at 0°C) and $\Delta \omega$ is the change in humidity ratio (kg water/kg dry air).

Most real air-conditioning processes involve both sensible and latent heat transfer simultaneously. The sensible heat ratio (SHR) quantifies the fraction of total heat transfer that is sensible:

$SHR = \frac{\dot{Q}_s}{\dot{Q}_s + \dot{Q}_l}$

The SHR defines the slope of the process line on the psychrometric chart. The ASHRAE comfort zone typically specifies dry-bulb temperatures of 20–26°C and relative humidity of 30–60%. Psychrometric charts are the central tool for analyzing all of these moist-air processes, letting you read off temperature, humidity ratio, relative humidity, enthalpy, and specific volume at any state point.