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Refrigeration cycle, performance indicators (COP, EER, IPLV), refrigerants, VRF technologies, energy audit and decarbonization.
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Fundamentals of Air Conditioning
An air conditioning system is never a standard product
An air conditioning system is a set of components that cools, dehumidifies, filters and circulates air in a space to maintain occupant comfort during hot periods. Each installation is sized for a specific building and use. In France, the air conditioning load represents 30 to 60% of the electricity bill of a commercial building during the summer period.
1.1 Comfort air conditioning, process air conditioning
In institutional, commercial and residential buildings, air conditioning systems primarily serve the health and comfort of occupants. This is referred to as comfort air conditioning. In industrial buildings, they serve the manufacturing process as well as operator comfort: this is process air conditioning. Cold rooms, on the other hand, address a third need: the preservation of foodstuffs.
Under these uses, three main technological families coexist. The choice between them depends on cost, the availability of a waste heat source and the target temperature.
- The most widespread family (95% of installations)
- Compressor circulates the refrigerant between high and low pressure
- Technical foundation of air conditioners, chillers and heat pumps
- Typical COP: 2.5 to 6.2 depending on technology
- Compressor replaced by a generator and an absorber
- Relevant when a waste heat source is available
- Higher initial investment cost but can be amortized
- Uses pairs like ammonia-water or lithium bromide-water
- Reserved for niche applications
- Aeronautics and cryogenics (very low temperature refrigeration)
- Not common in standard building or industry
1.2 The seven basic air treatment processes
Regardless of the climate, any air conditioning system combines up to seven elementary operations. The local climate determines which ones are actually needed. In summer, the key processes are sensible cooling, dehumidification and air cleaning.
| Process | Function | Summer priority |
|---|---|---|
| Sensible cooling | Removes heat from the conditioned space | High |
| Dehumidification | Removes water vapor from the air | High |
| Sensible heating | Adds heat to the conditioned space | Low |
| Humidification | Adds water vapor to the air | Low |
| Air cleaning | Removes dust, particles and contaminants | High |
| Air renewal | Exchanges air between inside and outside | High |
| Air movement | Controls air circulation in the space | High |
1.3 Components of an air handling unit
Behind every breath of conditioned air, a chain of components works in sequence. Understanding this chain means knowing where to look in case of performance drift.
Types of Systems & Thermal Comfort
Choose the right architecture, before choosing the equipment
Depending on the size, construction and intended operating mode, an air conditioning system belongs to one of five main families. Choosing the wrong architecture costs more than a bad brand choice.
2.1 Five architectures, five logics
- Window units, split or package air conditioners
- Outdoor unit separate from indoor unit
- Direct expansion (DX) cooling
- Typical capacity: 2 to 15 kWf
- Ideal for small spaces and residential
- Exploits cooling through water evaporation
- No compressor, much lower consumption
- Only effective in dry climates (humidity < 60%)
- Energy savings: 50 to 80% compared to a conventional system
- Compressors operate during off-peak hours
- Chilled water (4-6 °C) or ice stored for peak periods
- Always centralized type
- Reduces installed cooling capacity by 30 to 50%
- Critical particle control (ISO 14644)
- Temperature, humidity, pressure, noise, vibrations
- Directly impacts product quality (pharma, microelectronics)
- ISO class 5 to 8 depending on needs
2.2 Thermal comfort: a balance, not a temperature
Thermal comfort results from a heat balance between a person and their environment. Many parameters influence the feeling of comfort: activity (metabolism), ambient temperature and humidity, air movement, clothing. Comfort can be achieved at air temperatures between 20 °C and 26.6 °C, and relative humidity between 20% and 70%.
In summer, humidity control is crucial: excessively high relative humidity (> 70%) prevents sweating and worsens the feeling of heat. Effective dehumidification is therefore essential for summer comfort.
| Psychrometric term | Definition | Typical summer value |
|---|---|---|
| Dry-bulb temperature | Measured by a standard thermometer | 22-26 °C |
| Wet-bulb temperature | Measured by a thermometer with a wet wick | 16-20 °C |
| Relative humidity | Ratio of actual water vapor / water vapor of saturated air | 40-60% RH |
The 6 chapters that turn reading into an action plan
Refrigeration cycle and performance indicators, quantified savings levers, refrigerants, VRF and chiller technologies, audit methodology and decarbonization.
