8.3 Air-Conditioning Processes and Systems

Air conditioning is all about keeping you comfy. It's not just about cooling—it's a whole system that manages temperature, humidity, and air quality. Think of it as your indoor weather controller.

The process involves some cool science. Refrigerants, compressors, and heat exchangers work together to move heat from inside to outside. It's like a heat sponge, soaking up warmth and moisture to create your perfect indoor climate.

Air-conditioning system components

Main components and their functions

Top images from around the web for Main components and their functions

• 

  Heat pump and refrigeration cycle - Wikipedia View original

  Is this image relevant?

• 

  Direct Expansion Air Conditioning Systems – Basic HVAC View original

  Is this image relevant?

• 

  Air conditioning - Wikipedia View original

  Is this image relevant?

• 

  Heat pump and refrigeration cycle - Wikipedia View original

  Is this image relevant?

• 

  Direct Expansion Air Conditioning Systems – Basic HVAC View original

  Is this image relevant?

1 of 3

Top images from around the web for Main components and their functions

• 

  Heat pump and refrigeration cycle - Wikipedia View original

  Is this image relevant?

• 

  Direct Expansion Air Conditioning Systems – Basic HVAC View original

  Is this image relevant?

• 

  Air conditioning - Wikipedia View original

  Is this image relevant?

• 

  Heat pump and refrigeration cycle - Wikipedia View original

  Is this image relevant?

• 

  Direct Expansion Air Conditioning Systems – Basic HVAC View original

  Is this image relevant?

1 of 3

• Compressor compresses the refrigerant, increasing its pressure and temperature, allowing heat transfer in the condenser (R-410A, R-134a)
• Condenser releases heat from the high-pressure, high-temperature refrigerant to the surrounding environment (outdoor air, water), causing the refrigerant to condense into a liquid state
• Expansion valve (throttling device) reduces the pressure and temperature of the liquid refrigerant, enabling heat absorption in the evaporator (thermostatic expansion valve, capillary tube)
• Evaporator absorbs heat from the indoor air, cooling and dehumidifying it, as the low-pressure, low-temperature refrigerant evaporates back into a gaseous state (finned-tube heat exchanger)

Air distribution and refrigerant cycle

• Cooled and dehumidified air is distributed to the conditioned space through a system of ducts and fans (supply ducts, return ducts, air handling unit)
• Refrigerant, now in a gaseous state, returns to the compressor, and the cycle repeats, continuously transferring heat from the indoor space to the outdoor environment
• Additional components such as filters, dampers, and control systems ensure proper operation and maintain desired indoor conditions (thermostat, air filters, zone dampers)
• The efficiency and effectiveness of the air-conditioning system depend on the proper sizing, installation, and maintenance of all components working together seamlessly

Thermodynamic processes in air-conditioning

Vapor-compression refrigeration cycle

• Air-conditioning systems operate on the vapor-compression refrigeration cycle, consisting of four main thermodynamic processes: isentropic compression, isobaric heat rejection, isenthalpic expansion, and isobaric heat absorption
• Isentropic compression occurs in the compressor, where the refrigerant undergoes an adiabatic and reversible compression process, increasing pressure and temperature while maintaining constant entropy
• Isobaric heat rejection takes place in the condenser, where the high-pressure, high-temperature refrigerant releases heat to the surrounding environment at a constant pressure, changing phase from a superheated vapor to a saturated liquid
• Isenthalpic expansion happens in the expansion valve, where the pressure and temperature of the refrigerant decrease at constant enthalpy, in an adiabatic and irreversible process
• Isobaric heat absorption occurs in the evaporator, where the low-pressure, low-temperature refrigerant absorbs heat from the indoor air at a constant pressure, changing phase from a liquid-vapor mixture to a saturated or superheated vapor