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Refrigeration cycle & performance indicators
The vapor compression refrigeration cycle powers the majority of air conditioning equipment. Understanding it is the key to interpreting a data sheet, a supervision alert or a performance discrepancy in the field.
3.1 Subcooling and superheat
The fluid leaving the condenser is generally subcooled below the saturation temperature, which increases the refrigerating effect. At the compressor suction, the vapor is slightly superheated to ensure dry compression. These two settings are performance levers.
- Subcooling: the higher it is, the greater the refrigerating effect (typically 3 to 5 °C)
- Superheat: typically 5 to 10 °C, protects the compressor from liquid slugging
3.2 Performance indicators
| Indicator | Formula | Usage | Typical value |
|---|---|---|---|
| COP | Refrigerating effect (kW) ÷ input work (kW) | Manufacturer's reference conditions | 2.5 to 6.2 |
| EER | Refrigerating effect (BTU/h) ÷ electrical power (W) | Compressors, packages | 8 to 20 |
| IPLV | 0.01·A + 0.42·B + 0.45·C + 0.12·D | Weighted performance at 100/75/50/25% load | 3.5 to 8.5 |
| kW/ton | Input work (kW) ÷ refrigerating effect (ton) | Consumption per ton of refrigeration | 0.4 to 2.0 |
3.3 Three compressor families
- Capacity: 0.5 to 200 ton
- Low initial cost, frequent maintenance
- Ideal for small installations
- Capacity: 90 to 2,000 ton
- Very efficient at full load, compact
- Less efficient at partial load (except with inverter)
- Capacity: 20 to 1,000 ton
- Compact, lightweight, quiet
- Efficient at both partial and full load
Energy saving levers
An air conditioning system is custom-designed for a specific use: its audit is just as specific. The general methodology combines two complementary axes: reducing the load at source and optimizing operation.
4.1 CAV or VAV
A constant air volume (CAV) system always delivers the same air volume, regardless of the load. At partial load, it wastes energy by reheating the cooled air. A variable air volume (VAV) system delivers a variable air volume at constant temperature, significantly reducing waste. Converting a CAV fleet to VAV is a recognized energy saving measure, with typical gains of 20 to 40% on fan consumption.
4.2 Variable frequency drives (VFD)
📊 Example: 50 hp fan with VFD
A 50 hp fan operates 10 h/day, 250 days/year. At full speed, the annual cost is €5,559 (at €0.18/kWh). With a VFD and a realistic operating range (25% at 100%, 50% at 80%, 25% at 60%), the annual cost drops to €3,113.
4.3 Free-cooling
Free-cooling produces chilled water without running the compressor, by using cool outside air. In a data center in mainland France, free-cooling represents 50 to 75% of the annual time, i.e. 30 to 50% energy savings on cooling production.
- Direct free-cooling: dedicated heat exchanger, chiller off
- Indirect free-cooling: condenser cooling, chiller modulated
- Integrated free-cooling: additional module on air-cooled chiller
4.4 Replacing an existing chiller
📊 Replacement of an 800 kW chiller
An existing 800 kW chiller with an average seasonal COP of 3.5 is replaced by a chiller of the same capacity with an average seasonal COP of 4.5. Electricity cost: €0.18/kWh. Equivalent full-load operating hours: 1,000 h/year.
Before any replacement, five settings should be checked on the existing chiller:
- Set the chilled water to the highest possible temperature (increases COP)
- Lower the condensing water temperature (water-cooled condenser)
- Increase the heat exchange surface of the evaporator and condenser
- Enlarge refrigerant lines to reduce pressure drops
- Treat condensing water to prevent scaling
Refrigerants & environmental impact
The refrigerant absorbs and transmits heat in the vapor compression cycle. Its decisive properties are the pressure-temperature relationship, chemical stability, toxicity, ozone depletion potential (ODP) and global warming potential (GWP).
| Refrigerant | Formula | ODP | GWP | 2026 status |
|---|---|---|---|---|
| CFC-12 | CF₂Cl₂ | 1 | 10,600 | Banned |
| HFC-134a | CH₂FCF₃ | 0 | 1,300 | Phase-out in progress |
| HFC-152a | CH₃CHF₂ | 0 | 120 | Promising alternative |
| HC-290 (propane) | C₃H₈ | 0 | 3 | Strong growth (limited charge) |
| HC-600a (isobutane) | C₄H₁₀ | 0 | 3 | Domestic refrigerators |
| R454B | HFO/HFC | 0 | 466 | Main transition 2024-2030 |
| R1234ze | HFO | 0 | < 1 | Long-term reference > 200 kWf |
5.1 Vapor absorption
The vapor absorption system is appealing for its low operating cost and environmental friendliness, as soon as a waste heat source is available (industrial waste heat, cogeneration, solar thermal). The compressor is replaced by a generator (desorber) and an absorber. The most commonly used pairs are:
- Ammonia-water: for negative cooling, robust, but toxic
- Lithium bromide-water: for positive cooling, safer, but requires a high generation temperature (80-120 °C)
Advanced technologies: VRF, chillers & BMS
6.1 VRF: one outdoor unit, individualized needs
A variable refrigerant flow (VRF) system uses multiple evaporators of different capacities and configurations, enabling individualized comfort control, simultaneous heating and cooling according to zones, and heat recovery between zones. VRF saves 11 to 17% energy compared to conventional units.
6.2 Chillers (chilled water groups)
A chiller (chilled water group) is a thermodynamic machine that produces chilled water (5 to 18 °C) to supply cooling coils, radiant ceilings or process heat exchangers. It becomes essential when:
- The capacity exceeds 100 kWf
- Distribution distances are long (> 150 m)
- Operation must be centralized
- F-Gas requirements weigh on the DRV option
In 2026, the chiller market is marked by three converging transitions: refrigerant transition (low GWP), energy transition (partial load modulation) and digital transition (systematic IoT supervision).
6.3 BMS: the backbone of control
The building management system (BMS) monitors and regulates several systems at an optimal level: HVAC, electricity, security, maintenance. Protocols to impose in specifications:
- BACnet/IP: for HVAC equipment (chillers, controllers)
- Modbus RTU/TCP: for electricity meters and drives
- MQTT + TLS: for cloud data collection
- OPC UA: for integration with industrial systems
Audit methodology & decarbonization
7.1 Field audit checklist
- Reduce operation when space is unoccupied
- Turn off unoccupied areas (vestibules, meeting rooms)
- Adjust pre-cooling schedules
- Adjust thermostats according to season (summer: 24-26 °C)
- Calibrate them regularly
- Install an enthalpy-based economizer cycle
- Heat recovery wheel: 50 to 70% energy recovered
- Insulate ducts and chilled water pipes
- Avoid thermal bridges
- Clean coils
- Replace filters (every 3-6 months)
- Repair duct leaks
7.2 Decarbonization
Adopting decarbonization technologies can save 30 to 40% of a building's total energy. The priority levers for air conditioning are:
- Free-cooling: maximum use of cool outdoor air
- Natural refrigerants: R290 (propane), R744 (CO₂), R717 (ammonia)
- Heat recovery: desuperheater for DHW
- IoT supervision: continuous commissioning, drift detection
- Thermal storage: load shifting to off-peak hours
FAQ & glossary
FAQ
Glossary
- COP: Coefficient of Performance = Refrigerating effect ÷ compression work
- EER: Energy Efficiency Ratio = Refrigerating effect (BTU/h) ÷ electrical power (W)
- IPLV: Integrated Part Load Value: Weighted performance at 100/75/50/25% load
- ODP: Ozone Depletion Potential
- GWP: Global Warming Potential
- CAV: Constant Air Volume
- VAV: Variable Air Volume
- VRF: Variable Refrigerant Flow
- BMS: Building Management System
- Free-cooling: Cooling production without a compressor using outside air
- Subcooling: Liquid cooled below its saturation temperature
- Superheat: Vapor heated above its saturation temperature
- Dry bulb / wet bulb: Temperatures measured with and without a wet wick
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Refrigeration cycle, performance indicators, quantified savings levers: this guide gives you the method. Our teams can support you in implementing it on your installation.
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