Analyzing the air-conditioning cycle

• The performance of the air-conditioning cycle can be analyzed using pressure-enthalpy (P-h) and temperature-entropy (T-s) diagrams
• P-h diagrams show the relationship between pressure and enthalpy, allowing for the visualization of the refrigerant's thermodynamic states at different points in the cycle (saturated liquid line, saturated vapor line, constant entropy lines)
• T-s diagrams display the relationship between temperature and entropy, helping to identify the heat transfer processes and the work input required in the cycle (isentropic compression, isobaric heat rejection, isenthalpic expansion, isobaric heat absorption)
• By understanding the thermodynamic processes and utilizing these diagrams, engineers can optimize the design and operation of air-conditioning systems for improved efficiency and performance

Air-conditioning system performance

Performance and efficiency metrics

• The coefficient of performance (COP) is the ratio of the cooling capacity (heat removed from the conditioned space) to the work input (electrical energy consumed by the compressor), with a higher COP indicating better performance and efficiency
  • $COP = \frac{Cooling capacity}{Work input}$
• The energy efficiency ratio (EER) is similar to the COP but expressed in British thermal units (BTUs) per watt-hour, calculated by dividing the cooling capacity in BTUs per hour by the power input in watts
  • $EER = \frac{Cooling capacity (BTU/hr)}{Power input (W)}$
• The seasonal energy efficiency ratio (SEER) measures the average performance of an air-conditioning system over a cooling season, considering variations in outdoor temperature and the system's part-load performance
  • $SEER = \frac{Total cooling output during the season (BTU)}{Total electrical energy input during the season (Wh)}$

Factors affecting performance and efficiency

• Choice of refrigerant impacts the system's efficiency, environmental impact, and safety (R-22 phaseout, R-410A, R-32)
• Proper sizing and design of system components, such as the compressor and heat exchangers, ensure optimal performance and avoid issues like short-cycling or insufficient cooling
• The efficiency of the compressor (scroll, reciprocating, rotary) and heat exchangers (fin spacing, tube diameter) directly influence the overall system efficiency
• Control strategies, such as variable-speed compressors, multi-stage systems, and smart thermostats, help optimize performance and efficiency under varying load conditions
• Regular maintenance, including cleaning heat exchanger coils, replacing filters, and checking refrigerant charge, is crucial for maintaining the system's performance and efficiency over time

Principles of heating vs cooling

Sensible heat transfer

• Heating and cooling processes in air-conditioning systems involve the transfer of sensible heat, which changes the temperature of the air without affecting its moisture content
• Heating can be achieved through electric resistance heating, heat pumps, or other means, transferring heat to the air and increasing its temperature (electric furnace, gas furnace, heat pump)
• Cooling is accomplished by the evaporator, which absorbs heat from the air, reducing its temperature (direct expansion coil, chilled water coil)
• Sensible heat transfer is governed by the equation: $Q_s = m \cdot c_p \cdot \Delta T$, where $Q_s$ is the sensible heat transferred, $m$ is the mass of the air, $c_p$ is the specific heat of the air, and $\Delta T$ is the change in temperature

Latent heat transfer and humidity control

• Humidification and dehumidification processes involve the transfer of latent heat, which changes the moisture content of the air without affecting its temperature
• Humidification adds moisture to the air, increasing its humidity ratio, using methods such as steam humidifiers or evaporative humidifiers (isothermal humidification, adiabatic humidification)
• Dehumidification removes moisture from the air, decreasing its humidity ratio, by cooling the air below its dew point temperature in the evaporator, causing water vapor to condense on the evaporator surface
• Latent heat transfer is governed by the equation: $Q_l = m \cdot h_{fg} \cdot \Delta W$, where $Q_l$ is the latent heat transferred, $m$ is the mass of the air, $h_{fg}$ is the latent heat of vaporization, and $\Delta W$ is the change in humidity ratio
• The combination of sensible and latent heat transfer processes determines the overall comfort level in the conditioned space, with the ideal comfort zone typically defined by a range of temperature and relative humidity values (ASHRAE comfort zone)
• Psychrometric charts are used to analyze and visualize the properties of moist air, including temperature, humidity ratio, relative humidity, and enthalpy, aiding in the design and optimization of air-conditioning processes for desired comfort conditions (sensible heating, sensible cooling, humidification, dehumidification)